Leidenfrost Effect

[V-man]

I noticed this in a message about Pop cans and it peaked my intrest I have to know what is the Leidenfrost Effect and why did someone want to shove their hand into molten lead because of it???

[Ursa Major]

The Leidenfrost Effect is best demonstrated by throwing a bit of water on a hot frying pan. The water is unable to spread over the pan because of an air cushion created by the water vapor. I imagine if you thrust your wet hand in and out of molten metal real quick there is a chance that this cushion might protect you from serious injury.(I wouldn't try it, though.) I think this is the same concept employed by fire walkers.

[Cabbage]

Yeah, I've seen someone on Jay Leno stick his hand into molten lead by using this effect.

[Zor]

That was probably Jearl Walker, Professor of Physics at Cleveland State University, Scientific American writer, supposed buddy of Master Cecil, the nut who thrust his hand in molten lead, and the only person whose paper occupies 4 pages of the physics text book I have on my shelf

Ursa Major's description of the Leidenfrost effect is basically correct, except it's actually quite reliable if you ever want to take a quick dip in molten lead. You do however, need to make sure the heated material, liquid or solid, is above 200 degrees Celcius if you plan to use water vapor as insulation. I wouldn't suggest you test this out with a frying pan though. If it's one of none-stick pans, you might destroy it without good control of the temperature. I'd say pouring a few drops of liquid nitrogen in your palm would be a good demonstration, although it might be hard to come by for you.

Incidentaly, Jearl Walker has also made 5 walks over hot coal himself too. He was unharmed the first four times, which he partially attributes to the fact that his feat were sweaty due to fear. On the fifth occasion, he was so comfortable that he didn't sweat as much, and his feet got badly burned. This doesn't mean that the Leidenfrost effect is the only or even main reason that protects firewalkers, but it certainly seems like one of the contributing factors.

And here's the last paragraph from Jearl Walker's article in my textbook...

I have long argued that degree-granting programs should employ "fire-walking" as a last exam. The chairperson of the program should wait on the far side of a bed of red-hot coals while a degree candidate if forced to walk over the coals. If the candidate's belief in physics is strong enough that the feed are left undamaged, the chairperson hands the candidate a graduation certificate. The test would be more revealing than traditional final exams.

Glad you aren't going to study physics at Cleveland State University, eh?

[Kat]

Cecil's take on the sizzle effect aka the Leidenfrost effect: Can you walk on hot coals in bare feet and not get burned?

Memorable quote: "any time you've got a phenomenon that lets you work in a Jearl-Walker-tempts-death story, as was the case with the Leidenfrost effect, in my book you want to cut it a little slack."

------------------
Your Official Cat Goddess since 10/20/99.

SD chat:
<Jophiel> Kat's here! the board must be down!

[Irishman]

Cecil's column hits it pretty well.

Here are a couple links for those of you interested. I have not tried this myself yet, but would like to sometime.

Firewalking: Myth vs Physics http://www.pitt.edu/~dwilley/fire.html

Instructions for holding your own firewalk http://www.pitt.edu/~dwilley/firedemo.html

Skeptic's Dictionary http://www.dcn.davis.ca.us/~btcarrol.../firewalk.html

[HeadlessCow]

Jearl Walker also wanted to check to make sure it was the water on his hand that was protecting him from the molten lead so he decided to reach in with his bare hand. Apparently that wasn't as successful and actually quite painful.

[The Ryan]

quote:
I have long argued that degree-granting programs should employ "fire-walking" as a last exam. The chairperson of the program should wait on the far side of a bed of red-hot coals while a degree candidate if forced to walk over the coals. If the candidate's belief in physics is strong enough that the feed are left undamaged, the chairperson hands the candidate a graduation certificate. The test would be more revealing than traditional final exams.


But if it's sweat that protects people, wouldn't students that don't trust physics, are afraid, and therefore sweating, get through fine.

[Zor]

There's supposed to be a wet grass lawn before the coal trail... so...