Predicting How And When Materials Jam

Whether in the business of jamming granular materials into a solid (manufacturing pills, for instance) or keeping materials from solidifying (as in wastewater treatment), it's crucial to understand the processes of how particle systems make the transition from a jammed state to an unjammed one, and vice versa.

Shedding light on that is a new paper from the lab of Prof. Corey O'Hern, which describes a theoretical framework that can be used to predict the strain required to bring these materials into a jammed state (strain is the movement of the boundary that causes deformation of the material). It was published today in Physical Review E. Thibault Bertrand, a graduate student in O'Hern's lab, is the lead author.

The study looks specifically at two of the main protocols for jamming particle-based systems: isotropic compression, in which pressure is applied from all directions; and shear, in which forces are applied from opposite directions (similar to how a deck of cards is spread across a table).

Generally, it's been thought that each process generates an entirely different set of jammed packings with different structural and mechanical properties. O'Hern's study, however, shows that the sets of jammed packings for these two processes are actually the same. The protocol, however, determines the probability with which each jammed packing will be obtained, and thus the structural property you're mostly likely to find.

"There's this idea that these processes are fundamentally different and lead to different states of the system," O'Hern said. "It's true, but not in the way that people were thinking. The probabilities are different, which causes the average properties of the system to be different."

O'Hern noted that shear forces are often used in industry to keep particle-based systems flowing. But these same forces can also cause the systems to jam. To that end, the paper also describes a model for predicting the average amount of shear strain needed to jam a particle system.

For the study, O'Hern's lab developed computer simulations to model the motion of particulate systems as they evolve toward jammed packings. For simplicity, the particles in the simulations were idealized (spherical and frictionless). In a small system, the shear protocol can induce linear chain-like structures of particles that experience larger than average forces.

Larger systems of frictionless particles lose this effect, however, as they're more likely to slip - the packing structure then becomes more isotropic.

The good news is that in the real world, particles do have friction, so it's easier to build and maintain shear-induced force chains in these systems. O'Hern said this study will allow him and his research team to apply the same approach to predict the transition to the jammed state in frictional, non-spherical particles.