I don't know if you have the experience of buying a pan in the mall, some salespeople may sell you "uncoated non-stick pan", and use oil-free fried eggs to show, if you don't understand the Leiden Frost effect and the principle of non-stick pan, it is easy to be fooled, because you can use ordinary iron pans to do oil-free fried eggs. Let's take a look at the magic of the Frost Effect in Leiden.
The Frost effect in Leiden was first discovered in 1732 by the Dutch botanist and physician Herman Boerhaave. It was not until 1756 that the German physician Johann Gottlob Leidenfrost conducted an in-depth study, who described a phenomenon in his essay "On the Nature of Ordinary Water": when water comes into contact with a surface much higher than its boiling point, the droplets immediately evaporate, creating an insulating layer of vapor that suspends the droplets above the surface. This is the Leiden-Frost effect.
For example, if we take a high-temperature iron plate and start to sprinkle water droplets from above, these water droplets can stay in place and roll on the iron plate for a period of time after touching the iron plate. Anyone who sees this phenomenon for the first time will be surprised, but in fact, this is where the Leyden-Frost effect comes into play. The water vapor produced by vaporization, which has poor thermal conductivity, acts as a protective film and effectively insulates the heat.
However, when he himself did this experiment, he used not an iron plate, but an iron spoon that burned red. It is reported that during the experiment, the water droplets were "suspended" in the iron spoon for about 30 seconds, after which they boiled and disappeared. It is worth mentioning that the Leidenfrost effect does not occur at random, and if the temperature difference between the boiling point of a hot object and the liquid is small, it cannot cause violent vaporization to occur, so there is no protective film. In this case, the droplets of water dripping into the iron plate or pot will quickly boil and evaporate.
From the above description, we know that the Leiden Frost effect has very high requirements for temperature difference, if the boiling point of water is 100 degrees Celsius, then we have to heat the iron pot to about 200 degrees Celsius in order to witness this magical phenomenon. Some of the low-quality non-stick pan vendors we mentioned at the beginning also take advantage of this to make everyone mistakenly think that the pans they sell do have good "non-stick" performance. But in fact, when you add a different substance to the pot, or directly change the type and purity of the liquid dripping into the pot, it will affect the final result. It can be seen that the use of the Leiden-Frost effect to turn an iron pan into a non-stick pan is only a superficial effort, and in fact, the iron pan has not really become a "non-stick pan".
Although we already know about the Leiden-Frost effect for more than 200 years, it continues to surprise physicists. In recent years, many new studies have also discovered more characteristics of this miraculous phenomenon, including the critical temperature at which the water vapor layer forms and dissipates, the triple Leiden Frost effect, and more.
Some theories suggest that the critical temperature range for the formation and dissipation of water vapor layers depends on the different types of metal surfaces and even related to the salt content of the water. In 2021, a team of physicists at Emory University developed a new electrical technique to study the phenomenon. Using this new technology, they demonstrated that the Frost water vapor layer in Leiden can be maintained at temperatures well below the temperature required for its formation. Specifically, they found that the Leiden Frost water vapor layer formed at around 240°C and dissipated at about 140°C.
The process by which the water vapor layer around a metal cylinder immersed in water heats fails
In a study published in Physical Review Letters, a team of researchers analyzed what happens when two different droplets are added to a hot surface, and found that when two different drops of liquid are placed on top of a hot surface, they do not merge with each other, but instead bounce back and forth. Researchers call this new phenomenon the triple Leidenfrost effect.
To explain this rebound, the researchers tested 11 different liquids. They dropped the liquid in pairs on an aluminum plate heated to a high temperature, and then used a high-speed camera to film the interaction between the different droplets, and each set of tests was repeated at least five times. They observed that when two droplets dripping on a high-temperature aluminum plate came from the same liquid, or from two liquids with similar boiling points, the droplets merged directly within a few milliseconds, while when the two droplets dripping on the high-temperature aluminum plate belonged to two liquids with very different boiling points, there was a rebound phenomenon that lasted from a few seconds to a few minutes.
The boiling points of ethylene glycol (transparent) and chloroform (blue) are very different, and when these two droplets collide, a vapor layer can be seen forming between them.
The researchers believe that the reason for the rebound in this new scenario is that when two droplets on the same surface have different boiling points, the hotter droplet becomes the "second hot surface" of the lower warm droplet, which heats the edges of the cooler droplets, creating an additional "Leidenfrost layer" that causes the droplets to bounce off each other.
The existence of the Frost effect in Leiden also poses a challenge to the problem of high-efficiency liquid cooling on extremely high temperature walls. In 2022, with the cooperation of Chinese and foreign scientists, the Leiden-Frost characteristic (LFP) of high-efficiency liquid cooling failure has been increased from below 300°C to above 1150°C, and the LFP only depends on the temperature resistance limit of the material, and is no longer limited by design. The research results solve the long-standing scientific and engineering bottlenecks in the field of thermals, and are expected to solve a major problem in the heat dissipation of high-precision equipment in the field of extreme high temperature in the mainland, and help the new generation of high-tech in the mainland continue to develop rapidly.
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