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Re: Missing term



On 08/29/2003 01:03 PM, Bernard Cleyet wrote:
>
> Regarding freezing, I think one should differentiate between the
> increase in viscosity when glasses are cooled and other liquids
> crystallize. Glasses are still liquids when called solids because
> they still flow and there is not a demarcation in their properties.
> i.e. their viscosity smoothly increases on cooling and there is no
> "heat" of fusion. "Heat" must be "extracted", of course, for this
> viscosity increase to occur, but there is no "discontinuity". There
> are other differences, e.g. how they fracture.

I know it is conventional in some circles to say
things like that, but it has never made any sense to
me, and it appears that thoughtful usage is trending
away from calling glass a liquid. The term amorphous
solid will do nicely.

When handling a piece of glass, normal people are
going to think of it as a solid. They don't really
care whether there were or weren't any "discontinuities"
in the phase diagram during its manufacture.

Also, there are plenty of crystalline solids that
exhibit vitreous fracture, so that's hardly a
defining characteristic.

The original meaning is also the most common and
most sensible meaning: A liquid is something that
flows easily. Liquid assets are not required to
be amorphous, just flowable.

If you want to make this sound more sophisticated,
you can say that a solid will support a static shear,
whereas a liquid will not.

This definition is not infinitely sharp, because all
solids (amorphous *or not*) will creep if subjected
to high temperatures and pressures. That is, we
may need to say that *all* solids support shear on
a very long (but not quite infinite) timescale.

One is reminded of Feynman's definition of equilibrium,
when all the fast things have happened but the slow
things have not. In normal everyday situations,
liquids flow on a fast timescale and solids creep
on a tremennnnndously longer timescale, so the
distinction is useful and unambiguous in practice.