Abstracts
International Electrokinetics Conference 2004
Pittsburgh, Pennsylvania

MIRJAM LEUNISSEN¹, Paddy Royall¹, Anand Yethiraj², Alfons van Blaaderen¹;  ¹ Utrecht University, Debye Institute, Soft Condensed Matter, Princetonplein 1, 3584 CC Utrecht, The Netherlands; ² University of British Columbia, Chemistry Dept., 2036 Main Mall, Vancouver, V6T1Z1, Canada

We study monodisperse colloidal dispersions as model system for condensed matter physics, because their phase behaviour is analogous to molecular systems. Until now, no quantitative 3D real space studies of systems with long-ranged repulsions exist, as it is impossible to achieve double layers of more than 300 nanometers in aqueous dispersions, which are studied most. Recently, we developed a new model system consisting of charge- and sterically stabilized PMMA-latex in apolar (dielectric constant ~5-8) organic solvents, which are index and density matched. In this system the Debye screening length can be as large as several micrometers. The particle interactions are tunable from very soft to hard sphere-like by addition of salt. Moreover, we can independently add a dipolar contribution by applying an electric field, ranging from a small perturbation to the point where it completely determines the phase behaviour (the dispersion behaves as a electrorheological fluid). These two independent tuning parameters give rise to a rich phase behaviour, which is accessible in real space by means of confocal microscopy on fluorescently labeled particles. By extracting particle coordinates in 3D we obtain the radial distribution function which is uniquely determined by the interactions. Fitting it with Monte Carlo simulation we estimate the colloidal charge and Debye length, which are consistent with electrophoresis and conductivity measurements. Besides the normal phase behaviour we also observe 'anomalous' behaviour, such as the 'coexistence' of high- and low-density crystals, 'superheated' crystals, void formation and relatively extended zones at the container walls that are depleted of particles. This is strongly reminescent of previous observations for deionized aqueous dispersions, but can now be studied in a different system without ion exchange resin.

BORIS KHUSID¹, Andreas Acrivos²; ¹ New Jersey Institute of Technology, University Heights, Newark, NJ 07974;

² City College of New York, 140th St., New York, NY 10031

We consider field- and shear-driven phenomena in a suspension flowing through microfluidics whose origin is dielectrophoresis accompanied by the field-induced phase separation. As a result, a suspension undergoes a field-driven phase separation leading to the formation of a distinct boundary between regions enriched with and depleted of particles. The theoretical predictions are consistent with experimental data even though the model contains no fitting parameters. It is demonstrated that the field- induced dielectrophoresis accompanied by the phase separation provides a new method for concentrating particles in focused regions and for separating biological and non-biological materials, a critical step in the development of miniaturizing biological assays.

W. D. Ristenpart, I. A. Aksay, D. A. SAVILLE; Department of Chemical Engineering, Princeton University, princeton NJ 08544

Nonhomogeneous electric fields in the polarization layer near an electrode arise from the presence of colloidal particles. These fields generate electrohydrodynamic flows that carry particles toward one another and, under appropriate conditions, 2-dimensional arrays form. Here we present experimental results and theoretical interpretations applicable to assembly in ac fields. Tracking the behavior of large numbers (~1000) of monodisperse particles using video microscopy provides the rate of disappearance of singlets. Scale analysis of one and two-particle configurations how the coagulation rate scales on the applied field and its frequency, in agreement with experiments over a range of conditions.  Experiments with suspension of two sorts of particles display other interesting characteristics, including the formation of square and hexagonal superlattices.

MARTIJN BROUWER, Stefan Kooij, Herbert Wormeester, Bene Poelsema; Solid State Physics, MESA+ Research Institute, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

The deposition kinetics of the irreversible adsorption of citrate-stabilized, nanocolloidal gold particles on Si/SiO2 surfaces, derivatized with (aminopropyl)triethoxysilane, is investigated in situ using single wavelength reflectometry. A well-defined flow of colloids toward the surface is realized using a radial impinging jet cell geometry. The saturation coverage after prolonged deposition can be analyzed in terms of random sequential adsorption (RSA) and depends on the ionic strength of the solution, in good agreement with DLVO theory. Atomic force microscopy measurements indicate that for higher coverages, the formation of particle clusters gives rise to a deviation from DLVO behavior. The dynamics of the deposition process is at first mass transport limited. Surface blocking effects determine the adsorption kinetics in the final stage. The entire deposition process can be adequately described in terms of a generalized adsorption theory, which combines the effects of mass transport and the actual adsorption onto the surface.

Currently we are investigating particle deposition under the influence of external electric fields. Both the deposition kinetics and the spatial distribution are expected to be affected by an externally applied potential, allowing us to tune the ordering within the nanocolloidal monolayers.

J.C. BAYGENTS; Department of Chemical & Environmental Engineering, The University of Arizona, Tucson, AZ 85721 USA

In the proposed presentation, it will be demonstrated that oscillatory electric fields induce steady flows in the neighborhood of colloidal particles.  These flows vary in magnitude with the square of the imposed field strength and die off as (a/r)², where r is the distance measured from the center of a particle of characteristic size a.  Understanding the nature of these steady flows is important for at least two reasons.  First, they can be shown to contribute to the dielectrophoretic translations of colloidal particles.  Second, they contribute to the electrically directed assembly of particles near electrode surfaces.  This latter point is illustrated as follows.  Electric dipole-dipole interactions drive a relative motion between neighboring particles that varies as (a/d)⁴, where d is the inter-particle separation.  The hydrodynamic interactions, due to steady electrokinetic motion, vary as (a/d)².  Electrokinetic flows can thus contribute significantly to the relative motion between neighboring particles.

The theory to be presented is for a spherical colloidal particle, immersed in an unbounded ionic solution.  Solutions for the electric potential, ion concentrations and the fluid velocity are obtained from the standard electrokinetic model, albeit the analysis is extended to account for effects that are nonlinear in the electric field strength.

Jeffrey A. Fagan,  DENNIS C. PRIEVE, Paul J. Sides; Dept. of Chemical Engr., Carnegie Mellon University, Pittsburgh, PA 15217

In an effort to understand the mechanism of 2-D aggregation of colloidal particles previously observed on a/c electrodes, we have used Total Internal Reflection Microscopy (TIRM) to monitor the elevation of a single 6 mm polystyrene latex particle very near the horizontal electrode undergoing oscillation in voltage at frequencies up to 10 kHz.  TIRM can detect changes in elevation of the particle as small as 1 nm. 

Application of the alternating electric field causes the the average height to decrease in KOH but to increase in NaHCO3.  In HNO3, the average height is decreased for frequencies below about 200 Hz and is increased for larger frequencies.  These trends observed with single particles correlate strongly with trends observed by others with multiple particles moving together or apart on the surface of the electrode.  This correlation is expected if the same electroosmotic flow is causing both aggregation of multiple particles and normal motion of single particles. 

If the particle is at its terminal electrophoretic velocity at each instant, oscillations in the elevation of the particle far from the electrode surface are expected to be exactly 90 degrees out of phase with oscillations in the current density (or electric field).  Observations on particles near the electrode reveal a phase difference of less than 90 degrees with the measured current in NaHCO3 and HNO3 but more than 90 degrees in KOH.

Armand AJDARI; Laboratoire de Physico-Chimie Theorique, ESPCI-CNRS Paris

Many groups have evidenced and discussed in the last years the possibility of inducing the reversible aggregation of colloidal particles in the vicinity of a conducting surface using an AC field. I will first present experimental results where we have varied the size and surface charge of the particles and the field strength and frequency.

I will then propose a survey of the possible mechanisms responsible for this field-induced attraction. Based on our experimental study, we concluded that AC electro-osmosis on the electrode surface is the best candidate. We have tried to check our understanding of the underlying phenomena by conducting an experimental and theoretical study of the flow in a simple model geometry, where the colloid is replaced by an insulating stripe. A discussion and an outline of the (numerous) questions that remain open will be attempted.

If time permits I will then point out possible uses of this mechanism of flow generation for a few microfluidics applications.

RICHARD W O'BRIEN, William N Rowlands; Colloidal Dynamics Pty Ltd, Unit 145, The National Innovation Centre, Australian Technology Park, Eveleigh NSW 1430, Australia

This talk is focussed on aqueous suspensions of particles with nanometer-sized pores. Such suspensions are frequently encountered in the catalyst industry. These systems exhibit very interesting electroacoustic effects because the internal flow in the pores can have a strong influence on the particle motion. As a result, the electroacoustic spectra of such suspensions are very different to those of solid particles. In this talk I will present a theory and measurements of electroacoustic effects for microporous particles. The aim is to find out what such measurements tell us about the state of charge inside the pores.

CONSTANTINO GROSSE¹, Jose Juan Lopez Garcia², Jose Horno²; ¹ Departamento de Fisica, Universidad Nacional de Tucuman, Av. Independencia 1800, (4000) San Miguel de Tucuman, Argentina, and Consejo Nacional de Investigaciones Cientificas y Tecnicas, Argentina; ² Departamento de Fisica, Universidad de Jaen, Facultad de Ciencias Experimentales, Campus de las Lagunillas, Ed. B-3, 23071, Jaen, Spain

The standard electrokinetic equations for a spherical uncharged insulating particle suspended in a binary electrolyte solution with an applied AC electric field are analytically solved in the general case when both the ion diffusion coefficients and valences have arbitrary values. It is shown that under these conditions, the field-induced ion density profiles extend at low frequencies at far larger distances from the particle than in the case when both diffusion coefficients have the same value. The corresponding induced charge density modifies the dipolar coefficient leading to an additional low-frequency dielectric dispersion.

A.V. DELGADO¹, F. Carrique², F.J. Arroyo¹, S. Ahualli¹; ¹ Department of Applied Physics, Faculty of Science, University of Granada, 18071 Granada, Spain; ² Department of Applied Physics, Campus Teatinos, Faculty of Science, University of Málaga, 29071 Málaga, Spain

Among the different electrokinetic techniques, both low-frequency dielectric relaxation and electroacoustics are gainig interest because of their applicability to concentrated colloidal suspensions. This is so partly because they do not require the observation or tracking of individual particles by optical methods. Nevertheless, the problem of electrokinetic phenomena in concentrated systems is far from being fully solved. Cell models are often used to account the hydrodynamic and electrostatic interactions between particles in an approximate and simple fashion. We have previously applied this kind of model to the evaluation of the electric permittivity of suspensions of spheres with volume fractions anywhere between dilute systems and 50 percent. In the present work, we describe our calculations of the dynamic electrophoretic mobility for the same systems and compare them with the predictions of O'Brien's calculations, that take into account particle-particle interactions in an entirely different way. Additionally, this presentation includes experimental data on the dynamic mobility (obtained by means of an Electrokinetic Sonic Amplitude, ESA, device) of alumina suspensions with different volume fractions, pH, and ionic strengths. A systematic comparison is carried out between these results and the two model predictions. Finally, the suitability of the cell calculations is also checked against dielectric dispersion measurements in the same set of suspensions.

S. DURAND-VIDAL¹, P. Turq¹, J. B. Rosenholm²; ¹ LI2C-UPMC, Paris, France; ² Dpt of Physical Chemistry Abo Akademi University, Turku, Finland

Conductivity and acoustophoresis are two experimental methods that can be used to determine the effective charge of nanocolloids. An exceptionally large colloidal solid volume fraction can be used because these two techniques are not based on light detection. In the case of conductivity of spherical colloids, the signal   is approximately proportional to  iciZi2/Ri, where ci, Zi and Ri are respectively the concentration, the algebraic charge and the radius of the species i. Concerning acoustophoresis, the UVP (Ultrasonic Vibration Potential) is proportional to  iciZiRi2. Usually, for colloidal suspensions, the signal from simple ions is negligible in acoustophoresis whereas the signal from colloids is negligible in conductivity measurements.

These approximations are not valid for nanosuspensions (Ri < 10 nm) where each ionic species must be taken into account individually. In order to consider the contribution of all the species, we describe these two electrokinetic phenomena combining Onsager's continuity equation with MSA (Mean Spherical Approximation) equilibrium functions using a Green's function formalism. Nanocolloids are treated as big ions like in our previous description of the conductivity of micellar systems. We applied this approach to experimental results for: i- simple ions, ii- spherical nanomagnetic colloids (Ri = 5 nm) and iii-  plate like colloids (Laponite).

J.J. LOPEZ-GARCIA¹, C. Grosse², J. Horno¹;  ¹ Departamento de Física, Facultad de Ciencias Experimentales, Universidad de Jaén, Spain; ² Departamento de Física, Universidad Nacional de Tucumán, San Miguel de Tucumán, Argentina and Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina

It is well known that due to the presence of the particles, the complex dielectric permittivity of a suspension differs from that of the supporting electrolyte solution, this fact being often used to characterize colloidal suspensions. Classically, the dielectric properties of the system are deduced from the induced dipole coefficient, which is obtained, either analytically or numerically, assuming that the system is electroneutral outside the electrical double layer.

 In most of the literature, it is accepted that the thickness of the electric double layer is of the order of a few Debye lengths. Therefore, further away from the particle surface, the electroneutrality condition is fulfilled and the dipole coefficient can be obtained and used to calculate the dielectric permittivity increment.

 In this work we show that this is indeed the case for DC and AC electric fields when the diffusion coefficients of the two ionic species are identical. However, the situation drastically changes when different diffusion coefficients are considered since then, electroneutrality is only attained at much larger distances that further increase when the frequency is lowered. These changes should be taken into account in order to avoid substantial errors in the evaluation of the dielectric permittivity increment.

TOMMASO BELLINI¹, Francesco Mantegazza²; ¹ University of Milano, Dpt. of Chemistry, Biochemistry and Biotecnology, via Cervi 93, 20090 Segrate (MI), Italy; ² University of Milano-Bicocca, Dpt. of Experimental Medicine, Via Cadore 48, 20052 Monza (MI), Italy

We have measured the birefringence induced by alternate fields in mixed colloidal dispersions of rods and spheres in which the signal is almost entirely due to the orientational distribution of the rod-like particles.  We find that, whenever an insulated charged rod particle is surrounded by a mildly concentrated population of (typically smaller) charged spheres and a low frequency field is applied, the electric torque favors alignment perpendicular to the applied field. This behavior, still unreported in the literature, is anomalous since it contrasts the universally observed field induced parallel alignment of diluted charged rods. We find that this anomalous orientation is universal, since it does not depend on the properties of the rods - we worked with bioparticles, latex rods and inorganic crystals - nor on those of the spheres, provided their charge is large enough. The dependence of the frequency range of the anomaly on the size and concentration of the species reveals that the anomaly is due to the onset of a local fluctuation in the concentration of spherical particles asymmetrically distributed around the rod. It is not clear how this local clustering, expected by the current electrokinetic models if the spherical particles are considered as multivalent co-ions, could in turn produce an anomalous torque.

Thibault Roques-Carmes, Robert A. Hayes, B.J. Feenstra, LUC J.M. SCHLANGEN; Philips Research Laboratories Eindhoven, Prof Holstlaan 4 (WA11), 5656 AA Eindhoven, The Netherlands

The electrowetting effect allows control over the two-dimensional movement of a water interface across hydrophobic insulator coated electrode surfaces. Consider an insulator coated electrode, covered with a thin colored oil film that is laterally confined by polymer walls, immersed in water. The application of a voltage between the water and the electrode moves the water into contact with the insulator thus reducing the insulator area that is covered with oil. The resulting electrowetting behavior and micro fluidic motion is investigated as a function of parameters like addressing voltage, oil film thickness, oil type and confinement dimensions.

