Signatures of granular microstructure in dense shear flows

Abstract

Granular materials and ordinary fluids react differently to shear stresses. Rather than deforming uniformly, materials such as dry sand or cohesionless powders develop shear bands--narrow zones of large relative particle motion, with essentially rigid adjacent regions. Because shear bands mark areas of flow, material failure and energy dissipation, they are important in many industrial, civil engineering and geophysical processes. They are also relevant to lubricating fluids confined to ultrathin molecular layers. However, detailed three-dimensional information on motion within a shear band, including the degree of particle rotation and interparticle slip, is lacking. Similarly, very little is known about how the microstructure of individual grains affects movement in densely packed material. Here we combine magnetic resonance imaging, X-ray tomography and high-speed-video particle tracking to obtain the local steady-state particle velocity, rotation and packing density for shear flow in a three-dimensional Couette geometry. We find that key characteristics of the granular microstructure determine the shape of the velocity profile.

Dilatancy in semi-solid steels at high solid fraction

“…Note that the pushplate moves vertically upwards from the bottom right of each image. It can be seen that, as the pushplate intrudes into the sample, the space between the grains increases and fills with liquid, seen in lighter grey in Figure 2 The opening of spaces between the grains as the pushplate shears the sample is the phenomenon of shear-induced dilation or Reynold's dilatancy [27] which is widespread in particulate materials [28][29][30] and has been shown to occur in semi-solid alloys [13,31,32]. This is shown at higher magnification in Figure 3 The shear-induced dilation deformation mechanism can be understood from the group of four grains shown in Figure 4(a) (which is from the region highlighted in Figure 3 The lack of grain-grain cohesion indicates that most grains are separated by liquid films and not coalesced grain boundaries.…”

Dynamical fluctuations in dense granular flows

“…We note that these are different from sheared systems (Losert et al 2000;Mueth et al 2000) or vibrated systems (Knight et al 1997) in that the energy input to the system is not from a boundary. As a result, these flows are also relatively homogeneous.…”

Rheology of a confined granular material

“…Dynamical behavior of slowly driven granular materials was investigated recently by different groups both in compression and/or in shear experiments [3], [5], [8], [14], [15]. Here, we investigate the rheology of a granular assembly confined in a cylindrical column and pushed vertically from the bottom.…”

“…. Another source of difficulty is the dynamical contribution of contact forces and there is, so far, only few studies on the macroscopic emergence of these aging properties due to the slow plastic deformation of contacts and the influence of surrounding humidity [6], [11],[12], [13].Dynamical behavior of slowly driven granular materials was investigated recently by different groups both in compression and/or in shear experiments [3], [5], [8], [14], [15]. Here, we investigate the rheology of a granular assembly confined in a cylindrical column and pushed vertically from the bottom.…”