Investigation of the drag reducing effect of hydrophobized sand on cylinders

Abstract

Superhydrophobic surfaces show strong potential for drag reducing applications. If such a surface supports a Cassie-Baxter state with low solid surface fraction and when immersed it retains a plastron air layer, large slip can occur across its surface and a consequent reduction in drag. In this work we report a facile method for creating hydrophobic cylinders an d hydrophobic flat surfaces with varying surface roughness able to support a Cassie-Baxter state . Cylinders of 12mm diameter were coated in hydrophobised sand with grain sizes in the ranges 50-100μm, 212-300μm 425-600μm and 600-710μm to produce the different roughness. A Laser Doppler Anemometer was used to measure the velocity profile of the water across their wake in a large water circulating flow chamber. The hydrophobic cylinders in the Cassie-Baxter state show drag reductions of up to 28% compared to the same sample in the Wenzel state for flows with Reynolds number s of 10,000 to 40,000. These drag reduction results , in combination with confocal microscopy images of the plastron air layer and feature height , show that the thickness of the plastron and the protrusion height of the features combine to give a drag reduction or drag increase depending on the ratio of the two.