Re: Shake some supercooled water and you get ice, why?

On Feb 18, 8:00 am, "Edward Green" <spamspamsp...@xxxxxxxxxxx> wrote:
On Feb 18, 10:28 am, andy everett <vze2q...@xxxxxxxxxxx> wrote:

Edward Green wrote:
I don't know if anybody has elucidated a detailed mechanism since 1948.

We have all Sunday, lets figure it out!

Oish. The problem with continually pontificating "let's stop
insulting each other, and discuss some physics", is that sooner or
later somebody will call your bluff, and actually want to discuss some
physics. This requires work.

First we might start by reviewing the general theory of nuceation and
phase transformations. Then we should do an extensive literature
review, probably by finding articles citing Dorsey. This may take
more than one Sunday. :-)

Anyway, as you probably know, it is widely accepted that condensed
phases are unstable below a certain threshold, even when
thermodynamically favored in the bulk, and hence must first nucleate,
meaning that a sufficiently large region of the new phase is formed so
that it can grow by adding molecules to essentially the bulk phase.

One thing I've noticed is that supercooled liquids, on freezing, tend
to form slurries. This means there is not one or two nucleation sites
(which would lead to dendritic growth), but many appearing almost
simultaneously, creating a porridge of tiny crystals. Whatever
happens, at least sometimes, seems to happen throughout the bulk.

Excellent point. Seems to be the case in the many YouTube
demonstrations...

Knocking the liquid achieves this. Do microbubbles form nucleation
sites?

ISTR degassed water does this as well.

OTOH it is often claimed that liquid water always contains a small
proportion of clusters of molecules in the gas and solid phases; one
wonders why the solid clusters don't support nucleation.

Freezing water that's just below the freezing point may release just
enough heat of fusion to prevent complete solidification.

Or, it may be due to the relative degree of supercooling; if it's
cold enough it ought to freeze completely. (Yes, I know, read the
cited paper. But it's Sunday, and I construe that as "work".)

Has the effect been seen in _moving_ water? I mean bulk motion, even
though it's intuitively apparent that supercooled water probably
supports little to no internal convection currents.

If it cannot happen if the water is moving because turbulence
supplies the "knock", in what sense can supercooled water be said to
be liquid?


Mark L. Fergerson

.

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