Materials scientists find better model for glass creation

Nov 04, 2009 BY STEVE BRADT
“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 of Engineering and Applied Sciences (SEAS) and the Department of Physics. Photography by Stephanie Mitchell/Harvard Staff Photographer

(PhysOrg.com) -- Harvard materials scientists have come up with what they believe is a new way to model the formation of glasses, a type of amorphous solid that includes common window glass.

Glasses form through the process of vitrification, in which a glass-forming liquid cools and slowly becomes a solid whose molecules, though they've stopped moving, are not permanently locked into a . Instead, they're more like a liquid that has merely stopped flowing, though they can continue to move over long stretches of time.

"A 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 at Harvard's School of Engineering and Applied Sciences (SEAS) and the Department of Physics. "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."

Other than window glass, made from or , Weitz said many sugars are glasses. Honey, for example, is not a glass at room temperature, but as it cools down and solidifies, it becomes a glass.

Scientists like Weitz use models to understand the properties of glasses. Weitz and members of his research group, together with colleagues at Columbia University and the University of North Texas, report in this week's Nature a new wrinkle on an old model that seems to improve how well it mimics the behavior of glass.

The model is a colloidial fluid, a liquid with tiny , or colloids, suspended evenly in it. Milk, for example, is a familiar colloidial fluid. Scientists model solidifying glasses using colloids by adding more particles to the fluid. This increases the particles' concentration, making the fluid thicker, and making it flow more slowly. The advantage 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 can be observed with a microscope.

"They're big; they're slow. They get slower and slower and slower and slower," Weitz said. "They don't behave like a fluid. They don't behave 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 most glasses, which continue to flow ever more slowly as they gradually solidify. Weitz and colleagues 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 it 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 particles," Weitz said. "The fact you can visualize things gives you tremendous insight you can't get with molecular glass."

Source: Harvard University (news : web)

Explore further: Synthesis of a new lean rare earth permanent magnetic compound superior to Nd2Fe14B

add to favorites email to friend print save as pdf

Related Stories

Physicist opens new window on glass puzzle

Aug 09, 2007

When most people look at a window, they see solid panes of glass, but for decades, physicists have pondered the mysteries of window glass: Is glass a solid, or merely an extremely slow moving liquid" An Emory University research ...

A plane with wings of glass?

Jun 22, 2008

Imagine a plane that has wings made out of glass. Thanks to a major breakthrough in understanding the nature of glass by scientists at the University of Bristol, this has just become a possibility.

Crystal to glass cooling model developed

Feb 22, 2006

University of Tokyo scientists have discovered why cooling sometimes causes liquid molecules to form disordered glasses, rather than ordered crystals.

Researchers clear way to stronger glass

Dec 07, 2006

Look at your window - not out it, but at it. Though the window glass looks clear, if you could peer inside the pane you would see a surprising molecular mess, with tiny particles jumbled together any which ...

Recommended for you

Three-dimensional metamaterials with a natural bent

1 hour ago

Metamaterials, a hot area of research today, are artificial materials engineered with resonant elements to display properties that are not found in natural materials. By organizing materials in a specific way, scientists ...

Wild molecular interactions in a new hydrogen mixture

Oct 20, 2014

Hydrogen—the most abundant element in the cosmos—responds to extremes of pressure and temperature differently. Under ambient conditions hydrogen is a gaseous two-atom molecule. As confinement pressure ...

Atomic trigger shatters mystery of how glass deforms

Oct 18, 2014

Throw a rock through a window made of silica glass, and the brittle, insulating oxide pane shatters. But whack a golf ball with a club made of metallic glass—a resilient conductor that looks like metal—and the glass not ...

User comments : 0