Varsity Science & Technology -- The Leidenfrost Effect

The Leidenfrost Effect
or how to walk over hot coals

By Raymond Ho



Browsing through my physics textbook (odd as it may sound) I happened upon something called the Leidenfrost Effect. The phenomenon, named for the German physicist who studied it, explains how it is possible for people to walk over hot coals.

Leidenfrost heated a spoon red-hot over a fireplace. He placed a drop of water in the spoon, and noted that it lasted for about 30 seconds. After the drop had evaporated, it left a "dull" spot, where the spoon had cooled considerably. The next drop of water lasted for about 10 seconds, and subsequent drops of water lasted for only a few seconds. Thus, at lower temperatures, the drops of water evaporated more quickly than at higher temperatures. How was this possible?

For many liquids, there is a temperature well above its boiling point called the Leidenfrost point. Water has a Leidenfrost point of over 200 degrees Celsius. Consider a simple experiment where a droplet of water is placed on a hot surface (hot: a temperature above the boiling point of water). If the temperature of the surface is below the Leidenfrost point, then the droplet starts to spread out and vaporises rather quickly. At or above the Leidenfrost point, however, the bottom layer of the droplet vaporises almost immediately on contact, effectively creating a cushion of vapour that repels the rest of the droplet from the surface. The droplet does not make contact with the surface, and thus no heat can be transferred directly from the surface.

At such high temperatures, one might expect that the vapour layer would quickly transfer enough heat to the rest of the droplet to vaporise it. Water vapour, however, is a very poor conductor of heat at these temperatures. Hence, the vapour layer actually acts as an insulator.

Being curious, I tried conducting this experiment myself. I took a frying pan and began heating it on my stovetop. I sprinkled a few droplets of water on it, and watched as they quickly fizzled away. As the pan grew hotter, the droplets took longer and longer to vaporise. After some time, the pan was hot enough so that the droplets of water remained for well over a minute. This "experiment" is actually a common practise in the kitchen, to determine whether or not your skillet is hot enough for making pancakes.

Intrigued by the effect, I continued. Eventually, I managed to produce a globule of water, approximately two centimetres in diameter, that vaporised at an extremely slow rate. It remained on the pan for about three minutes before disappearing.

At this point, I grew even more curious, and decided to conduct more experiments. I had read that it is possible to plunge wet fingers into molten lead for a split-second. The insulating vapour layer would allow very little heat to be transferred to the flesh. A variation of this: holding liquid nitrogen in one's mouth.

I wasn't about to try my hand at these activities, but I did try an experiment that relates to walking over hot coals. With the frying pan still hot, I wet my fingers and went to touch the hot surface. After conjuring up enough courage to go through with this, I put my finger on the pan for a split-second, and marvelled as I felt no heat. I did this several times, with the same result. By this time, my finger tips had become slightly charred, but I didn't feel any pain. I tried touching a dry finger to the surface, and noticed that it became quite hot.

I still wasn't satisfied. I took the frying pan off the stove and stared at the red-hot element, wet my finger, and touched it. As expected, I didn't feel anything. I repeated this several times. And as if this wasn't enough, I went one step further. Being a brave fool, I wet my entire hand, and placed it across the element for an instant. The laws of physics did not fail me; I was unharmed, but I noticed afterwards that there were white marks on the palm of my hand, roughly in the shape of a stove element. It was probably just some burnt dead skin. Nothing to worry about.

The Leidenfrost Effect explains how people can walk over hot coals without burning themselves. Either they wet their feet prior to the stunt, or they become so nervous that the perspiration on their feet is sufficient. In addition, the heat capacity of coal is relatively low. So although the coals may have a high temperature, they will transfer little heat to the person's feet. If walking briskly, then the time that one's foot is in contact with the coals is very short, thus decreasing further the amount of heat that can be transferred.

The Leidenfrost Effect has long been observed in various carnival stunts. Personally, I would not recommend doing any of the more ambitious experiments that I mentioned above. These stunts have been performed by many people, but not without a few accidents. Though if you're more foolish than I am, and you really, really, have faith in physics, then who's going to stop you?