Dr. Sidney R. Nagel of the University of Chicago Physics Department Physics Department is awarded the APS Medal for Exceptional Achievement in Research from astrophysicist Dr. Robert Rosner, also of the U. of C., at the American Physical Society's leadership meeting in Washington, Jan. 26

Dr. Sidney Nagel, a University of Chicago physicist and Kenwood resident, has won the 2023 American Physical Society Medal for Exceptional Achievement in Research for his work in soft matter physics.

"All we really know about in physics well is what the properties of a system that's in thermal equilibrium are," he said. "If you took a box of gas and you let it sit there for awhile, after awhile I could tell you everything that you want to know about the box and then some. I could bore you stiff with all the properties. Why? Because it's in thermal equilibrium."

But few things are in thermal equilibrium. The atmosphere, with its fluctuations of weather and climate, isn't. People who are in equilibrium would be dead.

A glass of water or a solid is in equilibrium; external contact will not change its density. But Nagel illustrated his work via a cylinder filled with sand with a funnel at the bottom; when he struck it, the sand flowed through the funnel.

"Here is something that is so trivial: a sandpile. Yet it's behaving in ways that are so counterintuitive to all that we've learned about how to treat solids, because solids and liquids are typically in equilibrium, and here is a system which is blatantly out of equilibrium. And because of that, you don't even really know what the tools are that you need to describe the behavior," he said.

Nagel produced some mustard seeds and asked how it would flow. When the surface they're on is tilted, they flow like a liquid, but they can jam a funnel, which liquids wouldn't do. But if the funnel is shaken, they would flow again.

"Much of our research is not geared particularly towards applications. But as with so many things in science, if you find something that's interesting, it turns out that it does have applications of some kind," he said. "Every industry that uses powders, and factories of granules and powders, you have jamming all the time. You're trying to get powder from one side of the factory floor to another, and what do you see? Coming in, you see a lot of dents. Why? Because it got stuck in there, and someone took a hammer and dashed it, trying to get it to flow again."

He has also researched splashes — in terms of how liquid droplets splash in different amounts of air, for instance — which has applications to pharmaceuticals, wherein it's important that things do not contaminate each other.

Nagel has also researched why dried liquid stains, like coffee stains, have a darker ring at their perimeter — away from the point of impact at the center of the stain. That has had applications to medical testing, like testing a drop of blood from someone with diabetes.

"First you have to recognize this problem, then you have to understand the physics behind it, and only then can you control for it," he said.

Nagel thanked his students for their work during experiments and the university for its support.

"It wasn't what people particularly thought was important to physics, but yet the university, my physics department and institute really supported me in doing this," he said. "I don't think many universities would have felt this way about that kind of stuff. They would have dismissed it as not important, not relevant research."

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