Leidenfrost phenomenon

The Leidenfrost phenomenon was first discussed in 1756 (LI). This phenomenon is the occurrence of a repulsion between a liquid and a very hot solid. For example, a drop of water on a hot plate will dance around noisily for some time before evaporating. On a moderately warm plate the phenomenon does not appear, and evaporation is very rapid. Nukiyama s test shed some light on this mystery. 

Leidenfrost phenomenon—The observation that a drop of water on a red-hot metal plate will not evaporate at once, since it is protected by its own layer of steam. 

Liquid Marbles, Fig. 2 The evaporation time of liquid marbles and water droplets at different surface temperatures. Liquid marbles feature long evaporation time at all tested temperatures. Water droplets feature the same long evaporation time only when the temperature is beyond the Leidenfrost point. If the temperature is below the Leidenfrost point, water droplets usually evaporate within seconds. A theoretical curve for Leidenfrost phenomenon is also plotted for comparison (Reproduced with permission from Aberle et al. [5])... 

The interest in the Leidenfrost phenomenon rises from an explosive behaviour associated with non-equilibrium of pressures in dynamic change of phases. That affects all the industrial sectors where hot temperatures are used (for example in metal industry quenching of metals [3]). This generates the unsteadiness of distillation plants in petroleum industry and many accidents in nuclear engineering (Tchemobyl 1986) [4]. 

The Leidenfrost effect describes the hovering of a droplet above a hot surface. In the case of release agents for aluminum demolding, the emulsion droplet may not touch the metallic mold surface because of the rapid evaporation of the water contained in the droplet. This phenomenon occurs above a certain temperature, called the Leidenfrost tenqjerature. The formulations are optimized for a maximum Leidenfrost ten jerature, so that the product efficiency is maximized even at elevated temperatures. 

J.G. Leidenfrost first experimented the process in 1756 a little water poured on a red hot spoon, does not damp the spoon and takes the same shape as mercury. A strong motion of vapour laying between liquid and metal supports liquid masses and causes fast vibrating motion in the liquid bulk. This is the film boiling phenomenon [1]. The phenomenon was the subject of numerous studies during the nineteenth century [2]. 

On fig. 1, the solution of our equations matches exactly the film boiling phenomenon a liquid bulk, a very thin interface, then a film of vapour where p decreases a little bit until p reaches the value for which temperature is equal to that of the plate. It must be larger than the maximum temperature in the interface. That leads to the evaluation of the temperature from which the film boiling occurs (Leidenfrost s temperature). If the temperature on the plate was smaller, the solution would have stopped in the interface, the fluid would have damped the surface and would have come to the boil immediately. On the contrary, in the case of film boiling, the thickness of the vapour film has an order of size more important than the one of interface. 

Finally, there is a third category of pearls—the so-called Leidenfrost drops. The method to create them involves placing a water drop on a very hot plate (typically 300°C). The drop will retain a spherical shape because of the vapor film that supports it. Needless to say. the drop evaporates, but because the vapor film is a good thermal insulator, the evaporation is slow (of the order of a minute for a millimeter-size drop). This phenomenon has been known for a long time (Leidenfrost, 1756), and has been discussed by Boiiassc under the name spheroidal state. Nevertheless, many questions persist (such as the thickness of the underlying film and the lifetime of the drop). 

Leidenfrost, who, as one knows, discovered the phenomenon of the spheroid state of fiquids, devotes a great portion of his Mdmoire where he expounds on this subject, to a detailed smdy of soap bubbles. This work, published in 1756, and about... 

We will use the same principles to explain a mysterious fact described initially by Hooke ( 318), then by Leidenfrost ( 321), and that Fusinieri sought to explain ( 324) on the basis of his theory I speak about the explosion of bubbles. It follows from observation that the phenomenon is more marked when the film is very thin, and I have seen that its burst can make a perceptible noise, and convert it into a kind of liquid dust which can carry more than one meter away. 

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