Forget road salt and other deicers, engineers have figured out how to stop chilled water in its tracks, before ice has a chance to form.
The scientists think their lab findings could lead to a new type of coating that can be directly integrated into various materials and commercialized in the near future. The result, they say, would be an ice-prevention method that's more efficient and environmentally friendly than the traditional deicing chemicals used on planes and roads.
"We wanted to take a completely different tact and design materials that inherently prevent ice formation by repelling the water droplets," said study researcher Joanna Aizenberg, an engineer at Harvard University. "From past studies, we also realized that the formation of ice is not a static event. The crucial approach was to investigate the entire dynamic process of how droplets impact and freeze on a supercooled surface."
The research was detailed online Nov. 9 in the journal ACS Nano.
To do this, Aizenberg, Harvard's Amy Smith Berylson, and colleagues first looked at how nature deals with water (the precursor to ice). They found, for example, that mosquitoes can defog their eyes, and water striders can keep their legs dry thanks to an array of tiny bristles that repel droplets by reducing the surface area each one encounters.
"Freezing starts with droplets colliding with a surface," Aizenberg explained. "But very little is known about what happens when droplets hit surfaces at low temperatures."
The team created surfaces to mimic some of those found in nature, with teensy bristles, blades and interconnected patterns, such as honeycombs and bricks. Then they used high-speed videos to observe supercooled droplets hitting these surfaces.
They saw that when a cold droplet hit one of their nanostructured surface, it first spread out, and then the process ran in reverse: The droplet retracted to a spherical shape and bounced back off the surface before ever having a chance to freeze.
By contrast, on a smooth surface without the structured properties, the droplets remained spread out and eventually froze.
The nanostructured materials prevent the formation of ice even down to temperatures as low as low as minus 13 to minus 22 degrees Fahrenheit (minus 25 to minus 30 degrees Celsius). Below that, due to the reduced contact area that prevents the droplets from fully wetting the surface, any ice that forms doesn't adhere well and is much easier to remove than the stubborn sheets that can form on flat surfaces.
"We see this approach as a radical and much needed shift in anti-ice technologies," Aizenberg said. "The concept of friction-free surfaces that deflect supercooled water droplets before ice nucleation can even occur is more than just a theory or a proof-of-principle experiments."
In fact, the team has just begun testing the technology in real-world settings.
The research was funded by DARPA (Defense Advanced Research Projects Agency), the Wyss Institute for Biologically Inspired Engineering at Harvard University, and the U.S. Department of Homeland Security (DHS) Scholarship and Fellowship Program.
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