Minimal contact

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Nature 503, 385–388 (2013)

The design of a superhydrophobic surface can be thought of as an optimization problem: liquid droplets spend as little time as possible in contact with the perfect candidate material. Hydrodynamics would seem to offer a natural lower limit on this contact time, but James Bird and co-workers have gone below the theoretical minimum by engineering surfaces that redistribute the droplet mass, effectively changing the hydrodynamics of the problem.

Theory suggests that contact time is minimized by a surface with just enough texture to trap an air layer below the liquid. And because droplet dynamics look increasingly axisymmetric in this limit, common wisdom holds that axisymmetric recoil, in which the droplet's centre remains stationary, corresponds to the shortest contact time. But Bird et al. reasoned that if the liquid at the centre actually assisted the recoil, contact time might be reduced below the theoretical limit.

The team fabricated macrotextured surfaces designed to induce asymmetric recoil, resulting in a non-uniform velocity field — and contact times that were significantly shorter than those on smooth surfaces. As well as testing their engineered surfaces, they also observed centre-assisted recoil on the wings of the Morpho butterfly, and on nasturtium leaves, both of which have similarly textured superhydrophobic surfaces.

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Klopper, A. Minimal contact. Nature Phys 9, 753 (2013). https://doi.org/10.1038/nphys2840

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