Diffusiophoresis: from dilute to concentrated electrolytes.

Abstract

Electrolytic diffusiophoresis is the movement of colloidal particles in response to a concentration gradient of an electrolyte. The diffusiophoretic velocity v_DP is typically predicted through the relation v_DP = D_DP ∇log c_s, where D_DP is the diffusiophoretic mobility and c_s is the concentration of the electrolyte. The logarithmic dependence of v_DP on c_s may suggest that the strength of diffusiophoretic motion is insensitive to the magnitude of the electrolyte concentration. In this article, we emphasize that D_DP is intimately coupled with c_s for all electrolyte concentrations. For dilute electrolytes, the finite double layer thickness effects are significant such that D_DP decreases with a decrease in c_s. In contrast, for concentrated electrolytes, charge screening could result in a decrease in D_DP with an increase in c_s. Therefore, we predict a maximum in D_DP with c_s for moderate electrolyte concentrations. We also show that for typical colloids and electrolytes , where D_s is the solute ambipolar diffusivity. To validate our model, we conduct microfluidic experiments with a wide range of electrolyte concentrations. The experimental data also reveals a maximum in D_DP with c_s, in agreement with our predictions. Our results have important implications in the broad areas of electrokinetics, lab-on-a-chip, active colloidal transport and biophysics.