Mysterious boiling phenomenon during experiment

[aseeb.syed]

Greetings,

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?

 

[mfb]

A guess: As long as the metal is hot (compared to the nitrogen), you get the Leidenfrost effect - nitrogen gas forms an insulating layer between metal and liquid.
When the temperature drops, this could get disrupted and lead to a different way of boiling. Not sure how this would appear to look like more boiling. More smaller bubbles maybe.

 

[aseeb.syed]

A guess: As long as the metal is hot (compared to the nitrogen), you get the Leidenfrost effect - nitrogen gas forms an insulating layer between metal and liquid.
When the temperature drops, this could get disrupted and lead to a different way of boiling. Not sure how this would appear to look like more boiling. More smaller bubbles maybe.

Thank you kindly. A plausible reason.

 

[christopher.s]

The leidenfrost effect is the correct explanation. The way I've had it explained to me is that as the metal cools to a point where the temperature difference does not create as effective of a vapor shield, the heat transfer from the metal to the LN2 drastically increases for a moment. Basically the LN2 is able to actually "touch" the metal at this point instead of nitrogen vapor.

 

[Doug Huffman]

I think better terms are transition from film boiling to nucleate boiling to departure from nucleate boiling.
https://en.wikipedia.org/wiki/File:Boiling_Curve.jpg

 

[aseeb.syed]

The leidenfrost effect is the correct explanation. The way I've had it explained to me is that as the metal cools to a point where the temperature difference does not create as effective of a vapor shield, the heat transfer from the metal to the LN2 drastically increases for a moment. Basically the LN2 is able to actually "touch" the metal at this point instead of nitrogen vapor.

Thank you for the reassurance.

 

[aseeb.syed]

I think better terms are transition from film boiling to nucleate boiling to departure from nucleate boiling.
https://en.wikipedia.org/wiki/File:Boiling_Curve.jpg

Thank you, Sir.