Electrodiffusiophoresis: Particle motion in electrolytes under direct current

Abstract

Colloidal particles in electrolytes move in response to electric fields (electrophoresis) and salt concentration gradients (diffusiophoresis), and related flows also occur at fixed surfaces (electro-osmosis and diffusio-osmosis, respectively). In isolation, these electrokinetic phenomena are well understood, e.g., electrophoresis without far-field concentration gradients and diffusiophoresis without applied electric fields. When the electrolyte passes direct current, however, concentration gradients accompany the bulk electric field (concentration polarization) and the resulting particle motion, called “electrodiffusiophoresis,” involves a nonlinear combination of electrophoresis and diffusiophoresis, depending on ion transference numbers and particle properties. In this work, we analyze the electrodiffusiophoresis of spherical particles in the limit of thin double layers, neglecting surface conduction (Du<<1) and convection (Pe<<1), considering both nonpolarizable (fixed charge) and ideally polarizable (induced-charge) surfaces. Via asymptotic approximations and numerical solutions, we develop a physical picture to guide potential applications in electrochemical cells, such as analyte focusing, electrophoretic deposition, and microfluidic mixing near membranes or electrodes. By controlling the mean salt concentration, particle size, current, and concentration gradient, significant motion of particles (or fluid) is possible toward either electrode and toward high or low concentration.