Experiments indicate that hydrodynamic drag may be reduced by using ultrahydrophobic substrates to induce slip at the solid-liquid interface. The connection between drag-reduction and slip is explored, and these previously isolated fields are joined into a single, more comprehensive area of research.

We present data from two experimental techniques indicating the role of hydrophobicity and size scale in drag-reduction. In one series of studies, water was directed through a channel containing two rectangular plates of smooth silicon. Flow rates for smooth non-hydrophobic plates and smooth hydrophobic plates were identical.

In another series of studies, a rheometer was used to measure the relative drag experienced by a polypropylene-coated disk as it was rotated in water. The polypropylene (PP) was ultrahydrophobized (water contact angles were θa/θr = 167°/166°) by argon plasma etching in the presence of poly(tetrafluoroethylene). Drag was reduced by over 20% by using the roughened PP instead of the virgin PP.

Contact angle hysteresis is isolated as a critical factor in the drag-reduction mechanism proposed.

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