Flow and Packing of Dense Granular Materials

Packings of frictional, bidisperse spheres can attain packing fractions larger than the values attributed to the corresponding random loose and close packing limits: nominally identified over the range, 0.55 – 0.64 in packing fraction. It is also known that excitation of a given packing, such as through tapping, can lead to further compaction. Here, we explore just how high can the packing fraction for frictional, bidisperse packings go, using large-scale, pressure-controlled discrete element simulations. Our binary sphere packings span a wide range in particle size ratio, up to 40:1, over several orders of magnitude in particle friction coefficient. Starting from the densest stable state, these packings are subject to small amplitude and low frequency oscillations, then allowed to slowly evolve over time (of the order of 10⁷ simulation timesteps). We compare and contrast the structural evolution of the different packings in an effort to identify common behavior during the vibration process.