Energy conversion by surface-tension driven charge separation

TL;DR

This paper investigates how surface tension gradients can induce charge separation and generate streaming potentials in electrolytic flows, analyzing effects of electric double layer properties, surface charge, slip, and temperature or surfactant variations.

Contribution

It provides a detailed analysis of shear-induced electrokinetic streaming potential in free-surface flows, including a proposed method for surface-tension driven energy conversion with efficiency comparisons.

Findings

Streaming potential depends on Debye length and surface charge.

Surface tension variations can induce significant streaming potentials.

Destillocapillarity can achieve efficiencies comparable to pressure-driven flows.

Abstract

In this work, the shear-induced electrokinetic streaming potential present in free-surface electrolytic flows subjected to a gradient in surface tension is assessed. Firstly, for a Couette flow with fully resolved electric double layer (EDL), the streaming potential per surface stress as a function of the Debye parameter and surface potential is analyzed. By contrast to the Smoluchowski limit in pressure-driven channel flow, the shear-induced streaming potential vanishes for increasing Debye parameter (infinitely thin EDL), unless the free surface contains (induced) surface charge or the flow at the charged, solid wall is permitted to slip. Secondly, a technical realization of surface-tension induced streaming is proposed, with surface stress acting on the free (slipping) surfaces of a micro-structured, superhydrophobic wall. The streaming potential is analyzed with respect to the slip parameter and surface charge. Finally, the surface tension is assumed to vary with temperature (thermocapillarity) or with surfactant concentration (destillocapillarity). The maximal thermal efficiency is derived and compared to the Carnot efficiency. For large thermal Marangoni number, the efficiency is severely limited by the large heat capacity of aqueous solvents. By contrast, destillocapillary flows may reach conversion efficiencies similar to pressure-driven flow.