Fluids are pretty amazing in their behavioral nature. For example, if you heat up a container filled with water, what will happen? It will begin to boil and if heated further it will eventually evaporate. But have you ever thought if we pour droplets of water on a heating container? If not let me tell you about the observation you are likely to observe.
- Until 100° C, the droplets will get collected in the container. As they begin to boil.
- Between 100° C & 220° Cthe droplets will boil and evaporate as soon as they come in contact with the container.
- But, 220° C after you will observe something bizarre behavior of the droplets. Something as shown below.
Amazing isn’t it? The droplets begin to float on the surface of the container after ( known as the Leidenfrost point of water ). This phenomenon is known as the Leidenfrost Effect.
What is the Leidenfrost Effect?
The Leidenfrost effect is a physical phenomenon in which a liquid, in near contact with a mass significantly hotter than the liquid’s boiling point, produces an insulating vapor layer keeping that liquid from boiling rapidly. Because of this ‘repulsive force’, a droplet hovers over the surface rather than making physical contact with it. This is most commonly seen when cooking: one sprinkles drops of water in a pan to gauge its temperature: if the pan’s temperature is at or above the Leidenfrost point, the water skitters across the pan and takes longer to evaporate than in a pan with a temperature below the Leidenfrost point but still above boiling. It is named after Johann Gottlob Leidenfrost, who discussed it in A Tract About Some Qualities of Common Water in 1751.
The most common occurrence of the Leidenfrost effect can be found in the kitchen when a droplet of water is placed on a hot frying pan. If the pan is heated to the Leidenfrost point, about 200°C, the water evaporates so quickly that a thin layer of vapor forms between the pan and the water droplet, protecting the water from the burning hot pan. The vapor also causes the water droplet to “dance” around in the pan as the vapor propels it in different directions. The droplet will, of course, evaporate over a certain amount of time, but it will take significantly longer than if the pan were heated to a temperature above boiling, but below the Leidenfrost point.
Demonstrations of the Leidenfrost Effect
While seeing a droplet of water dance around on a pan is an apt example of the Leidenfrost effect, there are other demonstrations available on the web that are a little edgier and entertaining. These include sticking a hand into a bucket of molten lead or liquid nitrogen and even sending water droplets through a metal maze.
You can dip your hand in a bucket of water and then in a bucket of molten lead. If performed properly, your hand will remain unharmed. As with the pan example I described above, if heated to a high enough temperature, the water will evaporate into a layer of steam that creates a barrier between your hand and the lead and briefly protect your hand from getting burned. Conversely, in the liquid nitrogen example, your hand is the “high-temperature” object, relatively speaking of course. With the boiling point of liquid nitrogen being -195.8°C, an insulating vapor forms when your hand, at 37°C, is dunked into it. For both of these examples, the protection is short-lived, and the hand must be immediately removed from the liquids to avoid injury.
Video Courtesy – ” Sci Show “
Further Reading – https://www.reed.edu/physics/courses/Physics332.s18/pdf/Leidenfrost.pdf
More about the Leidenfrost point – https://pdfs.semanticscholar.org/7ada/7ce805ff50b9a9b2e64cb3b50d95ed54b984.pdf