To test the temperature of a frying pan, people often dribble a few drops of water onto the surface. If the pan is cold, the water sits placidly on the surface. But if the metal is hot, the droplets will skitter around like deranged dancers. What makes them move?
When the water hits the hot skillet, it immediately begins to boil off, but it doesn’t evaporate entirely. Instead, the released vapor forms an insulating layer around the still-liquid droplet. This vapor coat not only slows the boiling, it also reduces friction and makes the droplet hydroplane over the metal. The whole process, called the Leidenfrost effect and demonstrated in the video, can occur when a drop of any liquid sits on a surface hotter than its boiling point.
A Leidenfrost droplet and its reflection over time, as the
droplet shrinks down and flies away
If you watch a skittering water droplet in slow motion, you’ll realize there’s one more puzzle—why do the droplets shrink to a certain size and then fly off the surface, rather than gradually evaporating to nothing? To find out, researchers recently created a model of the forces at work in the Leidenfrost effect, and then compared their theoretical work to high-speed films of water and ethanol droplets on a heated surface.
The researchers observed that the water typically fell to the surface, bounced, and then sat comfortably cushioned by its vapor coat, with the upward push of the vapor beneath it balanced by the weight of the droplet. But as the droplet slowly evaporated, its vapor coat actually grew thicker, until the force exerted by the vapor exceeded that of the drop, and the underlying vapor layer shoved the droplet into the air. For ethanol, which boils at a lower temperature than water, droplets didn’t even have a chance to bounce before the vapor propelled them up, up, and away.
[via the American Physical Society]