Role of Electro-Osmosis in Microchannel-Nanochannel Impedance Response

Electrical Impedance Spectroscopy (EIS) has been used extensively for the experimental study and characterization of ion-exchange membranes. The impedance characteristics and the appearance of a tangent line at an apparently universal 45 degree angle for the region near the cut-off of the so-called Warburg branch of the impedance plane (Nyquist) plot are predicted and understood in terms of equivalent circuit models and have been derived from fundamental electro-diffusion.  However, despite the myriad of current applications, a similarly comprehensive study of microchannel-nanochannel interfacial impedance response has not been undertaken. Here we present some fundamentally new experimental results showing similar Warburg response to membrane systems at zero bias and a deviation from the constant-angle Warburg response seen in membrane systems, resulting in a decrease in the tangent angle increasing with applied current in microchannel-nanochannel devices.  Curious features in the Bode plots distinct from those of membrane systems are also shown. The experimental results are analyzed in terms of a fundamental theory relating some of these differences in response to the presence and AC perturbations of electro-osmotic flow in the microchannel-nanochannel system, resulting in modification of the standard Warburg-type impedance term as well as streaming contributions to the impedance.