Mysterious boiling phenomenon during experiment

[aseeb.syed]

I am a student in my third year of engineering (materials science) and I was performing a lab experiment in which I was asked to calculate the heat capacity of metals using thermodynamic relationships.

The experiment was done by placing a metal sample into a foam container filled with liquid nitrogen. I had to wait until the liquid nitrogen stopped boiling (this indicated that the temperature of the metal had reached the temperature of the liquid nitrogen. The amount of liquid nitrogen evaporated was measured and used to calculate how much energy was transferred from the metal to the liquid. And from there, the heat capacity was calculated. (That was the experiment in a nutshell).

So the weird thing I noticed was this: During the time I had to wait until the liquid nitrogen stopped boiling, I noticed that RIGHT BEFORE the liquid nitrogen ceased boiling it actually boiled VERY FURIOUSLY and THEN it ceased.

I don't how to explain how to explain this clearly but picture this:

the metal is dropped in the container; you see the liquid nitrogen boiling as usual
you wait and wait
all of sudden it starts to boil a LOT more furiously
then, about a second later, it suddenly calms down and reaches equilibrium.

What I was expecting was that it would just boil then calmly cease to boil but this clearly was NOT what I saw.

My mind was blown and I STILL can't figure out what happened. Would anyone please be kind enough to explain this strange phenomenon?

 

[sophiecentaur]

I think it could be what's going on particularly on the lower surface of the metal block. Whilst the metal is supplying a lot of heat (it is 'hot') there is a bubble of gaseous N2 underneath it and also, bubbles all around it, insulating the surface from the liquid and limiting the rate of heat transfer. Once the metal cools down to near the liquid N2 temperature, there rate of heat transfer is not enough to sustain that insulating bubble and the block falls to the bottom. The shock of hitting the bottom, releases bubbles and there is a bigger surface of liquid N2 and the heat transfer rate goes up and increases the rate of bubbling briefly and the block vibrates against the bottom, increasing the rate of heat transfer.

This sort of effect can be seen when drops of water are dropped onto a very hot frying pan. The local boiling produces a layer of steam which keeps the drops from contacting the pan. Once the pan cools and the rate of steam production goes down, the drops spread over the pan and there is much more 'boiling activity'.

You could check this by lowering the metal on a string and see if you get the same effect. Or you could see if flat sided blocks behave differently from irregular shaped blocks.

 

[aseeb.syed]

I think it could be what's going on particularly on the lower surface of the metal block. Whilst the metal is supplying a lot of heat (it is 'hot') there is a bubble of gaseous N2 underneath it and also, bubbles all around it, insulating the surface from the liquid and limiting the rate of heat transfer. Once the metal cools down to near the liquid N2 temperature, there rate of heat transfer is not enough to sustain that insulating bubble and the block falls to the bottom. The shock of hitting the bottom, releases bubbles and there is a bigger surface of liquid N2 and the heat transfer rate goes up and increases the rate of bubbling briefly and the block vibrates against the bottom, increasing the rate of heat transfer.

This sort of effect can be seen when drops of water are dropped onto a very hot frying pan. The local boiling produces a layer of steam which keeps the drops from contacting the pan. Once the pan cools and the rate of steam production goes down, the drops spread over the pan and there is much more 'boiling activity'.

You could check this by lowering the metal on a string and see if you get the same effect. Or you could see if flat sided blocks behave differently from irregular shaped blocks.

Thank you very much for your explanation.

 

[Bystander]

See also "film or sheet boiling."

 

[sophiecentaur]

Thank you very much for your explanation.

Does it 'agree' with what you have seen? Next time you're in the kitchen, try the water experiment. I reckon you'll see much the same dynamic at work.

 

[aseeb.syed]

Does it 'agree' with what you have seen? Next time you're in the kitchen, try the water experiment. I reckon you'll see much the same dynamic at work.

Yes, indeed, this leidenfrost effect seems to explain what I saw in lab. And yes, I will definitely try this experiment for myself when I get the time.