Geometric Cohesive Granular Hexapods

A heap of dry sand collapses after being released from a bucket, because cohesion is necessary to maintain stability for convex particles. However, non-cohesive, elongated or non-convex granular `building blocks’ can form stable structures. Particles such as rods and staples are known to be geometrically cohesive, demonstrating that microscopic cohesive at contacts is not a necessary condition for effective macroscopic cohesion. Here, we show that non-cohesive and inherently non-convex hexapods have macroscopic cohesion. The hexapods are plastic particles that consist of three orthogonal sphero-cylinders. The cross sectional diameter of the sphero-cylinders is 3 mm and the length varies from 10 to 60 mm. We pack particles inside a pair of tubes and perform direct shear tests by displacing the cylinders. Data for the shear strength vs. pressure following the classical Coulomb relation yields the internal friction from the slope and the cohesion from the intercept. We determine the internal friction and cohesion as a function of hexapod geometry. We also perform shear tests inside an X-ray CT scanner. These studies yield particle centers and orientations as well as contacts.