ME Seminar Series: Staying dry under water
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Monday, October 29, 2012 - 11:30am to 1:00pm
Prof. Neelesh Patankar - Northwestern University
Superhydrophobicity of rough surfaces has attracted global interest through the past decade and a half. There are naturally occurring instances of such surfaces, e.g., lotus leaves, which led to the popular term ¿lotus effect.¿ Researchers worldwide are exploring numerous applications such as drag reduction, self-cleaning, heat exchangers, anti-icing, textile, etc. Maintaining superhydrophobicity of rough surfaces has, hitherto, relied on the presence of air in roughness grooves. Keeping these surfaces dry (non-wetting) under water has been challenging because the plastron layer of trapped air is found to deteriorate. This technological barrier limits the utility of these surfaces. Here it is demonstrated that an immersed surface remains at least partially dry and retains its non-wetting properties if hundreds of nanometer scale or smaller textures are used on hydrophobic solids. The proposed mechanism is the stabilization of the vapor phase of the liquid and of trapped gases in roughness grooves. There is a critical roughness scale below which these mechanisms are effective. Theoretical predictions are found to be consistent with the viable roughness length scales observed in our experiments as well as on ¿air-retaining¿ insect surfaces. Our approach is passive and does not involve active generation of gas. Vapor stabilization by surface roughness can also lead to fundamentally novel phenomena in heat transfer.