A microchannel based study of drag on deformable superhydrophobic walls
A superhydrophobic surface combines chemical hydrophobicity with roughness. Due to the surface tension of water, the water cannot penetrate between the roughness elements, which remain filled with dissolved gas; the water surface thus remains suspended over the gas, creating shear free flow regions. In this study, we conducted experiments to study drag in flows over deformable superhydrophobic walls. Our superhydrophobic walls were fabricated by lithographically defining pores on 1-um-thick silicon nitride membranes; the membranes were then treated with silane to make them hydrophobic. For the flow experiments, we fabricated microchannels in which one wall was the porous and deformable superhydrophobic membrane and the other wall was a rigid hydrophilic surface. We measured the pressure drop and the flow rate in these microchannels; we simultaneously used white light interferometry to visualize the deformations of the superhydrophobic membrane. From both sets of measurements, we determined the relevant quantities, including the slip length at the liquid-solid interface. The results from these measurements suggest that the drag onthese deformable superhydrophobic walls is due to a complex interplay between wall deformation and shear-free flow; in most cases, it is not possible to directly observe the drag reduction due to the reduction in the solid-liquid interfacial area.
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