On the coupling between buoyancy forces and electroconvective instability near ion-selective surfaces

TL;DR

This paper investigates how buoyant forces influence electroconvective instability near ion-selective surfaces, revealing that gravity can significantly modify flow patterns and transport rates when the Rayleigh number exceeds about 1000.

Contribution

It provides a detailed analysis of the interaction between gravitational convection and electroconvection, highlighting the conditions under which buoyancy affects electroconvective transport.

Findings

Buoyancy significantly alters electroconvection when Rayleigh number > 1000.

Gravity can stabilize or destabilize electroconvective patterns.

Flow patterns include upward and downward fingers of different densities.

Abstract

Recent investigations have revealed that ion transport from aqueous electrolytes to ion-selective surfaces is subject to electroconvective instability that stems from coupling of hydrodynamics with electrostatic forces. Electroconvection is shown to enhance ion mixing and the net rate of transport. However, systems subject to electroconvection inherently involve fluid density variation set by salinity gradient in the bulk fluid. In this study we thoroughly examine the interplay of gravitational convection and chaotic electroconvection. Our results reveal that buoyant forces can significantly influence the transport rates, otherwise set by electroconvection, when the Rayleigh number $Ra$ of the system exceeds a value $Ra \sim 1000$. We show that buoyancy forces can significantly alter the flow patterns in these systems. When the buoyancy acts in the stabilizing direction, it limits the extent of penetration of electroconvection, but without eliminating it. When the buoyancy destabilizes the flow, it alters the electroconvective patterns by introducing upward and downward fingers of respectively light and heavy fluid.