Jam Session

Physicist Andrea Liu discusses research into how systems of particles transition to jammed states.
June 2009

In collaboration with her colleagues in the physics department and researchers at the University of Chicago, Andrea Liu is trying to understand how certain systems of particles come to behave like solids when they jam.

“The question we’ve been asking,” explains Liu, “is how do things that get stuck act like solids? That is, how do they have the mechanical properties of solids when their structure is disordered? And can we think of them in a common framework? Can we unify all of these different systems that jam?”

Liu and her colleagues took an important step closer to answering these questions with a recently completed study—published in Nature—that found the first experimental evidence of a vestige of the zero-temperature jamming transition in a system of particles where thermal energy is important. The jamming transition, explains Liu, is what takes place when particles become so densely packed that they transition from large loose objects, such as grains of sand, to rigid solids such as sand dunes.

“Imagine you put a pile of sand on a table,” says Liu. “It would stay there; it would hold its shape—it’s acting like a solid. But if you started jiggling the table, it would flow like a liquid and spread around. So at high shaking amplitude, it behaves like a liquid. But if you decrease the amplitude of shaking, it will eventually jam up into this disordered arrangement of sand grains that is behaving like a solid again.”

“The question we’ve been asking is how do things that get stuck act like solids? That is, how do they have the mechanical properties of solids when their structure is disordered?” - Andrea Liu

Systems of large particles like sand grains can be considered zero-temperature systems, explains Liu, because the energy associated with a typical temperature is negligible compared to what would be required to shift the particles. “Thermal energy is not going to cause jiggling of sand grains, because they’re just too big,” she says. “The energy supplied by room temperature is tiny compared to what it takes to lift a grain up and move it around, so effectively it’s zero temperature. But what about small things, like the micron-sized particles in paint, or molecules in a liquid? There thermal jiggling is really important.”

Seeking to discover whether the zero-temperature jamming transition shows up in a system that is not at zero temperature, Liu and her colleagues combined mathematical computer simulations with an experiment run by James M. Skinner Professor of Science Arjun Yodh.

Their analysis focused on the separation between neighboring spheres in a system—in particular, on how this separation evolved as the system became jammed. By confining soft microspheres that changed size relative to changes in the temperature of the system—and that were small enough so that thermal motion is important—they were able to study how separation distance changed as the volume occupied by the spheres varied through the jamming transition. What they discovered was a vestige, at non-zero temperature, of one of the important structural signatures that arise at the zero-temperature jamming transition.

“It was a very clever system,” says Liu. “By changing the temperature just a little bit, they were able to change the size of these particles a lot, and that’s a very nice knob to turn to tune it through this transition. And because the particles were fairly large—between molecules and big things which we know are at zero temperature—you could see them under a microscope.”

Liu, Yodh and the study’s other authors will continue to work collaboratively as they look for connections between the zero-temperature jamming transition and the glass transition.