Isochoric ideality in jammed random packings of non-spherical granular matter

Randomly packed particles in granular matter, ubiquitous in nature and industry, usually defy simple predictions for the optimal amorphous packing density because jammed granules are strongly correlated. While mixing different granular shapes seems to be a further complication, we discovered in simulations that binary rod-sphere mixtures harbor a surprisingly simple dependence of packing volume fraction on mixture composition. This isochoric ideality covers the entire composition range and is experimentally validated by mixtures of sphero-cylindrical TicTac sweets and spherical beads: their joint random packing volume is indeed independent of the segregation state. Isochoric ideality occurs in a rod-shape range that includes the unique aspect ratio, which universally maximizes rod-sphere packing densities and suggests a novel amorphous analog of a plastic crystal, namely rod-sphere mixtures with completely uncorrelated particle orientations.