Although getting things to stick to surfaces is useful, there is also value in making surfaces more slippery — for instance to keep them clean or dry or to reduce drag. Mechanical engineer Kripa Varanasi develops nano-engineered surfaces to be hydrophobic so that water runs off. Varanasi found inspiration in a nasturtium plant in his office at the Massachusetts Institute of Technology in Cambridge. “I was one day struck by the fact that the veins were on the top of the leaf, not the bottom,” he says.
Until then, plant scientists thought that nature’s most water-repellent — also called superhydrophobic — surface belonged to the lotus leaf. Lotus leaves are coated with wax, which chemically repels the water. The wax is arranged in microscopic bumps; the roughness means that there is less contact between the water and the leaf, so a droplet stays rounder and rolls off more easily. Such a surface is also self-cleaning, because the water carries away contaminants; this makes the lotus effect popular among researchers developing surfaces that need to resist moisture or bacterial build-up. But Varanasi wondered whether the nasturtium had veins on the top of its leaves to do the same job as the lotus leaf’s waxy bumps.