Shear alignment and orientational order of macroscopic rodlike grains

This contribution establishes a link between very different soft matter systems that exhibit orientational order. Granular matter research has been focused so far mainly on ensembles of spherical or irregularly shaped grains. Recently, several studies of anisometric particles have been reported, but still, many phenomena in such materials are little understood. Quantitative experiments are scarce. We report shear experiments with macroscopic shape-anisotropic particles [Borzsonyi et al., Phys. Rev. Lett., 108, 228302 (2012)] and discuss induced orientational order and alignment. Optical observations of the top layer are accompanied by X-ray computed tomography [Wegner et al., Soft Matter, in press (2012)], where positions and orientations of each individual grain in the bulk can be resolved. We establish quantitative relations between shear alignment and aspect ratio. The induced orientational order influences local packing and other macroscopic properties like the shear resistance. A comparison is drawn with molecular liquid crystals (LC). Many observations are qualitatively and even quantitatively comparable to the well-understood nematic phase of rodlike molecules, even though the types of interactions are completely different.