(96a) Desalination Shocks in Microstructures

Transport in quasi-neutral bulk electrolytes is governed by linear diffusion and convection. As first noted by J. J. Bikerman in the 1930s, the proximity of charged walls increases the importance of surface conduction and electro-osmotic flow, driven by diffuse ionic charge. In nano-channels and membranes with overlapping double layers, these interfacial effects dominate diffusion, resulting in linear electrokinetic phenomena. In microchannels and porous media, however, bulk and interfacial transport compete with each other, and the dynamics of concentration polarization become nonlinear. Mani, Zangle and Santiago recently showed that nearly discontinuous shocks in the conductivity can propagate from a microchannel/nanochannel junction, analogous to shock waves in gases, even if the double layers are thin. This talk describes the basic physics of such ?desalination shocks? and develops a mathematical theory of concentration polarization in non-uniform microstructures or porous media. The theory predicts a new mechanism for super-limiting current to membranes or electrodes, driven by surface conduction, even in the absence of convection.