Physicists crack another piece of the glass puzzle

By Carol Clark | eScienceCommons | Oct. 17, 2012

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The experiment showed how particles on the verge of becoming a glass move like cars trying to get out of a grid-locked parking lot.

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When it comes to physics, glass lacks transparency. No one has been able to see what's happening at the molecular level as a super-cooled liquid approaches the glass state – until now. Emory University physicists have made a movie of particle motion during this mysterious transition.

Their findings, showing how the rotation of the particles becomes decoupled from their movement through space, are being published in the Proceedings of the National Academy of the Sciences..

"Cooling a glass from a liquid into a highly viscous state fundamentally changes the nature of particle diffusion," says Emory physicist Eric Weeks, whose lab conducted the research. "We have provided the first direct observation of how the particles move and tumble through space during this transition, a key piece to a major puzzle in condensed matter physics."

Weeks specializes in "soft condensed materials," substances that cannot be pinned down on the molecular level as a solid or liquid, including everyday substances such as toothpaste, peanut butter, shaving cream, plastic and glass.

Scientists fully understand the process of water turning to ice. As the temperature cools, the movement of the water molecules slows. At 32 F, the molecules lock into crystal lattices, solidifying into ice. In contrast, the molecules of glasses do not crystallize.The movement of the glass molecules slows as the temperature cools, but they never lock into crystal patterns. Instead, they jumble up and gradually become glassier, or more viscous. No one understands exactly why.

The phenomenon leaves physicists to ponder the molecular question of whether glass is a solid, or merely an extremely slow-moving liquid.

This purely technical physics question has stoked a popular misconception: That the glass in the windowpanes of some centuries-old buildings is thicker at the bottom because the glass flowed downward over time.

"The real reason the bottom is thicker is because they hadn't yet learned how to make perfectly flat panes of glass," Weeks says. "For practical purposes, glass is a solid and it will not flow, even over centuries. But there is a kernel of truth in this urban legend: Glasses are different than other solid materials."

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