That's how it always is, Roger. It's fascinating to watch that boundary
sweep across the surface.
I have a thermocouple in the pool and watch the temperature drop from
room temperature. It goes to about -2 C or so, holds steady for a bit
and then poof - it's all over. Vent the system and pass the ice chunk
around.
Now do that same experiment, but replace the water with liquid nitrogen!
That is amazing to watch. Then try to pass the solid air chunk (sponge?
Mesh?) around... Try it... and let me know what you think.
You do have a ballast valve on your pump when you pump on water, right??
If so, open it up, it will save your pump!
Sam
-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Roger
Haar
Sent: Monday, May 07, 2007 4:18 PM
To: Forum for Physics Educators
Subject: [Phys-l] sudden Vacuum freezing
Hi,
For our final lab for or sophomore physics majors, we try to
show them
some odd things like rattlebacks. We show them how to freeze water in a
vacuum. One uses a shallow dish (1/2 inch of water) on a good insulator
in a vacuum belljar. and start pumping. (This is hard on the pump.) The
vapor pressure drops, and the water boils. The water cools to its
freezing point with the removal of thermal energy.
I saw something surprising, at least to see. The water
supercooled,
and while I was watching it froze in about a second. It was slow enough
that I could see a the water-ice boundary move across the dish like
almost like a wave. The dish was then frozen to at least half of its
depth.
I understand what happened, but am amazed that the water was
supercooled to a temperature such that so much water would freeze, and
that I happened to be looking at the right moment.