[tt] NYT: The Nature of Glass Remains Anything but Clear

[Premise Checker]

What I want to know is why it seems that old glass is discolored. I had 
thought it was due to its being a liquid. The glass on exhibit at a 
terrific exhibit of mostly decorative arts in Afghanistan from about 2000 
BC to early AD at the National Gallery of Art was astonishingly clear. I 
asked the best of the National Gallery's guides, Russell Sale, about this 
at his superbly organized and informative talk yesterday, and he thought 
it was because these glass objects had been buried most of the time since 
they were made. He admitted that this explanation might not be correct.

The article below is excellent.

The Nature of Glass Remains Anything but Clear
http://www.nytimes.com/2008/07/29/science/29glass.html

By KENNETH CHANG

Correction Appended

It is well known that panes of stained glass in old European
churches are thicker at the bottom because glass is a slow-moving
liquid that flows downward over centuries.

Well known, but wrong. Medieval stained glass makers were simply
unable to make perfectly flat panes, and the windows were just as
unevenly thick when new.

The tale contains a grain of truth about glass resembling a liquid,
however. The arrangement of atoms and molecules in glass is
indistinguishable from that of a liquid. But how can a liquid be as
strikingly hard as glass?

"They're the thickest and gooiest of liquids and the most disordered
and structureless of rigid solids," said Peter Harrowell, a
professor of chemistry at the University of Sydney in Australia,
speaking of glasses, which can be formed from different raw
materials. "They sit right at this really profound sort of puzzle."

Philip W. Anderson, a Nobel Prize-winning physicist at Princeton,
wrote in 1995: "The deepest and most interesting unsolved problem in
solid state theory is probably the theory of the nature of glass and
the glass transition."

He added, "This could be the next breakthrough in the coming
decade."

Thirteen years later, scientists still disagree, with some
vehemence, about the nature of glass.

Peter G. Wolynes, a professor of chemistry at the University of
California, San Diego, thinks he essentially solved the glass
problem two decades ago based on ideas of what glass would look like
if cooled infinitely slowly. "I think we have a very good
constructive theory of that these days," Dr. Wolynes said. "Many
people tell me this is very contentious. I disagree violently with
them."

Others, like Juan P. Garrahan, professor of physics at the
University of Nottingham in England, and David Chandler, professor
of chemistry at the University of California, Berkeley, have taken a
different approach and are as certain that they are on the right
track.

"It surprises most people that we still don't understand this," said
David R. Reichman, a professor of chemistry at Columbia, who takes
yet another approach to the glass problem. "We don't understand why
glass should be a solid and how it forms."

Dr. Reichman said of Dr. Wolynes's theory, "I think a lot of the
elements in it are correct," but he said it was not a complete
picture. Theorists are drawn to the problem, Dr. Reichman said,
"because we think it's not solved yet -- except for Peter maybe."

Scientists are slowly accumulating more clues. A few years ago,
experiments and computer simulations revealed something unexpected:
as molten glass cools, the molecules do not slow down uniformly.
Some areas jam rigid first while in other regions the molecules
continue to skitter around in a liquid-like fashion. More strangely,
the fast-moving regions look no different from the slow-moving ones.

Meanwhile, computer simulations have become sophisticated and large
enough to provide additional insights, and yet more theories have
been proffered to explain glasses.

David A. Weitz, a physics professor at Harvard, joked, "There are
more theories of the glass transition than there are theorists who
propose them." Dr. Weitz performs experiments using tiny particles
suspended in liquids to mimic the behavior of glass, and he ducks
out of the theoretical battles. "It just can get so controversial
and so many loud arguments, and I don't want to get involved with
that myself."

For scientists, glass is not just the glass of windows and jars,
made of silica, sodium carbonate and calcium oxide. Rather, a glass
is any solid in which the molecules are jumbled randomly. Many
plastics like polycarbonate are glasses, as are many ceramics.

Understanding glass would not just solve a longstanding fundamental
(and arguably Nobel-worthy) problem and perhaps lead to better
glasses. That knowledge might benefit drug makers, for instance.
Certain drugs, if they could be made in a stable glass structure
instead of a crystalline form, would dissolve more quickly, allowing
them to be taken orally instead of being injected. The tools and
techniques applied to glass might also provide headway on other
problems, in material science, biology and other fields, that look
at general properties that arise out of many disordered
interactions.

"A glass is an example, probably the simplest example, of the truly
complex," Dr. Harrowell, the University of Sydney professor, said.
In liquids, molecules jiggle around along random, jumbled paths.
When cooled, a liquid either freezes, as water does into ice, or it
does not freeze and forms a glass instead.

In freezing to a conventional solid, a liquid undergoes a so-called
phase transition; the molecules line up next to and on top of one
another in a simple, neat crystal pattern. When a liquid solidifies
into a glass, this organized stacking is nowhere to be found.
Instead, the molecules just move slower and slower and slower, until
they are effectively not moving at all, trapped in a strange state
between liquid and solid.

The glass transition differs from a usual phase transition in
several other key ways. Energy, what is called latent heat, is
released when water molecules line up into ice. There is no latent
heat in the formation of glass.

The glass transition does not occur at a single, well-defined
temperature; the slower the cooling, the lower the transition
temperature. Even the definition of glass is arbitrary -- basically
a rate of flow so slow that it is too boring and time-consuming to
watch. The final structure of the glass also depends on how slowly
it has been cooled.

