|Measurement of Colloidal Forces|
We have developed a new optical technique called Total Internal Reflection Microscopyfor monitoring the instantaneous separation between a Brownian microsphere and the nearby wall. Changes in distance as small as 1 nm can be detected. From the equilibrium distribution of elevations sampled by Brownian motion, we can determine the potential energy profile. Thus we have measured the net bouyant weight of single particles (as small as 0.1 pN), double-layer (electrostatic) repulsion, retarded van der Waals attraction, depletion attraction (arises from nonadsorbing polymer), steric repulsion (arises from adsorbed polymer), the optical force exerted by a focussed laser, and unexpectedly long-range attraction between receptor-ligand pairs. From the dynamics of Brownian motion, we can determine separately the mobility or diffusion coefficient of the sphere, which are greatly reduced by hydrodynamic hinderance afforded by proximity to the wall.
Recent Article: S. Biggs, D.C.Prieve and R.R.Dagastine, “Direct Comparison of AFM and TIRM Measurements in the Presence of Non-adsorbing Polyelectrolytes”, Langmuir 21, 5421-5428 (2005). DOI: 10.1021/la050041e
Review Article: D.C. Prieve, “Measurement of Colloidal Forces with TIRM,” Adv. Colloid Interface Sci. 82, 93 (1999). preprint
|2-D assembly of colloidal particles on a/c electrodes|
D/C current normal to a planar electrode causes particles next to the electrode to aggregate as a result electroosmotic flow around the particles; reversing the polarity of the d/c field causes disaggregation. a/c current normal to the electrode can also cause aggregation if the frequency, amplitude and electrolyte are correctly chosen. Aggregation is one step in producing multilayered nanodeposits for photonic devices. Paul Sides and I are using TIRM to observe the motion of single particles NORMAL to an electrode under the same conditions in which others have studied the TANGENTIAL motion of ensembles. A major advantage of using single particles is that any model is axisymmetric; a second advantage is that no other technique is able to make these measurements.
Recent Article: Fagan, J. A.; Sides, P. J.; Prieve, D. C., “Evidence of Multiple Electrohydrodynamic Forces Acting on a Colloidal Particle near an Electrode Due to an Alternating Current Electric Field,” Langmuir 21, 1784-1794 (2005). DOI: 10.1021/la048076m
|Determination of Zeta Potential on a Flat Plate Using a Rotating Disc|
In water, a charged particle attracts ions of opposite charge, which accumulate near the surfaces to form a diffuse cloud. The charge on microscopic particles is usually determined by measuring their terminal velocity in an electric field. Determining the charge on a flat surface is more difficult. By rotating a circular disk about its axis, we shear the ion cloud which generates a streaming current. Conservation of charge induces a streaming potential profile in the bulk solution which can be measured and used to deduce the surface charge density. We are working with Malvern Instruments (UK) to develop a commercial device to measure electrical properties of membranes, silicon wafers etc.
Recent Article: J. D. Hoggard, P.J. Sides and D.C. Prieve, "Measurement of the Streaming Potential and Streaming Current near a Rotating Disk to Determine Its Zeta Potential," Langmuir 21, 7433-7438 (2005). DOI: 10.1021/la050537w
|Stability of dispersions caused by nonionic polymers|
Nonionic water-soluble polymers are commercially available and used as dispersants to suspend fine particles in a liquid. For example, Pluronic(R) dispersants sold by BASF are even approved by the FDA for use in drug delivery. We now believe such polymers adsorb on the surface of much larger particles to produce a water-like gel film which acts sterically to prevent the particles from coming close enough together to stick by van der Waals attraction. Heating the solution causes the layer of Pluronic to collapse resulting in loss of stability at some critical temperature. We are now trying to understand why this critical temperature depends substantially on the size of the particle on which it adsorbed. I believe the thickness is a continuous function of temperature; i.e. its collapse is not a 1st order phase transition.
Recent Article: D.C. Prieve and M.A. Bevan, “Effect of Physisorbed Polymers on the Interaction of Latex Particles and Their Dispersion Stability,” in Polymers in Particulate Systems: Properties and Applications #(ed. by V.A. Hackley, P. Somasundaran and J.A. Lewis), Vol. 104 of Surfactant Science Series, Marcel Dekker, New York, (2001).
|Electrostatic repulsion between particles in liquids of low dielectric constant|
Electrostatic repulsion across water is quite well understood. A charged particle is surrounded by a cloud of counterions which make it appear electrically neutral from a distance. When two particles are close enough together for their clouds to overlap, repulsion is experienced which decays exponentially with separation (rather than the inverse-square expected from Coulomb's law); the decay length is the Debye length from the Debye-Huckel theory. Electrolytes tend to weakly dissociate in low-dielectric fluids like dodecane. Mechanisms of charging as well as electrostatic forces resulting from charge are much less well understood in nonaqueous media. These are quite important (for example) for display technology in which electric fields are used to turn pixels on and off.