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When a granular material consisting of macroscopic elongated grains is exposed to shear, the individual grains align. We determine the particle distribution functions and orientational order parameters and study the collective dynamics as well as individual particle motion during shearing. X-ray computed tomography (CT) is used to obtain three-dimensional images of the shear zone. All individual particle positions and orientations are extracted by image processing software and the complete order tensor is determined. We compare the behavior of our ensembles of macroscopic grains with well-known continuum models for shear alignment and director dynamics of anisotropic liquids. Irrespective of the completely different particle interactions and size scales, analogies are found even on a quantitative level. Measurements of the local packing densities inside and outside the shear zone reveal a shear dilatancy, irrespective of the more efficient packing that can be expected for ordered ensembles of cylinders compared to randomly oriented samples.
Wegner et al. (Sun,) studied this question.