Wetting of textured surfaces and their application to drag and friction reduction

Abstract

The presence of a layer of gas trapped at a submerged surface leads to significant drag reductions in laminar and turbulent flows. However this gas is easily lost. The focus of this work is to characterise the wetting properties of a range of surfaces made of hydrophobic cavities and where possible to modify the surfaces to increase the lifetime of the gas and optimise for drag and friction reduction. For the majority of this work surfaces are made using a templated electrodeposition method which can produce a range of cavity widths and heights in a close packed arrangement.

The wetting properties of hydrophobic surfaces consisting of micron sized cavities are investigated which shows the surface displays the ‘petal effect’; high contact angle and high contact angle hysteresis, and has the ability to form hexagonally shaped droplets due to the close packed arrangement of the cavities. This surface is then modified to allow the top up of the gas as it dissolves and the determination of the wetted area and using electrochemical methods. This work is built upon to allow full reintroduction of gas from a fully wetted state using much larger cavities.

The wetting of sub-micron sized cavities is investigated using atomic force microscopy which reveals the presence of nanobubbles within the cavities. These have an exceptionally long lifetime and the cavity structure is shown to prevent coalescence. The frictional properties of this surface, as well as flat hydrophobic and hydrophilic surfaces are investigated using a custom built tribometer which simulates conditions found in microelectromechanical systems.

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Identifiers

ORCID for Robert Wood: orcid.org/0000-0003-0681-9239

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