🚀 go-pugleaf

In article <1992Dec11.023731.17721@lynx.dac.northeastern.edu> mkagalen@lynx.dac.northeastern.edu (michael kagalenko) writes:
>In article <10DEC199219275429@trentu.ca> lchardon@trentu.ca (PETIT PAPA NOEL.) writes:
>>
>>	I'm sorry if this has been discussed here before, but I'm quite 
>>puzzled by the following:
>>
>>	In my thermodynamics class, we were all happily playing with liquid 
>>nitrogen, freezing squash balls, making erasers explode (BTW, why do 
>>plastic erasers explode when in liquid nitrogen?) and all that kind of 
>>things that you do when you have 10 litres of nitrogen to waste, when a 
>>prof dropped by and told us to try to immerge some piece of metal in the 
>>nitrogen. Sure enough, somebody pulled out a penny and dropped it in a 
>>full styrofoam cup. What happens is the following:
>>The nitrogen will boil for some time, like it boils when you drop anything 
>>in it, but then suddenly it will boil really hard, about 3 times more 
>>violently than it did before, for about half a second, and then it will 
>>just gently bubble away (When the metal is in thermal equilibrium with it, 
>>i guess...).
>>
>>	WHY ???????????????
>>	WHY JUST METALS ???
>>	
>This is an interesting question. You can do the same observation with
>the iron ;
>1. Heat it 
>2. Turn it off
>3. drop small amounts of water on the hot surface(make sure it's horizontal) 
>You'll see 2 different situations :
> - When iron is hot enough, water doesn't touch metal ! Steam production 
>allows drop of water to levitate. If you can fix an iron strictly 
>horizontally, even small drops can survive more than 1 minute.
> - When iron become cooler, the rate of steam production is too small 
>to isolate the drop from the hot surface; in that case water touchs the
>metal & evaporates in several seconds. It seems a bit paradoxical,
>but I'm unwilling to explain the details (surface tension of drop of
>water does matter). Liquid nitrogen + coin works the same way.
>

These are two different processes!

The situation is not the same.  The drop is levitating away from the heat
source on a cushion of steam.  And since steam doesn't conduct heat very
well, the steam acts as an isulator, allowing the drop to survive.
If you were to invert the iron and go from the
bottom (iron on top) you would not get this two step effect, as the steam would
escape immediatly, and the water would remain in contact (to a point, for
if the steam had enough pressure to overcome the weight of the iron, the iron
would levitate as well).  But my point is is that they are two entirely
situations.

The situation in the LN2 is that the LN2 is touching the object over all
parts and there is no isolation of the LN2 from the object, for as soon as the
LN2 turns into N2 vapor, the bubble breaks off and heads for the surface of
the dewar.  So there is no "levitation" or isolation of the LN2 from the
object.

I have made a post where I given the (correct) answer.