🚀 go-pugleaf

jeff@mama.minmet.uq.oz (Jeff Gates) writes:

>Now, since the subject of this article was "the Nature of Glass",
>I would like to see if I can start a bit of controversy on the net!
>
>It is often said that
>	" Glass is a liquid "
>or that
>	" Glass is a supercooled liquid "
>
>I have my own opinions on this, but first I want to see what comes out.

I would argue that it is not a liquid.  Consider two "window panes", one
made of vitreous silica, one made of monocrystalline quartz.  In the
glass (the vitreous silica), the argument for being a liquid is that it
still can "flow", even at room temperature.

It is clear that this "flow" process in the glass at room temperature
takes place through diffusion.  Gravity would add a nearly imperceptible
bias for diffusion in the downward direction, which conceivably over
millions of years would be measurable through extra thickness at the
bottom.  (See the calculations in the related posting.)

Now consider the crystalline "window".  Vacancies are naturally created
at the surface, and because they are lighter than an ion, their
diffusion has a nearly imperceptible bias upwards.  This would result in
thickening at the bottom also.

In short, if a glass "flows" at room temperature, it is through
diffusion; but amorphous structures have no monopoly on diffusion.
Therefore a crystal can be considered to "flow" at room temperature as
much as a glass, but clearly the crystal is not a liquid; therefore,
vitreous silica at room temperature can't be considered to be one
either.  Therefore I reject the definition of a glass as a supercooled
liquid, at least at room temperature.  However, it is a useful concept.

As a working definition for *inorganic* glasses, I would offer "a
metastable supercooled melt".  Whether or not certain amorphous polymers
can be considered glasses, I have no idea.  What do you polymer people
think?

				- Rob