Out of the frying pan

You may have noticed droplets of water skittering around on the film of hot oil when you’re frying something in a skillet. Now MIT researchers have analyzed that seemingly trivial phenomenon and its implications for the first time.

A droplet of boiling water on a hot surface will sometimes levitate on a thin vapor film, a well-studied phenomenon called the Leidenfrost effect. If the surface is coated with hot oil, which has much greater friction than a vapor film, the droplet should be expected to move much more slowly—but the MIT experiments showed that it actually zooms away 10 to 100 times faster.

Kripa Varanasi, a professor of mechanical engineering, and graduate student Victor Julio Leon used extremely high-speed photography to understand the surprisingly complex interplay of factors at work. Under the right conditions, it turns out, the oil will coat the outside of each droplet, and tiny bubbles of vapor will form along the interface between the water and the oil as it heats. Because these bubbles accumulate randomly along the droplet’s base, asymmetries develop, and the lowered friction under the bubble loosens the droplet from the surface.

Without the oil cloak, the vapor bubbles would flow out of the droplet in all directions, but the cloaking effect holds them in like the skin of a  balloon. When the bubbles burst through, however, they impart a force, and “the balloon just flies off because the air is going out one side,” Varanasi says.

Once the researchers figure out how to control the process, it could potentially be used for self-cleaning or de-icing systems, or to propel tiny amounts of liquid through microfluidic devices.