  Overlimiting current (OLC) through electrolytes interfaced with
perm-selective membranes has been extensively researched in recent years for
understanding the fundamental mechanisms of transport and developing efficient
applications from electrochemistry to sample analysis and separation.
Predominant mechanisms responsible for OLC include surface conduction,
convection by electro-osmotic flow, and electro-osmotic instability depending
on input parameters such as surface charge and geometric constrictions. This
work studies how a network of microchannels in a non-uniform array, which
mimicks a natural pore configuration, can contribute to OLC. To this end,
micro/nanofluidic devices are fabricated with arrays of parallel microchannels
with non-uniform size distributions. All cases maintain the same surface and
bulk conduction to allow probing the sensitivity only by the non-uniformity of
the channels. Both experimental and theoretical current-voltage relations
demonstrate that OLCs increase with increasing non-uniformity. Furthermore, the
visualization of internal recirculating flows indicates that the non-uniform
arrays induce flow loops across the network enhancing advective transport.
These evidences confirm a new driving mechanism of OLC, inspired by natural
micro/nanoporous materials with random geometric structure. Therefore, this
result can advance not only the fundamental understanding of
nanoelectrokinetics but also the design rule of engineering applications of
electrochemical membrane.
