Science Saturday – Leidenfrost Effect Explained

Science Saturday - Leidenfrost EffectThe Leidenfrost Effect has nothing to do with frost or the Dutch city of Leiden. Rather, it’s named after a German physicist, Johann Gottlob Leidenfrost, who published a scientific paper on the phenomenon back in 1756.

You can witness it for yourself, if you sprinkle water onto a really hot frying pan or the rings of an electric cooking hob. That moment when the hob gets really hot and water droplets go whizzing off as if on a cushion of air is the Leidenfrost effect in its simplest manifestation.

It usually occurs when the surface temperature that the water droplets touch is greater than 200C. Past the Leidenfrost point, the bottom surface of the water droplet turns to vapour so quickly that it creates a little insulating pocket under the drop. Pressure from the vapour keeps the droplet aloft, like a tiny little hovercraft.

As it scoots around on the heated surface, molecules of water keep turning into vapour from the underside of the droplet, in a process known as film boiling. But the part of the Leidenfrost effect that’s the most interesting to scientists is how the droplets move.
With so little friction between the liquid and the surface, even slight disturbances in the vapour pockets – caused by escaping molecules of gas – are enough to send the droplets ricocheting all over the place. And the bigger the droplet, the less stable its vapour cushion is, so the more it moves around.

It’s speed, if controllable, could be used in various different applications, from pharmaceuticals to ink-jet printing. That being said, the temperatures required for are so high that it could prove hazardous and costly in terms of heat generation.

 

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