Impact of Particle Roughness on Packing

The quest to the control packing fraction has a long pedigree, from early insights into dense sphere packings and their modern proofs to contemporary efforts that extend beyond spheres to frictional, anisotropic, and non convex grains. However, a unifying picture that ties particle roughness across scales to macroscopic packing remains incomplete. We address this gap via Discrete Element Method simulations with a multi sphere representation to quantify particle roughness at three scales: microscopic (surface friction), mesoscopic (furrow depth set by the number of spheres used to approximate a rod), and macroscopic (overall shape: rod, cross and star). After gravity deposition, we measure the pile height, packing fraction, coordination number, and force networks. The results clearly show scale separation. Reducing mesoscopic roughness promotes sliding and compaction, reducing pile height following a linear trend while leaving the coordination numbers and force distributions essentially unchanged. In contrast, macroscopic roughness controls pile architecture: progressing from rods to crosses to stars increases the pile height and alters the coordination numbers and the tails of force distributions. We propose a compact semi empirical framework that links the packing fraction to the three roughness scales.