Laminar drag reduction in hydrophobic microchannels

Abstract

The apparent slip effects of laminar water flow in smooth hydrophobic microchannels and patterned hydrophobic microchannels were investigated. A series of experiments were performed to demonstrate the drag reductions for laminar water flow in hydrophobic microchannels. These microchannels were fabricated from silicon wafers using photolithography and were coated with hydrophobic octadecyltrichlorosilane (OTS). To generate a larger drag reduction, the patterned hydrophobic microchannels were fabricated to allow the liquid to flow over a region of trapped air in the cavity between the microridges. With the geometrical dimensions used, pressure drop reductions ranging from 10 to 30% were found in the smooth microchannels and patterned microchannels. The pressure drop reduction was shown to increase with increasing microridge spacing and decreasing microchannel width. Using micro-particle image velocimetry (PIV), we measured an apparent slip velocity at the wall of approximately 8% of the centerline velocity, yielding a slip length of approximately 2 lm in the smooth hydrophobic microchannel. Theoretically, the analytical solution derived for three-dimensional flow in a rectangular duct is presented to predict the slip velocity and slip length at the wall based on the pressure drop measurement. These results are in agreement with the experimental data obtained using micro-PIV. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.