Local Mechanical Properties of Granular Media

Granular materials consist of athermal conglomerates of macroscopic particles which interact via dissipative short-range potentials. The global behaviour of large granular systems away from a transition can be described quite well by conventional continuum mechanics. For smaller system sizes, unusual properties appear which originate from the macroscopic scales of the constituent particles. In this thesis, we will investigate the micromechanical response of granular media in different physical conditions. We use techniques to track the structural and mechanical properties of the packings on the grain scale. The dynamics of granular media strongly depends on the external driving conditions. We built setups to expose granular particles to different states of aggregation. We probe the micromechanical response of the systems to external stresses. In a first experiment, a macroscopic stress field is applied to a granular solid through isotropic compression. We evaluate the micromechanic response by refining current techniques of stress-birefringence measurements. This provides access to the distribution of contact forces in the system. We also analyse the linked distribution of local moduli in a coarse-graining approach which relates local structural and mechanical properties of the material. For crystalline structures we find a narrow distribution of moduli. In contrast, amorphous media shows heterogeneous mechanical properties on microscopic scale. The distribution becomes more homogeneous for higher compression states. As we vary mixture compositions we can attribute the level of heterogeneity to the amount of disorder in the packing. In a second experiment, we expose granular particles to strong external driving which induces fluidlike dynamics. We probe the micromechanic response by a local pertubation with an intruder. We measure the viscosity of the system by tracking the intruder’s motion. We demonstrate access to three viscosity regimes: linear response, shear thinning and shear thickening; depending on the shear rate. We attribute the effect of shear thinning to a decrease of entropic forces from the bath when the intruder is fast enough to break cages. Shear thickening appears due to the formation of particle clusters in front of the intruder for very high velocities. In a third experiment, granular particles show gaseous dynamics when exposing them to a zero gravity environment. The long time cooling behaviour is in accordance with Haff’s law.