To Slip or Not to Slip: Water Flows in Hydrophilic and Hydrophobic Microchannels

Abstract

The slip effects of water flow in hydrophilic and hydrophobic microchannels of 1 and 2 μm depth are examined experimentally. Fabrication processes for silicon/Pyrex microchannels were chosen to ensure good control of the channel height and to obtain atomically smooth surfaces. Hydrophilic surfaces were prepared with an RCA-1 cleaning, while hydrophobic surfaces were created by coating the channel surface with the self-assembled monolayer of octadecyltrichorosilane (OTS). The flow rates of pure DI water at various applied pressure differences for each surface condition were measured using a high precision flow metering system and it was observed that the flow rates in hydrophobic channels is larger than in the same hydrophilic channel. The increase of the flow rate can be explained by assuming a slip velocity at the wall. The slip effects become more pronounced as the channel height decreases and the wall shear rate increases. The slip length was found to vary as approximately the square root of the shear rate and had values of approximately 40 nm in the hydrophobic channels and 15 nm in the hydrophilic channels at a shear rate of 105 s−1.