Why Do Water Droplets Bounce?

By Bryan Menegus on at

When a droplet of water impacts a larger body of water it has a tendency to bead up and bounce. It happens constantly, but you might not have focused closely enough to consider its strangeness—and it is very strange, especially when viewed on a high-speed camera.

As Dustin from Smarter Every Day discovered with some help from astronaut Don Pettit, the answer to why water bounces is air. As the droplet falls, a layer of air gets caught between the two impacting surfaces which keeps them from becoming one singular body of water. Surface tension from the standing water launches the water bead back upward, each time with less energy, until eventually enough air is forced out and the bead coalesces with the larger mass.

On a vibrating surface — a speaker cone, for instance — the droplets survive even longer. And some truly unusual things happen when water, air, and a cello get together in space. But I’ll leave the heavy science lifting to Dustin.