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Interrogating a granular material : force networks, acoustics, and state variables

Karen Daniels

Mercredi 9 mai 2012

à 11h en salle C. Brot

Granular materials are inherently heterogeneous, and continuum models of properties such as the shear modulus and sound speed often fail. A promising alternative is to build an understanding of bulk behaviors from measurements at the particle scale, perhaps by analogy with the statistical mechanics of thermal systems. I will describe acoustics experiments in a photoelastic granular material, in which the internal stress state of the system takes the form of force chains. We utilize two complementary techniques, high-speed imaging and piezoelectric transduction, to provide spatiotemporal measurements of the dynamics of compressional waves. We observe that the sound wave amplitude is on average largest within particles experiencing the largest forces, particularly in those chains radiating away from the source. The force-dependence of this amplitude in qualitative agreement with a simple nonlinear (Hertzian-like) model of particle contact area. In addition, we find that the particle-scale vibration spectrum, driven by white noise, scales with the degree of compression in the system. These particle-scale acoustics measurements allow us to calculate a thermal-like density of states for the granular material.