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 above boiling temperature, but below the temperature of the Leidenfrost point. It has also been used in some dangerous demonstrations, such as dipping a wet finger in molten lead or blowing out a mouthful of liquid nitrogen, both enacted without injury to the demonstrator. 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.

The effect can be seen as drops of water are sprinkled into a pan at various times while it is heating up. 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 the Leidenfrost point, 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 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, when water touches the hot plate, the bottom part of the water vaporizes 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.

It should be noted that the temperature at which the Leidenfrost effect begins to occur is not easy to predict. Even if the volume of the drop of liquid stays the same the Leidenfrost point may be quite different with a complicated dependence on the properties of the surface as well as any impurities in the liquid. Some research has been conducted into a theoretical model of the system, but it is quite complicated.[1] As a very rough estimate, the Leidenfrost point for a drop of water on a frying pan might occur at 160 °C.

The effect was also described by the eminent Victorian steam boiler designer, Sir William Fairbairn, in reference to its effect on massively reducing heat transfer from a hot iron surface to water, such as within a boiler. In a pair of lectures on boiler design[2], he cited the work of one M. Boutigny & Professor Bowman of King's College, London in studying this. A drop of water that was vaporized almost immediately at 334 °F (168 °C) persisted for 152 seconds at 395 °F (202 °C). Lower temperatures in a boiler firebox might evaporate water more quickly as a result. An alternative approach was to increase the temperature substantially, beyond the Leidenfrost point. Fairbairn considered this too, and may have been contemplating the flash steam boiler, but considered the technical aspects insurmountable for the time.

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

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