ANYTHING BUT

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?

Scientists disagree about the nature of 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. 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 organised 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. 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.

Peter G. Wolynes, a professor of chemistry at the University of California, San Diego, thinks he solved the glass problem two decades ago based on ideas of what glass would look like if cooled infinitely slowly. 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.

Wolynes and his collaborators came up with a mathematical model to describe this “ideal glass”. Based on this, they said the properties of real glasses could be deduced, although exact calculations were too hard to perform. Not everyone found the theory satisfying. 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.

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.

David A. Weitz, a physics professor at Harvard, joked, “There are more theories of the glass transition than there are theorists who propose them.” Weitz performs experiments using tiny particles suspended in liquids to mimic the behaviour of glass.

For scientists, 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 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.
(New York Times)