Collectively handled spheres self-organize: How friction and dynamics limit the jamming phase diagram

We show that the network of interparticle contacts in collectively handled jammed frictional and polydisperse sphere assemblies exhibits protocol-dependent self-organization. Here, by collective handling, we refer to boundary-controlled sphere assemblies where the creation of a jammed state from a stress-free state and the transition between jammed states are controlled only from the boundaries like in experiments and practical applications. We use the discrete element method and follow protocols similar to previous experiments to create 2d jammed states and quantify the self-organization of the network of interparticle contacts using the k-core decomposition method in graph analysis. The degree of self-organization depends on the method of preparation or protocol followed and affects the state attained in the jamming phase diagram. We explain the micromechanical origin and detail how the protocol followed can be tuned to limit self-organization. However, in contrast to algorithmic protocols where either the dynamics is not considered or particle features like size or interaction details like friction are tuned to attain dense states, we find that only a subregion of the phase diagram is attainable in collectively handled spheres and, remarkably, the jamming phase boundary of frictionless spheres marks the inaccessible region. We conjecture that the frictional interaction-dependent dynamics-driven formation of the 4-core subnetwork underlies the existence of the inaccessible region and propose the existence of a collective dynamical jamming region in the phase diagram of granular materials.