In glass, fast crowds boogie to brittle end

Glass is clear, but the process by which it forms remains maddeningly opaque.

As a liquid hardens into a glass, its molecules become extremely sluggish—more so than expected at the temperatures or conditions of molecular crowding under which the process takes place. Moreover, the molecules remain disorderly even after solidification, unlike those in a crystal, where the molecules occupy a tightly bonded lattice. Something else must cause the molecular gridlock in a glass.

Studying suspensions, or colloids, of tiny plastic spheres that can form glasses, Eric R. Weeks of Harvard University and his colleagues have now observed a striking coordination that may play a role in creating gridlock.

Crowded together, individual spheres apparently must wait for neighbors to change position in order to move themselves, Weeks explains. When the researchers scanned their sample, they found that the spheres moving most rapidly tended to be in clusters. This bunching reveals that sphere motion requires coordination, Weeks says.

Sharon C. Glotzer of the National Institute of Standards and Technology in Gaithersburg, Md., and her colleagues have witnessed similar behavior in supercomputer simulations of atoms in a liquid. The coordinated particles “follow one another, kind of like in a long conga line,” she says. Glotzer adds that she’s “really excited” to see that the predicted clustering appears to be borne out by the experiment at Harvard.

Weeks and his coworkers found that as the suspension gets closer to forming a glass, the average cluster size grows from about 10 to about 50 beads. “If you need all the particles to cooperate in order for any of them to move, you have a solid,” Weeks extrapolates. “That’s what’s exciting about this.”

Weeks and other scientists experiment with colloids because the spheres move much more slowly than molecules or atoms of a liquid and are big enough to be seen through an optical microscope (SN: 4/12/97, p. 224:

Using a microscope that can focus to different depths in suspensions, Weeks and his coworkers tracked the three-dimensional positions of 5,000 particles in each sample over many hours.

Their findings are “certainly interesting,” comments C. Austen Angell of Arizona State University in Tempe. He cautions, however, that colloid studies and simulations probe only an intermediate stage of glass formation.

Weeks and his colleagues describe their findings in the Jan. 28 Science. Willem K. Kegel and Alfons van Blaaderen, both of the University of Utrecht in the Netherlands, reported a similar experiment in the Jan. 14 Science. They also saw “regions of high mobility.” However, because their equipment limited them to watching, one at a time, two-dimensional slices of a suspension, they neither directly observed full clusters nor counted particles in them.

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