Granular materials, those made up of individual pieces, whether grains of sand or coffee beans or pebbles, are the most abundant form of solid matter on Earth. The way these materials move and react to external forces can determine when landslides or earthquakes happen, as well as more mundane events such as how cereal gets clogged coming out of the box. Yet, analyzing the way these flow events take place and what determines their outcomes has been a real challenge, and most research has been confined to two-dimensional experiments that don’t reveal the full picture of how these materials behave.

Now, researchers at MIT have developed a method that allows for detailed 3D experiments that can reveal exactly how forces are transmitted through granular materials, and how the shapes of the grains can dramatically change the outcomes. The new work may lead to better ways of understanding how landslides are triggered, as well as how to control the flow of granular materials in industrial processes. The findings are described in the journal PNAS in a paper by MIT professor of civil and environmental engineering Ruben Juanes and Wei Li SM ’14, PhD ’19, who is now on the faculty at Stony Brook University.1

Scientific understanding of these materials really began a few decades ago, Juanes explains, with the invention of a way to model their behavior using two-dimensional discs representing how forces are transmitted through a collection of particles. While this provided important new insights, it also faced severe limitations.

In previous work, Li developed a way of making three-dimensional particles through a squeeze-molding technique that produces plastic particles that are free of residual stresses and can be made in virtually any irregular shape. Now, in this latest research, he and Juanes have applied this method to reveal the internal stresses in a granular material as loads are applied, in a fully three-dimensional system that much more accurately represents real-world granular materials.

• 1. From soil and sand to flour and sugar, granular materials are ubiquitous. “It’s an everyday item, it’s part of our infrastructure,” says Li. “When we do space exploration, our space vehicles land on granular material. And the failure of granular media can be catastrophic, such as landslides. One major finding of this study is that we provide a microscopic explanation of why a pack of angular particles is stronger than a pack of spheres,” Li says. Juanes adds, “It is always important, at a fundamental level to understand the overall response of the material. And I can see that moving forward, this can provide a new way to make predictions of when a material will fail.”