Leidenfrost effect

The Leidenfrost effect is a physical phenomenon in which a liquid, close to a surface that is significantly hotter than the liquid's boiling point, produces an insulating vapor layer that keeps the liquid from boiling rapidly. Because of this “repulsive force”, a droplet hovers over the surface rather than making physical contact with the hot surface.
A heat engine based on the Leidenfrost effect has been prototyped. It has the advantage of extremely low friction.