Leidenfrost effect


Leidenfrost effect

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 and 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. [Bernardin and Mudawar, "A Cavity Activation and Bubble Growth Model of the Leidenfrost Point," Transactions of the ASME, (Vol. 124, Oct. 2002)] As a very rough estimate, the Leidenfrost point for a drop of water on a frying pan might occur at 160 C.

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

References

ee also

*Critical heat flux

External links

* [http://www.wiley.com/legacy/college/phy/halliday320005/pdf/leidenfrost_essay.pdf Essay about the effect and demonstrations by Jearl Walker] (PDF)
* [http://www.uoregon.edu/~linke/climbingdroplets/index.html Site with high-speed video, pictures and explanation of film-boiling] by Heiner Linke at the University of Oregon, USA
* [http://news.bbc.co.uk/1/hi/sci/tech/4955398.stm "Scientists make water run uphill"] by BBC News about using the Leidenfrost effect for cooling of computer chips.
* [http://www.abc.net.au/catalyst/stories/s1647557.htm "Uphill Water"] - ABC Catalyst story


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