Identifying bridges from asymmetric load-bearing structures in tapped granular packings

The presence of gravity and friction results in the formation of cooperative bridge structures in granular systems, where neighboring particles support each other for mutual stability. These bridge structures serve as unique mechanical backbones in both static granular packings and dynamic granular flows, significantly influencing their mechanical responses and rheological behaviors. Using high-resolution x-ray tomography, we experimentally investigate the bridge structures in tapped granular packings composed of particles with varying friction coefficients. We find that gravity can induce subtle structural changes on the load-bearing contacts, allowing us to identify the correct load-bearing contacts based on structural information alone. Using these identified load-bearing contacts, we investigate the cooperative bridge structures which are mechanical backbones of the system. We characterize the geometric properties of these bridges and find that their cooperativity increases as the packing fraction decreases. The knowledge of bridges can enhance our understanding of the mechanical stability and rheological properties of granular materials, since bridges represent localized, mutually stabilizing configurations that bear load collectively.