Advective species separation in combined steady and unsteady flows: Taylor dispersion analysis

Counter flow mass transfer provides viable routes for species separation in microchannels by effectively imposing different mass transfer resistances on different species, e.g. counter flow electromigration. Here, we propose a separation technique that exploits the competition between unsteady advective flows and diffusive mass transfer. We show that this method discriminates between species having different diffusion coefficients at high Peclet number flows. A Taylor dispersion approximation reveals that two Peclet numbers appear: one based on the overall length of the channel and the other on its radius. Increasing the radial Peclet number enhances the diffusive mass transfer in the duct, which reduces selectivity, while increasing the longitudinal Peclet number enhances selectivity through advective counterflow effects. Both Peclet numbers are species-dependent, and even a modest difference in Peclet numbers introduces a significant selectivity for species separation. We perform parametric studies to identify parameter space regions that maximize species separation.