Earlier this year, we told you about a slippery material system developed by researchers at Harvard's Wyss Institute for Biologically Inspired Engineering, which can start and stop liquid drops rolling down a vertical surface. The system can be tuned to change the amount of water or oil it repels, depending on its transparency.
That material system, which both infuses and coats a nonporous elastic substrate, was the next-generation version of the Slippery Liquid-Infused Porous Surfaces (SLIPS) material platform designed by the senior member of the current research team, Joanna Aizenberg, professor of materials science at Harvard's SEAS and a Wyss Institute core faculty member. SLIPS was originally inspired by the carnivorous pitcher plant, coated with an ultra-slippery surface that slides insect victims into its depths. Unlike earlier materials that repel water, SLIPS also repels oils, and resists the formation of bacterial biofilms and ice.
An ultraslippery coating that repels oil and water even on vertical surfaces is created with a glass honeycomb-like structure with craters (left). This is coated with a Teflon-like chemical (purple) that binds to the honeycomb cells to form a stable liquid film. The film repels droplets of both water and oily liquids (right). Because it's a liquid, it flows, which helps the coating repair itself when damaged.
(Source: Nicolas Vogel, Wyss Institute)
Now that synthetic super-slippery surface has become a durable, completely transparent coating that can turn regular glass into a material that doesn't stain, resists scratches, and repairs itself. Potential applications include self-cleaning windows, stronger and scratch-resistant eyeglass lenses, medical diagnostic devices, and more durable solar panels. The team describes its work in an article (purchase or subscription only) in Nature Communications.
The new coating is just as slippery as the previous material system, but the research team has improved its transparency and durability. Aside from water and oil, it also repels wine, ketchup, and octane, as well as sticky liquids like honey.
The coating works because of its structure, a sturdy honeycomb-like formation that holds the slippery lubricant in many tiny, tightly packed cells. After liquid glass is poured on tiny spheres of polystyrene, the spheres are burned away to form the honeycomb structure. The lubricant binds to the cells to form a stable liquid film, which helps the coating clean and repair itself. Transparency is achieved due to cell diameter being smaller than the wavelength of visible light.
The research team's next steps are making the coating better able to coat curved pieces of glass and to coat plastics such as Plexiglas. Other goals include making the process manufacturable. Research was supported by the Advanced Research Projects Agency-Energy, the Air Force Office of Scientific Research, and the Wyss Institute.