By Mary-Russell Roberson
Why is glass solid? No one knows, despite the fact that people have been making glass for 3,500-plus years.
It's a mystery, because glass has the molecular structure of a liquid. As Duke's Patrick Charbonneau, a professor in the chemistry and physics departments, likes to say, a wineglass is virtually indistinguishable from wine if you could look at them at the atomic level. Yet the glass holds its shape while the wine sloshes.
“How is it that something that looks like a liquid becomes solid enough that I can build with it?” asks Charbonneau. “This problem is reasonably simple to formulate, yet so hard to solve.”
Standard window glass is made by heating silica (sometimes in the form of quartz sand) with an alkaline material (often soda ash). As the molten glass cools, it hardens before its molecules have settled into the regular, repeating patterns characteristic of most solids. But glass as a class of materials includes many plastics, certain metallic alloys and more.
The disordered structure of the molecules makes it hard to study or simulate glasses. “We don't have an easy reference model,” Charbonneau says. “If you want to understand a crystal, I can draw a perfect crystal and then look at how small deviations from this arrangement lead to different properties of the crystal. But if you have something that is disordered, how do you know where to start? There's not a single way to draw this.”
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A deeper understanding of glass formation could shed light on other materials that have a disordered structure but are not liquid, from gels to bodily tissues. It could also open up the possibility of manipulating the properties of a glass by tweaking the formation process, with the goal of improving the performance of glass used for a specific purpose. For example, the performance of the LIGO observatory, which detects gravitational waves, is only as good as the quality of the glass mirrors used in the facility. In any case, the work could set the stage for a wide range of new applications of old materials, perhaps leading to as-yet-undreamed-of technologies.
To gain insight into glass formation, Charbonneau creates complex and lengthy computer simulations. (How complex? They take place in universes with up to 13 dimensions. How lengthy? They take up to a year to run.)
Even though Charbonneau hasn't cracked the glass problem, he's made progress. He has discovered commonalities between the disordered system of glass and the disordered system of jammed particles, such as sand grains stuck in an hourglass funnel. In both cases, once particles are packed tightly enough — whether those particles are sand grains or molecules — they stop moving.
Charbonneau says the model he is creating is an archetype that could be applied to other disordered systems. “The problem is generic enough that I don't need to be concerned with what the particle represents — it could be an atom or a molecule or a grain,” he says. “I'm just looking at something that does not crystallize. If I can understand this super simple system, I should be able to understand many more systems.”
Already, his glass and jamming simulations have informed Nobel-prize winning work. Charbonneau's long-time colleague, Giorgio Parisi of the Sapienza University of Rome, won the 2021 Nobel Prize in physics for his theories related to complex and disordered systems. Charbonneau's simulations demonstrated the robustness of key parts of Parisi's theories. In fact, two of Charbonneau's papers are cited in the scientific background of Parisi's work provided by the Nobel Prize committee.
But he's not resting on his laurels. Charbonneau still wants to be able to describe exactly what happens in the moment when glass transitions from liquid to solid. Is he confident he will one day be able to do so? “I know a pathway to get there,” he says, “but it's a hard one.”
Solving the millennia-old mystery of glass formation likely won't happen anytime soon, but that doesn't discourage Charbonneau. His motivation comes from the research process itself — continually refining the questions, the models, the simulations. “I find joy in that experience,” he says.