The Leidenfrost effect is a physical phenomenon in which a liquid, in near
contact with a mass significantly hotter than the liquid's boiling point,
produces an insulating vapor layer keeping that liquid from boiling
rapidly. Due to this ‘repulsive force,’ the droplet hovers over the surface
rather than making physical contact with it. This is most commonly seen when
cooking; one sprinkles drops of water in a pan to gauge its temperature: if the
pan's temperature is at or above the Leidenfrost point, the water skitters
across the pan and takes longer to evaporate than in a pan below the
temperature of the Leidenfrost point. The effect is also responsible for the
ability of liquid nitrogen to skitter across floors. It has also been used in
some potentially 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 latter is potentially
lethal, particularly should one accidentally swallow the liquid
nitrogen. It is named after Johann Gottlob
Leidenfrost, who discussed it in A Tract About Some Qualities of Common Water in
1756. 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 100
°C, the water flattens out and slowly evaporates, or if the temperature of the
pan is well below 100 °C, the water stays liquid. As the temperature of the
pan goes above 100 °C, the water droplets hiss when touching the pan and
these droplets evaporate quickly. Later, as the temperature exceeds the
Leidenfrost point, the Leidenfrost effect comes into play. 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.
This is because at temperatures above the Leidenfrost point, the bottom part
of the water droplet vaporizes immediately on contact with the hot
plate. 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
plate. As steam has much poorer thermal conductivity, 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. 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 193 °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, he
cited the work of Pierre Hippolyte Boutigny and Professor Bowman of King's
College, London in studying this. A drop of water that was vaporized almost
immediately at 168 °C persisted for 152 seconds at 202 °C. Lower temperatures in
a boiler 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 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.
It has been demonstrated that it is possible to stabilize the Leidenfrost
vapour layer of water by exploiting superhydrophobic surfaces. In this case,
once the vapour layer is established, cooling never collapses the layer, and
no nucleate boiling occurs; the layer instead slowly relaxes until the surface
is cooled. Leidenfrost effect has been used for the
development of high sensitivity ambient mass spectrometry. Under the influence
of Leidenfrost condition the Levitating droplet does not release molecules out
and the molecules are enriched inside the droplet. At the last moment of
droplet evaporation all of the enriched molecules release in a short time domain
and thus increase the sensitivity. This finding was reported in Journal of The
American Society for Mass Spectrometry:341-7. doi:
10.1007/s13361-012-0564-y) A heat engine based on the Leidenfrost
effect has been prototyped. It has the advantage of extremely low friction.
Leidenfrost point The Leidenfrost point signifies the
onset of stable film boiling. 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 vapor. In 1756, Leidenfrost
observed that water droplets supported by the vapor film slowly evaporate as
they move about on the hot surface. As the surface temperature is increased,
radiation through the vapor film becomes more significant and the heat flux
increases with increasing excess temperature.
The minimum heat flux for a large horizontal plate can be derived from
Zuber's equation, where the properties are evaluated at
saturation temperature. Zuber's constant, C is approximately 0.09 for
most fluids at moderate pressures. Heat transfer correlations
The heat transfer coefficient may be approximated using Bromley's equation,
Where, is the outside diameter of the tube. The correlation constant C is 0.62
for horizontal cylinders and vertical plates and 0.67 for spheres. Vapor
properties are evaluated at film temperature.
For stable film boiling on a horizontal surface, Berenson has modified Bromley's
equation to yield, For vertical tubes, Hsu and Westwater
have correlated the following equation, Where, m is the mass flow rate in at
the upper end of the tube At excess temperatures above that at the
minimum heat flux, the contribution of radiation becomes appreciable and
becomes dominant at high excess temperatures. The total heat transfer
coefficient can be is thus a combination of the two. Bromley has suggested the
following equations for film boiling boiling from the outer surface of
horizontal tubes. If ,
The effective radiation coefficient, can be expressed as,
Where, is the emissivity of the solid and is the Stefan-Boltzmann constant.
The Pressure Field in a Leidenfrost droplet
The equation for the pressure field in the vapor region between the droplet and
the solid surface can be solved for using the standard momentum and
continuity equations. For the sake of simplicity in solving, a linear
temperature profile and a parabolic velocity profile are assumed within the
vapor phase. The heat transfer within the vapor phase is assumed to be through
conduction. With these approximations, the Navier-Stokes equation can be solved
to get the pressure field. Leidenfrost Temperature and Surface
Tension Effects The Leidenfrost temperature is the
property of a given set of solid-liquid pair. The temperature of the solid
surface beyond which the liquid undergoes Leidenfrost phenomenon is
termed as Leidenfrost temperature. The calculation of Leidenfrost temperature
involves the calculation of minimum film boiling temperature of a fluid. Berenson
obtained a relation for the minimum film boiling temperature from minimum heat
flux arguments. 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. One critical parameter to
consider is the surface tension. 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. Since film boiling occurs after nucleate boiling,
the minimum temperature for film boiling should have a proportional dependence on
the surface tension. Henry developed a model for Leidenfrost
phenomenon which includes transient wetting and microlayer evaporation.
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.
While the Leidenfrost temperature is not directly related to the surface tension
of the fluid, it is indirectly dependent on it through the film boiling
temperature. For fluids with similar thermophysical properties, the one with
higher surface tension usually has a higher Leidenfrost temperature. For
example, for saturated water-copper interface, the Leidenfrost temperature
is 257 C. The Leidenfrost temperatures for glycerol and common alcohols are
significantly smaller due to their lower surface tension values
Reactive Leidenfrost Effect 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. Detailed characterization of the reactive Leidenfrost effect was
completed for small particles of cellulose on high temperature polished
surfaces by high speed photography. Cellulose was shown to decompose to
short-chain oligomers which melt and wet smooth surfaces with increasing heat
transfer associated with increasing surface temperature. Above 650 °C,
cellulose was observed to exhibit transition boiling with violent bubbling
and associated reduction in heat transfer. Liftoff of the cellulose
droplet was observed to occur above about 750 °C associated with a dramatic
reduction in heat transfer. High speed photography of the reactive Leidenfrost
effect of cellulose on porous surfaces 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 effect' was
characterized by a dimensionless quantity, 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