A model describing the two-dimensional movement of a confined oil/water interface has been developed. The model is based exclusively on physical parameters such as the thickness of the insulating coating, the oil film thickness, the confinement shape and size and the oil/water interfacial tension. The model accounts for the experimentally observed threshold voltage and accurately describes the electro-optic behavior of an electrowetting based, switchable optical element. Such an element can be used as the pixel engine in a bright full color reflective display with video speed.

A model describing the two-dimensional movement of a confined oil/water interface has been developed. The model is based exclusively on physical parameters such as the thickness of the insulating coating, the oil film thickness, the confinement shape and size and the oil/water interfacial tension. The model accounts for the experimentally observed threshold voltage and accurately describes the electro-optic behavior of an electrowetting based, switchable optical element. Such an element can be used as the pixel engine in a bright full color reflective display with video speed.

HIROYUKI OHSHIMA; Faculty of Pharmaceutical Sciences, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278-8510, Japan

The electrophoretic mobility of a spherical charged colloidal particle in an electrolyte solution with thin double layers tends to a nonzero constant value in the limit of high zeta potentials. It is demonstrated that this is caused by the fact that counterions condensed near the highly charged particle surface do not contribute to the electrophoretic mobility and coions govern the mobility. The present theory is also applied to cylindrical particles, showing that the leading term of the limiting electrophoretic mobility of a cylindrical particle in a transverse field with thin double layers is the same as that of a spherical particle. The electrophoretic mobility of a cylindrical particle in a tangential field, on the other hand, is proportional to the particle zeta potential and dose not exhibit a constant limiting value for high zeta potentials.

R QIAO¹, N. Aluru²; ¹ Beckman Institute and Dept. of Mechanical Engineering, Univ. of Illinois, RM 3213 Beckman Institute, 405 N. Mathews Ave, Urbana, IL, 61801; ² Beckman Institute and Dept. of Mechanical Engineering, Univ. of Illinois, RM 3265 Beckman Institute, 405 N. Mathews Ave, Urbana, IL, 61801

Electroosmotic (EO) transport is an important fluid transport  mechanism in micro and nanofluidic systems. However, EO transport  in nanometer scale channels is not fully understood yet. The major  challenge is that as the critical dimension of the channel is  comparable to the size of the fluid molecules and ionic species,  and the classical continuum theories may no longer be valid. Instead, the molecular nature of fluid atoms and ions must be accounted for in the modeling of EO transport.

The EO transport in slit nanochannels of various widths was studied by using molecular dynamics simulation. Simulation  results indicated that the Poisson-Boltzmann equation can not predict the ion distribution near the channel wall accurately as it fails to account for the non-electrostatic interactions between ion-surface and ion-water. Simulation results also indicated that the Navier-Stokes (NS) equation can be used to analyze the flow in channel as narrow as 2.2 nm provided the viscosity variation in the channel is accounted for. However, the NS equation breaksdown completely when the channel is 0.95 nm wide, i.e., approximately 4 water molecule diameter wide. Analysis shows that these observations are mainly caused by the finite size of ion and water molecules.

Uwe Freudenberg¹, Ralf Zimmermann¹, Kati Schmidt², Sven-Holger Behrens²,

Helmut Auweter², Wolfgang Pompe³, CARSTEN WERNER^1,4; ¹ Department Biocompatible Materials, Institute of Polymer Research Dresden and The Max Bergmann Center of Biomaterials Dresden, Hohe Strasse 6, 01069 Dresden, Germany; ² BASF Aktiengesellschaft, Polymer Physics, Ludwigshafen, Germany; ³ Technische Universität Dresden, Department of Materials Science and The Max Bergmann Center of Biomaterials Dresden, Dresden, Germany; ⁴ Department of Mechanical and Industrial Engineering, University of Toronto, Toronto ON, Canada

Electrosurface characteristics of biopolymer-based materials influence the performance of a variety of demanding products. To clarify the charging of different types of collagen model layers were reconstituted from dissolved collagen I in vitro and covalently attached to glass carriers coated with poly(octadecene-alt-maleic acid anhydride) films. The collagen films were analyzed by streaming potential/ streaming current experiments to determine zeta potential and surface conductivity in aqueous solutions of varied pH and KCl concentration. The results indicate that tropocollagen layers and immobilized collagen fibrils formed out of the very same tropocollagen exhibit distinct differences in the acid-base properties. While the tropocollagen layers showed only a minor shift of the isoelectric point from pH 5.5 to 5 the isoelectric point of the collagen fibrils was found to be shifted from pH 7.5 to 5.1 upon increase of the background electrolyte concentration from 10^-4 M to 10^-2 M KCl. The very explicit and reversible ionic-strength dependent switching of the acid-base behavior of the collagen fibrils reminds of earlier reports on the salt-induced structural changes of collagen materials (Steinberg et al. Nature 1966 210 568) and provokes further analysis by means of complementary methods.

HAO ZHOU¹, Lee R. White¹, Robert D. Tilton^1,2; Departments of ¹Chemical Engineering and ² Biomedical Engineering, Center for Complex Fluids Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213

With the ability to differentially manipulate different types of particles based on their dielectric properties, ac electric field patterning is envisioned as a promising strategy to produce patterned arrangements of cells and/or biofunctionalized colloids for biosensing or for microfluidic bioreactor and cell separation technologies. We describe the patterning behaviors of micron sized colloidal particles in nonuniform ac electric fields.  Carboxylated polystyrene (PS) colloidal particles, yeast cells and sheep red blood cells (RBC) were studied as model systems of non-biological particles, fungi and mammalian suspension cells, respectively. Particles and/or cells were place in a chamber surrounded on top by a uniform electrode and beneath by a 16-strip patterned electrode. The electric field frequency range was controlled in the region between 5 Hz and 20 MHz.  PS particles and yeast cells, by themselves, were successfully patterned into straight lines with a preferred interparticle separation distance that was determined by the dimension of bottom electrode strips.  The patterning of the sheep RBC on the bottom electrode, however, could only be observed at 1kHz.  Based on these observations, defined micron-scale lateral separation of PS/yeast and RBC/yeast mixtures was realized by tuning the electric field frequency.  Both vertical and lateral separations of RBC and yeast cells were obtained by tuning the electric field strength and frequency.  The patterning mechanism involved in this nonuniform field system was considered to be a combination of dielectrophoresis and AC electroosmosis.  The differentiation in the patterning behavior observed for these three model colloidal systems indicates the potential for developing microfluidic separation technologies and for developing complex ordered structures.

N.A.MISHCHUK, L.L.Lysenko; Institute of Colloid Chemistry and Chemistry of Water of National Academy of Sciences of Ukraine, Vernadskogo avenue, 42, Kyiv-142, Ukraine, 03142

Electroosmosis plays an important role in various technologic processes, including desalting of water, drying of different materials, consolidation of soils and their purification from heavy metals and radionuclides. However, very often the velocity of electroosmosis is insufficient and needs special conditions for its intensification. Such possibility could be created by electroosmosis of the second kind caused by an induced space charge near a spherical or cylindrical surface of ion-exchange materials. Its velocity could be 10-100 times larger than the velocity of classical electroosmosis. It is also possible to create similar electroosmotic flow near the surfaces with other geometry, for example, at combination of spherical ion-exchange granules and a porous diaphragm.

Theoretical model of concentration polarization of such system supposes the appearance of an induced space charge not only near a surface of granules, but also inside of cylindrical pores of a diaphragm. The analytical expressions for extent and density of induced charge and the velocity of electroosmotic flow through a diaphragm, caused by this charge, is obtained. The theoretical model is compared with the results of the experimental study of electroosmotic flow through the uncharged diaphragm and through the same diaphragm with an adjacent layer of cation exchange resin.

Emilij K. Zholkovskij¹, Jacob H. Masliyah¹, Vladimir N. Shilov², Subir Bhattacharjee³; ¹ University of Alberta, Department of Chemical and Materials Engineering, 536 Chemical- Mineral Engineering Building, Edmonton, Alberta, Canada T6G 2G6; ² Institute of Bio-Colloid Chemistry of Ukrainian Academy of Sciences, Vernadskogo,42, 03142, Kiev, Ukraine; ³ University of Alberta, Department of Mechanical Engineering, 4-8C Mechanical Engineering Building, Edmonton, Alberta, Canada T6G 2G6

The presentation is concerned with comparative analysis of the Levine-Neale and Shilov-Zharkikh boundary conditions that are set at the cell border while describing electrokinetic phenomena using the spherical cell model approach. A derivation is proposed to interrelate the electric field strength, which is applied to a disperse system, and the potential distribution within the cell. The proposed derivation leads to the Shilov-Zharkikh boundary condition. Accordingly, a conclusion is made that, using the spherical cell model approach, the correct analysis should be based on the Shilov-Zharkikh boundary condition. It is discussed why, for some particular cases, the Shilov-Zharkikh and Levine-Neale boundary conditions, which contradict each other, lead to common results.

Manuel Medrano, ALBERTO T PEREZ, Carlos Soria; Departamento de Electrónica y Electromagnetismo Facultad de Física. Universidad de Sevilla Avda. Reina Mercedes s/n 41012 Sevilla. Spain

We have measured the conductivity of suspesions of TiO2 in a hydrocarbon mixture (Isopar L, supplied by Exxon) up to 20

 volume concentration. We used two techniques: a commercial conductivity meter (by Irlab) and an alternative system built in our laboratory. We compare both series of measurements and discuss the contribution of particles to the suspension conductivity.

BRUNO F. MARQUES, James W. Schneider; Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890

Liposomes filled with fluorescent material can be useful signal-amplifying tags in biosensing applications. Here, capillary zone electrophoresis (CZE) is used to monitor the mobility of DNA-hybridizing liposomes after interaction with different DNA oligomers. We are able to detect binding by measuring mobility shifts in the order of 10^-5 cm² V^-1 s^-1, which other techniques, such as Laser Doppler Velocimetry, are unable to achieve because of polydispersity in the liposomal population. These liposomes contain a small amount of peptide nucleic acid (PNA), a synthetic DNA mimic with superior DNA binding abilities. Under these conditions, liposomes bind approximately 0.83 DNA strand/PNA strand. When a single base mismatch is introduced in the DNA sequence, the binding efficiency drops four-fold. We have also been able to bind DNA strands containing overhanging bases, but have found that the binding efficiency is considerably lower (0.33 DNA/PNA) and apparently depends on the position of the extra bases. An explanation for the latter may lie in the formation of PNA clusters on liposomal surfaces, which we are currently investigating with images of deposited lipid monolayers under an epifluorescence microscope.

O.SHRAMKO, V. Shilov; Institute of Biocolloidal Chemistry, NAS of Ukraine, 42, Vernadskogo Blvd., 03142 Kiev, Ukraine

In classic theory of dielectrophoresis  the velocity of particle is expressed in terms of force which acts on the particle's induced dipole moment in applied non-homogeneous field E. However for the case of colloid particle in electrolyte solution one should take into account two additional sufficient circumstances:

1. Induced dipole moment is formed with the contribution of ionic space charge distributed in surrounding liquid rather then at the particle surface and hence, the force applied to induced dipole moment is not a force applied to the particle.

2. The interaction between ionic space charge which is non-linear (quadratic) with respect to E and the charge of equilibrium double layer contributes to the dielectrophoresis.

We took into account both of these factors in the new theory available for arbitrary thickness of double electric layer. The space induced charge distribution was found with taking into account of essential influence of ionic convective flow.

Hsien Chen Ma, HUAN J. KEH; Department of Chemical Engineering, National Taiwan University,

Taipei 106-17, Taiwan, ROC

The steady diffusioosmotic flow of an electrolyte solution in a fine capillary slit with each of its inside walls covered by a layer of adsorbed polyelectrolytes is analytically studied.  In this solvent-permeable and ion-penetrable surface charge layer, idealized polyelectrolyte segments are assumed to distribute at a uniform density.  The electric double layer and the surface charge layer may have arbitrary thicknesses relative to the gap width between the slit walls.  The Debye-Huckel approximation is used to obtain the electrostatic potential distribution on a cross section of the slit.  The macroscopic electric field induced by the imposed electrolyte concentration gradient through the slit is determined as a function of the lateral position rather than taken as its constant bulk-phase quantity.  Explicit formulas for the fluid velocity profile are derived as the solution of a modified Navier-Stokes/Brinkman equation.  The effect of the lateral distribution of the induced axial electric field in the slit on the diffusioosmotic flow is found to be of dominant significance in most practical situations.

V. I. KOVALCHUK¹, E. K. Zholkovskiy¹, M. P. Bondarenko¹, D. Vollhardt²; ¹ Institute of Biocolloid Chemistry, Vernadski str., 42, 03142, Kiev, Ukraine; ² Max-Planck-Institute of Colloids and Interfaces, 14424 Potsdam/Golm, Germany

The present paper predicts a new electrokinetic effect: during the Langmuir-Blodgett deposition of a monolayer of dissociating surfactant molecules, the electric field and ionic concentration profiles occur around the three-phase contact line. Mechanism of the effect is associated with a non-zero magnitude of the convective electric current which, in the case of charged monolayer, is transferred toward the three-phase contact line. To provide zero value of the total electric current, the electric field is formed around the three-phase contact line. As well, to provide steady state continuity of individual ionic fluxes, the concentration profiles are formed in the system. These processes affect the morphology, composition and structure of the deposited monolayer.

JUNHYUNG KIM¹, John L. Anderson¹, Steve Garoff², Luc Schlangen³; ¹ Carnegie Mellon University, Dept. of Chemical Engineering 5000 Forbes Ave, Pittsburgh, PA 15213; ² Carnegie Mellon University, Dept. of Physics 5000 Forbes Ave, Pittsburgh, PA 15213; ³ Philips Research the Netherlands Prof. Holstlaan 4 (WA 11) 5656 AA Eindhoven, the Netherlands

Electrophoretic movement of particles can be used to create thin-film optical filters that provide gray-scale adjustment at the pixel level. The goal is to reversibly control particle distributions over the appropriate length scales at 100 msec time scales. The particles are carbon black with a mean diameter of about one micron. Dodecane containing a charge transfer agent (PIBS) is used to provide a low-conductivity medium. Here we report experimental results for the electrophoretic mobility of carbon black, and video images of particle motion in a thin liquid film between two glass slides with parallel strip electrodes on one slide. The electrophoretic mobility data are interpreted with the O'Brien-White model, and the jectories of the particles in the liquid film are compared with predicted motion based on the measured electrophoretic mobility. The issues are 1) are the measured electrophoretic mobilities in a weakly conducting (nonaqueous) liquid consistent with the classical theory for electrophoresis? and 2) do the particles move in the thin liquid film by simple electrophoresis - or are other phenomena such as electroosmosis and dielectrophoresis important transport mechanisms.

