Porous Superhydrophobic Membranes: Hydrodynamic Anomaly in Oscillating Flows

TL;DR

This study introduces a novel porous superhydrophobic membrane that exhibits unexpected hydrodynamic behavior during oscillations, linked to air layer percolation, with potential implications for fluid dynamics and material design.

Contribution

The paper reports the fabrication and characterization of a new porous superhydrophobic membrane system and uncovers a hydrodynamic anomaly related to air layer percolation in oscillating flows.

Findings

Hydrodynamic mass remains constant for solid area fraction 0.9 to 1.

Mass drops sharply when solid area fraction drops below 0.9.

Viscous friction decreases significantly after an initial slow decrease.

Abstract

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.