Now this is something very cool we observe when water is put on a very hot surface. We can see little droplets bouncing around which should actually be evaporated instantly because of such high temperatures. This is due to a phenomenon called Leidenfrost Effect (named after Johann Gottlob Leidenfrost).

When liquid droplets come into contact with a hot surface (around boiling point temperatures) it evaporates almost immediately. If the temperature is high enough such that the layer of liquid drop which comes into contact with hot surface vaporises immediately then this layer of vapour acts like a cushion. This layer of vapour cushion prevents heat transfer to rest of the liquid droplet as steam has poorer thermal conductivity than the pan/hot surface. The steam cushion levitates the droplet and it skitters around without any friction and thus surviving for much longer times. This “high enough” temperature is called the Leidenfrost Temperature.

For instance water has boiling temperature of 100° C. If we put water drops on a pan with temperature of 100°C the droplets will just hiss and spread out vaporising rapidly. But when the pan temperature reaches around 193°C, which is the Leidenfrost point for water, it levitates and skitters around.

So the Leidenfrost temperature is different for different liquids. There’s also something called as inverse Leidenfrost effect where hot liquid droplets are levitated on cold liquid surfaces as the liquid vaporises levitating the hot droplet. For example Anaïs Gauthier’s team at University of Twente have studied this effect by depositing a room temperature droplet of alcohol on top of pool of liquid nitrogen at – 196 degrees celcius.

There’s one application for which we can use this property of liquids. Water droplets just skitter around on a flat hot surface, but if we change the texture of the hot surface we can give a direction to the motion of droplet undergoing Leidefrost effect. If the texture if made of sharp steps at an angle / ratchet like texture , some vapour from below the droplet exits and propels the droplet.

 

This also allows the droplet to move uphill against gravity. This can be used to make Thermostats with no moving parts. Thermostat is a device which senses the temperature of a system and then can be used to control or regulate the temperature of the system. The Leidenfrost thermostat works by using the cooling power of water droplets. It moves the water droplets in one direction to cool the system when the temperature is too high, but discards the drops by moving them in the opposite direction when the temperature is too low, allowing the system to heat up to the correct temperature. This would be better understood by watching this video.

Published in the Journal of Heat Transfer, the thermostat is demonstrated in a short film made by undergraduate students:

Leidenfrost effect is also the explanation for some bizzare stunts some people perform without harming themselves like hitting a stream of molten metal or dipping wet finger in molten lead or blow out a mouthful of liquid nitrogen. The drastic temperature differences creates an heat insulating layer between the skin and materials for a very short duration.