Thursday, August 30, 2007

Supercooling is Super Cool

So, if you've never heard about supercooling liquids, you should listen up, because it is a fantastic physical phenomenon, first introduced to me by the inimitable Ray Hively. As we all know water usually freezes at 32 degrees Fahrenheit, but this is clearly not an unbreakable law. You can have a rushing river that is colder that 32 degrees, and adding substances like salt can lower the freezing point. What we don't usually hear, though, is that if you keep fluids really, really still, you can achieve the same effect. Crystalline structures like ice need an imperfection or seed crystal to start forming, and without motion, the fluid can resist its temptation to freeze. Your refrigerator jostles too much when the compressor turns on, but if it didn't, you'd be able to see the effect in a tray of would be ice cubes. In laboratories, they've been able to cool water to below negative 40 degrees.

Well, that's all well and good, but the exciting part is when you decide to disturb the water after it's been supercooled. Then, depending on how cold it is, it can freeze in a matter of seconds. Freezing rain is a common example of this phenomenon. The rain is fluid until it strikes a surface, where it freezes. Check out this video of someone pouring out a bottle of supercooled water as it freezes:



It is also possible to superheat water, so that it boils immediately after it is disturbed. Check out this video:

Evidently, this can be a problem if you put a still cup of water in a microwave for too long. When you take the mug out, it gets disturbed and it can severely scald you. I would hazard a guess that it's only really a danger in old microwaves that don't rotate, so you probably are not in danger.

There are lots of other really neat phenomena out there surrounding the change of phases of materials (especially water). Apparently if you cool water really fast, as in, at least 1 million degrees, per second, you can turn water into a glass, which is such a common substance that we often forget how neat it is. More on that later.

-Seth

4 comments:

Matt Grosso said...

Wicked cool. And punderful.

AdamB said...

isn't there something where if you apply a lot of pressure you can prevent phase changes as well?

Angie said...

This is neat! I have noticed that water heated in the microwave sometimes boils alot right when I start to remove the container from the oven. Now I know why!

Seth said...

Oh yeah, good point Adam. A pressure cooker can also raise the boiling temperature of a fluid. That's because

Pressure = Temperature * Constant

in this case. So, they're directly proportional, and if you up the pressure, the temp will increase as well without a phase change.