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Possible mechanisms of overlimiting current in unsupported electrolytes,exceeding diffusion limitation, have been intensely studied for theirfundamental significance and applications to desalination, separations,sensing, and energy storage. In bulk membrane systems, the primary physicalmechanism is electro-convection, driven by electro-osmotic instability on themembrane surface. It has recently been predicted that confinement by chargedsurfaces in microchannels or porous media favors two new mechanisms,electro-osmotic flow (EOF) and surface conduction (SC), driven by largeelectric fields in the depleted region acting on the electric double layers onthe sidewalls. Here, we provide the first direct evidence for the transitionfrom SC to EOF above a critical channel height, using in situ particle trackingand current-voltage measurements in a micro/nanofluidic device. The dependenceof the over-limiting conductance on channel depth (d) is consistent withtheoretical predictions, scaling as d^-1 for SC and d^4/5 for EOF with atransition around d=8um. This complete picture of surface-driven over-limitingcurrent can guide engineering applications of ion concentration polarizationphenomena in microfluidics and porous media.

Experimental Verification of Overlimiting Current by Surface Conduction and Electro-osmotic Flow in Microchannels