Overlimiting current in non-uniform arrays of microchannels*

Overlimiting current (OLC) through electrolytes interfaced with perm-selective membranes has been extensively researched in recent years. Identified mechanisms responsible for OLC include surface conduction (SC), convection by electro-osmotic flow (EOF), and electro-osmotic instability (EOI). In each system the dominant mechanism depends on input parameters such as surface charge and geometric constrictions. This work studies how a network of microchannels in a non-uniform array can induce OLC. To this end, micro/nanofluidic devices were fabricated with arrays of parallel microchannels with either uniform or non-uniform size distributions. All cases maintained the same fixed surface and bulk conduction. The current-voltage measurement demonstrated that OLC increased with increasing size non-uniformity up to a certain threshold. A visualization of internal flows indicates that parallel channels with non-uniform size induce flow loops across the network enhancing advective transport. These results can provide an understanding about ion transport in natural micro/nanoporous materials with random geometric structures.