Dimos Poulikakos, ETH Zürich
Enhancing Superhydrophobicity and Icephobicity through Surface Flexibility Inspired by Butterfly Wings
Written by Aashutosh Mistry
Butterfly wings have incited a bit of scientific interest for fluid dynamists as they are very hydrophobic. Surface investigations have revealed that the surface itself is superhydrophobic with contact angles in the range 155°–165° (with 5° hysteresis). Since butterfly wings are flexible structures, an associated question is the role of flexibility on hydrophobicity (i.e., if it promotes or discourages the intrinsic hydrophobicity of the wings). Dimos Poulikakos and his research group at ETH Zürich have been investigating the importance of surface flexibility on hydrophobicity as well as icephobicity. They found that when water drops impact the solid substrate at a finite velocity, resulting rebound is higher for a more flexible surface, thus highlighting a positive correlation between flexibility and hydrophobicity. Even the harmonics of drop oscillations are found to differ based on the extent of surface flexibility. Based on their experiments, they found that the mass weighted relative velocity of drop and the surface is an important velocity scale. Use of this relative velocity collapses the results with both rigid and flexible surfaces onto a unique scaling relation. When extending such studies for supercooled surfaces (surface at temperatures below freezing point of water), they made some interesting observations. The entire water drop does not solidify instantaneously. Rather in many instances they found that the drop rebounds (again depending on surface flexibility) and freezes mid-air. A spontaneous formation of minute icicle network is observed inside the drop upon contact. Icephobicity is practically quite relevant because ice formation on operating surface can largely hinder their performance. The present set of results show that if the surface is assigned a control motion, solidifying drops will not adhere to the surface. Thus, it is observed that the surface elasticity and wetting properties have a collaborative effect that is tunable through altering substrate areal density, stiffness, and damping.