How to make a better raincoat with tiny “water bowls”

A FEW YEARS ago Kripa Varanasi, a researcher at the Massachusetts Institute of Technology, made the news with a ketchup bottle that could be emptied without leaving any of the ketchup behind. Instead of sticking to the bottle’s interior, the sauce was repelled by it.

Superhydrophobicity, as physicists call this effect, involves peppering a surface with microscopic structures that contain pockets of fluid. That reduces the area of contact between the surface and any water droplets which fall on it. This, in turn, diminishes the surface tension that would otherwise cause the droplet to cling on, so it instead falls off. In nature, using air as the fluid, lotus leaves and insect cuticles are both famously good at superhydrophobicity. In lotuses the air pockets are created by minute lumps of wax. In insects they are the result of tiny hairs. Dr Varanasi’s ketchup bottle improved even on these paragons, by replacing the air with an oil.