By Carol Clark
Emory physics professor Justin Burton received a $625,000 award from the National Science Foundation’s Faculty Early Career Development (CAREER) Program. The five-year CAREER grants, among the NSF’s most prestigious awards, support scientists who exemplify the role of teacher-scholars through outstanding research integrated with excellence in education.
Burton will apply the award to his research into amorphous matter, or substances made up of granules in jumbled, irregular states. These substances include everything from the foam on your cup of cappuccino to the vast, slushy mélange of a glacier as it breaks down and flows into the sea. Amorphous matter also encompasses soft condensed matter such as toothpaste, shaving cream, plastic and glass, which are collectively known as “glassy” materials.
“Amorphous material is everywhere, it’s among the most common states of solid matter,” Burton says, “and yet, there’s a lot that we don’t understand about it.”
Crystalline material, by contrast, is relatively rare but well understood by physicists. Crystals have a structural order that makes them easier to conceptualize and define mathematically. “Research into the thermodynamic behavior of crystals at ultra-low temperatures led to our understanding of how they conduct heat,” Burton says. “That’s one of the fundamental triumphs of quantum mechanics. It helped lay the foundation for a lot of important tools of the modern world, from computers to cell phones.”
Lacking the well-defined order of crystals, amorphous materials often behave in peculiar, unpredictable ways. Burton uses the example of a pile of sand at the bottom of an hourglass. “What seems stable enough can suddenly avalanche upon the addition of a few extra grains,” he says. “Or even a traffic jam: What determines the boundary between a flowing state and a rigid one? Our world is full of similar examples where systems exist in a region near marginal stability.”
Burton’s lab is creating model systems to simulate the dynamics of the microscopic granules of amorphous, glassy matter at ultra-low temperatures of below 1 degree Kelvin. That’s colder than the deepest reaches of space.
In a vacuum chamber, filled with argon gas, the lab conducts experiments. The chamber is filled with ionized argon gas. “It’s a plasma, or a gas that has had its electrons ripped away from its atoms,” Burton explains. “The electrons are constantly being ripped away and resembling.”
Colloidal particles, tiny as dust specks, are suspended in the plasma of the vacuum chamber, to stand-in for the molecules of an amorphous material. By altering the gas pressure inside the chamber, and varying the size of the particles, the lab members can study how the particles behave as they move between an excited, free-flowing state into a jammed, stable position.
They can also simulate how molecules in a stable position react to a disturbance. “We want to create a wave, like dropping a pebble into a still pond to make ripples, and study that dynamic,” Burton says. “That could help us understand, for instance, how sound moves through a glassy material.”
Burton’s lab will use another model, involving polymer hydrogel particles that expand or shrink in response to salt concentrations, to study Casimir forces, a special type of long-ranged force that can arise between objects in a highly fluctuating medium.
In addition to opening a window into the molecular motions common in glasses, the research could shed light on the connection between the dynamics and disorder in a broad range of physical systems, Burton says.
In parallel to his research effort, the CAREER award will also fund the creation of an after-school science club at an elementary school in Dekalb County. Burton and his graduate students will lead children in hands-on activities and experiments that give insights into basic principles of physics.
Related:
The physics of falling icebergs
Physicists crack another piece of the glass puzzle
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