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Rotation and ordering of elongated particles under shear
Ordering and alignment of elongated objects in shear flows can be observed
at all length scales, in log jams on rivers, in seeds, nanorods, viruses,
and even at molecular scales in nematic liquid crystals. We show,
that for dry granular materials in steady shear flow the time and
ensemble averaged direction of the main axis of the particles encloses a
small angle with the streamlines. This shear alignment angle is
independent of the applied shear rate, and it decreases with increasing
grain aspect ratio. The effective friction of the granular material is
considerably reduced due to the alignment. At the grain level the steady
state is characterized by a net rotation of the particles, as dictated by
the shear flow. We compare the dynamics observed in a dry granular
material to non-Brownian suspensions under shear.
