Acoustic Probes of Granular States

****Technical Abstract****

It is difficult to assess the stability of a granular material, or to determine whether failure is imminent. The ability to non-invasively characterize changes in the mechanical state of a granular material would aid our understanding of the transition to failure. In both ordinary atomic/molecular systems and idealized jammed systems, the density of states provides a wealth of information about the state of the system. Acoustic measurements are a promising route to a similar characterization for granular materials, due to their ability to transmit vibrational energy into the bulk of the material, and to gather information in return. We will develop such techniques both in static and sheared systems where internal stresses are visible, as well as in more realistic three-dimensional materials which are of natural and industrial importance. Objectives are: (1) Simulations and analytics predict that an increased abundance of low-frequency modes is associated with an impending loss of rigidity. Do real granular materials exhibit this feature as a universal hallmark of incipient failure, e.g. under shear? (2) Simulated jammed materials with different shapes (circles vs. ellipses vs. dimers) each have a characteristic density of states. Do real granular materials, for instance those with corners, exhibit similar shape-dependent features? (3) The properties of force and contact networks may depend on the dimensionality of the system. Results from shear experiments may improve our understanding of earthquake nucleation and rupture. Connections to geophysics are possible by analyzing results in light of current ideas about foreshocks, tremors, triggering, and monitoring fault damage.

****Non-Technical Abstract****

Many energy-related industries rely on granular materials: extraction from oil sands, the handling of pellets within manufacturing facilities, the stability of rocks surrounding oil, gas, or carbon dioxide sequestration reservoirs, or flows within fluidized-bed reactors. It is difficult to assess the stability of a granular system: is failure imminent? what is the most stable direction to load the pile to avoid failure? The ability to non-invasively characterize changes in the mechanical state of a granular material would provide a means to evaluate such questions. Physicists have built a quantitative understanding of the mechanical stability of a broad class of materials such as foams, emulsions, and granular materials. Despite a decade of measurements within computer simulations, experimental progress has been slowed by the difficulty of making measurements in the interior of an opaque granular material. Acoustic techniques are a promising route due to their ability to penetrate the interior of opaque materials. We will develop new acoustical measurements which will open avenues for non-destructive testing of granular materials, research which will be conducted by undergraduate and graduate students. These new techniques will allow us to seek acoustical signatures of how close a system is to slipping, the direction of largest applied forces, or the degree of particle alignment/disorder. Beyond applications in the energy industry, the new techniques may be able to improve our understanding of how the granular material between tectonic plates becomes unstable and triggers earthquakes. The research team will also develop a related set of hands-on activities for use with local Girl Scouts troops.