RALF ZIMMERMANN¹, Willem Norde², Martien A. Cohen Stuart², Carsten Werner^1,3;

¹ Department Biocompatible Materials, Institute of Polymer Research Dresden & The Max Bergmann Center of Biomaterials Dresden, Hohe Strasse 6, 01069 Dresden, Germany; ² Department of Physical Chemistry and Colloid Science, Wageningen University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands; ³ Department of Mechanical and Industrial Engineering, University of Toronto, 5 King's College Road, Toronto, Ontario, Canada, M5S 3G8

Two hydrophilic, brush-like surfaces were prepared from poly(styrene)-poly(acrylic acid)- and poly(styrene)-poly(ethylene oxide)- diblock copolymers on top of poly(styrene) coated planar glass carriers by means of the Langmuir Blodgett technique and subsequently analyzed with respect to electrical charging in aqueous electrolyte solutions of varied pH and KCl concentration using streaming potential/ streaming current measurements for the combined determination of zeta potential and surface conductivity. Charge formation at the poly(acrylic acid) brushes was confirmed to be caused by the dissociation of carboxylic acid groups giving zeta potential values of about -60 mV at full dissociation (10^-3 M KCl solutions). However, surface conductivity data revealed that this corresponds to 0.5

 of the mobile ions at the interface only which indicates that the surface charge is almost completely compensated within the hydrodynamically immobile layer. For the poly(ethylene oxide) brushes no charge formation was found to occur at the grafted polymers which, in contrary, effectively screened the charge of the underlying substrate. Accordingly, very low surface conductivity data were detected in this system. The hydrodynamic layer thickness of the poly(ethylene oxide) brushes was estimated from the dependence of the degree of screening on the ionic strength of the electrolyte solution according to Cohen Stuart & Mulder (Colloids Surf. 1985, 15, 49).

MUSABEKOV K.B., Omarova K.I.; Al-Farabi Kazakh National University

Using elektrokinetic method it is possible to value indirectly the absorption of the synthetic polyelectrolytes (SPE) on the surface of the solid different nature. In the process of adsorption on the hydrophobic surface prevail hydrophobic interaction contacting phase. At the same time absorption polyelectrolyte determine the properties of the hydrophobic surface, this means that PE forms it's own double electrical layer (DEL).

The elektrokinetic properties of teflon were systematic studies in the solution of polyacids- polyacrylic (PAA), polymetacrylic (PMAA) and polybasis-polyethylene imine (PEI), poly-2-methyl-5-vinilpyredine (PMVP). The concentration dependence ζ -potential of teflon in SPE solutions is a curve with a maximum at 10-2  The sing of ζ -potential is similar to the sing charge of adsorption macroion. The maximum of ζ -potential curve correspond to concentration of conformational transformations (CCT) of macromolecules.

The adsorption polyelectrolytic complex of ionic SAS, signed opposite SPE on the surface of teflon makes the inversion of the charge sing of elektrokinetic potential of teflon, accordingly to the charge of the sing ζ  -potential of the complex with increasing of relative concentration SAS. The complex SPE-ionic SAS formation by electrostatic interaction of components, which lead to neutralization of the SPE charge and with the increase of SAS concentration in the mixture supperequivalent (hydrophobic) binding of SAS and change sing charge of polycomplex.

The dependence of ζ  -c in mixture of SPE and nonionic SAS correspond to the form dependence of electroforetical moving of complexes SPE-nonionic SAS from potential. The maximum points of ζ  -n and Uef-n are the same. In the region of high n the curve ζ  -n approaches the plato similar to the dependence ζ  -c for nonionic polymer -SPE complex-nonionic SAS asquires the properties of nonionic polymer. The thickness (h) of adsorption layer complex PAA (PMAA)-OP-7 were counted using the value of ζ  -potential. The values h=(450-500) 10-10 m are similar to molecular constants of polyacids.

MATTHEW A. PRESTON¹, Ralph Kornbrekke², Lee R. White¹; ¹ Center for Complex Fluids Engineering, Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh PA 15213; ² Lubrizol Corporation, 29400 Lakeland Boulevard, Wickliffe OH 44092

Recent technological advances such as electroacoustic devices and advanced electronic imaging processes have highlighted a need for a detailed understanding of high-frequency colloidal electrokinetics.  In this study, we formulate a numerical procedure appropriate for the solution of the standard equations of electrokinetics that is stable and accurate over a frequency range from 1 Hz to over 100 GHz.

Charges may be stabilized in low dielectric media through the formation of inverse micelles, which can have a radius 100 times greater than a simple-salt ion found in aqueous media.  The size of this charge-carrier results in a diffusion length that is much less than the double-layer thickness in the megahertz frequency range.  Similarly, the ion diffusivity length in aqueous electrolyte systems can also be much less than the double-layer thickness, at frequencies greater than 10 MHz.

Previous numerical solutions have rested on an implicit assumption that the relevant length scales were all larger than the double-layer thickness.  We have relaxed this assumption so that a numerical description of the electrokinetics may now be applied to a greater variety of non-aqueous as well as aqueous colloidal systems and a wider frequency range than was previously available.

MARVI A. MATOS¹, Robert D. Tilton², Lee R. White²; ¹ Carnegie Mellon University, 700 Technology Drive, Pittsbugrh Technology Center – 4216, Pittsburgh, PA 15219; ² Carnegie Mellon University, Doherty Hall - Chemical Engineering, 5000 Forbes Ave, Pittsburgh, PA 15213

Crosslinked gels are frequently employed as biomolecule immobilization media and/or as anti-fouling barriers in biosensors and microdiagnostic devices. Although the polymer gel network allows for the protection and stabilization of the biomolecules that provide the sensing element of the biosensor, the gel creates a stagnant barrier that hinders mass transport and limits the response dynamics of the sensor.  We are investigating novel internal pumping strategies based on electroosmotically driven convection as a way to accelerate mass transfer of analytes in polyacrylamide gels. The gels are doped with charged silica colloids that drive local electroosmotic flow in response to externally applied electric fields.  Because of their size (7 to 25 nm), the particles are immobilized in the gel.  We use fluorescence spectroscopy to measure the mass transport of an intrinsically fluorescent dye, amino-methylcoumarin.  The use of an uncharged (at pH 8) dye allows separation of electrophoretic and electroosmotic effects.  We have studied the effects of silica particle size and concentration in the gel. For a fixed volume fraction of particles in the gel, we observe a greater mass transport enhancement for smaller silica particles.  We attribute this to the higher number fraction of the smaller particles that serve as point sources impulses for the fluid flow. By examining dye diffusion in the absence of an applied electric field, we have verified that the silica particles do not significantly perturb the structure of the gel. Thus, we conclude that the mass transport enhancement is due to electroosmotic flow, rather than on particle induced changes in the gel microstructure. Currently we are systematically investigating buffer, electric field, and pore size effects on the mass transport of dye through gels with stationary charged particles.   

A.G.ZHYGOTSKY¹, L.I.Tertykh¹, G.I.Chalyuk¹, Ye.F.Rynda¹, V.P.Klimenko²; ¹ Institute of Colloid and Water Chemistry, Ukrainian National Academy of Sciences, 03680, Kiev, Ukraine; ² Institute of Material Science, Ukrainian National Academy of Sciences,03180, Kiev, Ukraine

The electrodeposition process of heterodispersions on the base of micropowders ZrO2 and BaTiO3 and anionactive filmforming polybutadiene (PBO) and uralkyd (UAO) oligomers depending on their correlation, methods of their compatibility and parameters of deposition has been investigated. The  particle sizes of ZrO2 and BaTiO3 are changed from 100 to 300 nm and micelle sizes of UAO and PBO are changed from 50 to 200 nm. The electrodeposition of the investigated systems is realized at electric field strength E of 3-10 kV/m and deposition time of 0,5-2,0 min and temperature of (20&#61617;1) ^oC.

It was found the dependence of mass, thickness of deposits and content of oxides in them from nature and dispersibility of oxide particles and filmforming substances, composition of dispersed systems and conditions of their obtaining and electrodeposition.

The correlative dependency of oxide powder content in an electrophoretic deposit from its quantity in colloid system is established.

The obtained results are explained on the base of the conception of adsorption mechanism of formation of electrical charge on the oxide particles, their stabilization and transportation to an electrode, heterocoagulation processes near the electrode area, electrode reactions, endoosmosis condensation of deposits and following formation of dense electrophoretic deposits and composite coatings.

RALF ZIMMERMANN¹, Oliver Birkert², Günter Gauglitz², Carsten Werner^1,3; ¹ Department Biocompatible Materials, Institute of Polymer Research Dresden and The Max Bergmann Center of Biomaterials Dresden, Hohe Strasse 6, 01069 Dresden, Germany; ² Institute of Physical and Theoretical Chemistry, University of Tübingen, Auf der Morgenstelle 8, 72076 Tübingen, Germany; ³ Department of Mechanical and Industrial Engineering, University of Toronto, 5 King's College Road, M5S 3G8 Toronto, Canada

Thin polymer films (aminodextran, carboxylated dextran, diamino (poly)ethylene glycol, dicarboxy (poly)ethylene glycol and biomolecular ad-layers (biotinylated diamino (poly)ethylene glycol and streptavidin on biotinylated diamino (poly)ethylene glycol) as utilized to constitute the sensitive elements of biosensors were characterized by the determination of the zeta potential z and the surface conductivity K^s

Zeta potential vs. pH data (in 10^-3 M KCl solutions) indicate that the charge formation was controlled by hydroxide and hydronium ions. The isoelectric points of the polymer layers were attributed to the chemical constitution of the polymers, i.e. to the presence of an excess of acidic or basic functionalities.

Surface conductivities of the polymer layers in 10^-3 M potassium chloride solutions at pH 6 were determined in the range between 6.1 nS (carboxylated dextran) and 69.7 nS (streptavidin on biotinylated diamino (poly)ethylene glycol) and compared with the conductivities of the diffuse part of the double layer. Differences between both values indicate that in all cases only 6 or less of the mobile ions at the interfaces were located in the hydrodynamically mobile part of the electrical double layer. The magnitude of K^s was found to correlate with the z-extension of polymer film and their ionization, respectively.

MONICA TIRADO¹, Constantino Grosse^1,2; ¹ Departamento de Física, Universidad Nacional de Tucumán, Av. Independencia 1800, (4000) San Miguel de Tucumán, Argentina; ² Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina.

Dielectric spectra of monodispersed polystyrene particle suspensions were measured at 25ºC over a frequency range extended from 100 Hz to 10 MHz, using a HP 4192 A Impedance Analyzer. In view of the large particle diameter (1 micrometer) and the low electrolyte conductivities used (order of 0.005 S/m) both, the Low- and the High-Frequency dispersions were well-separated and fit inside the frequency range used.

The instrument was coupled to a variable spacing cell with parallel platinum black electrodes calibrated by using the quadrupole method. The measurement current was kept constant at all spacings. These measurements were complemented with determinations of the electrophoretic mobility by using a Rank Brothers Microelectrophoresis Apparatus MK II with a cylindrical cell.

The aqueous electrolyte solutions were prepared using equal molar concentrations of NaCl, KCl, NaAc, and KAc, keeping constant the Debye screening length and the Zeta potential while the conductivities changed. The polystyrene particles used (IDC, surfactant-free white sulfate latex) have a surface charge density that is essentially independent of the pH.

The dielectric spectra obtained were analyzed and compared with the existing theoretical models for the counterion polarization and for the Maxwell-Wagner-O'Konski dispersion processes, and with numerical calculations.

TOSHIHISA OSAKI¹, Manuela Markowski¹, Carsten Werner^1,2; ¹ Department Biocompatible Materials, Institute of Polymer Research Dresden & The Max Bergmann Center of Biomaterials Dresden, Hohe Str. 6, 01069 Dresden, Germany; ² Department of Mechanical and Industrial Engineering, University of Toronto, 5 King's College Road, Toronto, Ontario, Canada, M5S 3G8.

Fibronectin (FN) layers adsorbed on top of thin films of two different maleic acid copolymers (poly(octadecene-alt-maleic acid): POMA and poly(propylene-alt-maleic acid): PPMA) were investigated by means of the Microslit Electrokinetic Set-up (MES) combined with in-situ Reflectometric Interference Spectroscopy (RIfS) to reveal the impact of substrate properties for the characteristics of the adsorbed protein layers. Compared to the hydrophobic POMA surface, the trends of zeta potential and optical layer thickness at similar FN solution concentrations showed lower adsorbed amounts and more desorption on the hydrophilic PPMA. The surface conductivity after adsorption of FN on PPMA was found to decrease, indicating that the adsorbed FN compresses the extended polymer chains while a slight increase of the surface conductivity was observed after FN adsorption on POMA. The in-situ detection of the optical layer thickness performed in combination with the electrokinetic experiments confirmed the decrease of the layer thickness during the FN adsorption on PPMA. The isoelectric points after adsorption of saturating amounts of FN and rinsing with phosphate buffered saline (desorption) also showed distinct pH values for each surface: While on FN-coated POMA the IEP was rather close to the 'intrinsic' IEP of FN the IEP of PPMA after FN coverage remained significantly more acidic.

Martin Z. BAZANT¹, Todd M. SQUIRES²; ¹ Department of Mathematics and Institute for Soldier Nanotechnolgies, Massachusetts Institute of Technology, Cambridge, MA 02139; ² Departments of Applied and Computational Mathematics and Physics, California Institute of Technology, Pasadena, CA 91125

We give a general, physical description of ``induced-charge electro-osmosis'' (ICEO), the nonlinear electrokinetic slip at a polarizable surface, in the context of some new techniques for microfluidic pumping and mixing. ICEO generalizes ``AC electro-osmosis'' at micro-electrode arrays to various dielectric and conducting structures in weak DC or AC electric fields. The basic effect produces micro-vortices to enhance mixing in microfluidic devices, while various broken symmetries --- controlled potential, irregular shape, non-uniform surface properties, and field gradients --- can be exploited to produce streaming flows. Although we emphasize the qualitative picture of ICEO, we also describe the mathematical theory (for thin double layers and weak fields), apply it to various model problems, and discuss more general  nonlinear electrokinetic phenomena which may occur at larger voltages.

L.L.Lysenko, N. A.MISHCHUK; Institute of Colloid Chemistry and Chemistry of Water, Vernadskogo av., 42, Kyiv-142, Ukraine, 03142

The developed method of soil recovering by application of an electric field is based on a mass transfer of polluting substances caused by their electromigration and electroosmotic transportation with subsequent extraction from a cathode chamber. The efficiency of heavy metals removal directly depends on their mobility in soil; therefore they should be desorbed from complexes with soil components and transferred into water-soluble form. Removed heavy metals can be deposited as hydroxides in cathode chamber. This process is caused by a generation of hydroxyl ions on a cathode and, correspondingly, by high pH of water in the cathode chamber and in the soil adjacent to it.

The received experimental data concerning the purification of a sod-podzol soil, polluted with nickel and cadmium compounds, have shown that the high degree of remediation is reached in electrodialyzer at wetting of the treated soil by sulfuric acid solutions.

Special scheme of electroosmosis intensification, developed during our investigations, accelerates the process of soil purification. As a consequence of intensification of water flush of soil, at the same energy consumption and time of soil treatment the degree of soil decontamination increased twice as much.

N.A MISHCHUK, N.O.Barinova; Institute of Colloid Chemistry and Chemistry of Water of National Academy of Sciences of Ukraine Vernadskogo avenue, 42, Kyiv-142, Ukraine, 03142

The electroosmosis of the second kind was predicted theoretically more than ten years ago, however till now it is explored experimentally only near the surface of single granule of cation- and anion-exchange materials /1/. Analysis of this phenomenon in more composite systems therefore is of interest.

