Diffusiophoresis: from dilute to concentrated electrolytes. Issue 30 (21st July 2020)

• Record Type: Journal Article
• Authors: Gupta, Ankur; Shim, Suin; Stone, Howard A.
• Is Part Of: Soft matter. Volume 16:Issue 30(2020)
• Page Start: 6975
• Page End: 6984
• Publication Date: 2020-07-21
• Subjects: Soft condensed matter -- Periodicals; 530.413
• DOI: 10.1039/d0sm00899k
• Languages: English
• ISSNs: 1744-683X

Abstract

Abstract : The underappreciated effect of ion concentration on diffusiophoretic mobility is investigated, both theoretically and experimentally. It is demonstrated that diffusiophoretic mobility possesses a maximum with ion concentration. 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.