An MRI study on the disentangling of a nest-like column of long rods under vertical shaking

Highly anisotropic granular particles can form very loose and stable piles with a large repose angle due to the geometric cohesion effect. The random geometric features of such structures and the origin of their stability against external mechanical perturbations remain to be explored. Here, using magnetic resonance imaging techniques, we perform an accurate structural reconstruction of columns built with long granular rods. We study the structural evolution during the gradual disentangling and collapse of a randomly poured free-standing pile of long rods under consecutive vertical shaking. The average height of the column decreases exponentially upon shaking, accompanied by a nonmonotonic evolution of the average packing fraction and an increasing negative nematic order of the rod orientations. Yet, the distributions of the relative position and orientation of contacting rod pairs remain roughly unchanged during the disentangling process. Distribution and spatial correlation between contact points as well as the evolution of contact network have also been analysed, providing insights into the micromechanics of nest-like structures.