WCU Physicist’s Research Reported in Nature

West Chester University physicist Kevin Aptowicz is part of a team of researchers who are trying to unravel one of the most baffling mysteries in science – the transition of glass from liquid to solid. Their work was recently published in the journal Nature (May, 2009).

Why is the transition of glass so puzzling? “Typically, when a substance changes from liquid to solid – the change we see on a macroscopic level – there is a corresponding change on the microscopic level,” explains Aptowicz.

“If you looked at water microscopically, you would see particles that are randomly oriented and free to move around. When water freezes and becomes solid, the microscopic view changes as well. The particles become highly organized and locked in place.

“But when glass makes a dramatic change on the macroscopic level – from molten to solid – there is no corresponding change on the microscopic level. The glass molecules are randomly oriented just as they were in the molten state. It feels hard to the touch, it stays in your windows and keeps the weather out, but it remains essentially a liquid.”

Or, as scientists say, the glass has entered a jammed state. It’s not a solid, but it is behaving as one. Jamming happens to other substances as well – sugar that solidifies and won’t pour from the package, sand that forms dunes, even cars caught up in traffic grid lock.

Aptowicz and the team, led by Andrea Liu of the University of Pennsylvania and Sidney Nagel of the University of Chicago, have been studying jamming based on a theory proposed by Liu and Nagel that the physics of the glass transition may be the same as that in other jammed systems. “If we could understand what’s going on in the larger systems and connect them to smaller ones,” Aptowicz says, “we could eventually understand what’s going on in glass.”

The study presented in Nature offered the first evidence that, despite the size of the particles involved – molecules, grains of sand, boulders, cars – the process by which they turn from moving objects to solid objects is basically the same.

In their experiment, the researchers used colloids – polymer spheres approximately one micron in diameter. “Compared to glass molecules, colloids are large,” Aptowicz says, “but compared to grains of sand, they’re quite small. They provided a bridge system.”

This was important because of a characteristic of larger jammed systems that had been documented, the uniformity of the “nearest neighbor distance” of the system particles. “In the larger systems, scientists had observed that the distance between any two particles was the same,” says Aptowicz.

In their colloid experiment, Aptowicz and his colleagues saw the same behavior. As the material transitioned into a jammed state, the nearest neighbor distance of the particles became uniform. “We saw the same structural signature in our jammed system that we see in larger systems.”

Will this structural signature carry over to glass molecules and thus solve the ages-old puzzle? Aptowicz is cautious. “That’s a future experiment,” he says. “It’s unclear how much of what we’ve learned will carry over.”

Long-term, there are practical applications of this work such as improved time-release medications (medications contained in substances that slowly transition from solid to liquid) and, perhaps, even better traffic flow.

Aptowicz, who holds a Ph.D. in applied physics from Yale, has been a member of the Department of Physics at West Chester since 2005 and has spent summers working with the Penn-based research team since 2006. He has included West Chester undergraduates in this work. Student Sean Gossin is currently part of the team. Aptowicz credits physics department chairman Anthony Nicastro and arts and sciences dean Lori Vermeulen for supporting this work.

The study was funded by the Department of Energy and the National Science Foundation.