Taking the Ice Bucket Challenge With Liquid Nitrogen

Nerval's Lobster writes As a trend, the ALS Ice Bucket Challenge seems a bit played out—who hasn't yet dumped a bucket of icy water over his or her head for charity? But that didn't stop Canadian chemist Muhammad Qureshi from executing his own sublimely scientific, potentially dangerous variation on the theme: After donating to the ALS Association, he proceeded to douse himself with a bucket of liquid nitrogen. Anyone who's taken a chemistry class, or at least watched the end of Terminator 2, knows that liquid nitrogen can rapidly freeze objects, leaving them brittle and prone to shattering. Pouring it on your skin can cause serious frostbite. So what prevented that bucketful of liquid nitrogen from transforming Qureshi into a popsicle? In two words: Leidenfrost effect. Named after 18th century scientist Johann Gottlob Leidenfrost, the effect is when a liquid comes near a mass that's much warmer than the liquid's boiling point, which (in the words of Princeton's helpful physics explainer) results in an insulating vapor layer that "keeps that liquid from boiling rapidly." In other words, the vapor makes the liquid "float" just above the surface of the object, rather than coming into direct contact with it.

Comments

it tingles

[sayfawa]

Yeah, liquid nitrogen is pretty safe. Dip your hand in it, throw it at people, put it in your ice cream; all valid uses. Unless you drink it or jump in a pool of it, it's mostly harmless

Re:it tingles

[93 Escort Wagon]

In my old job we used a lot of liquid nitrogen - mainly to transfer carbon dioxide around in various scientific apparatus (a mass spectrometer, for one).

Anyone who has worked extensively with the stuff will tell you it is NOT safe unless you are careful. The Leidenfrost effect works... for a relatively short duration. But the co it used application of liquid nitrogen to a specific area rapidly cools the immediate surroundings, and then the effect stops working - especially if the nitrogen doesn't have a way to skitter away on that layer of gas (if you were to pour it into a cupped palm, for example).

Also, small droplets (such as are generated from the stuff boiling when you're freezing carbon dioxide into a cold finger) don't seem to have much difficulty reaching one's skin, Leidenfrost or no. Most of us in the lab frequently had small burns on the thumb sides of our hands.

Re:it tingles

[phantomfive]

In my physics book, the author told about his own personal experiments with the Leidenfrost effect. He would plunge his hand into a bucket of molten lead (after dipping it in water), and pretty soon had advanced to putting liquid nitrogen in his mouth and breathing it out.

He stopped the last one after it went slightly wrong, and all his teeth cracked. His dentist suggested he not do it any more.

I'd have preferred

[Rick in China]

If he were to turn into a popsicle.

Re:Is it really the Leidenfrost effect?

[Anonymous Coward]

It really is this time. The Leidenfrost effect comes into play when a surface is much hotter than a given material's boiling point. The Leidenfrost effect explains safe contact of hot skin with liquid nitrogen, wet hands surviving molten lead, and why water skitters on a hot skillet. Any time a liquid contacts a surface much hotter than its boiling point, such that it can be suspended in the air by convection currents, the Leidenfrost effect is responsible. In the LN2 case, your skin is far enough above the -195C boiling point that the nitrogen boils off before it touches your skin. In the molten lead case, the water on your hands must evaporate before your hands can start burning, this creates a temporary steam bubble that insulates your hand much like a winter coat. The water on a hot skillet case is the simplest case, where radiative and convective heat transfer is so intense that hot air and water vapor form a convective bubble underneath the boiling water bubble and instabilities in the air bubble then cause the water bubble to flow towards a theoretical edge and skitter around the pan.

The firewalking claim is a little dubious, it seems more likely that the short contact time combined with the small surface area exposed during normal walking is responsible for the undamaged feet. Most firewalkers don't seem to sit around getting their feet good and sweaty before firewalking.

Re:so the T-1000 shouldn't have frozen?

[Anonymous Coward]

No, it works for a few seconds, the evaporation of the nitrogen still extracts energy from the skin. If you pour it for more than a few seconds your hand will freeze and then crack.

Re:I'm starting to wonder...

[StormCrow]

So IOW, don't give your money to the ALS foundation, since only around a quarter of it at best will go there.

Demonstrably false with about 5 seconds of Google searching.

http://www.snopes.com/politics...