Shear-induced pressure anisotropy in granular materials of nonspherical particles

When a granular material composed of elongated grains is sheared in a split-bottom shear cell, a pressure difference develops within the material. This pressure difference depends on the interparticle friction ($\mu$), which affects shear localization and particle alignment. For large $\mu$, alignment is confined to a narrow shear band, leading to localized increases in packing density and pressure. For small $\mu$, particles align over a wider region, leading to a nearly uniform packing density and pressure throughout the material. In contrast, spherical particles, regardless of $\mu$, maintain a uniform packing density and pressure throughout the material. We observe a phenomenological similarity to the Weissenberg effect in non-Newtonian fluids, where normal stress differences induce radial pressure gradients, unlike the uniform pressure in Newtonian fluids.