The slip effects of water flow in hydrophilic and hydrophobic microchannels of 1 and 2 mum depth are examined experimentally. High-precision microchannels were treated chemically to enhance their hydrophilic and hydrophobic properties. The flow rates of pure water at various applied pressure differences for each surface condition were measured using a high-precision flow metering system and compared to a theoretical model that allows for a slip velocity at the solid surface. The slip length was found to vary approximately linearly with the shear rate with values of approximately 30 nm for the flow of water over hydrophobic surfaces at a shear rate of 10(5) s(-1). The existence of slip over the hydrophilic surface remains uncertain, due to the sensitivity of the current analysis to nanometer uncertainties in the channel height. (C) 2003 American Institute of Physics.
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