| Abstract: |
Inspiration for man-made superhydrophobic surfaces comes from Nature: the Lotus leaf [1–3], for instance, exploits superhydrophobicity to keep its surface dry and clean from contaminants. Superhydrophobicity emerges due to roughness on a chemically hydrophobic surface [4, 5]. As the roughness scale is increased, it becomes thermodynamically unfavorable for water to fill the spaces between the surface features. The liquid surface then remains suspended above the peaks of the rough solid with trapped air underneath [6, 7]. This reduction in the solid-liquid contact area gives rise to considerable drag reduction [8] on superhydrophobic surfaces in both laminar [9, 10] and turbulent flows [11], with important technological consequences. Here, we have fabricated and characterized a novel superhydrophobic system, a mesh-like porous superhydrophobic membrane with solid area fraction Φs, which can maintain intimate contact with outside air and water reservoirs simultaneously. Oscillatory water drag and added-mass measurements [12] on the porous membranes as a function of Φs reveal a surprising effect: the added water mass (oscillating in-phase with the membrane) initially stays constant, but drops precipitously for Φs . 0.9; similarly, the viscous friction drops anomalously after a slow initial decrease proportional to Φs. We attribute this effect to a percolation transition, which eventually leads to the formation of a complete Knudsen layer of air at the interface, sustained by constant influx of gas from the air reservoir. Porous superhydrophobic membranes may find applications in drag reduction; the observed transition may be relevant in shedding light on unexplained phenomena in bio-fluid-dynamics [13, 14]. |
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