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. But Harvard scientists say they have created materials that can prevent ice from forming on surfaces in the first place.
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.
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."
In tests, 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."
The nanostructures can be etched or molded into metal, rubber, or other substances, adds Aizenberg, whose group has applied for a patent on the method and plans to commercialize it. It's not clear how much it would cost to add ice resistance to a given surface, Aizenberg says, but she predicts it will be low enough not to discourage its use. Also still to be tested is whether a surface application of the nanostructures would require periodic refreshing. Her team is conducting studies in wind tunnels to see how durable the materials are, analyzing their ability to withstand high velocities and other real-world stresses.
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. "Many historic buildings in New York, Boston, and Chicago are built using ledges," Herman says. "They're beautiful for creating shadow and texture, but they allow snow to build up. Eventually you get enough water and ice sitting on a ledge that it can fall off, and you have to put up scaffolding or a canopy to protect the people below."
Howard Stone, professor of mechanical and aerospace engineering at Princeton University, says Aizenberg's "wonderfully creative" work presents multiple commercial opportunities. He wondered whether applying ice-resistant materials would alter the aerodynamic behavior of a surface such as a plane wing, but Aizenberg and her colleagues have considered the question in depth and say the aerodynamics likely will be unaffected. They plan to publish those data in a forthcoming paper.
By Adam Marcus
From Technology Review