I’ve been learning a bit about water and its boiling tendencies at my work lately. I thought I’d share with you the Leidenfrost effect.
The Leidenfrost effect describes the boiling of water on a very hot surface. If you take water and throw it onto a moderately hot pan, the water sizzles away very quickly. If you take water and throw it onto a VERY hot surface something else happens: the water forms into droplets and dances around the pan. Instead of boiling off quickly, it actually takes a significant amount of time.
What’s happening here?
It turns out that when the water first hits the pan there is a momentary “initial liquid contact stage”* during which the water that strikes the pan turns into a gas. The rest of the water droplet then sits on top of the gas instead of directly on the pan.
Heat transfer from metal to water is pretty good, but if you have to go through a vapor barrier first the heat transfer capabilities drop significantly.
How is this useful? Well, amongst other things, it helps explain things like walking on coals. When you’re nervous and antsy, the sweat on your feet will create little Leidenfrost vapor barriers between your feet and the hot coals. I wouldn’t recommend trying it though, because if you’re calm and the heat isn’t enough to cause you to start sweating, you will be seriously burned.
*ASM Vol. 4 Heat Treating Metals Handbook