Solid ice, flowing liquids, ethereal water vapor... We all know that water has three phases. When the ice is heated, it turns into water, and if it continues to heat, it becomes water vapor. Anyone who observes a cup of iced tea or boils a pot of boiled pasta on a hot day will not be unfamiliar with these commonplace basic principles.
But the interesting nature associated with water goes far beyond that. For example, when the heat source is large enough that the temperature is much higher than the boiling point (100 ° C), things may develop differently.
You may have noticed in the kitchen that if you drop water on the surface of a pan that is heated to the point of overheating, the water droplets can be suspended above the surface and slide over the bottom of the pan through steam. This interesting phenomenon is known as the Leiden Frost effect.
Recently, a new study has found an exciting addition to the Leiden Frost effect, the three-phase Leidenfrost effect. The researchers discovered the critical temperature of the Leiden Frost effect that levitates ice on the surface and revealed models of heat transfer within it. The study was recently published in The Physical Review fluids.
The Layton Frost Effect
In addition to the ordinary solid-liquid-gas triphases, when the heat source is hot enough, the behavior of water can also change dramatically. The emergence of the Leiden Frost effect is one example.
For example, on an aluminum plate that far exceeds the boiling point of water, the water droplets on its surface will no longer boil. Instead, the vapor that forms when the water droplet approaches the surface is "trapped" under the water droplet, forming a kind of cushion that prevents the liquid from coming into direct contact with the surface. Trapped steam causes the liquid to float and slide on a heated surface like an air puck. This phenomenon is named after the German physician and theologian Johann Gottlob Leidenfrost, who first described it in 1751.
Although we already know that the Leiden Frost effect has been around for more than 200 years, it continues to surprise physicists. In recent years, many new studies have also found more characteristics of this miraculous phenomenon, including the critical temperature of the formation and dissipation of the water vapor layer, the triple Leiden Frost effect, and so on. (For details, see "A New Discovery of a Common Physical Phenomenon" and "Bullets, Bullets, Bullets~". ）
This universally accepted scientific principle, which applies to liquid water suspended on an air cushion of water vapor, has also aroused the curiosity of some researchers. They wondered, if water were replaced by ice, could things develop in a similar way? In other words, is the three-phase Leiden Frost effect possible?
Enter the realm of ice
About five years ago, curiosity sparked the team's first study. What they observed was very fascinating.
If the water droplets are replaced with ice, even when the aluminum plate is heated to more than 150 ° C, the ice is not suspended in the water vapor like liquid water. The researchers continued to raise the temperature and observed the ice's behavior as the heat increased.
They eventually found that the temperature threshold for suspension was significantly higher, reaching about 550 °C. Below this threshold, the meltwater below the ice will continue to boil in direct contact with the surface and will not exhibit the Leiden Frost effect.
The three-phase Leiden Frost effect. | Video credit: Mojtaba Edalatpour / Virginia Tech Mechanical Engineering
What exactly happened beneath the ice that suppressed the Leiden Frost effect at very high temperatures?
Soon after, the team developed new methods of heat transfer and put that knowledge to work around the problem. The answer they found was the temperature difference in the meltwater layer below the ice.
The top and bottom of the meltwater layer are at two almost opposite extremes, its bottom is in contact with the hot surface, it is boiling, which fixes the temperature at about 100 ° C, but its top adheres to the remaining ice, that is, it stabilizes at about 0 ° C.
Simulations show that maintaining this extreme temperature difference consumes most of the heat on the surface. As a result, most of the heat from the hot surface must pass through the water layer to maintain this extreme temperature difference, so only a tiny fraction of the energy can be used to generate water vapor. This also explains why the Leiden Frost effect is more difficult to occur for ice.
Changes in the behavior of ice at different temperatures in the experiment. | Image credit: Mojtaba Edalatpour / Virginia Tech Mechanical Engineering
Scientists believe that from the perspective of the application of heat transfer, it is actually a good thing to suppress the Leiden Frost effect from occurring at high temperatures.
Because boiling water will maximize the heat from the lower part, once the suspension begins, the heat transfer will drop sharply, because the liquid will no longer boil. The higher the temperature of the Leiden Frost effect, the larger the temperature window of boiling, and the better the heat transfer will occur.
Heat transfer using ice
Heat transfer plays an important role in cooling computer servers or car engines. It requires a substance or mechanism capable of removing energy from the hot surface, quickly redistributing the heat, thereby reducing wear on metal parts.
The researchers believe that this discovery has a myriad of possible applications. For example, in nuclear power plants, the application of ice to induce rapid cooling can be an easy emergency measure to deploy in the event of a power failure, or it can become a routine practice for repairing power plant components.
It also has potential applications in metallurgy. In order to produce alloys, it is necessary to quench the already formed metal in a narrow time window to make the metal stronger and more brittle. If ice is applied, it will allow heat to be quickly unloaded through the three phases, quickly cooling the metal.
#创作团队:
Compile: M ka
Typography: Wenwen
#参考来源:
https://vtx.vt.edu/articles/2022/01/eng-boreyko-boiling-ice-012022.html
https://arstechnica.com/science/2022/01/study-leidenfrost-effect-occurs-in-all-three-water-phases-solid-liquid-and-vapor/
#图片来源:
Cover image: Max Pixel
First image: Max Pixel