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Author Topic: Why does liquid nitrogen bubble in this way?  (Read 3230 times)

Offline thedoc

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Why does liquid nitrogen bubble in this way?
« on: 22/09/2012 03:30:01 »
John Crum  asked the Naked Scientists:
   
Hi Chris,

I'm a cryo-electron microscopist and I use liquid nitrogen every day.

I've noticed a phenomenon that I'd like to understand. When an object is cooled in liquid nitrogen there's a lot of bubbling and boiling until the object comes just to liquid nitrogen temperatures. Then there is a violent surge of bubbles, like a volcano, then the bubbling stops almost completely. Why does the nitrogen bubble furiously just before the object is cooled to LN2 temperature?

Hope to hear an answer to this question soon.

Cheers,

John Crum
The Scripps Research Institute

What do you think?
« Last Edit: 22/09/2012 03:30:01 by _system »


 

Online yor_on

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Re: Why does liquid nitrogen bubble in this way?
« Reply #1 on: 22/09/2012 04:33:28 »
Could it be droplets inside the bubbles, releasing energy as new bubbles? Or maybe the material (the nucleation site) they form from? Weird stuff anyway :)
 

Offline damocles

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Re: Why does liquid nitrogen bubble in this way?
« Reply #2 on: 22/09/2012 11:29:33 »
When you first plunge an object into liquid nitrogen, it is at such a high temperature that the liquid nitrogen boils, and forms an insulating sheath of gaseous nitrogen all around the object. For a long time the liquid never touches the object. New liquid keeps steadily boiling because of the slowly warming nitrogen gas it is in contact with. However, as the object gets colder, the warming of the gas sheath is not so extensive, and the gas sheath gets thinner. Eventually the last of the gas sheath will collapse, and there will be direct contace between liquid and solid object. That is when the sudden burst of boiling will occur.
 

Online yor_on

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Re: Why does liquid nitrogen bubble in this way?
« Reply #3 on: 22/09/2012 17:49:15 »
So you could think of it as the liquid only getting to the nucleation site in its last stage Damocles? And as they finally do, as it is from there the bubbles gets their formation, you get a last outburst of bubbles before the temperatures equalizes?
 

Offline damocles

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Re: Why does liquid nitrogen bubble in this way?
« Reply #4 on: 22/09/2012 23:22:34 »
I had been thinking of it more in terms of rate of heat transfer, but there would certainly be more nucleation sites when the liquid hit the solid surface.

A description in terms of more efficient nucleation would also work, and might even be more important. Remember, though, that nucleation is not necessary for evaporation from a liquid surface, only for bubble formation within the bulk of a liquid not far above its boiling point.

Both effects will work in tandem; the only question is which is more important.
 

Offline evan_au

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Re: Why does liquid nitrogen bubble in this way?
« Reply #5 on: 23/09/2012 05:11:15 »
Damocles refers to the Leidenfrost effect - it seems a likely culprit:
http://en.wikipedia.org/wiki/Leidenfrost_effect
 

Offline damocles

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Re: Why does liquid nitrogen bubble in this way?
« Reply #6 on: 23/09/2012 07:45:36 »
Nice article!

Thanks evan
 

Online yor_on

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Re: Why does liquid nitrogen bubble in this way?
« Reply #7 on: 30/09/2012 16:56:22 »

 

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