Shear-induced alignment and dynamics of elongated granular particles

Abstract

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.

Granular materials composed of shape-anisotropic grains

“…[29] that the relations between orientational order of the rods and alignment angle are very similar to the values found in nematic liquid crystals (see e. g. [129][130][131][132]). Numerical simulations with DEM [29,128] are in good agreement with the experiments. Torque measurements of sheared elongated grains showed that the alignment of the particles reduces friction in the sheared zone.…”

“…Three-dimensional experiments require either the use of invasive observation methods (excavation) or tomographic tools. Extensive quantitative experimental characterizations of the reorientation of elongated particles under shear in a 3D geometry were recently performed by means of optical imaging and X-ray CT [28,29,128]. Grains with different aspect ratios and geometrical shapes were sheared in a cylindrical split-bottom container (Fig.…”

The behaviour of free-flowing granular intruders

“…In addition it is likely that θ avg for the intruder is increased because it is a singular entity within a mass of cylinders which are analogous to rods/teardrops with an aspect ratio of 1. If the angle of mixtures follows the previously observed trends of inverse proportionality between aspect ratio and θ avg [4][5][6] which was also observed within our tests, it would likely increase the value of θ avg .…”

“…The inset shows tests at four different macroscopic shear rates, with the same confining pressure, which shows the mean orientation's independence from the shear ratewhich has also been found in previous studies of masses of elongated particles [4][5][6]. The data presented in the results section was gathered at the highest shear rate shown here.…”

“…Many tests, both experimental and computational, have used spheres or cylinders due to the simplicity of those shapes [1] with glass beads often chosen for fundamental experiments. There have been studies conducted on systems entirely consisting of irregularly shaped or elongated particles [2][3][4][5], as well as the influence of mixtures of regular and elongated particles [6]. The behaviour of a single elongated free flowing intruder within a 2D layer of spheres has also been studied [7].…”