Leidenfrost effect
Drop water onto a hot surface and something counterintuitive happens. At moderate temperatures the droplets boil away in seconds. But above about 200 °C — the Leidenfrost point — they float on a cushion of their own vapor and survive for minutes, skittering across the surface like tiny hovercrafts.
The vapor cushion
Above the Leidenfrost point, the bottom of the droplet vaporizes so rapidly that it creates a thin layer of steam between the droplet and the surface. This vapor layer insulates the droplet, dramatically slowing further evaporation. The droplet hovers on its own vapor, nearly frictionless.
The characteristic curve
Droplet lifetime follows a striking non-monotonic pattern. From 100°C to about 150°C (nucleate boiling), lifetime decreases rapidly. Then at the Leidenfrost point (~200°C) it jumps dramatically upward as the vapor cushion forms. Above that, it gradually decreases again as radiation and convection take over.
Three regimes
Nucleate boiling (100–150°C): bubbles form at contact points; rapid evaporation. Transition boiling (150–200°C): unstable vapor patches form and collapse. Film boiling (>200°C): a stable vapor film supports the droplet — the Leidenfrost regime.
Real-world examples
Chefs test pan temperature by flicking water droplets — when they dance, the pan is above the Leidenfrost point. The same physics protects hands briefly dipped in liquid nitrogen, explains why molten lead can be poured over a wet hand (do not try this), and governs heat transfer in nuclear reactor cooling.