Diffusiophoresis: from dilute to concentrated electrolytes

Electrolytic diffusiophoresis is the movement of colloidal particles in response to a concentration gradient of an electrolyte. The diffusiophoretic velocity vDP is typically predicted through the relation vDP = D-DP del 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 vertical bar D-DP/D-s vertical bar less than or similar to 1, 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.