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Nanostructured Materials Repel Water Before It Freezes

Dec. 31, 2010

Harvard University researchers recently designed nanostructured materials that repel water droplets before they can freeze, a finding that could lead to ice-free airplane wings, buildings, power lines, and highways.

The team designed special surfaces inspired by natural solutions, including mosquitos, which can defog their eyes, and water striders, which keep their legs dry with tiny bristles that repel water droplets by reducing the surface area they contact.

Videos of super-cooled water droplets hitting the model surfaces showed that the droplets initially spread out when they hit the surface, but then retract back into their spherical shape and bounce off before they can freeze. On a regular surface, the water droplets would stay spread out and eventually freeze.

Structures composed of interconnected patterns, like honeycombs and bricks, were ideally suited for liquid-shunning surfaces that can withstand high-impact droplets, like driving rain.

"We wanted to take a completely different tack and design materials that inherently prevent ice formation by repelling the water droplets," says Joanna Aizenberg, the Amy Smith Berylson Professor of Materials Science at Harvard School of Engineering and Applied Sciences, who led the study. "The crucial approach was to investigate the entire dynamic process of how droplets impact and freeze on a supercooled surface."

The nanostructured material prevents ice from forming in temperatures as low as -13 to -22 degrees Fahrenheit. Below that, any ice that does form can’t adhere well due to the reduced contact area, which makes it easier to remove than the sheets of ice that can form on smooth surfaces.

The researchers believe their improved understanding of how ice forms could lead to new coatings mixed directly into structural materials.

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