Simulation of Colloidal Diffusiophoresis in a Microfluidic Flow Cell

Diffusiophoresis is the advective transport of colloids in a fluid driven by a concentration gradient of solute. For charged colloids in an electrolyte gradient, diffusiophoresis results from solute osmotic pressure gradients and the electric field induced by relative motion of the charged colloid and electrolyte ions. Diffusiophoretic fluxes can exceed colloidal diffusion fluxes, so that diffusiophoresis may be the predominant transport mechanism for colloids whenever there is an electrolyte concentration gradient, and diffusiophoresis may provide a mechanism for separation of colloids based on charge. The goal of this project was to simulate colloidal diffusiophoresis in an imposed laminar flow in a microfluidic channel, as microfluidics are well suited to conducting diffusiophoresis experiments. The coupled advective diffusion equations for the solute and the colloid were solved using the COMSOL finite element simulation package. Simulations addressed how the microfluidic channel influenced experimental measurements. Particular attention was paid to the formation of steady-state solute gradients and colloid concentration profiles near the channel inlets, the variation of colloid concentration across the depth of the channel, and the possible variation of colloid diffusiophoretic mobility in response to local electrolyte concentration. Simulation results replicated key experimental observations and indicated 1) that sharp solute gradients near the inlets cause an abrupt accumulation of colloids, 2) that colloid concentration variation across the channel depth had negligible effects, and 3) that colloid mobility variation with electrolyte concentration had a small effect on the steady state colloid concentration profile.