Understanding slow compression of frictional granular particles by network analysis

We will discuss frictional granular packings exposed to quasi-static compression rates. For frictionless packings, earlier work (Soft Matter, vol. 18, 1868 (2022)) has uncovered that the system evolution/response involves smooth evolution phases, intercepted by fast transitions (events). The general finding is that the force networks' static properties correlate closely with the pressure, while their evolution resembles the kinetic energy of the packings. The former represents reversible (elastic) particle deformations with affine and non-affine components, whereas the latter also involves much stronger, irreversible (plastic) rearrangements of the packings. Events are associated with jumps in the overall kinetic energy as well as dramatic changes in the force networks describing particle interactions. The frictional nature of particle interactions affects both their frequency and the relevant time scale. Often, events are followed by an unexpected slow-down during which the kinetic energy drops below its average value. We find that these slow-downs are associated with a significant decrease in the non-affine dynamics of the particles, and are strongly influenced by friction. Friction also has a strong influence on both static and dynamic properties of the force networks. Friction modifies the structure of the networks, both through influencing the typical number of contacts of a particle, and by influencing topological features of the resulting networks. Furthermore, friction modifies the dynamics of the networks, with larger values of friction leading to slower evolution of the more stable networks.