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November 15, 2010

Domed Cities and large structures designed to handle ice and snow and Harvard has developed super-hydrophobic material to prevent icing

Khan Shatyr is a 150 meter tall (50 stories) transparent tent in Kazakstan which can hold 100,000 people. The hub and ETFE (strong form of teflon) skin of the could have to withstand the weight of 20,000 tonnes of snow and ice.

MIT Technology review reports that Harvard scientists say they have created materials that can prevent ice from forming on surfaces in the first place. Ice is a hazardous fact of winter life, playing havoc with roads, utility lines, buildings, and air travel. Conventional methods of getting rid of the ice, such as direct heating, applying salt, or using chemicals to trigger melting, all have liabilities: they can corrode the materials they're applied to, and damage the environment, and they are only modestly or temporarily effective.
The researchers say their breakthrough, reported in the latest issue of ACS Nano, could apply not only to aviation but to road paving, construction, power transmission, and virtually any other industry for which chemical and physical deicing is a concern. "What we want to do is to have ice not form at all," says Joanna Aizenberg, a materials scientist and leader of the project.

When an incipient ice droplet hits a conventional surface, it spreads out and grips, becoming a base for the aggregation of more droplets and ultimately a sheet of ice. But Aizenberg's surfaces are "super-hydrophobic," which literally means "very afraid of water." They contain micron-sized geometric patterns, including posts, bricks, and other structures, that cause droplets to bounce away before they can adhere. "The key feature is that we design these structures to be nearly friction-free," Aizenberg explains. "The droplets are effectively deflected before ice formation can occur."

The nanostructures can be etched or molded into metal, rubber, or other substances. Although airplanes are an obvious destination for ice-blocking materials, Aizenberg says another important application would be in construction. The accumulation of ice on roofs can threaten their structural integrity. Roofing surfaces that shrugged off ice could avert catastrophic collapses.

Matthew Herman, a building physicist with the international engineering firm Buro Happold, says ice-repellent technology would be extremely useful for large edifices




Aizenberg and her colleagues have found that their materials resist ice accumulation until the temperature drops to about -30° C. That's far colder than nearly any industrial setting, she notes. Even at ultra-low temperatures, when the ice-repellency starts to break down, all's not lost. The ice that does form has a weak grip, requiring a small fraction—less than 10 percent—of the normal force needed to remove it from traditional surfaces. "It's very easy to strip off because it's only contacting the surface at the tips of these nanostructured features," says Aizenberg. "It can be removed just by the flow of air."

Hold the ice: Conventional surfaces allow ice droplets (left) to grab hold and form sheets (center), but nanoscale roughness at the surface prevents ice from forming. Credit: Joanna Aizenberg, Harvard University.

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