Origin of geometric cohesion in nonconvex granular materials: Interplay between interdigitation and rotational constraints enhancing frictional stability

We present a series of experiments investigating the local microstructure of cylindrical piles composed of highly concave particles. By systematically varying particle geometry-from spheres to strongly nonconvex polypods-as well as frictional properties and the number of branches, we explore how these parameters, together with the preparation protocol, shape the internal structure of the system. Using x-ray tomography combined with a dedicated image-analysis pipeline, we accurately extract the position, orientation, and contacts of every particle in each pile. This allows us to quantify the evolution of key structural observables as a function of particle geometry and preparation method. In particular, we measure the distributions of local packing fraction, coordination number, number of neighbors, and contact locations, along with particle-particle positional and orientational correlations. More importantly, we construct a new stability indicator that correlates perfectly with the observed pile stabilities, enabling us to identify the fundamental mechanisms responsible for geometrically induced cohesion in granular systems composed of noninterlocking particle shapes: interdigitation, rotational constraint, friction-mediated cohesion, and the ability of a pile to restabilize.