  The alignment, ordering and rotation of elongated granular particles was
studied in shear flow. The time evolution of the orientation of a large number
of particles was monitored in laboratory experiments by particle tracking using
optical imaging and x-ray computed tomography. The experiments were
complemented by discrete element simulations. The particles develop an
orientational order. In the steady state 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. At the grain level
the steady state is characterized by a net rotation of the particles, as
dictated by the shear flow. The distribution of particle rotational velocities
was measured both in the steady state and also during the initial transients.
The average rotation speed of particles with their long axis perpendicular to
the shear alignment angle is larger, while shear aligned particles rotate
slower. The ratio of this fast/slow rotation increases with particle aspect
ratio. During the initial transient starting from an unaligned initial
condition, particles having an orientation just beyond the shear alignment
angle rotate opposite to the direction dictated by the shear flow.
