An Analysis of Electrophoresis of Concentrated Suspensions of Colloidal Particles.

An analysis of the electrophoretic motion of charged colloidal particles in a concentrated suspension is developed to predict the electrophoretic mobility of the particles and the electrical conductivity of the suspension. The analysis is based on a unit cell model that takes into account particle-particle hydrodynamic interactions and includes relatively thick electric double layers. The fluid motion in the unit cell is treated by writing the relevant Navier-Stokes equation in terms of the stream function and vorticity. The governing equations were then solved by a finite-difference method. The calculated electrophoretic mobilities are in agreement with prior analytical solutions for moderately concentrated suspensions, and the theory reduces to the result of O'Brien and White for low to moderate zeta potentials and dilute suspensions and to the classical result of Smoluchowski for thin double layers and dilute suspensions. A parametric study shows that the electrical conductivity of the suspension relative to a free electrolyte solution is affected by the counterion to co-ion diffusivity ratio, the double-layer thickness, and the volume fraction of particles. For a dispersion of moderately charged particles (moderate zeta potentials) with thick double layers, the numerical model predicts the electrical conductivity in agreement with experimental values reported in the literature. Copyright 1999 Academic Press.

Electrodynamic Trapping of Aerocolloidal Particles: Experimental and Theoretical Trapping Limits

Aerocolloidal particles have been trapped from an uncharged source aerosol using an electrodynamic balance. Graphite and soot particles were charged photoelectrically using a Xe2 (172 nm) excimer lamp, while particles of titanium dioxide, sodium nitrate, and diethylhexyl sebacate (DEHS) were charged using a unipolar corona charger prior to injection into the chamber. It was found that the Stokesian drag force produced by convection in the balance chamber can destabilize the levitated microparticle when it exceeds the electrostatic force required to center the particle. Although the electrostatic restoring force can be increased by increasing either the particle charge or the ac field strength, charging of the particles is more difficult as the particle diameter is decreased, which gives rise to a trapping limit. Monodisperse DEHS particles were used to determine the experimental trapping limit for unipolar charging. For the experimental apparatus used in this study, a diameter of about 1 µm was found to be the trapping limit for DEHS. Results are compared to the theoretical trapping limit calculated by a force balance on a particle exposed to motion of the surrounding gas.