The Physics of Everyday Things: Two Swarthmore College Scientists Awarded NSF Grant
Swarthmore College physics professors Cacey Bester (right) and Amy Graves received an NSF grant to study granular materials. Here they are posing with everyday objects that can flow and jam. Photo: David H. Cohen
Professor Cacey Bester and her students made this video showing how force propagates through grains (in this case polymer disks) when they are compressed.
In the basement of the Science Center at Swarthmore College, directly underneath Eldridge Commons with its high ceilings and enormous windows, Cacey Bester is trying to figure out what happens when things flow. The physicist is not looking at water or other liquids, as you might expect, but at small solid pieces: grains.
“Could be grains of sand,” Bester says. “Or grains of rice.”
To study the phenomenon, though, Bester uses neither sand nor rice, but special grains manufactured out of soft plastic. These make it easier to study how granular material flows, as well as what happens when the grains jam.
In the world of soft-matter physics, “flow” and “jam” are technical terms.
Bester, who is in her first year as an assistant professor in Swarthmore’s Department of Physics and Astronomy, is an experimentalist who builds apparatus to help her answer questions. For this work on granular materials, she has teamed up with Amy Graves, a senior member of the physics department. Graves, a theorist, runs computer simulations of experiments that would mimic what one could find in the world, or guide what one could eventually build. The two of them — together with two physicists, Brian Utter and Katharina Vollmayr-Lee, at Bucknell University in Lewisburg, Pennsylvania — were recently awarded a grant from the National Science Foundation (NSF) to study properties of granular materials. The three-year grant of approximately $600,000 will fund not only salaries for students who are working with the professors, but also equipment, travel to meetings, and more.
Right now, though, the lab is largely empty. At one end, dark, floor-to-ceiling curtains cordon off a corner. “This is where the experiment will go,” Bester says.
“It’s gorgeous,” Graves says.
Bester laughs. “It’s just an empty corner.”
“That’s where everything starts,” Graves says.
Beginnings
Bester remembers the first time she learned about flow and jamming. She was a college student from Louisiana doing a summer research project at the University of Chicago. On the first day, the physicist who was to be her advisor, Professor Sidney Nagel, showed her a video of granular material in motion.
“I thought, this seems like an almost silly thing to study,” Bester remembers. “Why would anyone care about grains?” She had hoped to get involved in particle or astrophysics research that summer, but instead she found herself immersed in the science of soft matter. Before long, she was hooked. “I liked the challenge that came with something that seemed so simple.” She would go on to do her Ph.D. work on a related problem in the field.
Grains turn out to be both important and not very well understood. Their behavior is significant in contexts from geology to industry to medicine. For example, when will ashy land devastated by fire give way in a mudslide, and when will it hold fast? To answer this question, you have to understand granular materials. As Graves explains, “The characteristics of the individual grains…are very important in [predicting] whether there’s going to be a catastrophe.”
Experiencing Flow
Graves loves the way soft-matter science is elegant physics (“It can be very mathy”) while also being rooted in the everyday. “If people have ever walked on the beach,” she says — “or had a sock crawl into a comforter that’s tumbling in the drier” — they have experienced flow. The phenomenon of jamming is also all around us, for instance when gravel is poured into trenches like the one Graves stepped over earlier in the day to get to the college’s newest building, Singer Hall.
“And I’m not even touching on biology,” Graves adds.
Biology turns out to be full of materials flowing and jamming. After coughing spasms, for example, cells resettle themselves in airways by flow. Scientists have learned that this post-cough resettling happens differently in asthmatics than it does in people whose lung function is normal. Flow also governs the way cells cluster in cancer tumors. “What we do is very cross-disciplinary,” Graves says.
One way to study flow and jamming is to put grains under pressure and see what happens. To demonstrate, Bester takes an object shaped somewhat like a picture frame from a nearby table. Bead-sized polymer grains, held between two sheets of clear plastic like ants in an ant farm, are designed to light up when they experience stress. Slowly she pushes a plastic plate down between the sheets, compressing the beads, demonstrating what happens when the granular material is compacted. “Do you see how some of the grains are looking brighter than others as I compress them into a smaller area? The brightness of the grain gives us information about the amount of stress that [it is] experiencing.”
Fruitful Questions
NSF funding is not easy to obtain, and most grants go to large universities. Swarthmore College has done well, with five currently active grants of a type which fund research at undergraduate institutions (RUI). “We have this awesome team,” Graves says. “Four physicists, two institutions, a bunch of students.” She notes that three of the four primary investigators on the project are women: “Which is upside down from physics statistics.” Approximately 80% of practicing physicists are men, she says. “Also, in terms of Professor Bester being a woman of color: there have been about 100 black American women who’ve gotten Ph.D.’s in physics — ever.”
Some of the behavior of granular materials is understood, but there are new questions no one has answered yet. Bester’s experiments and Graves’s computer primarily involve putting obstacles in the way of grains to see what changes. “Does that help the grains to flow more rapidly?” Bester says. “Or does it cause the grains to clog?”
This, Graves explains, is what scientists do. They look for the place current knowledge ends and take the next step into the unknown. “You try to find out where the exciting questions that have been answered stop,” she says, “and then what new exciting question you can ask.” Part of what she and Bester hope to convey to their students is not just the specifics of the work itself, but how to recognize which questions are likely to be fruitful to pursue.
Both Graves and Bester have selected students to help with the research this summer. Bester’s will help her build the first iteration of a new experiment. “One thing you learn as an experimentalist is that it’s very unlikely that the first iteration will work perfectly,” she says. Graves’ students will also be “building” — at least virtually — using NSF’s “Extreme Science and Engineering Discovery Environment” of supercomputers and visualization tools.
The group at Swarthmore and the group at Bucknell will collaborate via email and teleconference, as well as probably doing some visiting back and forth. “[The collaboration] will be polymorphous,” Graves predicts. “Like the stuff we’re studying.”
