  We have fabricated and characterized a novel superhydrophobic system, a
mesh-like porous superhydrophobic membrane with solid area fraction $\Phi_s$,
which can maintain intimate contact with outside air and water reservoirs
simultaneously. Oscillatory hydrodynamic measurements on porous
superhydrophobic membranes as a function of $\Phi_s$ reveal surprising effects.
The hydrodynamic mass oscillating in-phase with the membranes stays constant
for $0.9\le\Phi_s\le1$, but drops precipitously for $\Phi_s < 0.9$. The viscous
friction shows a similar drop after a slow initial decrease proportional to
$\Phi_s$. We attribute these effects to the percolation of a stable Knudsen
layer of air at the interface.
