Harvard materials scientists have come up with what they believe is a new way to model the formation of glasses. During vitrification, a glass-forming liquid cools and slowly becomes a solid with molecules that, though they’ve stopped moving, are not permanently locked into a crystal structure. Instead, they’re more like a liquid that has merely stopped flowing, though they can continue to move over long stretches of time.

“A glass is permanent, but only over a certain time scale,” said David Weitz, Mallinckrodt professor of Physics and Applied Physics in Harvard’s School of Engineering and Applied Sciences (SEAS) and the Department of Physics. “It’s a liquid that just stopped moving, stopped flowing. A crystal has a very unique structure, a very ordered structure that repeats itself over and over. A glass never repeats itself. It wants to be a crystal but something is preventing it from being a crystal.”

Traditional Model

Weitz and members of his research group, together with colleagues at Columbia University and the University of North Texas, reported in a recent Nature that they’ve identified a new wrinkle on an old model that seems to improve how well it mimics the behavior of glass. The traditional model is a colloidial fluid, which is manipulated by adding more particles to the fluid to model solidifying glasses. This increases the particles’ concentration, causing the fluid to thicken and making it flow more slowly.

The advantage of this approach, as opposed to studying glasses directly, is size, Weitz said. The colloid particles are 1000 times bigger than a molecule of a glass and can be observed with a microscope. “They’re big; they’re slow. They get slower and slower and slower and slower,” said Weitz. “They don’t behave like a fluid. They don’t behave like a crystal. They behave, in many ways, like a glass.”

A New Approach

The problem with the traditional colloids used in these models, however, is that they often rapidly solidify past a certain point, unlike most glasses, which continue to flow ever more slowly as they gradually solidify. Weitz and his colleagues have created a colloid that behaves more like a glass in that way by using soft, compressible particles in the colloid instead of hard ones. This makes the particles squeeze together as more particles are added, making them flow more slowly but delaying the point at which the colloid solidifies, giving it a more glasslike behavior.

By varying the colloidal particles’ stiffness, researchers can vary the colloidal behavior and improve the model’s faithfulness to various glasses. “There’s this wealth of behavior in molecular glass and we never saw this wealth of behavior in colloid particles,” Weitz said. “The fact that you can visualize things gives you tremendous insight you can’t get with molecular glass.”

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