Overlimiting Conductance in Confined Systems

In the course of this project overlimiting conductance (OLC) in concentration polarization (CP) was studied for two kinds of confinements near a perm-selective element: single micro-channel and micro-nano-porous frit. As opposed to open systems in which OLC results from hydrodynamic instability, in confined systems OLC is due to the propagation of Desalination Shocks (DS). Depending on channel depth or pore size, DS forms through one of the following mechanisms. For small pores, the porous medium effectively acts as weakly-charged ion-exchanger. In this case DS formation is tantamount to salt exclusion from such an ion-exchanger adjacent to a perfectly perm-selective membrane in the course of CP. For wider pores, DS forms due to a subtle Taylor-dispersion-like effect resulting from the interaction of electro-osmotically driven flow along the pore walls with a pressure-driven back-flow along the pore axis. Both mechanisms were tested theoretically and experimentally. A practical desalination device based on these principles was designed and tested. A detailed analysis of the onset of the Taylor dispersion regime and its eventual breakdown upon the increase of the surface flow rate was analyzed in detail for an idealized channel setup. The effect of the non-perfect membrane perm-selectivity was investigated theoretically for the following three settings: (1) The effect of CP on perm-selectivity was analyzed for homogeneous and heterogeneous membranes; (2) The feasibility of the previously unknown equilibrium electro-convective instability was discovered; (3) The breakdown of a purely 1D quiescent conduction state into electroosmotic oscillations without a pre-imposed concentration drop was predicted and analyzed.