a b s t r a c tWe investigate 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. 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 the Nernst-Planck equation for ion transport and the Poisson equation for electric field. Vortical flow develops above the block when the surface potential of the block is in opposite sign to that of the surface potential of the channel. The strength of the re-circulating vortex grows as the surface potential of the block increases. Vortical flow also depends on the Debye length when it is in the order of the channel height. The combined effects due to the geometrical modulation of the channel wall and heterogeneity in surface potential is found to produce a stronger vortex and hence a stronger mixing, as compared with the effect of 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. The recirculating vortex does not appear when EDL is thick and the effect of over-potential of the block on electroosmosis is negligible. The loss of momentum near the obstacle is compensated by the electrostatic force near the EDL, which prevents flow separation upstream or downstream of the obstacle. The mixing performance of the present configuration is compared with several other cases of surface modulation. The impact of the imposed pressure gradient on the vortical flow due to surface heterogeneity is analyzed.
In this paper, we have studied the electrokinetics and mixing driven by an imposed pressure gradient and electric field in a charged modulated microchannel. By performing detailed numerical simulations based on the coupled Poisson, Nernst–Planck, and incompressible Navier–Stokes equations, we discussed 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. A numerical method based on the pressure correction iterative algorithm is adopted to compute the flow field and mole fraction of the ions. Since electroosmotic flow depends on the magnitude and sign of wall potential, a vortex can be generated through adjusting the patch potential. The dependence of the vortical flow on imposed pressure gradient is investigated. Formation of vortex in electroosmotic flow has importance in producing solute dispersion. The circulation of vortex grows with the rise of patch potential, whereas the pressure-assisted electroosmotic flow produces a reduction in vortex size. However, the flow rate is substantially increased in pressure-assisted electroosmotic flow. Flow reversal and suppression of fluid transport is possible through an adverse pressure gradient. The ion distribution and electric field above the potential patch are distorted by the imposed pressure gradient. At higher values of the pressure gradient, the combined pressure electroosmotic-driven flow resembles the fully developed Poiseuille flow. Current density is found to increase with the rise of imposed pressure gradient.
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