Nonlinear electrophoresis of a polarizable particle by incorporating electrostatic correlations and ion steric interactions

We have studied the nonlinear electrophoresis of a polarizable charged particle in a monovalent as well as multivalent electrolyte by considering the short-range steric interactions and ion–ion electrostatic correlations of finite-sized ions. At a moderate applied field, in which the voltage drop across the Debye layer becomes higher than the thermal potential, the impact of the dielectric particle polarization becomes significant, resulting in an induced surface charge, which has opposite polarity on the two sides of the particle. In this process, an ionic exchange occurs between the electric double layer, which surrounds the particle, and the bulk solution. The steric effect due to the finite size of ions and correlation among ions modifies the ion distribution in the electric double layer and consequently modifies the polarization of the particle. Most of the earlier studies on nonlinear electrophoresis were limited to conducting particles by imposing a constant ζ-potential for a monovalent electrolyte and were found to have several discrepancies from experimental analysis. By considering the modified electrokinetic model for a dielectric particle, we provide an intricate analysis and justification of several experimentally observed phenomena. We adopt a continuum model, which incorporates the correlations of finite-sized ions and hydrodynamic steric interactions. The viscosity of the suspension is considered to vary with ionic volume fraction. The valence asymmetric electrolyte generates a nonuniform induced surface charge density, which modifies the electric force on the particle. We have validated our numerical algorithm with existing experimental results of nonpolarizable particles and thin-layer analysis of dielectric particles. At a higher imposed electric field, the over-screening of the surface charge due to the correlations among multivalent counterions attenuates, leading to the suppression of mobility reversal. The ion correlations and saturation attenuate the impact of polarization in the modified model.