Hand vs. Liquid Nitrogen and the Leidenfrost Effect

Uploaded by NurdRage on 10.07.2009

Greetings fellow nerds.
In this video I'm going to put liquid nitrogen on my hand.
I'm not crazy.
To show you why I'm not crazy I'm going to first show something called the Leidenfrost effect.
I have here a standard laboratory heater plate and I'm going to put some water on it and turn on the heat.
As expected, it's going to boil. As it heats up it will boil water faster and faster.
Eventually it gets so hot that water instantly vaporizes upon hitting the plate.
Now when it gets really hot, an interesting effect takes over.
Water now just beads up and rolls off the plate.
What's happening is that on contact the water vaporizes so fast that it creates a cushion of steam
that lifts up the remaining water drop.
So the water is now riding around on a cushion of it's own steam.
This makes the water last longer and slows down the heat conduction needed to completely vaporize it.
This is called the Leidenfrost effect.
This effect can be seen at home with a frying pan if you heat it high enough.
The Leidenfrost effect also occurs with liquid nitrogen.
As you can see here the liquid nitrogen skirts away
as I pour it on the floor without actually soaking into the floor.
Liquid nitrogen boils at -196 Celsius.
And this laboratory floor is at room temperature.
So from the liquid nitrogen's perspective the floor is super hot.
Here are some drops of liquid nitrogen rolling around on the floor just like the water on the hot plate.
As you can see they're not soaking in.
The Leidenfrost effect protects the liquid nitrogen from being instantly vaporized,
and consequently the floor from being instantly frozen.
Now the Leidenfrost effect does not last forever.
With enough liquid nitrogen the floor is eventually cooled enough
that the temperature difference no longer supports the effect and the liquid nitrogen soaks in.
So if I do put it on my hand I can only do so for a moment.
Nonetheless, the Leidenfrost effect will protect my hand from being instantly frozen.
Alright, so let's try it.
Let me fill this Dewar with liquid nitrogen.
Now if you look very closely,
the Leidenfrost effect is happening here too
as the liquid nitrogen is not yet contacting the sides of the Dewar
but a gas barrier is insulating it.
However the effect will break down eventually.
Human tissue is much less tolerant of cold than the floor or this steel Dewar
so i won't be able to put my hand in there for longer than a moment.
Alright, the nitrogen is ready.
I'm really nervous here, give me a moment to psyche myself up.
Whoa, my hand is ice cold right now.
But as you can see the nitrogen did not soak in,
and it's completely fine with no frostbite or other injuries.
The plume of mist from the liquid nitrogen shows that a lot of it vaporized to form the gas barrier around my hand.
Turning around the experiment.
I can also pour liquid nitrogen on my hand in short spills as long as i give my hand a few moments to recover.
This is pretty cold but the Leidenfrost effect prevents the liquid nitrogen
from directly contacting my hand and freezing it.
As long as my hand is warm enough, I can keep doing this.
Although it's starting to sting right now.
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