The investigations of electroosmotic movement of liquid in the space between two and four granules and between the rows of many granules were conducted by visualization of electroosmotic flows with the use of submicron and micron particles of low electrophoretic mobility. The carried out investigations have shown not only quantitative, but also qualitative influence of granules position on the direction and the velocity of electrosmotic flow. The dependence of velocity on an electric field strength and time is investigated. The role of the type of ion-exchanger, concentration and type of electrolyte, distance between granules and their arrangement are analyzed.

The obtained values are compared with the velocities of classical electroosmosis under identical conditions.

1. N.A.Mishchuk, S.S.Dukhin. Electrokinetic phenomena of the second kind. In Interfacial Electrokinetics and Electrophoresis; Delgado A. Ed.; Marcel Dekker, 2001, p.241-275

JEFFREY A. FAGAN,  Dennis C. Prieve, Paul J. Sides;  Dept. of Chemical Engr., Carnegie Mellon University, Pittsburgh, PA 15217

The nature of the forces that act on colloidal particles near an electrode with the application of alternating electric fields is of substantial interest to microfluidic applications.  We have measured distinct vertical forces on single colloids due to the application of ac electric fields for frequencies from 10-10000 Hz.  Experimental data demonstrates that the directions of these forces are electrolyte dependent over the entire frequency range, and implies that different forces are induced on a single particle over different ranges of the frequency spectrum.

Yandong Hu^1,2, Dongqing Li¹, RALF ZIMMERMANN², Carsten Werner^1,2; ¹ Department of Mechanical and Industrial Engineering, University of Toronto, 5 King's College Road, M5S 3G8 Toronto, Canada; ² Department Biocompatible Materials, Institute of Polymer Research Dresden and The Max Bergmann Center of Biomaterials Dresden, Hohe Strasse 6, 01069 Dresden, Germany

To clarify the relevance of surface microroughness for electrokinetics in microchannels we considered model surfaces with well-defined rough elements, i.e. rectangular grooves in different dimensions on one of the channel walls perpendicular to the flow direction.

A numerical model for electrokinetic flow through rough microchannels has been developed based on the theories for smooth channels. Simulations to analyze the potential in the electrical double layer and the net charge movement were conducted for different rough channel geometry parameters.

The experimental verification of the model predictions was based on microchannels formed between smooth (reference) and microrough walls fabricated out of silicone oxide surfaces. Flow resistance and streaming potential/streaming current measurements were performed across the smooth and the partially roughened microchannels.

The results showed that for the selected examples of rough surfaces a dramatic increase of the flow resistance and a significant reduction of the streaming potential/streaming current -under a given pressure drop- occurs in comparison to the smooth channels.

The experimental data (volume flow rate, streaming potential/streaming current) and the results of the simulation (pressure field, local current density in flow direction and local streaming potential per unit length) are interpreted to provide a quantitative explanation for the limits of applicability of the electrokinetic theories developed for smooth channels.

MYUNG-SUK CHUN, Tae Seok Lee, Nak Won Choi; Complex Fluids Research Lab, Korea Institute of Science and Technology (KIST), PO Box 131, Cheongryang, Seoul 130-650, South KOREA

Understanding the electrokinetic microflow is a key aspect in the research of microfluidic chip technology, which is rapidly growing.  With the fused silica rectangular microcapillary of 52 micrometers width, a hydrophobic channel surface is obtained by coating with a monolayer of octadecyltrichlorosilane [Tretheway and Meinhart, Phys. Fluids, 2002].  From the measurements of average fluid velocity and apparent velocity slip, we found a higher apparent viscosity would be predicted if the slip is neglected.  We examined the effects of the electric double layer and the zeta potential of channel surface upon the velocity as well as the electroviscous behavior.  To verify experimental results, we present a finite difference solution for electrokinetic flow with Navier's slip condition via the under-relaxation technique.  The externally applied body force originated from the electrostatic interaction between the nonlinear Poisson-Boltzmann field and the flow-induced electric field is employed in the equation of motion.  It is evident that liquid slip counteracts the effect by the electric double layer and induces a larger flow rate.  The velocity profile is clearly affected by the electric double layer for the cases of low ionic concentrations, where the average fluid velocity decreases as the solution ionic concentration decreases.

Viviana Zimmerman¹, CONSTANTINO GROSSE^1,2, Vladimir N. Shilov³; ¹ Departamento de Física, Universidad Nacional de Tucumán, Av. Independencia 1800, (4000) S.M. de Tucumán, Argentina; ² Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina; ³ Institute of Biocolloid Chemistry,

Ukrainian Academy of Sciences, Kiev, Ukraine.

The electric potential, ion number concentration, and fluid velocity distributions induced by an AC electric field applied to a biological cell suspended in an electrolyte solution are numerically calculated, using a network simulation method. The cell model consists of a conducting internal medium surrounded by an insulating membrane and an homogeneously charged ion-permeable cell wall. The distributions obtained are used to calculate the induced dipole coefficient, the dielectric spectra of a cell suspension, and the electrorotation velocity of the particles under the action of a rotating field.  Particularizing the results to simpler systems (ion exchange resin and soft particles) leads to good agreement with existing theories. The dependence of the low-frequency dielectric spectra on the internal medium properties, and the dependence of the rotation spectra on some of the parameters are analyzed.

MIGUEL MAYORGA, D. Osorio-González; Fac. Ciencias, Univ. Aut. Edo. Mex., Av. Instituto Literario 100, Toluca, Mexico, CP 50000

In this work we show the implications of changing the softness of the repulsive and the attractive part in the phase transitions of sterically stabilized colloids. We identify structural precursors of freezing in the pair correlation function using molecular dynamics simulations. Even if such precursors are similar to the one observed in the hard spheres systems,  we emphasize that the structural features can emerge from soft interactions too. The softness of the interaction can be tuned varying the solvent salt concentration and with an external electric field. We discuss the implications of such tuning in the phase transition in colloids.

C.Abbruzzese¹, B. Kornilovich², N. MISHCHUK², R.Klischenko², G.Pshynko; ¹ Institute of Environmental Geology and Geoengineering, Roma, Italy; ² Institute of Colloid Chemistry and Chemistry of Water, Vernadskogo av., 42, Kyiv-142, Ukraine, 03142

The phenomena of soil remediation by electric field include many factors: soil chemistry, ion exchange, polarization of soil particles and electrodes in electric field, electric-driven transport of contaminations etc. that have to be taken into account and optimized. Dependently on properties of soil one of two main physical processes prevails: electromigration for charged contaminations and electroosmosis for uncharged or weakly dissociated ones. Both processes strongly depend on structure of soil and polarization processes that limited application of large voltage.

Since soils have macroinhomogeneous structure and different chemical composition, polarization processes, electromigration and electroosmotic flows are inhomogeneous and change in time. Therefore the idea of application of unipolar pulse voltage was developed. The use of pulse regime of strictly determined parameters allows to affect the change of pH, decrease polarization phenomena and the average resistance of soil, and, consequently, to increase productive usage of applied voltage.  According to the obtained experimental data for electroremediation of model clay soils (kaolin, montmorillonite and its mixture with sand) from radionuclides (137Cs, 90Sr, U) and heavy metals (Co, Pb) the pulse regime decreases power imputes by at least 25-40.

Lee R.White¹, Mathew A.Preston¹, ROBERT O. JACK², Fraser McNeil-Watson², Carlos A.Rega²; ¹ Department of Chemical Engineering, Carnegie Mellon University,  Pittsburgh PA 15213-3890 , USA; ² Malvern Instruments Ltd, , Grovewood Road, Malvern WR141XZ, UK

The theory developed by O'Brien and White[1] to calculate the electrophoretic mobility of a solid spherical colloidal particle - the so called "standard electrokinetic model" - is now over a quarter of a century old. The model evaluates the electrokinetic potential at the plane of shear and neglects ionic transport processes within the Stern layer.

Whilst the model has had success, a number of authors[2] have discussed extensions to account for discrepancies between its predictions and experiment. Applying the techniques used by O'Brien and  White, White  and Mangelsdorf[3]  gave a detailed rigorous mathematic treatment to develop an extended standard model incorporating a general dynamic Stern layer model, as applied to electrophoresis.

In this paper we use the data of Borkovec , Behrens and Semmler[4] who although finding acceptable agreement between experiment and the standard model for larger latex particles, reported less satisfactory agreement for smaller particles. We investigate whether the use of a dynamic Stern layer as detailed by the theory of  White and Mangelsdorf can account for these discrepancies.  In particular we see if the reported surface charge densities of the samples obtained by conductometric titration can be reconciled with those predicted from electrophoretic mobility using this extended standard model.

[1] R.W.O'Brien and L.R.White, J.Chem.Soc.Faraday Trans. 2, 1978, 77, 1607

[2] C.F.Zukoski and D.A.Saville, J.Colloid Interface Sci 1985, 107, 322

[3] C.S.Mangelsdorf and L.R.White, J.Chem.Soc.Faraday Trans. 86, 2859

[4] M.Borkovec, S.H.Behrens and M.Semmler, Langmuir 2000, 16, 5209

VASILY N. MORARU; F. D. Ovcharenko Institute of Biocolloid Chemistry, National Academy of Sciences of Ukraine, 42 Vernadsky blvd., Kyiv 03142, Ukraine

The organoclay particles manifest very interesting and complicated electrokinetic behavior in organic liquids of different polarity. This behavior affects very strongly the stability and structure formation of corresponding dispersions. However, the interpretation of electrokinetic data and mechanism of EDL formation in organic media remains unclear. In this work the electrokinetic properties of alkylammonium montmorillonites (AM) in benzene and its derivatives (Cl-, CN-, NO₂, alkyl-substituted benzenes) were studied by means of the microelectrophoresis technique. It was found that :

1. The highly positive values of zeta potential of the AM in benzene and alkyl-substituted benzenes decrease and change sign in chlorobenzene, then the negative values of zeta increase for benzonitrile and nitrobenzene.

2. A increase in the surface modification degree higher cec of clay is accompanied by a pronounced increase of the zeta potential, which raises the dispersion stability but counteracts gel formation.

3. Zeta potential and structure formation of AM in hydrocarbons strongly depends on the moisture content (W). Zeta passes through minimum and dispersion viscosity reaches the maximum value at W * 2-3 mass as a result of superposition the potential of dipole jump of strongly adsorbed water molecules on the DEL of AM particles.

4. Addition of small amounts (0.5-5.0  of the polar additives not only increases the viscosity and yield stress values of AM-dispersions, but also changes the sign and value of the zeta potential and the enthalpy of wetting. Recharging of the AM surface in benzene occurs at decreasing concentration with increasing electron donor ability (or proton affinity).

The mechanisms of recharge of the AM particles in the mixtures of hydrocarbon with polar additives have been discussed.

Paddy Royall, Mirjam Leunissen, Alfons van Blaaderen;

Utrecht University, Debye Institute, Soft Condensed Matter, Princetonplein 1, 3584 CC Utrecht, The Netherlands

Charged colloids have received much attention, due to their technological importance and fundamental relevance as a model system. Close agreement has been found with the Derjaguin Landau Verwey and Overbeek (DLVO) theory, which predicts a repulsive Yukawa pair-potential between the colloids. Recently this has been challenged by the observation of possible attractions and many-body interactions.

We used a new model system of 2-micron poly-methyl-methacrylate particles in a solvent of low dielectric constant, 5.75. The ionic strength is very low, of order nano-moles, so small ion correlations can be neglected, and Poisson-Boltzmann theory is valid. The very low ionic strength leads to a large Debye screening length in the micron length-scale.

Using confocal microscopy we find a rich and unusual phase behaviour of re-entrant melting and subsequent freezing. We believe these are the first results of re-entrant melting as a function of volume fraction in charged colloids. The low-density fluid freezes to form a body-centered-cubic crystal at 0.042 volume fraction, but melts again at 0.118. The second freezing occurs around 0.5 volume fraction, consistent with hard-core interactions.

By extracting particle coordinates in 3D, we obtain the experimental radial distribution function (RDF), which is uniquely determined by the pair interactions. We fit the RDF with Monte Carlo simulation using the DLVO pair-potential, to yield the effective colloid charge and Debye length, which are consistent with independent electrophoresis and conductivity measurements. We find that the colloid charge falls as a function of concentration, and employ a Langmuir adsorption model, which accounts for the fall in charge and consequent re-entrant melting. We see no evidence of attractions, nor phase-separation, and many-body corrections to the pair-wise Yukawa interaction are very small.

PAUL W. REIMUS; Los Alamos National Laboratory, P.O. Box 1663, Mail Stop J534, Los Alamos, NM  87545

In recent years, numerous laboratory and field experiments have been conducted to assess and parameterize colloid and colloid-facilitated radionuclide transport for the Yucca Mountain Project and the Nevada Test Site (NTS) Environmental Restoration Project.  Radionuclide contamination of ground water currently exists within or near underground nuclear test cavities at the NTS, and the proposed Yucca Mountain high-level nuclear waste repository represents a potential future source of radionuclide contamination of ground water at the NTS.  Furthermore, recent field observations have indicated that small amounts of Plutonium, which normally adsorbs very strongly to mineral surfaces in aquifers, can transport quite rapidly and over significant distances in ground water when associated with inorganic colloids (Kersting et al., 1999).

Groundwater samples from all over the Nevada Test Site have been analyzed for colloid concentrations and size distributions, and it is clear that there are significant mass loadings of colloids in the ground water at some locations.  These colloids represent mobile surface area for potentially transporting strongly-adsorbed radionuclides.  Field transport experiments have involved the use of fluorescent-dyed carboxylate-modified latex (CML) microspheres in the 250- to 650-nm diameter size range as surrogates for natural colloids in forced-gradient tracer tests.  These experiments have indicated that effective colloid filtration coefficients appear to decrease as time and length scales increase.  They suggest that a small fraction of colloids may be able to transport significant distances in groundwater systems.  Laboratory experiments have been conducted to determine radionuclide sorption and desorption parameters onto inorganic colloids present in the groundwater systems and also to determine transport parameters for inorganic colloids in both fractured and porous media present at the Nevada Test Site.  More recent laboratory experiments have involved injecting inorganic colloids with radionuclides adsorbed onto them into fractured or porous media to determine the ability of the colloids to facilitate the transport of the radionuclides through the media.  Recent experiments have also involved comparing the transport behavior of CML microspheres and inorganic colloids so that more defensible inferences about inorganic colloid transport can be made from CML microsphere transport observations in field tracer tests. 

All of this experimental information has been collectively used to develop a modeling framework for evaluating sensitivities of predicted colloid-facilitated radionuclide transport to various colloid-transport and radionuclide-colloid-interaction parameters.  This modeling framework is helping to focus future experimental efforts on processes and parameters that have the greatest potential impact on colloid-facilitated radionuclide transport at the Nevada Test Site.

Reference:

Kersting, A.B., D.W. Erfund, D.L. Finnegan, D.J. Rokop, D.K. Smith, and J.L. Thompson, Migration of plutonium in groundwater at the Nevada Test Site.  Nature 397:56 (1999).

