Editing Leidenfrost effect (section)

== Details ==
[[File:Effet leidenfrost.ogg|thumb|A video clip demonstrating the Leidenfrost effect]]
[[File:Spherical harmonic in water drop.ogv|thumb|Excitation of [[normal modes]] in a drop of water during the Leidenfrost effect]]

The effect can be seen as drops of water are sprinkled onto a pan at various times as it heats up. Initially, as the temperature of the pan is just below {{convert|100|C|F}}, the water flattens out and slowly evaporates, or if the temperature of the pan is well below {{convert|100|C|F}}, the water stays liquid. As the temperature of the pan rises above {{convert|100|C|F}}, the water droplets hiss when touching the pan, and these droplets evaporate quickly. When the temperature exceeds the Leidenfrost point, the Leidenfrost effect appears. On contact with the pan, the water droplets bunch up into small balls of water and skitter around, lasting much longer than when the temperature of the pan was lower. This effect works until a much higher temperature causes any further drops of water to evaporate too quickly to cause this effect.

The effect happens because, at temperatures at or above the Leidenfrost point, the bottom part of the water droplet vaporizes immediately on contact with the hot pan. The resulting gas suspends the rest of the water droplet just above it, preventing any further direct contact between the liquid water and the hot pan. As steam has much poorer [[thermal conductivity]] than the metal pan, further heat transfer between the pan and the droplet is slowed down dramatically. This also results in the drop being able to skid around the pan on the layer of gas just under it.

[[File:Heat transfer leading to Leidenfrost effect for water at 1 atm.png|thumb|Behavior of water on a hot plate. Graph shows heat transfer (flux) vs temperature. Leidenfrost effect occurs after transition boiling.]]

The temperature at which the Leidenfrost effect appears is difficult 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.<ref name="Bernardin & Mudawar" >{{cite journal |doi=10.1115/1.1470487 |title=A Cavity Activation and Bubble Growth Model of the Leidenfrost Point |journal=Journal of Heat Transfer |volume=124 |issue=5 |pages=864–74 |date=2002 |last1=Bernardin |first1=John D. |last2=Mudawar |first2=Issam}}</ref>

The effect was also described by the Victorian steam boiler designer, [[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,<ref name="Fairbairn">{{cite book |title=Two Lectures: The Construction of Boilers, and on Boiler Explosions, with the means of prevention |author=William Fairbairn |date=1851 |url=https://books.google.com/books?id=VD5MAAAAMAAJ&q=fairbairn%20boiler&pg=PA1 |url-status=live |archive-url=https://web.archive.org/web/20171123164123/https://books.google.com/books?id=VD5MAAAAMAAJ&dq=fairbairn%20boiler&pg=PA1 |archive-date=2017-11-23 |author-link=William Fairbairn }}{{page needed|date=February 2014}}</ref> he cited the work of Pierre Hippolyte Boutigny (1798–1884) and Professor Bowman of [[King's College, London]], in studying this. A drop of water that was vaporized almost immediately at {{convert|168|C}} persisted for 152&nbsp;seconds at {{convert|202|C}}. Lower temperatures in a boiler [[Firebox (steam engine)|firebox]] might evaporate water more quickly as a result; compare [[Mpemba effect]]. An alternative approach was to increase the temperature beyond the Leidenfrost point. Fairbairn considered this, too, and may have been contemplating the [[flash boiler|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.<ref name="ReferenceA">{{cite book |last1=Incropera |last2=DeWitt |last3=Bergman |last4=Lavine |title=Fundamentals of Heat and Mass Transfer |edition=6th |year=2006 |pages=325–330 |isbn=0-471-45728-0 }}</ref>

It has been demonstrated that it is possible to stabilize the Leidenfrost vapor layer of water by exploiting [[superhydrophobic]] surfaces. In this case, once the vapor layer is established, cooling never collapses the layer, and no nucleate boiling occurs; the layer instead slowly relaxes until the surface is cooled.<ref name="vakarelski2012">{{cite journal |doi=10.1038/nature11418 |pmid=22972299 |title=Stabilization of Leidenfrost vapour layer by textured superhydrophobic surfaces |journal=Nature |volume=489 |issue=7415 |pages=274–7 |date=2012 |last1=Vakarelski |first1=Ivan U. |last2=Patankar |first2=Neelesh A. |last3=Marston |first3=Jeremy O. |last4=Chan |first4=Derek Y. C. |last5=Thoroddsen |first5=Sigurdur T. |bibcode=2012Natur.489..274V|s2cid=4411432 }}</ref>

