Why doesn't liquid nitrogen poured on your hand instantly cause severe frostbite?

Partly settled. Some of it is understood, some is still argued about.

Kapitza's problem #76 asks students to explain why pouring liquid nitrogen on the hand does not cause instant burns. The underlying physics, the Leidenfrost effect, was first described for water droplets on a hot skillet by Johann Gottlob Leidenfrost in 1756, and for cryogenic liquids on warm skin the same insulating-vapor-layer mechanism applies in reverse. The basic mechanism has been quantitatively understood since Biance, Clanet & Quere's 2003 hydrodynamic analysis of Leidenfrost drops, but active research continues into exactly what surface conditions, contact geometry, and exposure durations cause the vapor layer to collapse (leading to real cold burns), including 2020s work on how surface texture and thermal properties shift the critical 'Leidenfrost temperature/point'.

The same self-generated cushion of vapor that lets a water droplet dance across a hot skillet is what saves your skin from a splash of liquid nearly 200 degrees below freezing.

Explanations that circulated

• Skin is simply too warm to freeze from a brief splash regardless of the liquid's temperature (the real protective mechanism is the Leidenfrost effect: the skin's heat instantly vaporizes a thin insulating layer of nitrogen gas beneath the liquid, which is a poor heat conductor and drastically slows heat transfer, not merely a matter of contact being 'brief')

References

• Leidenfrost drops, A.-L. Biance, C. Clanet, D. Quere, Physics of Fluids, 2003
• How roughness and thermal properties of a solid substrate determine the Leidenfrost temperature: Experiments and a model, Various authors, arXiv preprint, 2023