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Experimental studies of superhydrophobic surfaces in flow
In recent years, research on superhydrophobic surfaces has exploded from more than five decades of relative obscurity. The use of superhydrophobic surfaces in flow is but a small part of renewed interest in the field. Superhydrophobic surfaces which have previously demonstrated the ability to reduce laminar regime drag in certain flows, are shown to be effective in reducing drag in the turbulent flow regime as well. In some cases, skin friction drag reductions of 50% were observed. Drag reductions result from slip at the gas-liquid interface that exists at the surface. In a turbulent flow, this slip has a significant influence on the viscous sublayer nearest the wall, and drag reduction performance appears to scale with viscous sublayer thickness. On stationary cylinders superhydrophobic coatings reduce the intensity and increase the frequency of vortices shed behind the body. Separation point and the structures in the wake are visually altered in the presence of superhydrophobic slip, especially slip in the flow direction. On elastically mounted cylinders, slip reduces the amplitude of vortex induced oscillations, but not their frequency, indicating the result is primarily the result of reduced fluid forcing. Experiments with superhydrophobic coated hydrofoils demonstrated slip can reduce drag and lift over a range of attack angles. Engineered and randomly patterned superhydrophobic surfaces were shown to be effective in generating slip, although considerable care must be exercised with randomly patterned surfaces to ensure the appropriate slip lengths exist; an experiment to measure slip length is presented. Unlike the previous flow studies where slip is the responsible mechanism, studies of the effect of contact angle and density on the orientation and stability of floating cubes are concerned only with superhydrophobic surfaces' high contact angles and resistance to wetting. These experiments show how the effect of high contact angles available from superhydrophobic surfaces can allow small objects, more dense than water, to "float" at the surface, a phenomenon observed with aquatic insects. A series of theoretical predictions are presented along with measurements of force and observations of floating cubes with known contact angles are presented. It is noted that cube size and contact angle determine the most stable orientation in which a cube of a given density will float, or if it will sink. Vertical edges and corners decreased the force and displacement a shape was able to bear before sinking, although the local shape and sharpness of the edge is likely to play a significant role.
Daniello, Robert J, "Experimental studies of superhydrophobic surfaces in flow" (2013). Doctoral Dissertations Available from Proquest. AAI3603073.