Leidenfrost effect

When a liquid touches a surface sufficiently hotter than its boiling point, rapid vaporization at the interface can create a layer of the liquid's own vapor beneath the remaining liquid. This film separates much of the liquid from direct contact with the solid. Because vapor conducts heat less effectively than liquid-solid contact, heat transfer and evaporation can be reduced; the vapor can also let a droplet glide over the surface. The droplet still evaporates, and the film is continuously sustained by vaporization.

The Leidenfrost point is the temperature threshold associated with stable film boiling for a particular liquid-and-surface setup. Below it, a drop may spread, boil vigorously or pass through transitional boiling; above it, a stable vapor film can support the drop. It is not a single universal temperature: experiments and models show dependence on fluid properties, pressure, surface roughness and wettability, surface thermal properties, and how the liquid is deposited. The Leidenfrost point is distinct from the ordinary boiling point, which is set by the liquid's equilibrium with its surroundings at a specified pressure.

The phenomenon is named after Johann Gottlob Leidenfrost, who described the persistence of water drops on strongly heated surfaces in an eighteenth-century treatise. Film boiling and the Leidenfrost point matter in heat-transfer engineering, including quenching and cooling, because a vapor film can sharply change heat flux. Researchers also study vapor-supported droplets for controlled motion and other fluid applications. The effect should not be treated as protection from burns: vapor films are condition-dependent and unstable, and demonstrations involving hot materials or cryogenic liquids are dangerous.