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 v is typically predicted through the relation v = D ∇log c, where D is the diffusiophoretic mobility and c is the concentration of the electrolyte. The logarithmic dependence of v on c may suggest that the strength of diffusiophoretic motion is insensitive to the magnitude of the electrolyte concentration. In this article, we emphasize that D is intimately coupled with c for all electrolyte concentrations. For dilute electrolytes, the finite double layer thickness effects are significant such that D decreases with a decrease in c. In contrast, for concentrated electrolytes, charge screening could result in a decrease in D with an increase in c. Therefore, we predict a maximum in D with c for moderate electrolyte concentrations. We also show that for typical colloids and electrolytes , where D 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 with c, 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.