Prehistoric peoples used naturally occurring glass to make some of the first cutting tools; modern humans began making this hard material at least 5,000 years ago. Today, a dazzling array of glasses can now be found on everything from spacecraft windows to your smart phone display. Nevertheless, the chemical mechanics of this ubiquitous material have remained elusive, so much so that it is seldom taught even in post-secondary education.
Glassmaking may be among humanity’s oldest industrial pursuits but we are still trying to understand its chemical dimension.
“You never learn about glass as an undergraduate, because they’re disordered condensed phases that are really hard to describe,” says James Forrest, a physics professor at the University of Waterloo. More specifically, chemists and physicists have struggled to describe key aspects of the glass transition, such as the concept of molecular “cages” that form as the temperature drops and the surface begins to harden. Forrest, who sometimes finds himself speaking about glass to experienced chemists who may know little about this field, spares these audiences from such complex and incomplete models. Instead, he described a simplified analogy to “jamming,” whereby molecular motion stops after a certain density is achieved, much the way a surplus of small particles will clog up the opening of a funnel.
It turns out that Forrest introduced more than just a convenient narrative device. When he and his colleagues from McMaster University and école supérieure de physique et de chimie industrielles in France began toying with a complete mathematical model based roughly around the concepts of jamming and caging, they discovered that it resolved many of the difficulties in describing glass formation. The pair recently published their findings in the Proceedings of the National Academy of Sciences, where they describe a “minimal” theory of glass formation that employs the dual concepts of crowding and string-like cooperative rearrangement to account for the constantly shifting interface that this material presents.
“People have been working on this for decades and we’re not claiming that this is the be-all, end-all theory,” Forrest says. “But it’s something that as far as we can tell is basically right. It doesn’t involve a lot of assumptions and it provides a framework that makes it accessible to more people than the glass transition was in the past.”
Above all, Forrest adds, a complex mathematical framework has been replaced by calculations any undergraduate could tackle, which could open up new avenues for investigating glasses of all kinds. “It’s not that we’re doing string theory and quantum information — this one involves algebra and a few derivatives,” he says. “Now people will start using it, people might start thinking about it more. It’s the beginning of a new stage of studying glasses. Maybe that’s a little bit grand but that’s what it is.”