Droplets of different liquids with different boiling temperatures will also exhibit a Leidenfrost effect with respect to each other and repel each other.<ref>{{cite journal |last1=Pacheco-Vázquez |first1=F. |last2=Ledesma-Alonso |first2=R. |last3=Palacio-Rangel |first3=J. L. |last4=Moreau |first4=F. |title=Triple Leidenfrost Effect: Preventing Coalescence of Drops on a Hot Plate |journal=Physical Review Letters |date=12 November 2021 |volume=127 |issue=20 |pages=204501 |doi=10.1103/PhysRevLett.127.204501 |pmid=34860033 |arxiv=2107.00438 |bibcode=2021PhRvL.127t4501P |s2cid=235694660}}
*{{cite magazine |author=Leah Crane |date=24 November 2021 |title=Watch droplets bounce off each other as they levitate on a hot plate |magazine=New Scientist |url=https://www.newscientist.com/article/2298528-watch-droplets-bounce-off-each-other-as-they-levitate-on-a-hot-plate/ |url-access=registration}}</ref>

The Leidenfrost effect has been used for the development of high sensitivity ambient mass spectrometry. Under the influence of the Leidenfrost condition, the levitating droplet does not release molecules, and the molecules are enriched inside the droplet. At the last moment of droplet evaporation, all the enriched molecules release in a short time period and thereby increase the sensitivity.<ref>{{cite journal | title=Leidenfrost Phenomenon-assisted Thermal Desorption (LPTD) and Its Application to Open Ion Sources at Atmospheric Pressure Mass Spectrometry| journal=Journal of the American Society for Mass Spectrometry |author=Subhrakanti Saha |author2=Lee Chuin Chen |author3=Mridul Kanti Mandal |author4=Kenzo Hiraoka | date=March 2013 | volume=24 | issue=3 | pages=341–7 | doi=10.1007/s13361-012-0564-y|bibcode = 2013JASMS..24..341S | pmid=23423791| s2cid=39368022 }}</ref>

A [[heat engine]] based on the Leidenfrost effect has been prototyped; it has the advantage of extremely low friction.<ref>{{cite journal|last1=Wells|first1=Gary G.|last2=Ledesma-Aguilar|first2=Rodrigio|last3=McHale|first3=Glen|last4=Sefiane|first4=Khellil|title=A sublimation heat engine|journal=Nature Communications|date=3 March 2015|doi=10.1038/ncomms7390|bibcode=2015NatCo...6.6390W|volume=6|pages=6390|pmid=25731669|pmc=4366496}}</ref>

The effect also applies when the surface is at room temperature but the liquid is [[cryogenic]], allowing [[liquid nitrogen]] droplets to harmlessly roll off exposed skin.<ref>{{cite news | url=http://bbc.com/news/magazine-19870668 | title=Who What Why: How dangerous is liquid nitrogen? | work=BBC News | date=8 October 2012 }}</ref> Conversely, the ''inverse Leidenfrost effect'' lets drops of relatively warm liquid levitate on a bath of liquid nitrogen.<ref>{{cite journal | url=http://pubs.acs.org/doi/10.1021/acs.langmuir.6b00574 | doi=10.1021/acs.langmuir.6b00574 | title=Inverse Leidenfrost Effect: Levitating Drops on Liquid Nitrogen | year=2016 | last1=Adda-Bedia | first1=M. | last2=Kumar | first2=S. | last3=Lechenault | first3=F. | last4=Moulinet | first4=S. | last5=Schillaci | first5=M. | last6=Vella | first6=D. | journal=Langmuir | volume=32 | issue=17 | pages=4179–4188 | pmid=27054550 | s2cid=21732968 }}</ref>