The Leidenfrost effect

Next academic year it looks likely that I will, for the first time, be teaching some thermodynamics. Then we will have new software that will allow us to offer an annotated list of links to books, webpages, YouTube videos, etc, integrated into the homepage of the course. One of my fave cool thermodynamic effects is the Leidenfrost effect:

I think that is quite cool. I should add a link to that to the course page.

A cute thing about the Leidenfrost is that it is bit counter intuitive. A water droplet dropped onto a hot plate evaporates rapidly, so you expect a water droplet dropped onto a very hot plate to evaporate very rapidly. Instead it sticks around for a while, zooming around the surface.

The reason is that when the plate is very hot (but not when it is not quite so hot), a bottom layer of water in contact with the plate vapourises very quickly and then you have a thin layer of steam between the hot plate and the droplet. Now, the rate at which the droplet evaporates is set by how fast heat flows from the plate into the droplet, and this depends on a thermodynamic property called the thermal conductivity.

This is low for gases, e.g., steam, which is why gases are a good thermal insulator, and so can be used to keep things warm (or cold). For instance a puffa jacket traps air which conducts heat poorly and so stops it flowing from your body to the outside world, keeping you warm.

The steam layer also acts as an insulator, and keeps the droplet cool, allowing it to survive for a relatively long time. The droplets scoot around because as the steam forms and expands it can be under pressure and so exert forces that push the droplet around.