The
Leidenfrost effect is a
phenomenonA phenomenon is any observable occurrence. In popular usage, a phenomenon often refers to an extraordinary event. In scientific usage, a phenomenon is any event that is observable, however commonplace it might be, even if it requires the use of instrumentation to observe it...
in which a liquid, in near contact with a mass significantly hotter than the liquid's
boiling pointThe boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
, produces an insulating
vaporA vapor or vapour is a substance in the gas phase at a temperature lower than its critical temperature....
layer which keeps that liquid from
boilingBoiling, a type of phase transition, is the rapid vaporization of a liquid, which typically occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure...
rapidly. This is most commonly seen when cooking; one sprinkles drops of water in a skillet to gauge its temperature—if the skillet's
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
is at or above the
Leidenfrost point, the water skitters across the
metalA metal is a chemical element that is a good conductor of both electricity and heat, forms cations and ionic bonds with non-metals. In chemistry, a metal is an element, compound, or alloy characterized by high electrical conductivity. In a metal, atoms readily lose electrons to form positive ions...
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
leadLead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air...
http://www.csicop.org/si/9911/willey.htmlor blowing out a mouthful of
liquid nitrogenLiquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density at its boiling point of 0.807 g/mL and a dielectric constant of 1.4...
, both enacted without injury to the demonstrator.
The
Leidenfrost effect is a
phenomenonA phenomenon is any observable occurrence. In popular usage, a phenomenon often refers to an extraordinary event. In scientific usage, a phenomenon is any event that is observable, however commonplace it might be, even if it requires the use of instrumentation to observe it...
in which a liquid, in near contact with a mass significantly hotter than the liquid's
boiling pointThe boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
, produces an insulating
vaporA vapor or vapour is a substance in the gas phase at a temperature lower than its critical temperature....
layer which keeps that liquid from
boilingBoiling, a type of phase transition, is the rapid vaporization of a liquid, which typically occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure...
rapidly. This is most commonly seen when cooking; one sprinkles drops of water in a skillet to gauge its temperature—if the skillet's
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
is at or above the
Leidenfrost point, the water skitters across the
metalA metal is a chemical element that is a good conductor of both electricity and heat, forms cations and ionic bonds with non-metals. In chemistry, a metal is an element, compound, or alloy characterized by high electrical conductivity. In a metal, atoms readily lose electrons to form positive ions...
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
leadLead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air...
http://www.csicop.org/si/9911/willey.htmlor blowing out a mouthful of
liquid nitrogenLiquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density at its boiling point of 0.807 g/mL and a dielectric constant of 1.4...
, 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 LeidenfrostJohann Gottlob Leidenfrost was a German doctor and theologian who first described the scientific phenomenon eponymously named the Leidenfrost effect.-Personal life and career:...
, 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.
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. 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 FairbairnSir William Fairbairn, 1st Baronet was a Scottish structural engineer.- Early career :Born in Kelso to a local farmer, Fairbairn showed an early mechanical aptitude and served as an apprentice millwright in Newcastle upon Tyne where he befriended the young George Stephenson...
, 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, 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 persisted for 152 seconds at . 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 boilerA flash boiler is a type of water-tube boiler, whose tubes are strong and close together with water pumped through the tubes. The tubes are kept very hot so the water feed is quickly flashed into steam and superheated...
, 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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