Investigation of the drag reducing effect of hydrophobized sand on cylinders

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 as well as a consequent reduction in drag. In this work we report a facile method for creating hydrophobic cylinders and hydrophobic flat surfaces with varying surface roughness able to support a Cassie-Baxter state. Cylinders of 12mm diameter were coated in hydrophobized sand with grain sizes in the ranges of 50-100, 212-300, 425-600 and 600-710 mu m to produce the varying degrees of 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 numbers of 10000 to 40000. 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.