Stabilization of electroconvective vortices by membrane corrugation: Thresholds, transitions, and impacts on ion transport

Abstract

In this work, the impact of geometrical membrane corrugations on ion transport and concentration polarization-induced electroconvection was investigated. A high-precision micro-particle-tracking velocimetry setup enables to visualize the temporal evolution of electroconvective vortices on different membrane surface geometries while simultaneously evaluating ion transport through the monitoring of current–voltage curves. A significant change in the electro-hydrodynamic response of a laboratory-scale electrodialysis cell was observed at overlimiting current densities for different corrugation sizes on the membrane. Experiments reveal that there is a relationship between the size of the corrugation features, the size of electroconvective vortices, and their effect on ion transport. We identified for the first time the coexistence of small and large electroconvective vortices, as foreseen by recent simulations, each moving at unique velocities. Additionally, it was found that the usually chaotic electroconvective vortices are stabilized by the electroosmotic forces induced by the membrane corrugation. More extensive membrane features led to more stable vortices, aligning their counter-rotation with the membrane’s geometry. A threshold for stable and unstable vortices was determined, and a characterization of transitioning to a chaotic and destabilized state was derived. Finally, the contribution of stable vortices to ion transport was evaluated, finding that stable, larger vortices lead to enhanced transport. These findings contribute to the proposal of optimizing membrane designs for electrodialysis application as a possible solution to overcome ion transport limitations at overlimiting current densities.