Microscopic ridges increase water-repellent effect

Nature : It was once thought that the most effective water-repellent materials force repulsed droplets into a symmetrical recoil shape. However, Kripa Varanasi of MIT and his colleagues have found that a textured surface that creates a controlled asymmetrical recoil is even more effective. They added 0.1-mm ridges to an already water-repellent material and filmed water droplets impacting the surface at 10 000 frames per second. The videos revealed that the asymmetrical recoil shapes yielded contact times—a measure of repulsion—that were 37% shorter than the non-ridged version of the material. Varanasi’s team also tested the material using molten tin. On the ridged version, the impact period was short enough that the metal droplets were repelled. On the non-ridged version the contact time was long enough that the droplets cooled and stuck to the surface. They also compared the material with ridged plant leaves and butterfly wings. Varanasi says that the superhydrophobic material should be easily commercialized.