Porous superhydrophobic membranes: hydrodynamic anomaly in oscillating flows.

We have fabricated and characterized a novel superhydrophobic system, a meshlike porous superhydrophobic membrane with solid area fraction Φ(s), which can maintain intimate contact with outside air and water reservoirs simultaneously. Oscillatory hydrodynamic measurements on porous superhydrophobic membranes as a function of Φ(s) reveal surprising effects. The hydrodynamic mass oscillating in phase with the membranes stays constant for 0.9≲Φ(s)≤1, but drops precipitously for Φ(s)<0.9. The viscous friction shows a similar drop after a slow initial decrease proportional to Φ(s). We attribute these effects to the percolation of a stable Knudsen layer of air at the interface.