Spheroidal state

This nondimensional group describes the spheroidal state of film boiling. 

It should be realized that the Leidenfrost superheat, A7 LDF = (TLDF - Tsat), is a function not only of pressure but also of droplet size, flow conditions, and force fields. Furthermore, experimental results obtained by Berger (Drew Mueller, 1937) for stagnant ether droplets falling on a horizontal, heated surface indicated a possible effect of surface material and roughness, as the minimum surface temperature necessary for the spheroidal state changes from 226°F (108°C) on a smooth surface of zinc to 240°F (116°C) on that of a rough surface, and from 260°F (127°C) on a smooth surface of iron to 284°F (140°C) on that of a rough surface. 

Heat a little sodium chloride in a platinum crucible to bright redness, and add a couple of drops of water to the hot crucible so that the water assumes the spheroidal state. In a moment, transfer the water to a beaker containing a faintly coloured soln. of blue litmus —the litmus is reddened, showing the presence of an acid—hydrochloric acid. The salt remaining in the crucible is dissolved in water, and it turns red litmus blue, showing the presence of an alkali—sodium hydroxide. There appears to be a reaction NaCl+H20 NaOH +HC1, and the water abstracts the more volatile hydrogen chloride. 

The sO Called spheroidal state, in which a drop of water or other volatile liquid rolls about on a hot metal plate (or on the surface of a boiling liquid) with only very slow evaporation, is mainly a consequence of the very slow transmission of heat from the solid through the thin layer of vapour separating it from the liquid drop. A beam of light may be passed between the drop and the plate. Poggendorff said that an electric current will not pass from the drop to the hot plate, but Buff showed that a weak current passes and that the drop oscillates, sometimes touching the support. Stark showed that the drop is supported by a layer of vapour but executes oscillations which may sometimes reach the support. 

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). 

Spheroidal State.—It is well known that when a drop of water is allowed to fall on a red-hot sheet of metal, such as platinum, it does not touch the metal, nor does it boil although rapid evaporation takes place, but it assumes the spheroidal state, moving about over the hot surface like a globule of mercury on a table. Careful experiments (4) have shown that the temperature of the water under these conditions does not reach 100, and the explanation of this fact... 

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... 

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