Life is just a theory

These time-lapsed images capture what happens above and below the surface when a water droplet falls into a pool of water. Check out the vortex below the surface! The droplet is dyed blue to make it easier to see. (Slow-motion videos can also provide a cool visual.)

The process seen here, called a droplet coalescence, occurs because the surface of the larger body of water has been coated in water-repelling oil. At first, the oil on the surface keeps the droplet afloat, but eventually a tiny hole forms in the oil layer, and capillary action sucks the water droplet into the larger body of water. But as its shape deforms to fit into that hole, the droplet pinches off a daughter droplet, which flies into the air and then falls back to land on the surface once more. And then the whole thing starts all over again.

But this particular image comes from a paper in Physical Review Letters. In this study, researchers from UC-Davis did more than take pictures. They managed to control the size of the daughter droplet by giving the water droplets an electrical charge, and then varying an external electrical field. It turns out that capillary action isn’t the only force in play during a droplet coalescence. The behavior of charged droplets also depends on electrostatic interactions between the droplets and the larger body of liquid.