Structure of plastically compacting granular packings

In this paper we present results of structural studies of compacting experimental systems of ductile grains in two and three dimensions. The high precision of our two-dimensional experiments enables a detailed study of the evolution of coordination numbers and local crystalline arrangements as a function of the packing fraction. The structure in both dimensions deviates considerably from that of hard disks and spheres, although geometrically, crystalline arrangements dominate on a local scale (in two dimensions). In three dimensions, the evolution of the coordination number is compared to experimental packings of hard and ductile grains from the literature. This comparison shows that the evolution of coordination number with packing fraction is not unique for ductile systems in general, but must depend on rheology and grain size.