Highways, airplanes, power lines and other critical surfaces may remain ice-free during future winters by integrating new anti-ice technology from Harvard University into coatings and other nanostructured materials.
A Harvard engineering team has designed and demonstrated the technology, which enables nanostructured materials to repel water droplets before they have the chance to freeze.
The research, reported online Nov. 9 in ACS Nano, could lead to new ways to keep surfaces and structures free from treacherous, damaging ice without conventional treatments like chemical sprays, salt and heating—methods that can create corrosion and other environmental problems.
With better understanding of the ice forming process, a new type of coating integrated directly into a variety of materials could soon be developed and commercialized, the researchers say.
‘A Completely Different Tack’
The goal of the research was on preventing ice buildup, rather than finding new ways to fight it, said team leader Joanna Aizenberg, Amy Smith Berylson Professor of Materials Science at the Harvard School of Engineering and Applied Sciences (SEAS) and a Core Member of the Wyss Institute for Biologically Inspired Engineering at Harvard.
“We wanted to take a completely different tack and design materials that inherently prevent ice formation by repelling the water droplets,” said Aizenberg. “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.”
For initial inspiration, the researchers turned to some elegant solutions seen in nature. For example, mosquitoes can defog their eyes and water striders can keep their legs dry because of arrays of tiny bristles that repel droplets by reducing the surface area each one encounters.
“Freezing starts with droplets colliding with a surface,” explains Aizenberg. “But very little is known about what happens when droplets hit surfaces at low temperatures.”
To gain a detailed understanding of the process, the researchers watched high-speed videos of supercooled droplets hitting surfaces that were modeled after those found in nature. They saw that when a cold droplet hits the nanostructured surface, it first spreads out, but then the process runs in reverse: the droplet retracts to a spherical shape and bounces back off the surface before ever having a chance to freeze.
By contrast, on a smooth surface without the structured properties, a droplet remains spread out and eventually freezes.
“We fabricated surfaces with various geometries and feature sizes—bristles, blades, and interconnected patterns such as honeycombs and bricks—to test and understand parameters critical for optimization,” says Lidiya Mishchenko, a graduate student in Aizenberg’s lab and first author of the paper.
The use of such precisely engineered materials enabled the researchers to model the dynamic behavior of impacting droplets at an amazing level of detail and create a better design for ice-preventing materials.
Testing a wide variety of structures also allowed the team to optimize for pressure-stability. They discovered that the structures composed of interconnected patterns were ideally suited for stable, liquid-repelling surfaces that can withstand high-impact droplet collisions, such as those encountered in driving rain or by planes in flight.
Effectiveness to -30° C
The nanostructured materials prevent the formation of ice even down to temperatures as low as –25 to –30 degrees Celsius. Below that, due to the reduced contact area that prevents the droplets from fully wetting the surface, any ice that forms does not 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,” says Aizenberg. “We have begun to test this promising technology in real-world settings to provide a comprehensive framework for optimizing these robust ice-free surfaces for a wide range of applications, each of which may have a specific set of performance requirements.”
The research was funded by DARPA (Award Number HR0011-08-C-0114); the Wyss Institute for Biologically Inspired Engineering at Harvard University; and the U.S. Department of Homeland Security (DHS) Scholarship and Fellowship Program.