Leidenfrost effect
Because of this repulsive force, a droplet hovers over the surface, rather than making physical contact with it.
The effect is named after the German doctor Johann Gottlob Leidenfrost, who described it in A Tract About Some Qualities of Common Water.
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.
[1] 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,[2] he cited the work of Pierre Hippolyte Boutigny (1798–1884) and Professor Bowman of King's College, London, in studying this.
Lower temperatures in a boiler firebox might evaporate water more quickly as a result; compare Mpemba effect.
[3] It has been demonstrated that it is possible to stabilize the Leidenfrost vapor layer of water by exploiting superhydrophobic surfaces.
[5] The Leidenfrost effect has been used for the development of high sensitivity ambient mass spectrometry.
At the last moment of droplet evaporation, all the enriched molecules release in a short time period and thereby increase the sensitivity.
[6] A heat engine based on the Leidenfrost effect has been prototyped; it has the advantage of extremely low friction.
It represents the point on the boiling curve where the heat flux is at the minimum and the surface is completely covered by a vapor blanket.
Heat transfer from the surface to the liquid occurs by conduction and radiation through the vapour.
In 1756, Leidenfrost observed that water droplets supported by the vapor film slowly evaporate as they move about on the hot surface.
The minimum heat flux for a large horizontal plate can be derived from Zuber's equation,[3]
For stable film boiling on a horizontal surface, Berenson has modified Bromley's equation to yield,[10]
Bromley has suggested the following equations for film boiling from the outer surface of horizontal tubes:
While the equation for the minimum film boiling temperature, which can be found in the reference above, is quite complex, the features of it can be understood from a physical perspective.
The proportional relationship between the minimum film boiling temperature and surface tension is to be expected, since fluids with higher surface tension need higher quantities of heat flux for the onset of nucleate boiling.
Henry developed a model for Leidenfrost phenomenon which includes transient wetting and microlayer evaporation.
[13] Since the Leidenfrost phenomenon is a special case of film boiling, the Leidenfrost temperature is related to the minimum film boiling temperature via a relation which factors in the properties of the solid being used.
The Leidenfrost temperatures for glycerol and common alcohols are significantly smaller because of their lower surface tension values (density and viscosity differences are also contributing factors.)
Non-volatile materials were discovered in 2015 to also exhibit a 'reactive Leidenfrost effect', whereby solid particles were observed to float above hot surfaces and skitter around erratically.
Above 675 °C (1,247 °F), cellulose was observed to exhibit transition boiling with violent bubbling and associated reduction in heat transfer.
Liftoff of the cellulose droplet (depicted at the right) was observed to occur above about 750 °C (1,380 °F), associated with a dramatic reduction in heat transfer.
[14] High speed photography of the reactive Leidenfrost effect of cellulose on porous surfaces (macroporous alumina) was also shown to suppress the reactive Leidenfrost effect and enhance overall heat transfer rates to the particle from the surface.
The new phenomenon of a 'reactive Leidenfrost (RL) effect' was characterized by a dimensionless quantity, (φRL= τconv/τrxn), which relates the time constant of solid particle heat transfer to the time constant of particle reaction, with the reactive Leidenfrost effect occurring for 10−1< φRL< 10+1.
The reactive Leidenfrost effect with cellulose will occur in numerous high temperature applications with carbohydrate polymers, including biomass conversion to biofuels, preparation and cooking of food, and tobacco use.
[14] The Leidenfrost effect has also been used as a means to promote chemical change of various organic liquids through their conversion by thermal decomposition into various products.
[17] In Jules Verne's 1876 book Michael Strogoff, the protagonist is saved from being blinded with a hot blade by evaporating tears.
[18] In the 2009 season 7 finale of MythBusters, "Mini Myth Mayhem", the team demonstrated that a person can wet their hand and briefly dip it into molten lead without injury, using the Leidenfrost effect as the scientific basis.
