On mixed electroosmotic-pressure driven flow and mass transport in microchannels

Abstract

In this paper, we report the combined pressure-electroosmotically driven flow and species transport in micro/nano-channels. By performing a detailed numerical simulation based on the coupled Poisson, Nernst–Planck and incompressible Navier–Stokes equations (NP-model), we discuss the effects of imposed pressure gradient on the electrokinetic transport of electrolyte. The Reynolds number dependence of the mixed EOF is analyzed for both thin and overlapped EDL cases. Validity of the linear model based on the equilibrium Boltzmann distribution of ions and velocity governed by the linear superposition of Helmholtz–Smoluchowski velocity with the Poiseuille flow is examined by comparing with the non-linear NP-model. Both the models are found to be close when the EDL is sufficiently thin, but they differ when the bulk of the channel is not electrically neutral. Flow reversal and the suppression of electroosmotic flow are possible through regulating the imposed pressure field. At a sufficiently large value of the imposed pressure gradient the electrokinetic effect becomes negligible and flow resembles the fully developed Poiseuille flow. A similarity of the mixed EOF-pressure driven flow with the imposed pressure gradient, surface potential and imposed electric field is investigated. The mixed EOF characteristics is found to be strongly dependent on the electric double layer thickness. The advection effect dominates over the diffusion and electrophoretic migration in transport of charged species with favorable pressure gradient.