  Ion transport in perm-selective media has been extensively investigated in
recent years owing to its applications in advancements of fundamental
understanding of nanoscale electrokinetics as well as innovative engineering
applications. A key phenomenon in this context is ion concentration
polarization (ICP) that occurs near perm-selective nanoporous membranes or
nanochannels under dc bias. In classical settings, involving voltage-driven
transport of ions through perm-selective membranes, concentration polarization
is well understood as formation of steep concentration drop to reach near zero
concentration (depletion layer) at the anodic side of cation-selective
membranes. In contrast to this classical description, we demonstrate here that
when ICP is driven in a microchannel and coupled with electroosmotic flows a
long-tailed fluorescent layer in front of diffusion layer is formed, which can
be characterized as an additional plateau in the concentration profile. Using a
micro/nanofluidic platform, in situ visualizations, and local concentration
measurements by microelectrode arrays we quantify that this plateau layer has a
concentration of 60 % of bulk electrolyte concentration, regardless of the bulk
concentration itself. Using a mathematical analysis of conservation laws for
flow, charge, and salt, we demonstrate that this plateau region is related to
the formation of a flow stagnation point due to the competition between
electro-osmotic and induced pressure driven flows. Consistent with the
experiments, this analysis predicts the plateau concentration to be 60% of the
bulk electrolyte. Thus, this plateau concentration region completes the picture
of ICP in microchannels which has drawn significant attention from the fields
of electrokinetics and micro/nanofluidics.
