Winter often brings with it inevitable flight delays as ground crews scramble to de-ice all those aeroplane wings before takeoff on cold wintry days. Wouldn't it be awesome if all that moisture never had a chance to freeze, and instead just bounced right off the surface?
Swedish scientists think they may have found a way to achieve just such a solution, via specially engineered silicon surfaces that repel water droplets so vigorously, the drops literally bounce, as if on a trampoline. They published their findings in Nature last week.
Using a special pressurised chamber, researchers at ETH Zurich in Switzerland placed a small water droplet on a rigid surface made of silicon. They dropped the pressure in the chamber and recorded the droplets' subsequent motion with a high speed video camera. What they observed was surprising. Per the press release from ETH Zurich:
At first, the drop rested motionless on the surface, but at around a twentieth of normal atmospheric pressure it suddenly jumped up. After a short leap the droplet eventually landed on the surface again, only to jump up again — even higher than the first time. Just like an athlete on a trampoline jumping higher with every rebound from the elastic sheet, the water droplet was propelled higher after each contact with the surface, even though that surface was completely rigid.
Check it out in the video below, which includes an athlete jumping on a trampoline for comparison:
This really shouldn't happen: something falling on a rigid surface shouldn't magically gain sufficient momentum to bounce higher and higher over time. It should gradually bounce lower and lower as it loses energy. So what gives?
The Swiss scientists found this odd behaviour doesn't really defy the laws of physics. It's a combination of water evaporation and the unique water-repellent properties of the silicon surface that makes the droplets act like they're on a trampoline.
Ice crystals start forming as evaporating water on the silicon surface "super-cools" below the freezing point. When that happens, the droplet briefly heats up for a moment. "This heating happens in a few milliseconds, and as a consequence, gives rise to explosive evaporation," lead author Thomas Schutzius explained. And that release of heat cools the droplet back down again so the cycle can repeat, this time with even higher over-pressure between surface and droplet:
As for the silicon surface, there's a sweet spot whereby the surface must be sufficiently rough so that the droplet won't stick to the surface, yet smooth enough that water vapour can't see through tiny cracks and pores too quickly. The ETH team's microstructured surfaces have arrays of precisely organised tiny columns spaced about five micrometres apart. That structure seems to hit the sweet spot to get the water drops to bounce off.
The biggest challenge will be figuring how how to get this to work at normal air pressures. And then we could finally have nice things like ice-free power lines, roads that naturally repel water and ice — and perhaps an end to the time-consuming process of de-icing aeroplane wings, since water droplets would simply bounce off the wings before they had a chance to freeze.
Schutzius, Thomas M. et al. (2015) "Spontaneous droplet trampolining on rigid superhydrophobic surfaces," Nature 527: 82-85.
Vollmer, Doris and Butt, Hans-Jurgen. (2015) "Materials science: droplets leap into action," Nature 527: 41-42.
Image: Visualisation by Digit Works/ETH Zurich. Videos: Schutzias et al./Nature.