Droplets already bounce off water-repellent surfaces, but scientists have engineered materials that speed up the bounce to repel water faster.

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Even the brief contact time between a surface and a droplet that bounces off it can allow water to freeze or collect on the surface and cause damage. Scientists previously thought that contact time was shortest when droplets spread and recoil symmetrically on a material before bouncing off. But in a paper published today in Nature1, a team has shown that a textured surface can cause droplets to recoil with controlled asymmetry, and bounce away faster than was thought possible.

Kripa Varanasi, a mechanical engineer at the Massachusetts Institute of Technology in Cambridge, and his colleagues used a water-repellent material which they further engineered by adding tiny ridges 0.1 millimetres high.

They then recorded water droplets hitting the surface with a high-speed camera that filmed at 10,000 frames per second or more. The ridges forced the liquid to splash asymmetrically, so that it recoiled faster than on a macroscopically smooth surface. The time that the water spent in contact with the surface fell by 37% compared to the same material with no ridges, the authors measured.

The researchers repeated the experiment with droplets of molten tin. On a surface without ridges, the liquid metal quickly solidified. But the effect of the ridges was strong enough to make the droplets bounce off while still liquid.

Varanasi says that the approach could have a number of commercial applications, such as preventing ice forming on surfaces exposed to freezing rain, or keeping moisture from collecting on steam turbines.

Noticing that a nasturtium plant (Tropaeolum majus L.) in Varanasi's office had similar ridges on its leaves, the researchers also ran experiments on natural textured surfaces. They found that both nasturtium leaves and the ridged wings of the Morpho butterfly (Morpho didius) caused water droplets to recoil asymmetrically. The surfaces even repelled water faster than ridge-less surfaces such as lotus leaves, which are often considered the gold standard of superhydrophobicity.