The aviation industry, among others, seeks materials that actively repel water and ice very strongly. Researchers have figured out how to design the rigid surfaces of such materials: by teaching water droplets how to “jump” on a trampoline.
The findings could lead to a better way to keep snow and ice off airplane wings—preventing turbulent airflow during takeoff, which can lead to dangerous reduced lift.
Dimos Poulikakos, a professor at ETH Zurich, and his collaborators at the Laboratory of Thermodynamics in Emerging Technologies studied the behavior of water droplets on surfaces by placing a droplet measuring one millimeter on a specially treated rigid silicon surface and then continuously reducing the air pressure in the experimental chamber, with a high speed camera filming the droplet.
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.
This may seem like magic to the layperson, but the expert sees, above all, what appears as a violation of fundamental physical laws according to which a body falling on a rigid surface shouldn’t be able spontaneously to gain motional energy that allows it to bounce back higher. Still, the trampolining droplet seems to do precisely that.
Inside the droplet
To understand where the force propelling the droplets upwards comes from, Poulikakos and postdoctoral students Tom Schutzius and Stefan Jung performed a detailed analysis of the droplet’s motion and, using a thermal imaging camera, the temperature distribution inside it.
They found that the combination of natural water evaporation and the microstructure of the material’s surface are essential for the trampoline phenomenon. The overpressure between the droplet and the surface, resulting from evaporation, launches the drop upwards at each impact, just like a spring would do.
When a water drop that is cooled far below zero degrees centigrade (or “supercooled”) starts freezing, evaporation is enhanced by the so-called recalescence. This is a well-known effect in material processing, for instance in tempered iron that, as it cools down, spontaneously heats itself up to a red heat for a short time. The reason for that is the latent heat, which is released as the inside of the piece of iron, solidifies.
Something very similar goes on inside a water droplet: as evaporating water on its surface cools it below the freezing point, ice crystals start forming. The heat released in this phase transition from liquid to solid then quickly heats up the droplet to zero degrees.
“This heating happens in a few milliseconds,” Schutzius explains, “and as a consequence gives rise to explosive evaporation.” This cools the droplet down again, and the cycle repeats itself. The explosive evaporation produces an even higher overpressure between droplet and surface and causes it to shoot up like a rocket.
Rough but not too rough
The crux of the matter, however, lies in the surface itself. It needs to be rough so that the droplet doesn’t stick to it, but on the other hand it mustn’t be too rough as otherwise the water vapor would escape too quickly through the pores and cracks of the surface, and the rocket effect would vanish into thin air.
The microstructured silicon surfaces produced by the researchers exactly fulfill these requirements: They are made of a regular array of tiny columns (only a few micrometers wide) with a spacing of roughly five micrometers.
“From the results of our research we can deduce what qualities surfaces need to have in general in order for them to violently repel water and ice, and then design them accordingly,” says Poulikakos. In their experiments the scientists examined different materials including etched aluminum and carbon nanotubes.
In order to make the trampoline mechanism even more useful for real-world applications, one would first have to make it work at normal air pressure. Poulikakos and his collaborators hope to make progress in this direction in the next few years.
If this is achieved, a host of applications present themselves, ranging from ice-free power lines to road surfacing that repels water and ice. And, possibly, someday the de-icing of airplane wings may no longer be necessary.
The findings appear in Nature.
Source: ETH Zurich