Re: Missing term

[Bernard Cleyet <anngeorg@PACBELL.NET>]

Quite true (JD's ...) -- I lost my point in my writing, which (was) is
that there is a difference between solidification without
crystallization  and crystallization.

"Another term that could be used for "solidification" is "crystallizat=
ion".  Is there any reason to prefer one over the other?  Is "freezing
g" preferable to both?"






"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) rescale."


I suppose one can np forever on this.   I answer: microscopically, creep is different from flow (grain boundaries, defects, vacancies, etc.)


However, an English text I have arbitrarily defines a sold as having a viscosity > 1E15 Poise.  It also defines fluid flow as linear (shear rate and applied shear stress, limit of low stress).  This brings up thixotropy, Bingham flow, and other non newtonian effects.

Another informative definition of a solid is Maxwell's. i.e. a relaxation definition.

What term one uses is not important if the listener / reader understands the underlying mechanism or, at least, is not misled.  I suppose it's similar to philosophical reality, naive and enlightened.  One learns our reality may not be as we assume.  Once enlightened one realizes naive reality is OK. [heat is OK]


bc



John S. Denker wrote:

> 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.