AMaSiS 2018 Workshop: Abstracts

Electro-Convective Instability in Concentration Polarization

Isaak Rubinstein and Boris Zaltzman

Ben-Gurion University, Jacob Blaustein Institutes for Desert Research

DC ionic current in a binary electrolyte passing through a perm-selective interface (electrode, ion exchange membrane, micro-nano-channel junction) is a basic element of many electrochemical engineering and micro-fluidic processes, such as electrodeposition, electrodialysis and protein pre-concentration. Such current passage is diffusion-limited in the sense that it induces a decrease of electrolyte concentration towards the interface, known as Concentration Polarization, whose expression is the saturation of current upon increasing voltage at some value known as the limiting current. Upon further increase of voltage, this saturation is followed by a relatively rapid current increase - the so-called Over-Limiting Conductance regime. The mechanism of Over-Limiting Conductance had remained unexplained for a long time. Only recently was it shown that in open systems Over-Limiting Conductance is due to the destruction of the diffusion layer by a micro-scale vortical flow which spontaneously develops as a result of Electro-Convective Instability near the limiting current and provides an additional ionic transport mechanism. This instability continues to be attributed to non-equilibrium Electro-Osmosis related to the Extended Space Charge, which develops at the limiting current. One reason for this attribution is the fact that for a perfectly perm-selective solid of infinite electrical conductivity, neither equilibrium Electro-Osmosis nor bulk electric force can yield Electro-Convective Instability. On the other hand, it has been shown that non-equilibrium Electro-Osmosis can. Recently, we have reported that relaxing the assumption of infinite electrical conductivity allows for equilibrium Electro-Convective Instability at a perfectly charge-selective solid, such as ion-exchange membrane or metal electrode, irrespectively of any extended space charge effects. In my talk I will present a simple model of electroconvective diffusion of ions in the depleted diffusion layer in this system which predicts a supercritical transition to instability in the vicinity of the limiting current, as opposed to the subcritical transition for the previously studied nonequilibrium instability related to the Extended Space Charge. The linear stability analysis in this model yields the division of the parameter space into domains in which each instability mechanism with its characteristic signatures dominates. Identification of the particular instability mechanism for a given system requires a detailed experimental study of the vicinity of the instability threshold in terms of both the voltage versus current dependence and flow visualization.