Event

Granular materials are inherently heterogeneous, and continuum models of properties such as the shear modulus and sound speed often fail. One promising alternative is to build an understanding of bulk behaviors from measurements at the particle scale, by analogy with the statistical mechanics of thermal systems. We observe that the amplitude of propagating sound is on average largest within particles experiencing the largest forces, due to the increased particle contact area. In addition, we find that the particle-scale vibration spectrum, driven by white noise, exhibits systematic changes with the degree of compression and disorder in the system. These measurements allow us to calculate a thermal-like density of states for the granular material. A second approach is to consider a network representation in which the nodes (particles) are connected by weighted edges obtained from contact forces (visualized using photoelastic materials). Such network representations provide a mathematical framework which allows for the comparison of features at different spatial scales, and to test which are the most descriptive of sound propagation.