Diffusiophoresis of ionic catalytic particles

Abstract

A migration of charged particles relative to a solvent, caused by a gradient of salt concentration and termed a diffusiophoresis, is of much interest being exploited in many fields. Existing theories deal with diffusiophoresis of passive inert particles. In this paper, we extend prior models by focusing on a particle, which is both passive and catalytic, by postulating an uniform ion release over its surface. We derive an expression for a particle velocity depending on a dimensionless ion flux (Damkohler number Da) and show that a charged region is formed at distances of the order of the particle size, provided the diffusion coefficients of anions and cations are unequal. When Da becomes large enough, the contribution of this (outer) region to the particle velocity dominates. In this case the speed of catalytic passive particles augments linearly with Da and is inversely proportional to the square of electrolyte concentration. As a result, they always migrate towards a high concentration region and in dilute solutions become much faster than inert (non-catalytic) ones.