HARI VISWANATHAN; Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545

Colloid-facilitated migration of plutonium in fractured rock has been implicated in both field and laboratory studies.  Other reactive radionuclides may also experience enhanced mobility due to groundwater colloids. Colloids therefore may provide a mechanism to transport radionuclides which would otherwise be immobile due to their affinitiy to sorb to the subsurface. Model prediction of this process is necessary for assessment of contaminant boundaries in systems for which radionuclides are already in the groundwater and for performance assessment of potential repositories for radioactive waste. Therefore, reactive transport models with varying levels of complexity have been developed to study colloid facilitated transport at Yucca Mountain, the Nevada Test Site and the Hanford Site.  The goal of these numerical models is to identify and parameterize the physical and chemical processes that affect the colloid-facilitated transport of radionuclides in the subsurface. In this presentation, we will describe several different colloid and colloid facilitated transport models that have been developed at Los Alamos National Laboratory. The Hanford colloid facilitated transport model takes a mechanistic approach. The Yucca Mountain colloid model uses probability distribution functions and uncertainty in the model development. The Nevada Test Site model that will be presented is a combination of the Hanford and Yucca Mountain numerical models.

ARTURO KELLER, Sanya Sirivithayapakorn; 3420 Bren Hall, Bren School of Environmental Science and Management, University of California, Santa Barbara, Santa Barbara, California, USA

We present pore-scale observations of colloid transport in an unsaturated micromodel. The focus was to investigate the pore-scale processes of colloids as they interact with the air-water interface (AWI), as well as the release of colloids during imbibition. Colloids are attracted to the AWI and accumulate almost irreversibly, until dissolution of the air bubble reduces or eliminates the AWI. Near the end of the AWI dissolution process, the colloids can be transported as colloidal clusters. We used DLVO theory to explain observed results. The strength of the force that holds colloids at the AWI was estimated, assuming that the capillary force is the major force holding colloids at the AWI. The forces that hold colloids at AWI are larger than the energy barrier between colloids. It is likely that clusters of colloids are formed by colloids attached at the AWI at the end of the dissolution process. Coagulation at the AWI may increase their overall filtration. Just as important, colloids trapped in the AWI might be quite mobile at the end of the dissolution process, resulting in increased breakthrough. These pore-scale mechanisms play a significant role in the macroscopic transport of colloids in unsaturated media.

SAAD F. ALKAFEEF¹, Abdullah F. Alajmi², Meshal K. Algharaib²; ¹ College of Technological Studies, P.O. Box 32, Hadyia 52851 Kuwait; ² Kuwait University, P.O. Box 5969, Safat 13060, Kuwait.

In the previous meeting of ELKIN 2002 we present the hydrodynamic thickness d of an adsorbed petroleum (crude) oil layer into the pores of reservoir rocks through which the oil is flowing. The hydrodynamic adsorption thickness of the petroleum oil had been studied by Poiseluille's flow law and the evolution of (electrical) streaming current. The study concluded that the adsorbed layer thickness d stopped growing either with time or with concentration of asphaltene in the flowing liquid after increasing to around 30

 of the pore radius. The adsorption thickness is confirmed with the blockage value in rock pores area determined by the combination of streaming current and streaming potential measurements. This behaviour is attributed to the effect on the disjoining pressure across the adsorbed layer described by Derjaguin and Churaev of which the polymolecular adsorption films lose their stability long before their thickness has approached the radius of the rock pore.

In this paper we present the hydrodynamic thickness d of an adsorbed (deposition) asphaltene layer on the wall of oil well-tubing (wide pipe). A well monitoring technique was conducted in West Kuwait (Jurassic) deep wells to monitor the behavior of the well flowing pressure using programmable data logger. The data gathered from the logger have predicted the onset pressure that influenced asphaltene deposition in well tubing. The hydrodynamic deposition thickness of asphaltene in well tubing was estimated by the data and was found to be ~30

 of the well-tubing radius. The result was found to be agreeable with the results obtained by the caliper test. Similar results were also found that the hydrodynamic thickness of scale and salt depositions in wide pipes to be 1/3 of the pipe radius. The question here why the hydrodynamic adsorption/deposition thickness stopped at around 1/3 of the radius either in narrow pores or wide pipes? 

P.GOETZ, A.S.Dukhin; Dispersion Technology Inc., 364 Adams Street, Bedford Hills, NY 10507, U.S.A.

Water-in-kerosene emulsion stabilized with SPAN surfactants exhibit a slow (on scale of hours) evolution. We continuously monitor this transition in relatively concentrated samples (5 vl water), without dilution, using Acoustic, Electroacoustic and Conductivity measurements. Continuous stirring prevents sedimentation.

Acoustic measurements yield information about the droplet size evolution in time. The original droplets, having a size of about 0.5 micron slowly coalescence into larger droplets. After 10 hours the droplet size has increased to about 5 microns. At this point a fraction appears with a droplet size of only 25 nm and the droplet size distribution becomes bimodal. It takes about 24 hours longer for the emulsion droplets to completely transform into a state with small (25 nm) size. The conductivity exhibits a rapid change during first 10 hours during emulsion coalescence, but the rate becomes much slower as the micoemulsion fraction begins to grow. Electroacoustic measurements show a gradual decay in the measured electroacoustic signal. This indicates that original emulsion droplets carry a substantial surface charge that decays with time. We are able to calculate this surface charge using Shilov's theory for overlapped DLs. In the final state the droplets generate practically no electroacoustic signal and appear uncharged.

Conductivity and electroacoustic measurements indicate a strong role for electrostatic factors in emulsion stability and its transition to microemulsion state. We suggest a model, which might describe these correlations. We show that ion exchange between exterior and interior diffuse layers leads to a gradual collapse of the exterior DL and can explain all the experimental observations.

MENACHEM ELIMELECH; Department of Chemical Engineering, Environmental Engineering Program, Yale University, New Haven, CT 06520-8286

The influence of bacterial electrokinetic properties and surface bound lipopolysaccharides (LPS) on cell transport and adhesion are examined using three mutants of Escherichia coli K12 with well-characterized LPS of different lengths and molecular composition.  Two experimental techniques, a packed bed column and a radial stagnation point flow system, are employed to investigate bacterial adhesion kinetics onto quartz surfaces over a wide range of solution ionic strengths.  Although the two systems capture distinct deposition (adhesion) mechanisms because of their different hydrodynamics, similar deposition kinetics trends are observed for each bacterial strain.  Bacterial deposition rates are directly related to the electrostatic double layer interaction between the bacteria and quartz surfaces, in qualitative agreement with classic DLVO theory.  However, DLVO theory does not fully explain the deposition behavior for the bacterial strain with the lengthy, uncharged O-antigen portion of the LPS.  Neither the length nor the charge characteristics of the LPS molecule directly correlated to deposition kinetics, suggesting a complex combination of cell surface charge heterogeneity and LPS composition controls the bacterial adhesive characteristics.  It is further suggested that bacterial deposition behavior is determined by the combined influence of DLVO interactions, LPS-associated chemical interactions, and the hydrodynamics of the deposition system.

F. THOMAS¹, L. Hermitte²; ¹ Laboratoire Environnement et Minéralurgie UMR7569 CNRS/INPL - BP40 - F-54501 Vandoeuvre Cedex; ² Laboratoire Ingénierie et Fonctionnalisation des Surfaces - 36, Av. Guy de Collonges - F-69134 Ecully Cedex

Acrylic and methacrylic polymers are used as biomaterials for medical applications such as intra-ocular or contact lenses. Increasing the long-term biocompatibility and functionality of such materials requires comprehensive characterization of the surface properties of biomaterials.

The presented study was focused on the investigation of the relationships between formulation, surface properties, and epithelial cell adhesion for a series of copolymers prepared with hydroxyethylmethacrylate (HEMA), methylmethacrylate (MMA), and ethylmethacrylate (EMA) monomers, and on the homopolymers PMMA and PHEMA.

The biological response of epithelial cells to the surface properties of the biomaterials (proliferation and spreading) is directly correlated to the hydrophilic-hydrophobic balance and to the electrokinetic potential.

Periodic instabilities in the wetting cycles, similar to Haines jumps, are observed with HEMA copolymers and support a bidirectional relaxation of the hydrophilic groups between external water and capillary water. The origin of the electrokinetic potential of these non-ionizable polymers is attributed to specific adsorption of OH- ions and to the properties of the water molecules near the interface. The correlations between the above properties and the formulation of the copolymers clearly indicate the importance of the statistic length of the side chains in the cell adhesion process.

TOMMY S. HOROZOV, Robert Aveyard, Bernard P. Binks, John H. Clint; Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull, HU6 7RX, UK

Silica particle monolayers at oil-water interfaces have been studied. It has been found that the gradual change of particle hydrophobicity (wettability) leads to a rather abrupt change in the monolayer structure: from aggregated, disordered monolayers at low hydrophobicity to well ordered monolayers of very hydrophobic silica particles displaying large inter-particle separations. The effects of oil type, electrolyte concentration and pH have been studied and the possible reasons for the very long-range repulsion between adsorbed particles are discussed. The two-dimensional particle sedimentation observed in the case of vertical monolayers has been analysed and valuable information about the charge density at the particle-oil interface has been obtained. The pair interactions between particles at large distances in the case of a horizontal fluid interface have been analysed. The structure and stability of thin liquid films with particle monolayers at their surfaces have also been studied. It has been found that the films can be very stable due to formation of a dense particle monolayer bridging the film surfaces. The implications of the above findings for the stability of particle-stabilised emulsions in the absence of surfactants are discussed.

AMR I. ABDEL-FATTAH¹, Mohamed S. El-Genk², Paul W. Reimus³; ¹ Los Alamos National Laboratory, P.O. Box 1663,  Mail Stop J514, Los Alamos, NM 87545; ² Dept. of Chemical and Nuclear Engineering, The University of New Mexico, Albuquerque, NM 87131; ³ Los Alamos National Laboratory, P.O. Box 1663, Mail Stop J534, Los Alamos, NM 87545

There is no question that long-term kinetics of particle adsorption and desporption at a solid-liquid interface is a key to evaluating the migration of colloidal contaminants in subsurface environments.  This paper addresses some unanswered questions raised by previous investigations in this challenging field.  Among these questions are: a) do desorbed particles deactivate their former adsorption sites at the solid-liquid interface? b) do particles adsorbing at early times stay longer at the interface compared to those adsorbed at later times? c) are there surface sites on the interface that always get occupied by adsorbing particles? and d) what is the effect of ionic strength and flow rate on these hypotheses?.  By evaluating these hypotheses we stand to develop better predictive models for the long-term subsurface colloid transport of contaminants.  We therefore performed a set of microscopic visualization experiments in which the adsorption and desorption events of 330-nm polystyrene spherical particles at a glass-liquid interface were tracked and enumerated every ~ 90 s for up to 4 days.  The particles were suspended in synthetic groundwater at a constant pH of 8 and two ionic strengths of 0.025 and 0.05 M.  Flow rates ranged from 0 to 0.16 ml/hr (average flow velocities in the cell between 0 and 2.78×10-3 cm/s).  Particle adsorption and desorption events were tracked and enumerated at the top planner glass surface of 200-mm aperture parallel-plate flow cell, into which the particle suspension was continuously injected at a given flow rate.  Selected sites at the glass surface from which particles have desorbed were tracked throughout the entire duration of the experiments to see whether or not they became re-occupied at any subsequent times.  The adhesion time distribution function, defined as the probability that particles adsorbed onto the surface at any given time T remain adsorbed until time t, was also quantified at different adsorption times, T. The results of these experiments will be presen!

N.A.MISHCHUK;Institute of Colloid Chemistry and Chemistry of Water of National Academy of Sciences of Ukraine, Vernadskogo avenue, 42, Kyiv-142, Ukraine, 03142

Characteristics of disperse systems depend on the homogeneity, smoothness and purity of the surfaces of interacting particles and the purity and homogeneity of the disperse medium.

One of the factors strongly affecting surface characteristics is the presence of very small bubbles of gas or vapor on the surface of the particles and/or in the interparticle gap. The size, shape and location of these bubbles and, consequently, their influence on the electrostatic characteristics of interface and surface forces depend upon the prehistory of their appearance. Since usually investigated systems are in contact with the atmosphere, dissolved gas is inevitably present in aqueous phase. Besides the dissolution of gas in liquid the important role is played by incomplete wetting of hydrophobic surface, remainder of air in surface microfissures, cavitation in the space between hydrophobic bodies, chemical reactions caused by their interaction etc. 

The importance of tiny bubbles in characterizations of disperse systems could be diverse. Small influence on one characteristic, for example, on the measured electrokinetic potential, could be accompanied by very strong influence on other characteristics, for example, on stability of disperse system. The role of tiny bubbles in different processes will be analyzed.

SAAD F. ALKAFEEF¹, Abdullah F. Alajmi²; ¹ College of Technological Studies, P.O. Box 32, Hadyia 52851, Kuwait; ² Kuwait University, P.O. Box 5969, Safat 13060, Kuwait

The surface (zeta) potential of reservoir rocks plays a significant role in oil recovery. The calculation of the zeta potential by the measurements of streaming potential in low electrolyte concentrations, such as hydrocarbon system, can be in considerable error as a result of rock surface conductivity.

The aim of this study is to correct the calculation of zeta potential values of reservoir rocks from the streaming potential measurements in low electrolyte concentrations. To achieve this it's important to determine the specific conductance of the solution including the rock surface conductivity. The total specific electric conductivity of the rock is calculated with the knowledge of the ratio of A/L of the effective capillaries and the resistance of the rock. The A/L ratio for porous systems was determined, in previous publications, by combining streaming potential and current measurements.

The measurement of streaming potential of various sandstone core samples in KCl solutions and hydrocarbon solvents were carried out using high pressure/high temperature core holder.

The calculated zeta potential values by the means of streaming potential measurements in low electrolyte have showed a good agreement with the values obtained by the streaming current method. The later method is free from the surface conductivity error, but do however require the knowledge of the ratio area/length (A/L) of the rock effective capillaries.

YASUO KOIDE¹, Kazunori Yamada¹, Shinji Yamaguchi¹, Hiroshi Sasaki², Shin-nosuke Usui³; ¹ Aquatic Environmental Engineering Laboratory of Waseda University; ²  Professor of Waseda University; ³ Emeritus Professor of Tohoku University, Okubo 3-4-1,Shinjuku-ku Tokyo 169-8555,Japan

The Dorn method is one of the sedimentation potential methods used to measure zeta potential of bubble surface in column. The Dorn potential, which is derived from electric dipole moment induced by deviation of surface concentration from equilibrium arises in buoyant bubbles. Each potential shows peculiar value according to concentration of alcohol and electrolytes.

Cornelia Bellmann¹, Anja Caspari¹, Thi Thu Loan Doan¹, Edith Mäder¹, THOMAS LUXBACHER², Renate Kohl²;

¹ Institute of Polymer Research, Hohe Strasse 6, 01069 Dresden, Germany; ² Anton Paar GmbH, Anton-Paar-Strasse 20, 8054 Graz, Austria

Natural cellulose-based fibres are gaining increasing attention in engineering such as building material or light-weight structural parts for motor vehicles. However cellulose fibres require intensive surface treatment which may be controlled by electrokinetic investigations.

In this contribution we report on the streaming potential method for characterizing the swelling capacity of jute, linters, and banana fibres. Furthermore the change of surface properties has been monitored during fibre cleaning and the application of coupling agents.

The comparison with solvatochromism experiments shows a correlation between the isoelectric point, which is determined from zeta potential measurement, and the acceptor number of the fibre surface.

