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>From the New York Times Science Section July 29, 2008<br>
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<h1><nyt_headline version="1.0" type=" ">
The Nature of Glass Remains Anything but Clear
</nyt_headline></h1>
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src="cid:part1.09090106.06080902@glassblower.info" alt="" border="0"
height="370" width="600">
<div class="credit">Mark Interrante</div>
<p class="caption">
<strong>ENIGMA</strong> Molten glass being worked into an ornament.
Understanding glass could lead to better products and offer headway in
other scientific problems.
</p>
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<div class="byline">By <a
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<div class="timestamp">Published: July 29, 2008</div>
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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<br>
<br>
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.
<p>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?</p>
<p>“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.”</p>
<p>Philip W. Anderson, a <a
href="http://topics.nytimes.com/top/news/science/topics/nobel_prizes/index.html?inline=nyt-classifier"
title="More articles about Nobel Prizes.">Nobel Prize</a>-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.”</p>
<p>He added, “This could be the next breakthrough in the coming decade.”</p>
<p>Thirteen years later, scientists still disagree, with some
vehemence, about the nature of glass. </p>
<p>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.”</p>
<p>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. </p>
<p>“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.” </p>
<p>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.”</p>
<p>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. </p>
<p>Meanwhile, computer
simulations have become sophisticated and large enough to provide
additional insights, and yet more theories have been proffered to
explain glasses.</p>
<p>David A. Weitz, a physics professor at <a
href="http://topics.nytimes.com/top/reference/timestopics/organizations/h/harvard_university/index.html?inline=nyt-org"
title="More articles about Harvard University.">Harvard</a>,
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.”</p>
<p>For scientists, glass is not just the glass of windows and jars,
made of silica, sodium carbonate and <a
href="http://health.nytimes.com/health/guides/test/serum-calcium/overview.html?inline=nyt-classifier"
title="In-depth reference and news articles about Serum calcium.">calcium</a>
oxide. Rather, a glass is any solid in which the molecules are jumbled
randomly. Many plastics like polycarbonate are glasses, as are many
ceramics.</p>
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.<br>
<p>“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. </p>
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.
<p>The glass
transition differs from a usual phase transition in several other key
ways. Energy is released, what is called latent heat, when water
molecules line up into ice. There is no latent heat in the formation of
glass. </p>
<p>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.</p>
<p>By contrast, water, cooled
quickly or cooled slowly, consistently crystallizes to the same ice
structure at 32 degrees Fahrenheit. </p>
<p>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. </p>
<p>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.</p>
<p>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.”</p>
<p>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.</p>
<p>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.</p>
<p>“You can see them,” Dr. Weitz said. “You can see them so clearly.”</p>
<p>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.</p>
<p>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.”<br>
</p>
<p>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.</p>
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.
<p>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. </p>
<p>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.</p>
<p>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.</p>
<p>Mark D. Ediger, a professor of chemistry at the <a
href="http://topics.nytimes.com/top/reference/timestopics/organizations/u/university_of_wisconsin/index.html?inline=nyt-org"
title="More articles about University of Wisconsin">University of
Wisconsin</a>,
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.</p>
<p>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.</p>
<p>“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.”<br>
</p>
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<h2><a
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<h2>
<a
href="http://tierneylab.blogs.nytimes.com/2008/07/28/glass-does-not-flow-except-in-space/index.html?ref=science">TierneyLab:
Glass Does Not Flow. Except in Space?</a> (July 28, 2008)
</h2>
<h2><a
href="http://topics.nytimes.com/top/reference/timestopics/subjects/g/glass/index.html">Times
Topics: Glass</a></h2>
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