The Leidenfrost Effect: Nature’s Hovercraft

You may not realize it, but liquids can glide on cushions of vapor. It's called the Leidenfrost Effect, and it's made some pretty awesome superpowers possible - as well as some unfortunate accidents.

Back in the nineteenth century, Sir William Fairbairn gave a controversial lecture on steam boilers, the engines that drove mechanical devices at that time. He argued that super-hot boilers for heating steam were all well and good, but lowering the temperature of the boiler might be the way to get maximum steam. No doubt Sir William had to pause after that statement, unable to be heard over the mass dropping of monocles, rustling of petticoats, and un-capping of smelling-salts bottles.

How, oh how, his audience wondered, would reducing the temperature of a boiler increase the amount of steam produced?

The answer lies in the Leidenfrost Effect, something most people have observed without knowing it. If you were to sprinkle a few drops of water on a warm frying pan, the drops would probably just hiss at you sullenly. Fair enough. You just dropped them on a hot frying pan, after all. But as the pan continues to heat, the water will hiss and sputter more violently, and will also evaporate more quickly.

Until the pan hit the Leidenfrost Point. At that point, drops you splattered in the pan wouldn't just lie there. They would roll or skitter across the pan in beads, probably trying to enact vengeance upon you. And vengeance they might get, since the Leidenfrost Effect actually allows a drop of water more time on a hot pan than it would otherwise have. Why? The water isn't actually rolling across the surface of the pan. It's rolling on a thin cloud of water vapor between it and the metal.

When the very edge of the liquid hits the hot surface of the frying pan, it turns into vapor so quickly and energetically, that it actually manages to support the rest of the droplet, kind of like the hands of a thousand drunken concert-goers support a crowd-surfer. Exactly like that, in fact, since the process is unreliable and short-lived. The vapor escapes around the edges of the droplet, letting more and more of the droplet down onto the pan until nothing is left but a dampish feel to the air. For the few seconds that the droplet is aloft, though, it is protected on an insulated cushion of water vapor, and kept relatively cool, hovering above the skillet.

The Leidenfrost Effect: Nature’s HovercraftS

Astute followers of physics will be thinking, "That's all well and good, but how can this allow me to get away with something stupid that, by all rights, should hurt like hell?"

Glad you asked!

The most famous example of the Leidenfrost Effect was on an episode of Mythbusters, in which people dipped their hands first into water, then into molten lead, and lo, they were not burned. This example is like the our frying pan and droplet of water, only instead of sandwiching the thin water-vapor cloud between a frying pan and a droplet of water, you put it between a vat of molten lead and your entire hand. (If it's not your dominant hand, the other physicists will call you a sissy.)

The Leidenfrost Effect: Nature’s HovercraftS

Another test of the Leidenfrost Effect shows how it works across a range of temperatures and substances. Liquid nitrogen froze the second Terminator, Wesley Snipes in Demolition Man, and was once used by Jason to kill someone. Despite the fact that none of those deaths were even remotely possible in real life, since it takes liquid nitrogen quite some time to freeze even small sections of tissue, nitrogen remains the most badass of liquids. It only has one mortal enemy: Room temperature. A sixty-eight degree, climate-controlled room is more than enough to evaporate liquid nitrogen. Human body temperature is liquid nitrogen's hot skillet. The internet is full of videos showing people pouring liquid nitrogen over their hands, but it's possible to go them one better. Because of the Leidenfrost Effect, it is possible to spit liquid nitrogen.

Carefully pouring a cup of liquid nitrogen into your mouth will cause the liquid nitrogen that hits your skin to turn to vapor, protecting your mouth from the intense cold and allowing you to spit out a long stream of liquid nitrogen that should freeze an evil-doer's hand when they reach for their gun - or take the temperature of the wires of a bomb to absolute zero, thereby defusing it. Actually all that will happen is the nitrogen will turn to steam as it makes contact with the air, but any of the above things would work if you were in a movie with a fake-blood budget.

Perhaps you desire something a little flashier in your science. For that, I give you this gentleman, who uses the Leidenfrost Effect to break flaming boards.

Consider, for a moment, why I listed that gentleman last. (Hint: Consider the moment marked 1:23, as he tried, with admirable calm, to put out the fire on his own wrist.)

For one thing, I'm not sure the last one was the Leidenfrost Effect as much as it was a guy's willingness to hurt himself on camera. For another, as we have learned from Jurassic Park, not all things that can be done, should be done. Particularly not while drunk or unprepared. Leaving alone what might happen if you accidentally swallowed or inhaled the liquid nitrogen, all of these stunts can hurt you.

Search for ‘Leidenfrost Effect' online and you will first find that these examples are shown many times. If you bother to look at any of the text that accompanies the examples, you will see the words ‘very difficult to calculate' used extremely frequently. Depending on the purity level of the substance, the texture of the surfaces, the amount of liquid, and many other factors, the Leidenfrost Point can playfully jump from one temperature to another. Lead that is too cool to cause the Leidenfrost Effect in one case, will be too hot for the Leidenfrost Effect in another. Yes, the Leidenfrost Point is Lucy in the Peanuts comic strip of life. It will playfully hold the ball for you, assuring you that this time it can be trusted, only to whisk it away and leave you flat on your back with your arms on fire. So please, enjoy the physics, and leave the boiling to the professionals and the people with YouTube accounts.