Direct measurement of slip flows in superhydrophobic microchannels with transverse grooves

Slippage effects in microchannels that depend on the surface characteristics are investigated, taking into account hydrophilic, hydrophobic, and superhydrophobic wettabilities. Microscale grooves are fabricated along the vertical walls to form superhydrophobic surfaces, which enable both the visualization of the flow field near the walls and the direct measurement of the slip length. Velocity profiles are measured using microparticle image velocimetry and those in hydrophilic glass, hydrophobic polydimethylsiloxane (PDMS), and superhydrophobic PDMS microchannels are compared. For the hydrophilic glass surface, the velocity near the wall smoothly decreases to zero, which is consistent with the well-known, no-slip boundary condition. On the other hand, for the flow in the hydrophobic PDMS microchannel, the velocity profile approaches some finite value at the wall, showing a slip length of approximately 2 μm. In addition, to directly measure the velocity in the superhydrophobic microchannel, transverse groove structures are fabricated along the vertical walls in the microchannel. For this surface, the velocity profile approaches a value that is larger than that for the PDMS case. Incidentally, instabilities in the velocity profile are observed at the interface with the air gap. Furthermore, the velocity profile near the wall shows a larger slip length than for any of the other experimental setups. For groove structures that are high and wide, the liquid meniscus forms curves in the cavity so that a wavy flow is created beyond the grooves. Moreover, if the pitch-to-width ratio of the groove structure increases, meniscus penetration into the cavity is observed.