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