The Leidenfrost effect is a phenomenon where a vapor layer insulates a liquid from a surface, preventing rapid boiling. The insulating vapor makes liquid droplets hover over very hot surfaces. Similarly, a vapor layer insulates between very cold liquids and hot solids. The effect takes its name from the German doctor Johann Gottlob Leidenfrost, who noticed the way water droplets skitter across a hot skillet.
How the Leidenfrost Effect Works
The Leidenfrost effect works when the temperature of the hot surface is well above the boiling point of a liquid. Visualizing what happens to water on a hot pan makes the process easier to understand.
- Flicking water drops onto a cool pan coats the pan with liquid drops that slowly evaporate away.
- If you sprinkle water drops on a pan just below the boiling point of water (100 °C or 212 °F), the droplets flatten out and rapidly evaporate.
- Water droplets hiss and boil away into vapor as they touch at pan heated just at the boiling point of water.
- Heating the pan leads to hissing and boiling until the pan reaches a certain temperature, which is called the Leidenfrost point. At the Leidenfrost point and higher temperatures, water droplets bunch up together and skitter around above the surface of the pain. While they evaporate, the drops last much longer than they do at cooler (but still hot) temperatures.
- At a much higher temperature, drops vaporize so quickly that the Leidenfrost effect does not occur.
The Leidenfrost Point
The Leidenfrost point depends on multiple factors, so it is not easily predicted. Some of these factors are the vapor pressure of the different materials, the presence of impurities, and the smoothness or roughness of the surfaces. The Leidenfrost effect works best on very smooth surfaces, such as water droplets and flat skillets.
At the Leidenfrost point, the outer surface of a droplet vaporizes. The vapor (a gas) forms a thin layer of insulation between the two materials. In the case of a water droplet and a skillet, the vapor suspends the drop above the surface and minimizes heat transfer between the metal pan and the water. While separate droplets clump together, the Leidenfrost effect also affects this process. The vapor layers around separate droplets are like little cushions. Drops often bounce off each other before they coalesce together.
Leidenfrost Effect Examples
There are multiple examples of the Leidenfrost affect. Flicking water onto a hot skillet is a good demonstration, but other examples aren’t particularly safe.
Water on a Hot Skillet
Adding a few droplets of water to a hot, dry skillet is a great way of estimating the pan’s temperature. Below the Leidenfrost point, the water sizzles. When the pan is very hot, the droplets skitter around. However, avoid using this method on Teflon pans because the coating gets into the air as a toxic gas as the pan gets very hot. Stick with cast iron skillets.
Liquid Nitrogen and the Ground
Spilling a small volume of liquid nitrogen onto a floor works just like water on a hot skillet. The boiling point of nitrogen is −195.79 °C or −320.33 °F, so a room temperature floor is well above the Leidenfrost point.
Liquid Nitrogen and Skin
The Leidenfrost occurs with liquid nitrogen droplets and human skin. The temperature of skin is well past the Leidenfrost point for liquid nitrogen. So, if a few droplets of liquid nitrogen land on your skin, they bounce away without causing frostbite. In one demonstration, an experienced educator tosses a cupful of liquid nitrogen into the air well above an audience, so it disperses into droplets. However, if the nitrogen does not break apart or the volume is too high, skin contact causes potentially serious frostbite. An even riskier demonstration involves sipping a small amount of liquid nitrogen and blowing puffs of liquid nitrogen vapor. There is a danger of accidentally ingesting the nitrogen, which can be fatal. Vaporization of nitrogen produces nitrogen bubbles that can rupture tissues.
Skin and Molten Lead
If you touch molten lead, you’ll get burned. However, the Leidenfrost effect offers protection if you wet your hand before touching the metal. In one demonstration, a person wets their hand with water and quickly dips it into and back out of molten lead without getting burned. The effect offers protection against other molten metals, too, but lead is the best option because it has a relatively low melting point of 327.46 °C or 621.43 °F. This is well above the Leidenfrost point for water, yet not so hot that brief exposure causes a burn. It’s comparable to removing a very hot pan from an oven using an oven mitt.
Leidenfrost Effect and Lava
Discussions of what might happen if you touch lava or fall into a volcano often reference the Leidenfrost effect. Partly, this comes from a video a person passing their hand through molten metal that got mis-identified as lava. Lava does flow, but it’s highly viscous (unlike liquid metal).
Water skitters across lava via the Leidenfrost effect. But, a vapor layer won’t protect your skin. Reaching out for lava is much like touching a super hot stove. Wetting your hand might very slightly protect you, but probably not enough. This is because the temperature of lava is around 1100 °C or 2100 °F. That is a lot hotter than molten lead!
The molten rock is so dense that if you fall into a volcano, it’s basically the same as hitting a solid surface. However, hot air rises, so the air column over the lava causes burns before impact. Also, the gases are toxic.
References
- Bernardin, John D.; Mudawar, Issam (2002). “A Cavity Activation and Bubble Growth Model of the Leidenfrost Point”. Journal of Heat Transfer. 124 (5): 864–74. doi:10.1115/1.1470487
- Incropera, Frank; DeWitt, David; Bergman, Theodore; Lavine, Adrienne (2006). Fundamentals of Heat and Mass Transfer (6th ed.). John Wiley & Sons. ISBN: 978-0471457282.
- Pacheco-Vázquez, F.; Ledesma-Alonso, R.; Palacio-Rangel, J. L.; Moreau, F. (2021). “Triple Leidenfrost Effect: Preventing Coalescence of Drops on a Hot Plate”. Physical Review Letters. 127 (20): 204501. doi:10.1103/PhysRevLett.127.204501
- Quéré, David (2013). “Leidenfrost Dynamics”. Annual Review of Fluid Mechanics. 45 (1): 197–215. doi:10.1146/annurev-fluid-011212-140709
- Vakarelski, Ivan U.; Patankar, Neelesh A.; Marston, Jeremy O.; Chan, Derek Y. C.; Thoroddsen, Sigurdur T. (2012). “Stabilization of Leidenfrost vapour layer by textured superhydrophobic surfaces”. Nature. 489 (7415): 274–7. doi:10.1038/nature11418