Alignment and dynamics of elongated cylinders under shear

Abstract

When a granular material consisting of macroscopic elongated grains is exposed to shear, the individual grains align. We determine the particle distribution functions and orientational order parameters and study the collective dynamics as well as individual particle motion during shearing. X-ray computed tomography (CT) is used to obtain three-dimensional images of the shear zone. All individual particle positions and orientations are extracted by image processing software and the complete order tensor is determined. We compare the behavior of our ensembles of macroscopic grains with well-known continuum models for shear alignment and director dynamics of anisotropic liquids. Irrespective of the completely different particle interactions and size scales, analogies are found even on a quantitative level. Measurements of the local packing densities inside and outside the shear zone reveal a shear dilatancy, irrespective of the more efficient packing that can be expected for ordered ensembles of cylinders compared to randomly oriented samples.

The internal structure of suspensions in uniaxial elongation

“…A similar agreement in shear flow was recently reported in Ref. 23. A small mismatch of the prediction and the data can be attributed to the thread non-uniformity along the x-axis, and a possible effect of the restriction imposed by a distant free surface on the particle rotation, albeit the latter is a minor effect.…”

“…X-ray computed tomography was used to measure orientation of rods with aspect ratios in the range 2-5 in concentrated suspensions undergoing shear flow. 23 In relation to the uniaxial elongational flows arising in electrospinning of polymer nanofibers, WAXD, wide-angle X-ray scattering (WAXS) and small-angle scattering were used to characterize the axial orientation of polymer crystals and the embedded CNTs. [24][25][26][27][28] In a more general context, note that several other techniques which were used to study the internal structure of flowing suspensions of mostly spherical particles are reviewed in Ref.…”

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.…”

Shear alignment and orientational order of macroscopic rodlike grains

“…A technique that can achieve this and provide the complete 3D arrangement of the grains is X-ray CT [13]. Tomographic data can be analyzed separately at the surface, in the bulk far from container boundaries, and near container walls.…”