Research

Granular Materials

Granular materials consist of discrete macroscopic particles. The interactions between particles are solely due to classical interactions, such as normal contact forces and frictional forces between grains. Generally, when considering a real granular system there are also interactions with the container walls and gravity to consider. We are interested in the flows of real granular systems, and we study them with both experiment and simulation.

Granular Flow and Clogging

One system we study is the gravity-driven flow of grains through a opening. We study this partially out of pure interest, but this system is analogous to traffic flowing through a bottleneck, crowds exiting through a doorway, or blood cells moving through a constricted vessel. This geometry is typically called a silo or hopper. When the opening is smaller than some critical size, the likelihood of a clog grows astronomically. When the opening is above some critical size, the material will appear to flow freely. There is even an empirical law that describes the flow rate as a function of opening size. However, it is still not clear how this law comes about from the microscopic interactions of grains, or that the law is even that good. (For one, a common scaling argument that produces this law is actually unphysical, for two, the flow is not really continuous.) Further, it is thought that perhaps the clogging transition may be something like a phase transition, and we should seek evidence to support or refute this hypothesis.

By acquiring high-speed, high resolution videos of hopper flow we are able to understand how the individual particle motions and rearrangements create the bulk flow. For some experiments, we are able to extract the forces between particles by using photoelastic particles as our granular material. (In our simulations, we always have this information.) We are especially interested in collective measurements of motion, such as plasticity (i.e. non-affine motion) and cooperativity (i.e. dynamical heterogeneities). We also are studying the topology and evolution of the contact network during flow and clogging events.

Avalanche!

We also study flows of avalanching granular materials. When a granular material is tilted beyond its angle of repose, the system may avalanche. Alternatively, avalanches may occur intermittently when granular material is continuously added to a pile of grains. Our current experiment investigates dry granular avalanches of hollow glass spheres. Thus these spheres mimic sand in their contact interactions, but weigh less than solid spheres. This effectively reduces the force of gravity in the system, and so the material is an analog for sand on a reduced gravity planet. Indeed, some gully features on Mars that are often attributed to liquid flows may be under a false comparison to systems on Earth, which has higher gravity than Mars. It is possible that some of these features are actually due to dry granular flows.