Effects of Wall Roughness and Heterogeneity of Potential on Electroosmotic Flow Within a Nano-Channel

Abstract

Electroosmotic flow in micro- and nano-channels is characterized by low Reynolds number, so the inertial effects are negligible. The motivation of the present study is to generate strong convection by introducing channel wall roughness in the form of a block which has a step jump in ζ–potential on the upper face of the wall mounted block. The width and length of the channel is considered to be high compare to the height of the channel, thus a two dimensional EOF is considered in the present analysis. The present model is based on the Nernst-Plank model including convective effects. A numerical method based on the pressure correction iterative algorithm (SIMPLE) is adopted to compute the flow field and mole fraction of the ions. Potential field is computed through the SOR technique. The EOF is found to be predominantly two-dimensional with strong convectional effect near the wall roughness. The usual plug-like profile for velocity is absent near the wall roughness. The potential patch induces a strong recirculation vortex which inturn generates strong pressure gradient. The streamlines follow a tortuous path near the wall roughness. The volume flow rate enhanced significantly. We found that the ionic distribution follow the equilibrium Boltzmann distribution away from the wall roughness. But the solutions due to Boltzmann distribution and Nernst Plank model are different when inertial effects are significant. The strength of the vortex, which appears adjacent to the potential patch, increases almost linearly with the increase of overpotential above the patch.