Superhydrophobic surfaces and their potential application to hydrodynamic drag reduction

Abstract

Superhydrophobic surfaces appear frequently in the natural world, for example allowing insects to respire underwater and plants, such as the lotus leaf, to have self-cleaning properties. Attempts to mimic these superhydrophobic surfaces have been successful on nano- and micro-scales, with increased efficiency of water flowing through micro-channels when the walls are superhydrophobic. This thesis is focused on the proposed use of superhydrophobic surfaces to reduce drag on a much larger scale, applicable to small water craft such as canoes and yachts. The potential for drag reduction using superhydrophobic surfaces arises from the ability of such surfaces to retain an air-layer or plastron on the surface. The presence of a plastron results in slip and reduced shear at the surface, producing a drag reduction. This potential drag reduction is explored through numerical simulations and experimental testing. Computational Fluid Dynamics is used to explore the effect of slip on flow separation and viscous drag, allowing the potential drag reduction mechanisms to be explored. A range of superhydrophobic surfaces have been developed and characterised based on their roughness, contact angle and ability to retain a plastron. Confocal microscopy is used to generate the first high resolution 3D images of the air-water interface on a superhydrophobic surface over a large area. These images confirm the presence of a plastron on the surfaces and help contribute to the understanding of optimal design of superhydrophobic surfaces. These surfaces are explored experimentally in a towing tank with a repeatability of better than 1%. Refinement of the surface design leads to the presence of a plastron producing a relative drag reduction of up to 3% for hydrophobic sand, up to 10% for hydrophobic ridges and up to 15% for a hydrophobic mesh. Overall, superhydrophobic surfaces are shown to be capable of producing a relative drag reduction when a plastron is retained on the surface, although with the penalty of increased roughness-induced drag component. The drag reduction is shown to be linked to both the structure of the surface, and the quality and thickness of the plastron. It is demonstrated that it is difficult to retain a plastron over long immersion periods and manufacturing constraints currently limit applicability.

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ORCID for Neil Sandham: orcid.org/0000-0002-5107-0944

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