Leidenfrost effect

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The Leidenfrost effect is a phenomenon in which a liquid, in near contact with a mass significantly hotter than its boiling point, produces an insulating vapor layer which keeps that liquid from boiling rapidly. This is most commonly seen when cooking; one sprinkles drops of water in a skillet to gauge its temperature—if the skillet's temperature is at or above the Leidenfrost point, the water skitters across the metal and takes longer to evaporate than it would in a skillet that is hot, but at a temperature below the Leidenfrost point. It has also been used in some dangerous demonstrations, such as dipping a wet finger in molten lead and blowing out a mouthful of liquid nitrogen, both enacted without injury to the demonstrator. There have been claims that the effect is also involved when walking on fire, but this contention remains controversial. The effect is also responsible for the ability of liquid nitrogen to skitter across lab floors, collecting dust in the process.

It is named after Johann Gottlob Leidenfrost, who discussed it in A Tract About Some Qualities of Common Water in 1756.

To demonstrate the Leidenfrost effect at home, just take a normal (not nonstick) clean frying pan and heat over a gas stove. Have a bowl of clean water handy. Every now and then, dip your fingers into the bowl of water and sprinkle a few drops onto the pan. Initially, as the temperature of the pan is below 100 °C, the water just flattens out and slowly evaporates. As the temperature of the pan goes above 100 °C, the water drops hiss on touching the pan and evaporate relatively quickly. Later, as the temperature goes past 220°C / 428 °F, the Leidenfrost effect comes into play. On contact the droplets of water do not evaporate away so quickly. This time, they bunch up into small balls of water and skitter around, lasting much longer than when the temperature of the pan was much lower. This effect lasts pretty much until a much higher temperature causes any further drops of water to evaporate too quickly to cause this effect.

This works because, at temperatures above the Leidenfrost point (about 220 °C / 428 °F for water), when water touches the hot plate, the bottom part of the water vapourizes immediately on contact. The resulting gas actually suspends the rest of the water droplet just above it, preventing any further direct contact between the liquid water and the hot plate and dramatically slowing down further heat transfer between them. This also results in the drop being able to skid around the pan on the layer of gas just under it .

(The Leidenfrost point may also be taken to be the temperature for which the hovering droplet lasts longest.)