Laminar drag reduction in microchannels using ultrahydrophobic surfaces

A series of experiments is presented which demonstrate significant drag reduction for the laminar flow of water through microchannels using hydrophobic surfaces with well-defined micron-sized surface roughness. These ultrahydrophobic surfaces are fabricated from silicon wafers using photolithography and are designed to incorporate precise patterns of microposts and microridges which are made hydrophobic through a chemical reaction with an organosilane. An experimental flow cell is used to measure the pressure drop as a function of the flow rate for a series of microchannel geometries and ultrahydrophobic surface designs. Pressure drop reductions up to 40% and apparent slip lengths larger than 20 μm are obtained using ultrahydrophobic surfaces. No drag reduction is observed for smooth hydrophobic surfaces. A confocal surface metrology system was used to measure the deflection of an air–water interface that is formed between microposts and supported by surface tension. This shear-free interface reduces the flow resistance by allowing the fluid to contact only a very small effective area of the silicon surface. The impact of the surface topology on the drag reduction is explored in detail and the results are found to be in good qualitative agreement with the predictions of analytical theory.