Video: When water droplets misbehave

You might think we know all there is to know about raindrops, including how they form on windshields and windowpanes. But the physics of this process has long been a mystery. Now, a new study shows just how it happens, contradicting expectations of what a proper liquid should do. Most scientists assumed that the process of drop creation from a film would be a perfect reversal of what happens when drops spread out. These spreading droplets typically maintain a smooth, rounded shape—basically a series of ever wider drops—while they melt into a film. To find out what actually happens when that film fattens up into a drop, scientists designed a system that used electrodes to force a single droplet into a film. When they removed the electric field, the film reverted into a droplet, a process the scientists captured using high-speed imaging (above). Their finding? Liquid films that coalesce into drops first form a raised rim around their edges, looking a little bit like the stage at a one-ring circus, the researchers report today in Science Advances. This could mean an entirely new way of designing products that make use of thin-liquid films, like smartphone screens and self-cleaning paint; a better understanding should help manufacturers control the behavior of liquids.