Movers And Shakers : New Evidence For A Unifying TheoryOf Granular Materials

Press release from Emory University:

By Carol Clark
Emory University | Jan. 6, 2020

Understanding the dynamics of granular materials — such as sand flowing through an hourglass or salt pouring through a shaker — is a major unsolved problem in physics. A new paper describes a pattern for how record-sized events affect the dynamics of a shaken granular material as it moves from an excited to a relaxed state, adding to the evidence that a unifying theory underlies this behavior.

The Proceedings of the National Academy of Sciences (PNAS) published the work by Stefan Boettcher, an Emory theoretical physicist, and Paula Gago, an expert in modeling the statistical mechanics of granular matter in the Department of Earth Science and Engineering at the Imperial College of London.

“Our work marks another small step forward to describing the behavior of granular materials in a uniform way,” says Boettcher, professor and chair of Emory’s Department of Physics.

“A complete understanding of granular materials could have a huge impact on a range of industries,” he adds. “To name just a few examples, it’s relevant to the compaction of granules into pellets to make pills, the processing of grains in agriculture and to predict behaviors of all kinds of geophysical matter involved in civil engineering.”

Granular materials are ubiquitous in everyday life, from gravel (above) to sesame seeds (below).

Granular materials are ubiquitous in everyday life, from gravel (above) to sesame seeds (below).

Granular materials are disordered systems often found in a far-from-equilibrium state. Examples include everything from sand, rice and coffee grounds to ball bearings.

“They are kind of the ‘odd balls’ of matter because they behave differently from solids, liquids and gases,” Boettcher says.

While the phases of water, for example, can easily be described as either a liquid, solid or gas, depending on specific temperatures, the thermodynamics of non-equilibrium systems are not well-defined. A major complication is the fact that individual particles in most granular materials have different, distinct properties and exert frictional forces on each other. And changes in temperature do not produce significant motion in them. Gravity further complicates the behavior of granular materials, since it affects the density of different layers in a system of particles.

In 1997, researchers developed a way to shake granular materials in a controllable way for a series of experiments on what is known as the “Chicago pile.” They filled a glass beaker with millimeter-sized glass beads and “tapped” the material upward with a specific amplitude. They were then able to measure the resulting density of the material in the beaker as a function of the strength of the taps, or the energy pulsing through the system.

Boettcher and Gago wanted to gain a molecular-level understanding of the compaction dynamics of a granular pile through analysis of computer simulations.

This press release was produced by Emory University. The views expressed here are the author’s own.