The Leidenfrost Effect

I’ve been learning a bit about water and its boiling tendencies at my work lately. I thought I’d share with you the Leidenfrost effect.

The Leidenfrost effect describes the boiling of water on a very hot surface. If you take water and throw it onto a moderately hot pan, the water sizzles away very quickly. If you take water and throw it onto a VERY hot surface something else happens: the water forms into droplets and dances around the pan. Instead of boiling off quickly, it actually takes a significant amount of time.

What’s happening here?

It turns out that when the water first hits the pan there is a momentary “initial liquid contact stage”* during which the water that strikes the pan turns into a gas. The rest of the water droplet then sits on top of the gas instead of directly on the pan.

Heat transfer from metal to water is pretty good, but if you have to go through a vapor barrier first the heat transfer capabilities drop significantly.

How is this useful? Well, amongst other things, it helps explain things like walking on coals. When you’re nervous and antsy, the sweat on your feet will create little Leidenfrost vapor barriers between your feet and the hot coals. I wouldn’t recommend trying it though, because if you’re calm and the heat isn’t enough to cause you to start sweating, you will be seriously burned.

*ASM Vol. 4 Heat Treating Metals Handbook

8 thoughts on “The Leidenfrost Effect

  • 1/11/2007 at 9:06 am

    I don’t know if nervous vs calm is the right… dichotomy in terms of coal walking…

    As someone who hasn’t walked on coals, I can see that I would be nervous about doing it but as soon as I did it, I’d probably be like “oh, that wasn’t that bad” and I would be less nervous each time and if that played a role, then I’d probably get hurt at some point and stop doing it.

    So how does that explain the people that walk on coals all the time*?

    * All the time refers to more often that just once, like the people that teach other people to walk on coals.

  • 1/11/2007 at 9:13 am

    Good question.

    In fact one of the articles I read about the process (Boiling and the Leidenfrost effect by Jearl Walker* of Cleveland State University) actually described how the author tried walking on coals. He describes himself during the first four times as being very fearful and caked in sweat, but on the fifth time he was confident and took his safety for granted… and then suffered extensive and painful burns on his feet.

    Perhaps once you burn your feet, you never walk across the coals calmly again.

    *the last name of ‘walker’ is surely just coincidental

  • 1/11/2007 at 10:58 am

    Interesting. I would have imagined* that it would be a gradual process and not so sudden as the fifth run is described.

    *I totally imagined it.

  • 1/11/2007 at 12:02 pm

    This reminds me of a heat question that’s bugged me for years.* My grandmother always put warm water into ice cube trays, saying that moderatly warm water freezes faster and more evenly than cold water. After I took high school chem and learned about heat transfer, I was pretty convinced she is wrong. Since you are vastly more knowledgeable about this subject, what do you think?

    *And kept forgetting to defy S. McQuaid with.

  • 1/11/2007 at 4:21 pm

    Let me dig back into APO* lore for a bit here. There was once a APO president who would were shiny pants, and never owned a computer through his years at WPI, even though he was an EE major. He had convinced multiple students taht boiling water freezes faster than normal water. He convinced them by comparing boiling water to a Kineteic/potential energy. Since the boiling water had “more” energy than room temp water it had farther to “fall” and when it got close to the freezing threshhold it would blast past it. Whereas the room temp wasn’t “going as fast” as the boiling waters “energy” at the threshold and would therefore freeze slower. Boy those kids were stupid.
    Look, if I was gonna have a party I wouldn’t boil water freeze, but would MikeD be invited to the party?

    * Alpha Phi Omega, Co-Ed Community Service Fraternity

  • 1/11/2007 at 7:24 pm

    Somewhat bizarrely, in some specific circumstances hot water can freeze faster than cold water. It’s called the Mpemba effect.

  • 1/13/2007 at 12:54 am

    I have heard this exact same theory. My dad, who used to play a lot of hockey, says the referees always repair chunks taken out of the ice during play with hot water. He claimed the same thing, that hot water freezes faster. I could never understand that, and came up with my own theory that the hot water melted some of the ice surrounding the patch, resulting in a smoother finished surface. But hearing the same thing from you, maybe there is something behind it?

  • 1/14/2007 at 12:35 pm

    I agree with your theory Ryan.

    I think the more important question is which ice tastes better? The hot frozen, or the lukewarm frozen, or the cold frozen? And which is better for crushing? These are things I think we’d all like to know.


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