2006-12-01

Freezing Water by Fast Hydrostatic Decompression

It's my understanding that freezing water fast enough to prevent crystallization and its negative effects on biological systems is a classical probem in cryobiology and cryonics. A more mundane application is food preservation, where flash-freezing has demonstrable advantages over slow freezing.

It makes sense that water is slow to freeze, because it has such large thermal mass. In a body of water of any appreciable size, the time it takes to conduct heat away from the central regions to the periphery is significant. There are inherent limits to the speed at which heat can be conducted out of a body of water. Freezing water by lowering its temperature is, obviously, limited by the speed at which you can move heat away, and the larger the body of water the slower that speed becomes.

But there's another way to freeze water. Consider the phase diagram for water here. The line from point M to point O represents the boundary between liquid and solid phases, and is either the freezing line or the melting line depending on which side you start from. Water is fairly unique among materials in that this line has a negative slope; among other things, this is an expanation for why ice is less dense than liquid water. For my purpose, please notice that at higher pressures the freezing point of water is lower than at ambient and lower pressures.

I propose to freeze water by first compressing it, so that its freezing point is lowered. Then it is cooled to a temperature below its freezing point at normal pressure. Because the elevated pressure will keep it in liquid form, it does not matter how fast the temperature is lowered, because the phase transition will be held off. Then, once it's cooled to, say, -5C, you rapidly release the hydrostatic pressure and the liquid, now under ambient conditions and well below its freezing point, should solidify very rapidly. Unlike temperature, the hydrostatic pressure of a liquid can be varied essentially instantaneously throughout its volume.

I read now here that this technique is actually in use to freeze food products. I haven't yet discovered if it has been applied to cryobiological problems, however. It's generally referred to as "Pressure Shift Freezing."

3 comments:

Mark said...

This should be easy to test with a small sample and some suitable single cell animals (so the effects can be easily observed). I understand that one problem with cryogenic effects on cells is the growth of large crystals domains, and this technique should result in very small crystals.
Perhaps unfreezing in the same manner would also be beneficial (for food and cells) -- less mechanical stress due to spatially variable contraction.
This might be the "hardware store" weapon of your 12/25/06 post.

Anonymous said...

Soooooooooo?
How did your experiments work out?

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