By contrast, water, cooled quickly or cooled slowly, consistently
crystallizes to the same ice structure at 32 degrees Fahrenheit.

To develop his theory, Dr. Wolynes zeroed in on an observation made
decades ago, that the viscosity of a glass was related to the amount
of entropy, a measure of disorder, in the glass. Further, if a glass
could be formed by cooling at an infinitely slow rate, the entropy
would vanish at a temperature well above absolute zero, violating
the third law of thermodynamics, which states that entropy vanishes
at absolute zero.

Dr. Wolynes and his collaborators came up with a mathematical model
to describe this hypothetical, impossible glass, calling it an
"ideal glass." Based on this ideal glass, they said the properties
of real glasses could be deduced, although exact calculations were
too hard to perform. That was in the 1980s. "I thought in 1990 the
problem was solved," Dr. Wolynes said, and he moved on to other
work.

Not everyone found the theory satisfying. Dr. Wolynes and his
collaborators so insisted they were right that "you had the
impression they were trying to sell you an old car," said
Jean-Philippe Bouchaud of the Atomic Energy Commission in France. "I
think Peter is not the best advocate of his own ideas. He tends to
oversell his own theory."

Around that time, the first hints of the dichotomy of fast-moving
and slow-moving regions in a solidifying glass were seen in
experiments, and computer simulations predicted that this pattern,
called dynamical heterogeneity, should exist.

Dr. Weitz of Harvard had been working for a couple of decades with
colloids, or suspensions of plastic spheres in liquids, and he
thought he could use them to study the glass transition. As the
liquid is squeezed out, the colloid particles undergo the same
change as a cooling glass. With the colloids, Dr. Weitz could
photograph the movements of each particle in a colloidal glass and
show that some chunks of particles moved quickly while most hardly
moved.

"You can see them," Dr. Weitz said. "You can see them so clearly."

The new findings did not faze Dr. Wolynes. Around 2000, he returned
to the glass problem, convinced that with techniques he had used in
solving protein folding problems, he could fill in some of the
computational gaps in his glass theory. Among the calculations, he
found that dynamical heterogeneity was a natural consequence of the
theory.

Dr. Bouchaud and a colleague, Giulio Biroli, revisited Dr. Wolynes's
theory, translating it into terms they could more easily understand
and coming up with predictions that could be compared with
experiments. "For a long time, I didn't really believe in the whole
story, but with time I became more and more convinced there is
something very deep in the theory," Dr. Bouchaud said. "I think
these people had fantastic intuition about how the whole problem
should be attacked."

For Dr. Garrahan, the University of Nottingham scientist, and Dr.
Chandler, the Berkeley scientist, the contrast between fast- and
slow-moving regions was so striking compared with the other changes
near the transition, they focused on these dynamics. They said that
the fundamental process in the glass transition was a phase
transition in the trajectories, from flowing to jammed, rather than
a change in structure seen in most phase transitions. "You don't see
anything interesting in the structure of these glass formers, unless
you look at space and time," Dr. Garrahan said.

They ignore the more subtle effects related to the
impossible-to-reach ideal glass state. "If I can never get there,
these are metaphysical temperatures," Dr. Chandler said.

Dr. Chandler and Dr. Garrahan have devised and solved mathematical
models, but, like Dr. Wolynes, they have not yet convinced everyone
of how the model is related to real glasses. The theory does not try
to explain the presumed connection between entropy and viscosity,
and some scientists said they found it hard to believe that the
connection was just coincidence and unrelated to the glass
transition.

Dr. Harrowell said that in the proposed theories so far, the
theorists have had to guess about elementary atomic properties of
glass not yet observed, and he wondered whether one theory could
cover all glasses, since glasses are defined not by a common
characteristic they possess, but rather a common characteristic they
lack: order. And there could be many reasons that order is thwarted.
"If I showed you a room without an elephant in the room, the
question `why is there not an elephant in the room?' is not a
well-posed question," Dr. Harrowell said.

New experiments and computer simulations may offer better
explanations about glass. Simulations by Dr. Harrowell and his
co-workers have been able to predict, based on the pattern of
vibration frequencies, which areas were likely to be jammed and
which were likely to continue moving. The softer places, which
vibrate at lower frequencies, moved more freely.

Mark D. Ediger, a professor of chemistry at the University of
Wisconsin, Madison, has found a way to make thin films of glass with
the more stable structure of a glass that has been "aged" for at
least 10,000 years. He hopes the films will help test Dr. Wolynes's
theory and point to what really happens as glass approaches its
ideal state, since no one expects the third law of thermodynamics to
fall away.

Dr. Weitz of Harvard continues to squeeze colloids, except now the
particles are made of compressible gels, enabling the colloidal
glasses to exhibit a wider range of glassy behavior.

"When we can say what structure is present in glasses, that will be
a real bit of progress," Dr. Harrowell said. "And hopefully
something that will have broader implications than just the glass
field."

This article has been revised to reflect the following correction:

Correction: July 31, 2008
An article on Tuesday about the nature of glass described
incorrectly the phase transition from water to ice. When water
molecules are lined up into ice, energy (called latent heat) is
released. The phase transition does not require energy to line up
the molecules. (In the phase transition for glass, there is no
latent heat.)