Rerouting warm engine air and pumping ice-melting chemicals onto the wings are a couple of ways to keep aircraft surfaces free of frost during flight, but researchers are looking for a more efficient technique. Taking their inspiration from a water-gathering desert beetle, scientists have developed a patterned surface on which the spread of ice can be controlled and prevented. They say the material could be scaled up and applied to not only aircraft parts, but also wind turbines, heat pump coils and car windshields.
The Namib Desert Beetle is a clever little critter that survives the dry African desert by way of a natural water-collecting system on its shell. This consists of small bumps where moisture from the air forms tiny droplets, and a smoother, water-repelling surface underneath which funnels the gathering water into the beetle's mouth. Material scientists at Virginia Tech were able to manipulate this combination of hydrophilic and hydrophobic surfaces to control the spread of frost.
"We made a single dry zone around a piece of ice," says Jonathan Boreyko, assistant professor of Biomedical Engineering and Mechanics at Virginia Tech. "Dew drops preferentially grow on the array of hydrophilic dots. When the dots are spaced far enough apart and one of the drops freezes into ice, the ice is no longer able to spread frost to the neighboring drops because they are too far away. Instead, the drops actually evaporate completely, creating a dry zone around the ice."
Using photolithography, the team layered a bumpy water-attracting pattern over a smooth water-repelling pattern on a small silicon wafer. As the proliferation of frost begins with a single frozen droplet, the researchers found that by altering the distance between the water-gathering bumps they could impact how quickly frost grew across the surface, or even prevent it altogether.
"The twist is how ice bridges grow," says Boreyko. "Ice harvests water from dew drops and this causes ice bridges to propagate frost across the droplets on the surface. Only a single droplet has to freeze to get this chain reaction started."
The technology follows a number of bio-inspired approaches to avoiding icy buildup on various surfaces. The rutted surfaces of lotus leaves, mosquito eyes and legs of water striders, all of which serve to minimize the available surface area where water can gather, are other natural wonders to capture the imaginations of materials scientists working in this area.
"Keeping things dry requires huge energy expenditures," says C. Patrick Collier, a research scientist at Oak Ridge National Laboratory and a co-author of the study. "That's why we are paying more attention to ways to control water condensation and freezing. It could result in huge cost savings."
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