X-Message-Number: 5128
From: 
Date: Mon, 6 Nov 1995 22:04:53 -0500
Subject: SCI. CRYONICS more fusion

Brian Wowk (# 5110) somewhat misrepresented my # 5099.  I did not "claim"
that sudden pressure applied to a block of ice at the freezing point would
make it all suddenly melt. In fact, I specifically questioned whether adding
a small amount of energy, by slightly increasing pressure, could accomplish
this. (Obviously it couldn't, if nothing else went on; sometimes I'm medium
stupid, but not stupid enough to forget energy considerations.) But then I
proceeded
to ask what happens on the assumption that somehow it does melt. (This was in
the context of Yvan Bozzonetti's assertion that adding pressure raises the
temperature, and I pointed out that this might only be true if there was no
phase change.) I came to no final conclusion, noting a few of several open
questions,  although I did, to my embarrasment, indicate that Yvan's
conclusion MIGHT be right.

The only reason I am posting this to SCI. CRYONICS is that Brian's post and
my previous one (misguidedly) were so posted, and I want to explain my
reasons for wasting everyone's time with this childishly simple stuff.
Sometimes--and I don't think I'm alone here--these Cryonet posts are
spur-of-the-moment, often late at night, stream-of-consciousness stuff,
 ramblings mainly to help one's own thoughts along, unedited.  (Omega Point
help us all if we didn't usually keep our editors turned on. I guess one
consolation is that such stuff may help our archivists reconstruct us.)
Here's the rest of it:

On previous occasions Brian has said that, when pressure is released on a
water solution at low temperature, it may partly freeze, but then the
released heat of fusion will melt it again, at least partly. Similarly, now
he says that adding a little pressure to ice at the freezing point will cause
only a little melting, the energy required coming from the internal energy of
the system and lowering the temperature, tending to cause refreezing.

This certainly sounds sensible,  but I was troubled by trying to picture the
mechanism, which is why I couldn't leave well enough alone. I usually try to
arrive at the same conclusion by at least two different routes before I am
satisfied.

How does added pressure lower the freezing point? One way to think of it is
that pressure tends to compress, and water is denser than ice. But then we
remember that the heat of fusion is sometimes thought of as the binding
energy of molecules in crystal formation, similar to the binding energy of
exothermic chemical reactions. If we push on the crystals, tending to break
the bonds between molecules, why should those or neighboring molecules
obligingly give up their thermal energy?

A fairly obvious answer is that the first molecules to break loose are those
that happen momentarily to have more than average energy (partly analogous to
evaporation). That would tend to produce some liquid and--at least
temporarily--reduce the temperature of the remaining ice so it doesn't melt
at the slightly raised pressure. We then have an inhomogeneous system, mostly
ice but partly liquid, at a slightly higher pressure than before and a
slightly lower temperature (if the thermal insulation doesn't leak). At that
temperature and pressure, liquid and solid can be in equilibrium. So that
seems to take care of that.

Three main practical questions remain, as well as several subsidiary ones.
 One is the temperature to which the body would have to be reduced, and the
pressure required, if it is suddenly to freeze completely upon release of
pressure. (In the neighborhood of - 100 C?) The second is whether, with this
procedure, conservation of information is clearly improved over other
procedures (even though damage by some criteria might be worse). The third is
cost.
 
Again, the hyperbaric procedure remains only a possible and not a probable
alternative, but I think it cannot yet be dismissed entirely.

Robert Ettinger


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