Pierre Turq, Virginie Marry, Natalie Malikova, Jean-Francois DUFRECHE; Laboratoire LI2C UMR CNRS 7612,

Liquides Ioniques et Interfaces chargees, BC 051 Universite P. et M. Curie, 4 place Jussieu, 75252 Paris Cedex 05, France

For the sake of simplicity, the description of electrokinetic phenomenona is commonly based on mesoscopic continuous models (Poisson -Boltzmann, Navier-Stokes or Smoluchowski approaches). We have compared these mesoscopic theories to microscopic descriptions (atomic and molecular level)  based on a Kubo type approach in the case of hydrated montmorillonite clays. Many equilibrium and transport properties have been calculated. The ion distributions are found to be in agreement as long as the interlayer spacing is large enough. The electro-osmotic flow has been calculated too: it is shown that a slip length has to be taken into account. Slipping may have significant consequences for the general theory of electrokinetic phenomenona. Some of them are briefly described.

MAREK KOSMULSKI^1,2, Jarl B.Rosenholm¹, Czesław Saneluta², Krystyna Marczewska-Boczkowska²; ¹ Department of Physical Chemistry, Abo Akademi University, 20500 Turku, Finland; ² Department of Electrochemistry, Lublin University of Technology, 20618 Lublin, Poland

The electroacoustic method made it possible to study electrokinetic phenomena at high ionic strengths. Interestingly, the zeta potential does not cease at ionic strengths as high as 3 molar in aqueous systems. Here we show that also in low temperature ionic liquids the electrokinetic phenomena do exist. Dispersions of anatase in 1-alkyl-3-methylimidazolium (alkyl=butyl, hexyl, octyl or decyl) trifluoromethanesulfonates, hexafluorophosphates, and mixed tetrafluoroborate-chloride ionic liquids were studied. These ionic liquids are molten salts at room temperature, and they can be considered as about 3 molar salt solutions in the absence of solvent. The electrokinetic potential of anatase determined by means of the electroacoustic method was negative for all ionic liquids studied, although for a few ionic liquids the standard deviation in a series 20 consecutive measurements was higher than the absolute value. Absorption of moisture by the ionic liquids has rather insignificant effect on the electrokinetic potential of anatase. Electroosmosis in specimens of porous alumina saturated with ionic liquids was studied, and the electrokinetic charge of alumina was also negative.

A.R.M. VERSCHUEREN¹, G.C. van Zandwijk², F. Strubbe³, L.J.M. Schlangen¹, P.H.L. Notten¹, K. Neyts³; ¹ Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands; ² Faculty of Physics and Astronomy, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands; ³ Electronics and Information Systems Department, Ghent University, Sint-Pietersnieuwstraat 41, B9000 Ghent, Belgium

A novel method for characterisation of charged species is presented. The method is completely based on measurement and interpretation of a set of transient current experiments. This allows simultaneous determination of concentration, absolute charge and mobility of charged species in low conductivity solvents.

A transient current measurement setup was designed and built, allowing precise measurements over a wide range of current (1pA-1muA) and time (1ms-10s). The electrical current waveforms are measured for a range of different voltages. Transient measurements have been performed on a series of plan-parallel cells (4-16 micron electrode-spacing) filled with n-dodecane and OLOA charging agent (0.01-4.0 weight percent). The voltage dependence of the total integrated current is used to determine the concentration and absolute charge (of the inverse micelles that dominate the conductance). When combined with the initial current, this also yields the mobility.

Transient electrical current waveforms of charged species have been calculated by 1D finite element method assuming basic migration and diffusion equations. The influence of three basic parameters (concentration, absolute charge and mobility) on the simulated electrical transients is investigated.

A very good correspondence between the transient measurements and simulations has been found. Consistent values were obtained for the n-dodecane/OLOA system, confirming the applicability/validity of the method.

ANDREW D. HOLLINGSWORTH¹, M.E. Leunissen², A. Yethiraj³, P.M. Chaikin⁴, W.B. Russel¹, A. van Blaaderen²; ¹ Dept. of Chemical Engineering, Princeton University, Princeton, NJ 08540; ² Soft Condensed Matter, Debye Institute, Utrecht Univ.; ³ Dept. of Chemistry, Univ. of British Columbia; ⁴ Dept. of Physics, Princeton Univ

Recent confocal microscopy studies of fluorescently labeled, monodisperse particles revealed a space filling, low volume fraction (phi ~ 0.01) crystal structure. To determine whether electrostatic repulsion with Debye screening lengths as large as several microns could be responsible, we performed complementary electrokinetic measurements (electrophoresis, dielectric spectroscopy, and conductivity). The micron size particles were sterically stabilized poly(methyl methacrylate) spheres suspended in near density matching solvents with dielectric constants between 5 and 8. A quaternary ammonium salt was used to screen charge (reducing the Debye length) and induce a charge reversal as determined previously by electrophoresis. From the particle mobility and suspension conductivity, we estimated the particle charge and surface potential. These results are consistent with parameters deduced from independent pair interaction energy measurements. Possible particle charging mechanisms are presented.

A.S.DUKHIN, P.J.Goetz; Dispersion Technology Inc., 364 Adams Street, Bedford Hills, NY 10507, USA

It is well know that the large differences in dielectric permittivity and conductivity between aqueous and non-aqueous solutions cause dramatic difference in double layer (DL) structure in heterogeneous systems based on these solutions. In aqueous systems DL is usually much thinner than particles size. In non-aqueous low conducting systems it is much thicker than particle size. Consequently in aqueous systems DLs are usually isolated even at very high volume fractions of particles. In non-aqueous systems they overlap even in rather dilute systems.

We show that these differences require different parameterization of the particles surface properties. In aqueous systems z-potential is accepted measurable parameter of the particles electric surface properties. Calculation of the surface charge from z-potential might become quite complex problem.

In low conducting non-aqueous concentrated dispersions with overlapped DLs situation becomes reverse. It turns out that electroacoustics yields information about surface charge directly, with no additional information about electrolyte required. Calculation of the z-potential does require information about ions properties. It is not available in non-aqueous systems in many cases. This makes z-potential much less reliable and useful property in non-aqueous concentrated dispersions.

We illustrate characterization procedure in non-aqueous systems using kerosene-alumina-SPAN dispersions as an example. Measurement technique is combination of conductivity and electroacoustic CVI method. This approach would enable us to establish properties of ions (size, diffusion coefficient, dissociation constant of SPAN surfactants) and electric surface properties of alumina particles: surface charge and z-potential.

JOHN L. ANDERSON¹, Junhyung Kim², Stephen Garoff³, Luc Schlangen⁴; ¹ Carnegie Mellon University, CIT Deans Office, Pittsburgh, PA 15213; ² Carnegie Mellon University, Department of Chemical Engineering, Pittsburgh, PA 15213; ³ Carnegie Mellon University, Departments of Physics and Chemical Engineering, Pittsburgh, PA 15213; ⁴ Philips Research Laboratories, Inorganic Materials Group, Eindhoven, The Netherlands

Current versus electrical potential relationships were measured for solutions of dodecane containing the charge control agent poly(isobutylene succinimide) (PIBS) at various concentrations. Both one-dimensional (parallel planar electrodes) and two-dimensional (strip electrodes) fields were studied. After an initial, approximately exponential decay of the current after the potentials were clamped (< 1 sec), the current remained at a low but finite value for the remainder of the experiment (hours) in both electrode configurations. While the initial current was ohmic, the residual current was not. The initial decay of the current in the one-dimensional configuration permitted estimation of the diffusion coefficient of the charge-carriers (micelles) in the solution, from which we determined a size that is consistent with value reported in the literature for PIBS. Our results for the initial current and the short-time transient are consistent with a model for the solution of isolated charge carriers of valence ±1 and a traditional double layer build-up of polarization charge on the electrodes;  however, the decay of current is faster than predicted from the classical Gouy-Chapman theory of the double layer. An explanation of the residual current is not obvious.  The geometric factor relating current to applied potential at time zero for the strip electrode cell agrees with our solution of LaPlace's equation for the electrical potential in this system.

V.N.SHILOV¹, Yu.B.Borkovskaja¹, A.S.Dukhin²; ¹ Institute of Biocolloid Chemistry, National Academy of Sciences of the Ukraine, 42 Vernadsky Blvd., Kiev 03142 ,Ukraine; ² Dispersion Technology Inc., 3 Hillside Avenue, Mount Kisco, NY 10549, USA

Existing theories of electroacoustic phenomena in concentrated colloids are valid for the "thin Double Layer", when the Double Layer thickness is much less than particle size. In this contribution we present a new electroacoustic theory, which removes this restriction. This would make this new theory applicable to characterizing variety aqueous nano-colloids and of non-aqueous dispersions and emulsions.

There are two versions of the theory leading to the analytical solutions.

The first version corresponds to strongly overlapped diffuse layers (so-called quasi-homogeneous model). It yields a simple analytical formula for CVI, which is valid for arbitrary ultrasound frequency, but for restricted range of small enough ka. This version of the theory, as well Smoluchowski theory for microelectrophoresis, is independent on the particles shape and polydispersity. This makes it very attractive for practical use.

In order to determine the ka range of the quasi-homogeneous model validity we develop the second version that limits ultrasound frequency, but applies no restriction on ka. The ultrasound frequency should substantially exceed the Maxwell-Wagner relaxation frequency. This limitation makes conductivity related current negligible comparing to the dielectric displacement current. This makes it possible to derive an expression for CVI in the concentrated dispersion as formulae inhering definite integrals with integrands depending on equilibrium potential distribution.

This second version allowed us to estimate the ranges of the applicability of the first, quasi-homogeneous version. It turns out that quasi-homogeneous model works for ka values up to almost 1. For instance, at volume fraction 30 the highest ka limit of the quasi-homogeneous model is 0.65. This makes this version of the electroacouistic theory valid for almost all non-aqueous dispersion and wide variety of nano-colloids, especially with sizes under 100 n

ALBERTO T PEREZ¹, Elisabeth Lemaire²; ¹ Departamento de Electrónica y Electromagnetismo

Facultad de Física. Universidad de Sevilla, Avda. Reina Mercedes s/n, 41012 Sevilla. Spain; ² LPMC - CNRS - Université de Nice, Parc Valrose - 06108 Nice cedex 2

 

Electrophoretic mobility of PMMA particles in a mixture of liquid hydrocarbons is measured. The suspension is contained by a cell mounted on a piezoelectric crystal. The device is placed on a microscope and a low frequency field is applied along the optical axis. The image of a particle is observed and the displacement of the piezoelectric crystal is adjusted to compensate for the particle motion, in such a way to make the particle image steady to the microscope. The piezoelectric displacement corresponds then to the particle displacement. The measurements are possible even at high volume concentration because the liquid mixture matches the index of refraction of the particles. The dependence of the mobility on the volume fraction of particles is then found experimentally. In order to increase the particle mobility we use a suitable chemical additive.

Ian D. Morrison; Cabot Corporation, 157 Concord Rd., Billerica, MA 01821

The presence of stable concentrations of ions and charged particles in nonpolar liquids has been known for over fifty years. Electrical conductivities are low, sometimes orders of magnitude less than deionized water. However, these conductivities are similar to semiconductors and so nonpolar solutions and dispersions find application in electronics such as electronic printing and electronic displays. Unfortunately, most studies have been described in the trade literature and not in scientific journals.

     A simple model for stable charge separation in nonpolar media is that ions are stabilized against neutralization by being held inside structures such as inverse micelles or coiled polymers. Ions inside an inverse micelle are actually

JOEL A. COHEN, Valentina A. Khorosheva; Department of Physiology, University of the Pacific, 2155 Webster Street, San Francisco, CA 94115, USA

Liposomes are colloidal particles composed of lipids.  Liposomes decorated with surface-grafted poly(ethylene glycol) (PEG) polymers, known as sterically-stabilized or Stealth^® liposomes, are in current therapeutic use for intravenous delivery of anti-cancer and anti-fungal drugs.  The "hairy" polymer coating retards recognition of the liposome by the host immune system, buying valuable time for systemic circulation.  We have measured electrophoretic mobilities of a series of such liposomes to determine their hydrodynamic drag under various conditions.  End-grafted PEGs range in size from 2 to 113 monomers; nominal grafting densities range from one PEG chain per 200 lipids to one PEG chain per 10 lipids; and salt ranges from 0.5 mM to 100 mM.  The liposome surface-charge density is held constant at one negative charge per 10 lipids.  Identically-charged liposomes with no grafted PEG serve as controls.  The observed drag effects are large: they can reduce mobilities by 90 percent.  The investigated polymer lengths and grafting densities span the mushroom-to-brush transition region.  The mobilities are analyzed to yield hydrodynamic coat thicknesses, polymer-water frictional coefficients, and polymer segment-density profiles.  These studies are of practical significance and also elucidate the physics of flexible polymers grafted on surfaces.

J. M. MÉNDEZ-ALCARAZ¹, O. Alarcón-Waess²; ¹ Departamento de Física, Cinvestav, Av. IPN 2508, Col. San Pedro Zacatenco, 07360 México D. F., Mexico; ² Departamento de Física y Matemáticas, UDLA-Puebla, Sta. Catarina Mártir, Cholula, 72820 Puebla, Mexico

We introduce a theory for the electrophoretic mobility  m of strongly interacting colloidal particles, which includes macroion and salt concentration effects at the same level of description. This theory explains the shift of the minimum of m, as a function of salt content, with increasing macroion concentration, the simultaneous enhancement of m and its following saturation for highly structured systems, as well as the charge inversion of the macroions in presence of asymmetric salt. In addition, it predicts a secondary macroion charge inversion and the universal value m=2m0 for highly structured systems, where m0 is the free particle mobility.

RALF ZIMMERMANN¹, Toshihisa Osaki¹, Günter Gauglitz², Carsten Werner^1,3; ¹ Department Biocompatible Materials, Institute of Polymer Research Dresden and The Max Bergmann Center of Biomaterials Dresden, Hohe Strasse 6, 01069 Dresden, Germany; ² Institute of Physical and Theoretical Chemistry, University of Tübingen, Auf der Morgenstelle 8, 72076 Tübingen, Germany; ³ Department of Mechanical and Industrial Engineering, University of Toronto, 5 King's College Road, M5S 3G8 Toronto, Canada.

The Microslit Electrokinetic Set-up¹, which permits the determination of the zeta potential and the surface conductivity of flat solid surfaces by streaming potential and streaming current measurements was recently combined with the Reflectometric Interference Spectroscopy² for the simultaneous determination of the optical thickness of interfacial layers³. By the simultaneous determination of this additional parameter information on structural variations of adsorbed or covalently bound polymers and on charge depended adsorption and desorption phenomena can be obtained.

To demonstrate the potentialities of this extended approach, the adsorption of the plasma protein fibrinogen at poly(octadecene-alt-maleic acid) films was investigated by electrokinetic and spectroscopic measurements at protein solution concentrations between 10 ng/mL and 1 µg/mL. The results obtained point at the high relevance of surface charge for the orientation of the adsorbed proteins.

