Communication: The phoretic drift of a charged particle animated by a direct ionic current

A charged colloidal particle which is suspended in an electrolyte solution drifts due to an external voltage application. For direct currents, particle motion is affected by two separate mechanisms: electro-osmotic slip associated with the electric field and chemi-osmotic slip associated with the inherent salt concentration gradient in the solution. These two mechanisms are interrelated and are of comparable magnitude. Their combined effect is demonstrated for cation-exchange electrodes using a weak-current approximation. The linkage between the two mechanisms results in an effectively modified mobility, whose dependence on the particle zeta potential is nonlinear. At small potentials, the electro-osmotic mechanism dominates and the particle migrates according to the familiar Smoluchowski mobility, linear in the electric field. At large zeta potentials, chemiosmosis becomes dominant: for positively charged particles, it tends to arrest motion, leading to mobility saturation; for negatively charged particles, it enhances the drift, effectively leading to a shifted linear dependence of the mobility on the zeta potential, with twice the Smoluchowski slope.