Electroosmotic flow in a slit nanochannel with superhydrophobic walls
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Electroosmotic flow (EOF) in a slit channel with alternating charged patches of vanishing slip velocity and uncharged patches of vanishing shear stress is studied by numerically solving the coupled Poisson–Nernst–Planck Navier–Stokes system. It is taken into account that there may exist an arbitrary shift between the patterns at the top and the bottom wall. The studied system should mimic a slit nanochannel with superhydrophobic walls. The results are expressed in form of a flow enhancement factor E f, representing the average flow velocity through the channel divided by the average flow velocity through a channel with planar, unstructured walls of the same zeta potential. Extensive studies are carried out showing how E f varies with the system parameters. While it has been shown that under the assumption of thin-Debye layer values of E f close to 1 should be expected for EOF along a superhydrophobic surface, a substantial flow enhancement is found for the scenario studied here. For the range of parameters considered, a maximum flow enhancement factor of about 7 is obtained. The main mechanism responsible for this flow enhancement is the proximity of regions in which an electric body force exists to gas–liquid interface patches. For the latter, the shear stress is much reduced compared to liquid–solid interface patches, reducing the net friction coefficient of the electrically driven flow.