Orientational Order and Alignment of Elongated Particles Induced by Shear

Abstract

Shear induced alignment of elongated particles is studied experimentally and numerically. We show that shear alignment of ensembles of macroscopic particles is comparable even on a quantitative level to simple molecular systems, despite the completely different types of particle interactions. We demonstrate that for dry elongated grains the preferred orientation forms a small angle with the streamlines, independent of shear rate across three decades. For a given particle shape, this angle decreases with increasing aspect ratio of the particles. The shear-induced alignment results in a considerable reduction of the effective friction of the granular material.

Particle alignment and clustering in sheared granular materials composed of platy particles

“…Due to dynamics in continuous shearing, this ordering phenomenon emerges even in systems composed of particles with very low platyness with a shape differing only slightly from spherical shape. This kind of ordering has previously been observed in experiments [27,22] and numerical simulations mainly with two-dimensional elongated particles [28,15,18]. Regarding the privileged direction of alignment, we found that, as platyness increases, this direction gradually changes from that of the major principal stress to a direction that is perpendicular to the shear direction.…”

“…This evolution of the privileged direction of alignment is consistent with several observations reported in Refs. [28,27,22].…”

“…elongated/prolate or flattened/oblate) have attracted increasing interest in recent years [22]. Oblate particles prefer facing along the major principal stress direction with the flat side, while prolate particles prefer orientation of their long axis along the minor principal stress direction [23][24][25][26][27][28]. It is also shown that, with increasing particle aspect ratio, the weak force network transforms from a passive stabilizing agent with respect to strong force chains to an active force-transmitting network for the whole system [18].…”

Granular materials composed of shape-anisotropic grains

“…There are several common methods used to characterize grain ensembles, irrespective of grain shapes. Modern experimental tools like X-ray Computed Tomography (X-ray CT) or Magnetic Resonance Imaging (MRI) are increasingly used for characterizing not only coarse grain packing [24][25][26], but also local packing structures and orientation at the grain level [27][28][29]. Grain sizes accessible with X-ray CT equipment range from several micrometers [30] to centimeters [28].…”

“…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.…”