1. Werner, C.; Körber, H.; Zimmermann, R.; Dukhin, S.S.; Jacobasch, H.-J. J. Colloid Interface Sci. 1998, 208, 329.

2. Gauglitz, G.; Krause-Bonte, J.; Schlemmer, H.; Matthes, A. Anal. Chem.1988, 60, 2609.

3. Zimmermann, R.; Werner, C.; Gauglitz, G.; Eichhorn, K.-J. Patent DE 102 05 775, February 6, 2002.

Reghan J. Hill; Department of Chemical Engineering, McGill University, 3610 University Street, Montreal, Quebec, H3A2B2

A detailed theoretical model is used to interpret electrokinetic experiments performed on liposomes with uncharged polymer coatings (Cohen & Khorosheva, 2001). The methodology provides a firm basis for future studies examining liposome surface chemistry and charge, surface-charge mobility, and the dynamics of adsorbed polymer on fluid-like membranes.

The model predictions are compared with experimental measurements of the electrophoretic mobility of stealth liposomes with terminally anchored poly(ethylene glycol) (PEG). The experimental data are interpreted by drawing upon self-consistent mean-field calculations of the polymer segment density distributions and numerically exact solutions of the governing transport equations (Hill, Saville & Russel, 2003, J. Colloid Interface Sci., 258). The approach leads to excellent agreement between theory and experiment with only one adjustable parameter, namely the hydrodynamic size (Stokes radius) of the statistical PEG segments.

The 'best fit' Stokes radius is remarkably small, and this will be demonstrated to be an artifact of the underlying Debye-Brinkman model of the polymer hydrodynamics, via a careful comparison of continuum (Debye-Brinkman and Kirkwood-Riseman) calculations and discrete (Monte Carlo) simulations yielding the hydrodynamic size of Gaussian polymer coils. Despite such shortcomings, the electrokinetic model is robust in its predictive capacity. For example, it quantitatively captures the effect of terminally anchored PEG on the mobility of PEG-derivatized lysozyme without any further fitted parameters.

KUNIO FURUSAWA¹, Hideo Matsumura²; ¹ Liaison-center, University of Tsukuba, Tennodai-1, Tsukuba, Ibaraki 305-8577, Japan; ² National Institute of Advanced Industrial Science and Technology, Central 2, Umezono-1, Tsukuba, Ibaraki 305-8568, Japan

Several preparation processes of composite particles relating with electric properties of particles are examined, i.e., 1) the heterocoagulation process of amphoteric polymer lattices on spherical silica particles, 2) composite formation comprised of silica/vesicle or silica/vesicle/silica particles, 3) the build-up process of polyelectrolyte or colloid particle multilayers on latex and silica surfaces., 4) the coating method of silica layers on the different polarized particles(hematite particle, anionic and cationic polymer lattices).  All these results indicate that the electrokinetic is a powerful technique for the formation of composite particles with different composition.  Especially in the process 4), the electrical charge on the templated shell plays an essential role on their porous nature of the resulted composites.

LEE P YEZEK, Herman P. van Leeuwen; Department of Physical Chemistry and Colloid Science,

Wageningen University, Dreijenplein 6, 6703HB Wageningen, The Netherlands

Streaming potential and conductivity measurements were carried out on thin films of porous polymer gels of varying fixed charge density.  The films consisted of gels of polyacrylamide-co-acrylic acid cross-linked with N,N'-methylenebisacrylamide.  The gel synthesis is based on the procedure developed by Zhang and Davison (1) for the synthesis of neutral gels for use in Diffusion Gradient in Thin Films (DGT) devices for metal ion speciation.  The gel samples were prepared in the form of thin films 0.25 mm thick. The equilibrium gel thickness varied as a function of fixed charge density due to osmotically driven swelling.  Streaming potential and conductivity measurements were carried out as a function of ionic strength for a variety of gel preparations of varying polymer composition.  Conductivity data allowed the estimation of the equilibrium Donnan potential difference which develops between the bulk gel phase and the electrolyte solution as a function of ionic strength.  The relationship between the experimentally determined Donnan potentials and the measured streaming potentials is examined in the context of a new theory of the electrokinetics of soft surfaces put forth by Duval and van Leeuwen (2), as well as with the model of Ohshima and Kondo (3).  The limitations of each model are discussed in the context of the experimental data for cross-linked polyacrylamide gels.

(1) H. Zhang and W. Davison, Anal. Chim. Acta, 398 (1999) 329.

(2) J.F.L. Duval and H.P. van Leeuwen, submitted to Langmuir.

(3) H. Ohshima and T. Kondo, J. Colloid and Interface Sci., 135, (1989) 443.

TOSHIHISA OSAKI, Carsten Werner

Alternating maleic acid copolymers dissolved in aqueous solutions exhibit characteristic two-step dissociation profiles and pH-induced structural transitions in dependence on the comonomer unit. In extension of these findings we analyzed a set of thin films of maleic acid copolymers (poly(octadecene-alt-maleic acid): POMA, poly(propylene-alt-maleic acid): PPMA, poly(styrene-alt-maleic acid): PSMA) randomly attached to planar glass surfaces by covalent binding. Streaming potential/streaming current measurements with an in-house developed Microslit Electrokinetic Set-up were applied to determine zeta potential and surface conductivity data of the polymer films in aqueous solutions of varied pH. The results confirmed the two-step dissociation behavior for all immobilized copolymers with pH variation. A dramatic increase of the surface conductivity was observed for PPMA and PSMA -but not for POMA- layers at alkaline pH values indicating that electrostatic repulsion of ionized groups controls the extension of the confined layers as long as hydrophobic interactions between the comonomers do not inhibit this effect. The isoelectric points (IEPs) indicate that the acidic functions dominate the interfacial charging and exhibit an enhanced acidity which can be explained by hydrogen bonds between the anion formed in the first dissociation and the proton of the adjacent undissociated group.

Ute Böhme, Bernd Fritzinger, ULRICH SCHELER

Dynamic NMR is applied to investigate electrophoretic motion and diffusion of molecules in solution and dispersed particles. From the diffusion coefficient the hydrodynamic size is inferred via the Stokes-Einstein equation. The force balance between the electrostatic force and the hydrodynamic friction yields the effective charge of the moving species free of any model. The condensation of counterions onto polyelectrolytes is directly monitored since the moving species are identified in the NMR spectrum correlated with the electrophoretic mobility. Polyelectrolyte complexes are characterized by size and effective charge. From the pH dependence of the effective charge the isoelectric point of proteins is derived. Dendritic molecules are used as model systems as well.

DUVAL Jerome, Lyklema Johannes, van Leeuwen Herman P.;  Laboratory of Physical Chemistry and Colloid Science, Wageningen University Dreijenplein 6, 6703 HB Wageningen, The Netherlands

The electrokinetic responses of electron-conducting surfaces and charged ion-permeable layers dramatically deviate from those expected on the basis of the classical Helmholtz-Smoluchowski formalism. For flat metallic surfaces in the presence of electroactive electrolytes, streaming potentials are greatly diminished due to the extensive electronic conduction in the substrate that results from the bipolar faradaic depolarization of the interfacial double layer.1 For gels or polyelectrolyte layers, the reduction of streaming potential is caused by ionic conduction in the bulk material.2

In the first part of the presentation, streaming potential data obtained for the gold/aqueous Fe(CN)6,3-/Fe(CN)6,4-;KNO3 solution interface will be presented and analyzed in terms of a recently developed theory,3 which takes into account reversible bipolar faradaic depolarization, the inherent non-linearity of the lateral electric field and the effects of flow on the rate of the faradaic reactions. It will be shown how critical the understanding of the bipolar electrochemical processes is for accurately determining the electrokinetic potential of metallic surfaces. In the second part, streaming potential data on crossed-linked polyacrylamide gels will be discussed on the basis of Ohshima's theory for soft surfaces.4 For low electrolyte concentrations, significant discrepancies between the theoretical predictions and the experimental results are observed and these are assigned to the heterogeneous spatial distribution of the fixed charged sites in the gel layer. We present an alternative model considering a linear profile for the volume density of gel polymer segments at the diffuse interface between the electrolyte solution and the bulk gel material. The non-linear Poisson-Boltzmann equation and Navier-Stokes equation are solved under the boundary conditions set by the profile of the diffuse interface. The resulting streaming potentials show encouraging agreement with experimental data over the entire range of electrolyte concentration investigated.

1. Duval, J.; Huijs G.K.; Threels, W.F.; Lyklema, J. and van Leeuwen, H.P. J. Colloid Interface Sci. 2003, 260 (1), 95.

2. Donath, E. and Voight, A. J. Colloid Interface Sci. 1986, 109, 122.

3. Duval, J.; van Leeuwen, H.P.; Cecilia J. and Galceran, J. J. Phys. Chem. B 2003, 107 (28) 6782.

4. Ohshima, H. Adv. Colloid Interface Sci.1995, 62, 189.

VOLKER RIBITSCH¹, Stefan Köstler¹, Angel V. Delgado², Karin Stana-Kleinschek³; ¹ University Graz, Institute of Chemistry, Graz, Austria; ² Departamento de Física Aplicada, Universidad de Granada, Granada, Spain; ³ University Maribor, Institute of Textile Chemistry, Maribor, Slovenia

Polyelectrolyte multilayers of poly(diallyldimethylammonium chloride), (PDADMAC) and poly(sodium 4-styrenesulfonate),(PSS) were deposited onto inorganic (silicon, glass) and pretreated polymer (PET, PTFE) substrates by consecutive adsorption from aqueous solution. The adsorption process, coverage layer stability as well as surface charge properties were characterised by means of streaming potential measurement. Morphology of the produced films was examined by AFM and ellipsometry.

Polyelectrolyte adsorption was indicated by a marked shift of the isoelectric point and the shape of zeta potential vs. pH curves. Regular layer-by-layer built up and an increase in surface coverage with the layer number could be shown. It was also shown that stability of the multilayer depends on whether PSS or PDADMAC formed the outermost layer. Surface thermodynamic properties and wetting behaviour were determined by contact angle measurements. Employing a thermodynamic model, we were able to determine apolar, electron donor, and electron acceptor components of the surface free energy. This discriminates between the individual free energy components and provides an opportunity to link wetting behaviour with electrokinetic, structural and chemical information. An unusual wetting behaviour of such multilayers was found and is explained by chain reorganisation.

ANDREI DUKHIN, Stanislav Dukhin; Electrokinetic Technology Inc., 12 Branch Street, Goldens Bridge, NY, USA 10526

This paper describes a new method of generating directed motion of the liquid in the microfluidic device by applying  "un-balanced" AC electric field for generating non-linear electroosmosis and related hydrodynamic flow in the chamber with any symmetry of the elements, including spherical or cylindrical symmetry, and any relative position of these elements. Direction of the flow depends on the phase of the "un-balanced" electric field, which opens a simple way to operate flow and create a desirable flow pattern

MARTIN Z. BAZANT¹, Jeremy Levitan¹, Todd M. Squires²; ¹ Department of Mathematics and Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, MA 02139; ² Department of Applied and Computational Mathematics, California Institute of Technology, Pasadena, CA 91125

``Induced-charge electro-osmosis'' (ICEO) refers to the nonlinear electrokinetic slip at a polarizable surface when an electric field acts on its own induced double-layer charge. Here, we develop a general physical picture of ICEO in the context of some new techniques for microfluidic pumping and mixing. ICEO generalizes ``AC electro-osmosis'' at micro-electrode arrays to various dielectric and conducting structures in weak DC or AC electric fields. The basic effect produces micro-vortices to enhance mixing in microfluidic devices, while various broken symmetries --- controlled potential, irregular shape, non-uniform surface properties, and field gradients --- can be exploited to produce streaming flows with small AC voltages.

E.J. VAN DER WOUDEN, T. Heuser, D.C. Hermes, J.G.E. Gardeniers, Albert van den Berg;

BIOS - Lab-on-a-Chip group, MESA+ Research Institute, University of Twente, The Netherlands

This paper described a robust method for the fabrication of glass microchannel networks with integrated insulated gate electrodes to control the Zeta-potential at the insulator surface and therewith the electro osmotic flow (eof). The fabrication method consists in a sequence of thin film deposition, photolithography and etching steps on a Pyrex glass substrate, followed by chemical mechanical polishing prior to bonding to a second glass plate to form closed microchannels. Electrical breakdown of the metal-insulator structures occurred at 250 V/cm. Experiments using fluorescent beads showed that the eof is linearly dependent on the applied gate voltage. The average eof could be stopped completely for longitudinal fields of 200 V/cm, by applying an appropriate gate potential, however local backflow in the channel above the gate electrode was observed. This is due to the fact that the gate electrode was present on only one side of the channel. The effect could be described quantitatively by computational fluid dynamics. The paper will also discuss the use of multiple gate electrodes to compensate for the longitudinal field gradient in the channel, as well as AC-drive of the gate electrodes and synchronization the eof with a longitudinal AC electric field.

EMILIJ K.ZHOLKOVSKIJ, Jacob H. Masliyah; University of Alberta, Department of Chemical and Materials Engineering,536 Chemical- Mineral Engineering Building, Edmonton, Alberta, Canada, T6G 2G6

Dispersion of a non-electrolyte solute due to the electroosmotic flow through a long straight microchannel is analyzed theoretically. Two sources of dispersion are considered: a non-uniformity of electroosmotic flow within a cross-section and a solute adsorption at the channel walls. Three types of electroosmotic velocity non-uniformity are taken into account: (i) the non-uniformity within the double layer region and the non-uniformities due to (ii) longitudinal and (iii) transverse variation of electrokinetic potential. The analysis is conducted using a thin double layer approximation which is valid when the Debye length is much smaller than the characteristic dimensions of the cross-section. Analytical expressions are obtained to address dispersion in terms of plate height for arbitrary magnitude of varying surface potential, electrolyte type and cross-section geometry. The results are presented for different cross-section geometries: parallel planes; circle; annulus; ellipse; arbitrary circumscribed polygon and rectangle. It is discussed how the predicted plate height depends on the cross-section geometry, transverse and longitudinal electrokinetic potential distributions, electrolyte content, adsorption isotherm and adsorption rate constant.

CHUN YANG, Yee Cheong Lam, Than Tun Naing, Huay Min Lee; School of Mechanical and Production Engineering Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798

Over the past decade, there has been a dramatically increasing interest in the development of microfluidic devices, ranging from pH and temperature sensors, to fluid actuators, such as pumps, mixers, and valves, as well as Lab-on-a-Chip systems for drug delivery, chemical analysis, and biomedical diagnosis. Many of microfluidic manipulations are based on electrokinetic transport of materials exploiting the phenomena of electrophoresis and electroosmosis. Fundamental understanding of electrokinetic delivery and manipulation of fluids in microchannels is crucial to the optimum design and process control of microfluidic devices.

In this work, we report an experimental study of electrokientic transport, breakup, and coalescence of gas bubbles in a microchannel. Thorough experiments were carried out to study bubble transport velocity under the influences of the strength of an applied electric field, the type of cations, the SDS surfactant concentration, and the ratio of bubble length to channel diameter. It was observed the bubble velocity increases with increasing electric field strength. Also the higher surfactant concentration (around or less than CMC) led to higher bubble velocities. Moreover, the effect of the type of cations showed scattering; it is associated to the applied electric field and the SDS surfactant concentration. Interestingly, it was found under the same electric field and water chemistry conditions, the bubble velocity exhibited a maximum peak with respect to the ration of bubble length to channel diameter. In addition, the electrokinetic manipulation of bubble breakup and coalescence in microchannels was demonstrated within a window of SDS surfactant and KCl concentrations.

