Concentration polarization effects in nanochannel induced-charge electro-osmosis

Concentration polarization (CP) has been observed in a variety of configurations: nanochannels, nanopores, ion-permselective membranes, and embedded conductors. It is believed, but not conclusively proven, that electric double-layer (EDL) overlap plays a significant role in CP. Consequently, further studies of fundamental electrokinetic effects related to CP are needed. We present theoretical and numerical studies of CP effects near an un-biased conductor placed in a straight nanoslit. In our model we combine the full Poisson-Nernst-Planck equations with Helmholtz-Smoluchowski slip, whereby we can construct large computational domains and at the same time resolve the EDL in the vicinity of the conductor. Based on our results we try to identify the basic physical mechanisms that lead to the CP. We show that when subjected to an external bias the axial symmetry is broken by the electro-osmotic flow. This leads to nonlinear interactions between flow, electric potential, and charged chemical species in the induced EDL on the conductor.