Engineers from Harvard University have created nanostructured materials that repel water droplets at low temperatures, preventing ice formation.
Engineers from Harvard University have created nanostructured materials that repel water droplets at low temperatures, preventing ice formation
The finding, reported online in ACS Nano on 9 November, could lead to a new way to keep aircraft wings, buildings, powerlines, and even entire highways free of ice during the worst winter weather. The researchers say that integrating anti-ice technology into a material is more efficient and sustainable than conventional solutions like chemical sprays, salt, and heating.
A team led by Professor Joanna Aizenberg at Harvard focused on preventing rather than fighting ice build-up.
"We wanted to take a completely different tack and design materials that inherently prevent ice formation by repelling the water droplets," says Aizenberg. "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, mosquitos 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," 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 modelled on 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 optimisation," says Lidiya Mishchenko, a graduate student in Aizenberg's lab and first author of the paper.
The use of precisely engineered materials enabled the researchers to model the dynamic behaviour of impacting droplets at a high level of detail, leading them to create a better design for ice-preventing materials.
An important benefit of testing a wide variety of structures, Mishchenko adds, was that it allowed the team to optimise 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.
The nanostructured materials prevent the formation of ice even down to temperatures as low as -25 to -30 degrees Celsius. Below that, ice may form but 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. "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 experiment. We have begun to test this promising technology in real-world settings to provide a comprehensive framework for optimising these robust ice-free surfaces for a wide range of applications, each of which may have a specific set of performance requirements."