Electrokinetics in Viscoelastic Liquid Electrolytes above the Diffusion Limit

Electrodeposition of metals at planar electrodes is known to be fundamentally unstable in liquid electrolytes driven at high ionic currents. The instability is thought to arise from unstable hydrodynamics at the electrode/electrolyte interface but may ultimately couple to underlying morphological instabilities to enhance the growth of ramified, three-dimensional structures, including dendrites. Here, we experimentally study the effect of high-molar-mass polymers on transport properties of liquid electrolytes and investigate the effect of electrolyte viscoelasticity in regulating the hydrodynamic instability known as electroconvection. Experiments performed in electrolytes bounded either by cation-selective polymer membranes or metallic electrodes make it possible to decouple hydrodynamic and morphological instabilities for the first time. Through indirect electrokinetic and direct tracer particle visualization studies, we find that high-molecular-weight polymers are effective in suppressing hydrodynamic instabilities in liquid electrolytes, particularly when the polymers are long enough to entangle. Direct visualization experiments reveal further that even at modest fields, electroconvection occurs on multiple length scales, and entangled liquid electrolytes reduce convection throughout the length scale range accessible to our experiments.