MYUNG-SUK CHUN, Sangwoo Lee; Complex Fluids Research Lab, Korea Institute of Science and Technology (KIST), PO Box 131, Cheongryang, Seoul 130-650, South KOREA

We have developed a particle tracking velocimetry technique to apply on microfluidic chips using fluorescence microscopy and digital image acquisition.  A Hele-Shaw flow channel was designed to allow for visualization in the microfluidic chip fabricated with glass substrate and hydrophobic polydimethylsiloxane.  For the exposure time in taking a photograph, moving fluorescent polystyrene latexes of 1 micrometer radius result in image streaks, in which the latex concentration is dilute.  Applying the relevant principle, the linear velocity of particles at lateral positions of the channel was determined in terms of the ratio of real distance to the number of pixels [Stroock et al., Phys. Rev. Lett., 2000].  The particle velocity profile has been obtained as a function of the solution environments such as solution pH and ionic strength, as well as latex concentration.  It is obvious that the velocity profile is modified depending on the surface character of microchannel walls.  We recognized the slip effect in velocity profiles treating the surfaces as hydrophobic.  Our experimental results are compared to the theoretical prediction based on a model for the colloidal particles under low-Reynolds number conditions.  We also present the implications in quantifying understanding of the transport in microfluidic chips.

JUAN G. SANTIAGO; Dept. of Mechanical Engineering, Stanford University, Stanford, CA 94305

Microfabrication technology has enabled the application of electrokinetics as a method of performing chemical analyses and achieving liquid pumping in electronically-controlled microchip systems with no moving parts.  This talk reviews progress at Stanford including novel methods for sample stacking and fundamental studies of electrokinetic flow instabilities.  Field amplified sample stacking (FASS) leverages conductivity gradients as a robust method of increasing sample concentration prior to CE separation. We have developed novel chip systems that can achieve signal increases of more than 1000 fold using FASS. Electrokinetic instabilities (EKI) present a major challenge to optimizing FASS devices, as well as an opportunity to achieve rapid on-chip mixing.  We have developed generalized models for heterogenous electrokinetic systems for both FASS and EKI, and validated these models with experiments. This work shows that electric body forces associated with the accumulation of charge in the bulk liquid are the cause of EKI.  Suppression and/or control of electrokinetic flow instabilities is directly applicable to sample stacking as conductivity-gradient-induced instabilities dramatically increase dispersion rates and thereby limit stacking efficiency.

JEREMY LEVITAN¹, Martin Z. Bazant¹, Todd Thorsen², Todd Squires³; ¹ Insitute for Soldier Nanotechnologies,

MIT; ²  Department of Mechanical Engineering, MIT; ³ Department of Physics, Caltech

Induced-charge electro-osmosis (ICEO) is a general mechanism for  nonlinear electrokinetic slip in a nearly uniform electrolyte at a polarizable surface (metal or dielectric). The special case of AC electro-osmosis at planar micro-electrode arrays  has been studied extensively in microfluidics. More generally, however, ICEO should also occur at non-electrode surfaces in DC or AC fields in more complicated geometries, although this remains to be demonstrated in experiments directly testing the theory. Here, we describe experiments on ICEO flows around metallic posts in polymer microchannels to study the effects of AC frequency, geometry, concentration, and voltage (nonzero Duhkin number). We also analyze and simulate various simple mathematical models and consider possible advantages and drawbacks of ICEO in microfluidics.

MIGUEL MAYORGA¹, O. Hernández-Flores², L. Romero-Salazar¹, J. M. Rubí³; ¹ Fac. Ciencias, Univ. Aut. Edo. Mex., Av. Instituto Literario 100, Toluca, Mexico, CP 50000;  ² Inst. de Física, Univ. Aut. de Puebla, CP 72570, Puebla, Mexico; ³ Departament de Física Fonamental, Univ. de Barcelona, E-08028, Barcelona, España

Combining a stochastic approach, the rules of non-equilibrium thermodynamics and integral equations, we obtain the density profile of macroions as is observed in a steady state electrophoresis experiment. Our approach permits us to observe the deviation of the density profile from an ideal case, when the pair interactions between the macroions are important, and yields to a better interpretation of the experiments. For the case of macroions with tuning interactions under a temperature gradient, we obtain the behavior of the collective diffusion and the thermal diffusion of the particles in terms of the volume fraction. We use this result to analyze the boosting of DNA in thermocapillary traps.

Stanislav S. Dukhin^1,2, Ralf Zimmermann², Carsten Werner^2,3; ¹ New Jersey Institute of Technology, Newark, NJ 07102-1982, USA; ²  Institute of Polymer Research Dresden, Department Biocompatible Materials, and The Max Bergmann Center of Biomaterials Dresden, Hohe Strasse 6, 01069 Dresden, Germany; ³ University of Toronto, Department of Mechanical and Industrial Engineering, 5 King's College Road, M5S 3G8 Toronto, Canada

During last decades the electrokinetic theory of Smoluchowski was extended to be applicable for soft surfaces (grafted polyelectrolyte layers (PL), biological and artificial membranes etc.) either using the Debye approximation or numerical solutions. In the theory of Ohshima the nonlinearized Poisson-Boltzmann (PB) equation for thick and uniform PL is solved analytically and a general hydrodynamic equation is derived in an integral form. These advantages of the theory of Ohshima provided a base for the further development of a generalized electrokinetic theory for soft surfaces. In his theory the final equation for the electroosmotic (electrophoretic) velocity is specified for the case of the complete dissociation of ionic sites within PL. Accordingly, the equation may be used only if the difference between pK and pH is very large. However, it turned out that an analytical solution of the nonlinearized PB equation for a thick PL is possible for any degree of dissociation. This was achieved using the approximation of excluded coions, if the absolute value of the reduced Donnan potential is larger than 2 and due to the simplification in the case of weak dissociation, when the absolute value of the reduced Donnan potential is less than 2. Combining this generalized DL theory for PL and the theory of Ohshima enables to obtain an analytical equation for electroosmosis for the general case of any degree of dissociation. This equation creates for the first time a theoretical base for the interpretation of electrokinetic fingerprinting (EF) for the characterization of soft surfaces. EF is especially important for soft surfaces, since many parameters (Donnan potential, surface potential, pK, volume fraction of segments, etc.) have to be determined for their comprehensive characterization.

DARRELL VELEGOL¹, Jason D. Feick²; ¹ Department of Chemical Engineering, Penn State University, University Park PA  16802; ² E Ink Corporation, 733 Concord Avenue, Cambridge, MA 02138

The classical models for colloidal forces have assumed uniform charge distributions on the particle surfaces.  However, recent papers have shown that a nonuniform charge distribution on two colloidal particles will significantly reduce colloidal stability from predictions based on a uniform charge distribution.  Thus, nonuniform charge distributions can cause unexpected suspension stability, structure, and rheology.  Using the technique of "rotational electrophoresis" (which combines video microscopy measurements of electrophoretic angular velocities with interpretations from electrokinetic theory), we have previously obtained data showing that particles are nonuniformly-charged on a nanoscale.  More recently we have shown that adding surfactants (e.g., Triton X 100) and polyelectrolytes (e.g., sodium polystyrene sulfonate) often reduces the charge nonuniformity on polystyrene latex particles with sulfate charge groups.  Current work is on charge nonuniformity of temperature-annealed polystyrene particles.  Experiments show that particle charge nonuniformity can be decreased by up to 80

 after temperature annealing.  This talk will show data comparing nanoscale charge nonuniformity before and after annealing, and it will present data that reveals changes in particle stability as a function of heat treatment.

M. QUESADA-PÉREZ¹, E. González-Tovar^2,3, A. Martín-Molina⁴, M. Lozada-Cassou³, R. Hidalgo-Álvarez⁵;

¹ Departamento de Física, Universidad de Jaén, Escuela Universitaria Politécnica de Linares, 23700 Linares, Jaén (Spain); ² Instituto de Física, Universidad Autónoma de San Luis Potosí, Álvaro Obregón 64, 78000, San Luis Potosí (Mexico); ³ Programa de Ingeniería Molecular, Instituto Mexicano del Petróleo, Eje Central Lázaro Cardenas 152, 07730 México, D.F. (Mexico); ⁴ Laboratoire de Physique Statistique de l'Ecole Normale Supérieure associée au CNRS et aux universités Paris VI et Paris VII, 24 rue Lhomond, 75231 PARIS CEDEX 05, France; ⁵ Grupo de Física de Fluidos y Biocoloides, Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Granada, Granada 18071 (Spain)

For many decades, the Gouy-Chapman model, whose cornerstone is the Poisson-Boltzmann equation, has been the traditional approach to describing the electric double layer (EDL). Since the early 80's, a great amount of theoretical work (mostly computer simulations and integral equation theories) has proved that this classical picture of the EDL presents severe failures in the case of electrolytes with multivalent ions as a result of neglecting ion size correlations. The phenomenon of overcharging is probably a representative example of such failures.

Nowadays, there exists a renewed interest in overcharging and other outstanding phenomena occurring the presence of multivalent counterions since they are intimately related to the fascinating behaviour of biological molecules (such as DNA) and its potential applications (e.g., gene therapy).

This work is a critical survey on the relevance of ion size correlations in real colloidal systems (focused mainly on solutions with multivalent counterions). Some previous conclusions (as well as theoretical predictions) of simulations and integral equation theories will be revised using realistic hydrated ion sizes. In addition, we will discuss to what extent ion size correlations contribute to overcharging in colloids of biological nature and other real colloids.

Toshihisa Osaki¹, Ralf Zimmermann¹, Rüdiger Schweiss¹, H. Jürgen Kreuzer², Michael Grunze³, CARSTEN WERNER^1,4; ¹ Department Biocompatible Materials, Institute of Polymer Research Dresden and The Max Bergmann Center of Biomaterials Dresden, Hohe Str. 6, 01069 Dresden, Germany; ² Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Canada; ³ Institute for Applied Physical Chemistry, University of Heidelberg, Germany; ⁴ Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Canada

Charge formation by unsymmetrical adsorption of OH^- and H₃O⁺ ions onto surfaces without dissociable sites determine the electrosurface characteristics of numerous solid/liquid interfaces. Although the variation of the solution pH provided the base of our current thinking it is limited to elucidate the process since the ion product of water is invariant at constant temperature. Therefore, we modulated the autodissociation of water by means of the temperature variation in electrokinetic experiments. Streaming current measurements were performed at 25 C and at 3 C, corresponding to pK_w of 14.0 and 14.8, across rectangular slit channels between macroscopic surfaces, coated with self-assembled monolayers of hexadecanethiol. The solution characteristics were altered by addition of 0.1 M HCl or 0.1 M KOH starting from degassed and de-ionized water. The results demonstrate that the absolute levels of OH^- and H₃O⁺ ions massively influence the interfacial charging of solids without dissociating surface sites in aqueous solution. The dominance of adsorbed OH^- ions over H₃O⁺ ions at neutral pH was confirmed for lower degrees of autodissociation. These new data are discussed in the context with recent theoretical work in which it was shown, on the basis of density functional calculations, that a self-assembled monolayers preferentially adsorb hydroxide ions.

A.M. POPA¹, J. Vleugels¹, J. Vermant², O. Van der Biest¹; ¹ Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium; ² Department of Chemical Engineering, Katholieke Universiteit Leuven, de Croylaan 46, B-3001 Heverlee, Belgium

A well-known polyelectrolyte salt, ammonium poly-methacrylate (Darvan-C) is used to stabilize ethanol based Al2O3 and Ce-ZrO2 suspensions with butylamine addition. The sequence in which n-butylamine and Darvan-C are added to the suspension greatly affects the properties of the suspension and the deposit obtained by electrophoretic deposition. To investigate this effect, electrical conductivity and the shear rate dependence of the viscosity are investigated. When n-butylamine is added first, the equilibrium in the suspension is almost immediately reached and a plastically deformable deposit is obtained over a large n-butylamine/Darvan-C ratio. The suspension has a shear-thinning viscosity and the deformable deposit is characterised by a high solvent content, which allows the rearrangement of particles during drying. When Darvan-C is added before the n-butylamine, the suspension has a lower viscosity and a near-Newtonian behaviour is observed. The deposit is smooth and rigid. The amount of n-butylamine has a stronger influence on the viscosity of the suspension than the amount of Darvan-C. A similar behaviour is observed for Al2O3 and Ce-ZrO2 suspensions. The density of the dried deposits is not influenced by the addition sequence and higher green densities are obtained for Al2O3 when compared to Ce-ZrO2.

VICTOR M STAROV; Department of Chemical Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK

The flow of a multicomponent electrolyte solution in fine cylindrical capillaries is considered. The well-known semi-empirical Osterle's method of the subdivision of electrical potential into quasi-equilibrium and streaming parts and the definition of streaming concentrations and pressure is discussed. The usefulness of this tool as a means of solving the electrohydrodynamic equations is shown and justified: the use of a small parameter enables a system of electrohydrodynamic partial differential equations to be reduced to a system of ordinary differential equations for streaming functions. Boundary conditions for streaming functions at both the capillary inlet and outlet are derived. The proposed model is developed for the flow of a multicomponent electrolyte solution with an arbitrary number of ions. This is coupled with: (i) the introduction of specific interactions between all ions and the pore wall, and (ii) the inclusion of the dissociation of water in both conservation and transport equations. A relation between the frequently used capillary and homogeneous models of nano-filtration membranes is discussed. An example of application of homogeneous model for interpretation of experimental data on nano-filtration separation of electrolyte solution is presented, which shows a reasonable predictive ability of homogeneous model.

D.Fairhurst¹, R. Rowell², R. Shattock³, A. Morfesis⁴; ¹ International Partnership for Microbicides, Silver Spring, MD; ² University of Massachusetts, Amherst, MA; ³ St.George’s Hospital Medical School, London, UK; ⁴ Malvern Instruments Inc, Southborough, MA

Electrokinetic methods provide information on the surface structure of biological cells without producing significant alteration of the cellular organization.  In addition, the course of any chemical, enzymatic, immunological or viral action that results in changes in the number of surface charge groups may be followed. 

A shortcoming of clinical/medical/biological zeta potential data published in the past is that the instrumentation available at the time was not truly capable of precise measurement under the high electrolyte conditions used, such as isotonic, phosphate-buffered saline.  Recent developments in instrumentation - phase analysis light scattering (PALS) - have removed this severe limitation. 

Electrophoretic fingerprinting (EF) is a real-time in-situ technique. An EF is obtained from three-dimensional templates of the mean electrophoretic mobility (the raw data from which the zeta potential is calculated) of a given particle versus pH and solution conductivity at a fixed temperature.  The EF thus represents a surface, described by isomobility lines, over all pertinent electrochemical conditions.  Although proposed as a powerful analytical tool, the technique has been limited because of the time taken to manually collect the necessary raw data.  Also, previous studies using EF have focused only on low electrolyte conditions. Addition of automated titration to PALS instrumentation has made the EF technique viable for biological sytems.

There is little understanding of the charge characteristics of whole virions and the relative contribution of viral and host-cell proteins to such charge. This information is vital in determining the charge basis of, for example, drug-virus interaction.  Since it is known that cells and viruses can have different “activity”, mapping the topography of the “particle” might provide insight to unravel potential interaction mechanisms.

  This paper will present preliminary EF data of human T-cells as part of a wider, in-depth, initial structure-function study.