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Trampolining Water Droplets Could Pave Way For Ice-Shedding Surfaces

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From their study of ice forming on superhydrophobic surface, Swiss scientists have demystified the process of "trampolining" water droplets - spontaneous bouncing and levitation in a display of defiance of physical laws.

Writing in the journal Nature, the team from the Swiss Federal Institute of Technology in Zurich, Switzerland, observed droplets making bounces on a grid of microscopic, Teflon like-coated silicon pillars for an extremely water-repellent surface.

What they found: water initially sitting on the surface "levitated" as the team progressively decreased the surrounding pressure and created a vacuum.

"[I]t's an example of what surface engineering can do for you," revealed study author Tom Schutzius, adding that it would be interesting to bring this phenomenon to real-world situations and explore it for anti-icing surfaces.

 Superhydrophobic surfaces exhibited [pdf] the opposite behavior that the researchers wanted. The team was vacuuming to try sucking the air out underneath the droplets, but the air remained hesitant to move. What happened was water boiled off from the edges of the water droplets.

"It starts to evaporate from anywhere that there's a liquid-air interface," Schutzius explained, citing that it included the behavior included the open-pillared structure.

He said that at a certain level, the droplet jumps off the surface spontaneously when the pressure gets too high. When he further lowered the surrounding pressure, evaporation accelerated and pressure between droplet and surface grew, leading to a higher bounce.

According to University of Oxford's Julia Yeomans, who was not part of the study, the findings showed the constantly surprising nature of drop impacts.

"In the past few years we have seen droplets that leave a surface when they coalesce, bounce in a pancake shape, run around ratcheted tracks and mimic quantum particles," she shared, adding that the research should next be able to translate it to practical applications for stay-dry, anti-icing surfaces.

The discovery may lead to novel ways of keeping car ice-free during freezing weather, no longer entailing time-consuming defrosting and scraping every morning. In addition, it may pave the way for new coatings for aircraft wings in order to keep them from icing up during a flight.

The challenge, said Schutzius, was that it is specifically tough to create sufficiently water-repelling materials. Superhydrophobic surfaces, he said, are not widely used because they may lack durability.

The team will continue to observe the trampolining effect in various contexts.

Photo: William Allen | Flickr

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