Here, investigations on some electrokinetic phenomena on the combined pressure-driven electroosmotic flow near a wall roughness in the form of a rectangular block mounted on one wall of an infinitely long microchannel have been reported. The insulated rectangular block has a constant surface potential which is different from the surface potential of the remaining part of the channel walls. The characteristics for the electrokinetic flow are obtained by numerically solving the Navier-Stokes equations coupled with Nernst- Planck equation for ion transport and Poisson equation for electric field. The electrokinetic transport and other hydrodynamic effects under the application of combined pressure and DC electric fields for different values of electric double layer thickness and channel patch potential have been discussed. Vortical flows developed above the block when the surface potential of the block is in opposite sign to that of the surface potential of the channel. The combined effects due to geometrical modulation of the channel wall and heterogeneity in surface potential is found to produce a stronger vortex, and hence a stronger mixing, compared with either of these. The recirculating vortex, which appears on the upper face of the block, grows and the average electroosmotic velocity increases with the increase of the electrolyte concentration.

Keywords: Electroosmotic flow, Surface potential, Pressure driven flow, Re-circulation, Transport.