Sometimes when you drop water on a very hot pan, it does not disappear right away. Instead, round drops dance around the pan. This is called the Leidenfrost effect.

When the bottom of a drop touches the hot pan, it turns into steam very quickly. The steam makes a thin cushion, and the drop floats on it. Because steam does not carry heat well, the drop lasts longer.

The Leidenfrost effect is named after a German doctor who described it in 1756. When a drop of water falls on a pan that is only a little above 100°C, it boils noisily and disappears quickly.

But when the pan is much hotter, the bottom of the drop turns into vapor at once, and this vapor lifts the drop. The temperature where this starts is called the Leidenfrost point, which is around 200°C for water, although it depends on the surface. Since vapor does not carry heat well, the drop can survive for tens of seconds or even minutes. In 2006, scientists found that drops on a saw-tooth surface move by themselves in one direction.

Described in detail in 1756 by the German physician Johann Gottlob Leidenfrost, the effect that bears his name appears when a liquid meets a surface far hotter than its boiling point. On a pan just above 100°C, a water drop spreads, boils violently and vanishes within moments. Beyond a threshold known as the Leidenfrost point, roughly 200°C for water but strongly dependent on the material, roughness and cleanliness of the surface, the drop's underside evaporates instantly, and the resulting vapor cushion levitates it. Because vapor conducts heat poorly, this layer acts as insulation, so, paradoxically, the drop survives longer on the hotter surface, sometimes for minutes, while gliding with almost no friction.

In 2006, researchers showed that on a heated surface machined into asymmetric saw-tooth ridges, drops of many liquids propelled themselves in one direction over a wide range of temperatures. They attributed this to vapor flowing along the ridges and dragging the drop, an explanation that later studies continued to test. The phenomenon also matters in industry. Vapor films can make the cooling of hot metal uneven, whereas a 2012 study found that a water-repellent coating kept the film stable even as the metal cooled, preventing explosive boiling.