Below are a series of images that show the failure of a thin layer of vapor surrounding a hot, metal, cylindrical rod, which is immersed in water. The vapor layer fails initially at the rod's tip. As the liquid wets and travels vertically up, bubbles quickly form. Between the first and the last images, approximately 1 ms has passed. The metallic finger measures 1.6 cm in size. Harvey, Harper & Burton.
The Leidenfrost phenomenon is a well-known physical phenomenon that was first discovered in 1756. This happens when liquids are near surfaces that are significantly warmer than their boiling point. This creates an insulating layer of vapor that stops the liquid from boiling quickly. This effect allows a droplet to hover above the surface rather than touching it.
Although the Leidenfrost phenomenon was discovered centuries ago, the temperatures at which vapor layers begin to form vary from one study to the next. This phenomenon has been studied by many physicists around the world to help them better understand how and when it happens.
Emory University researchers recently proved that Leidenfrost layers can be maintained at lower temperatures than the required for their formation. These findings, published in Physical Review Letters could have practical and theoretical implications for many areas of physics.
Justin C. Burton, one the researchers behind the study, said that he has been studying the Leidenfrost phenomenon for many years. "Our previous research focused on the fascinating dynamics of Leidenfrost droplets, including how they move and oscillate. This was done at extremely high temperatures. The thin vapor layer between the drop's hot surface and the drop is very strong, even though it is only about the thickness of a human hair.
Burton and his collaborators have provided valuable insight through past studies, but there is still one crucial question: What is the Leidenfrost temp? It was unclear what temperature is required to form a vapor layer on top of a surface, and how long it must last.
Credit: Harvey, Harper & Burton
Although physicists are still unsure how the vapor layer will eventually dissolve, they have observed that liquids touching solid surfaces and explosive boiling accompany its dissipation. These questions will not only inform physics research but also provide valuable information for many industries that use cooling hot objects, as well as for planetary sciences that study phenomena like phreatomagmatic explosions.
Burton explained that they set out to answer these questions by using an electric technique to monitor the thickness of vapor layers during formation and then as the hot material cools down, until the vapor layer spontaneously fell." "By adding some salt to the water, liquid functioned as an electrical circuit and thin vapor layer as a capacitor. This enabled us to monitor the vapor layer at high speed, milliseconds prior and after the moment it collapsed.
Burton and his associates took several measurements of the vapor layers. They also used high-speed video to see exactly when the layer collapses. Surprisingly they discovered that although it is possible to form a layer of vapor around a hot metal object in water, one must heat the object up to 240 degrees C. However, once the object has cooled down, the temperature at which the vapor layer collapses does not depend on the salt concentration or type of metal.
Burton stated, "I think that the most striking finding of our research is that there seems a lower temperature for maintaining the stability of Leidenfrost layers and that there is an upper temperature' for formation and an lower temperature' to fail." This is a practical finding that goes beyond basic physics.
The future could be informed by the research of this group of researchers. The physics of thin, liquid lubricating gas layers is a topic of ongoing inquiry in many areas. These include the study of friction, soft tissues, microfluidics, and nanoscale cooling.
Burton said that they are conducting numerical simulations to determine how the stability of vapor layers disappears at lower temperatures. It's a repeatable feature of the experiment and therefore must be based in basic hydrodynamics. It's like a raindrop falling on a leaf. As it grows in size, it eventually falls off. We don't yet know why the vapor layer became so unstable in our experiment. This is due to an excess of surface tension forces and gravitational forces.
Learn more about how boiling droplets can race over hot oily surfaces
More information: Dana Harvey et al, Minimum Leidenfrost Temperature on Smooth Surfaces, Physical Review Letters (2021). Information for Journal: Physical Review Letters Dana Harvey and colleagues, Minimum Leidenfrost temperature on Smooth Surfaces (2021). DOI: 10.1103/PhysRevLett.127.104501
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