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Abstract
We study force propagation and fluctuations in two dimensional (2D) static and quasi-static granular systems composed of photoelastic particles, with a particular focus on the role of stress chains in such systems. We explore these issues through two sets of experiments.
The first set of experiments is focused on static systems. In the 2D heap experiments, we examine the internal stress and structure characteristics of heaps formed from three different deposition procedures, two localized and one extended. We find that the history of the heap formation has a dramatic effect on the arrangement of particles (texture) and a weaker but clear effect on the forces within the sample. In the Green's function experiments, we explore a fundamental issue concerning how granular materials respond to applied forces. We experimentally obtain the ensemble averaged responses of point forces under a variety of conditions. We find that a range of important factors, including disorder, packing structure, friction and texture, can significantly affect force transmission in granular systems. On comparison to models, the responses of strongly disordered system are best described by elastic models, while our current data on weakly disordered systems does not distinguish between hyperbolic models and anisotropic elasticity models.
The second set of experiments is focused on quasi-static granular systems. In the first part of this set, we experimentally extract effective temperatures by measuring the diffusivity and mobility separately in a “stirred” granular system. Our experimental results confirm Brownian diffusion. However, we find that mobility depends on the tracer velocity. And the resulting effective temperatures are dependent on particle size. In the second part we investigate, through both experiments and simulations, the granular drag force and the nature of its fluctuations. We find that the mean drag force is a function of the drag velocity, packing fraction and the tracer particle size, and that fluctuations seem to be associated with the formation and breaking of force chains in the bulk of the system. Experimental observations on the mean drag force, the power spectra, the avalanches, and the “force chain force constant” can be reproduced qualitatively using a modified failure model.
