Why these water droplets behave like trampolines

Water is weird. It’s essential for life, but too much can wreck your cells. When frozen, it’s slippery, but physicists aren’t sure why. If you boil water in zero gravity, rather than make bubbles, it creates a mushroom cloud.

And when put under certain pressures, water behaves in unpredictable ways. Such was the case in a paper published Wednesday in the journal Nature. When scientists from ETH Zurich placed water at low pressure — one-twentieth of what’s typically felt at the Earth’s surface — the droplets behaved spontaneously like springboards.

High-speed video of a water droplet and a person trampolining. The droplet’s bounces are facilitated by a low pressure environment and a rigid surface made from an ultrahydrophobic silicon surface. Photo by Thomas M. Schutzius, Gustav Graeber, L. J. Yi and Dimos Poulikakos

The scientists were hitting the droplets against a homemade hydrophobic — water-repelling — material, so one might expect some push back, but what they got was quite a bounce.

High-speed video demonstrating how droplet trampolining can drive continuous motion of a cantilever. Photo by Thomas M. Schutzius and Dimos Poulikakos

The scientists said they want to adapt these hydrophobic surfaces, which are made from silicon, to coat airplanes. By doing so, the metal wings could spontaneously remove ice during flight. The researchers already have tested this idea of water removal with sheets of aluminum (see video below).

A high-speed video shows what happens to water during recalescence freezing, a physical phenomena where supercooling a substance leads to the sudden release of heat. The researchers found that the process can drive a vaporization, when combined with low pressure and a room temperature environment. The result is “de-wetting” of the droplet from a aluminum-based superhydrophobic surface.

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