Materials scientists find better model for glass creation
Harvard materials scientists have come up with what theybelieve is a new way to model the formation of glasses, a type of amorphoussolid that includes common window glass.Glasses form through the process of vitrification, in whicha glass-forming liquid cools and slowly becomes a solid whose molecules, thoughthey’ve stopped moving, are not permanently locked into a crystal structure.Instead, they’re more like a liquid that has merely stopped flowing, thoughthey can continue to move over long stretches of time.“A glass is permanent, but only over a certain time scale.It’s a liquid that just stopped moving, stopped flowing,” said David Weitz,Mallinckrodt Professor of Physics and Applied Physics in Harvard’s School ofEngineering and Applied Sciences (SEAS) and the Department of Physics. “Acrystal has a very unique structure, a very ordered structure that repeatsitself over and over. A glass never repeats itself. It wants to be a crystalbut something is preventing it from being a crystal.”Other than window glass, made from silica or silicon dioxide,Weitz said many sugars are glasses. Honey, for example, is not a glass at roomtemperature, but as it cools down and solidifies, it becomes a glass.Scientists such as Weitz use models to understand theproperties of glasses. Weitz and members of his research group, together withcolleagues at Columbia University and the University of North Texas, report inthis week’s Nature a new wrinkle on an old model that seems to improve how wellit mimics the behavior of glass.The model is a colloidial fluid, a liquid with tinyparticles, or colloids, suspended evenly in it. Milk, for example, is afamiliar colloidial fluid. Scientists model solidifying glasses using colloidsby adding more particles to the fluid. This increases the particles’concentration, making the fluid thicker, and making it flow more slowly. Theadvantage of this approach to studying glasses directly is size, Weitz said.The colloid particles are 1,000 times bigger than a molecule of a glass and canbe observed with a microscope“They’re big; they’re slow. They get slower and slower andslower and slower,” Weitz said. “They don’t behave like a fluid. They don’tbehave like a crystal. They behave in many ways like a glass.”The problem with traditional colloids used in these models,however, is that they often rapidly solidify past a certain point, unlike mostglasses, which continue to flow ever more slowly as they gradually solidify.Weitz and colleagues created a colloid that behaves more like a glass in thatway by using soft, compressible particles in the colloid instead of hard ones.This makes the particles squeeze together as more particles are added, makingthem flow more slowly, but delaying the point at which it solidifies, giving ita more glasslike behavior.By varying the colloidal particles’ stiffness, researcherscan vary the colloidal behavior and improve the model’s faithfulness to variousglasses.“There’s this wealth of behavior in molecular glass and wenever saw this wealth of behavior in colloid particles,” Weitz said. “The factyou can visualize things gives you tremendous insight you can’t get withmolecular glass.”