Geometrical parameters of binary granular mixtures with size ratio and volume fraction: experiments and DEM simulations

Binary mixtures represent the simplest case of polydisperse particulate systems which exhibit interesting and, in some cases, incomprehensive behavior. In this study, experimental and numerical investigations were conducted to examine the effect of particle size ratio and the ratio of volume fraction comprising small particles to the volume of all spheres (volume fraction) on geometrical properties of binary granular mixtures. The size ratio was chosen not smaller than 0.4 to prevent small particles from percolating through bedding and be trapped in the tetrahedron or octahedron made with large contacting spheres. Both, numerical tests and experiments showed an increase in the influence of the volume fraction of small particles on packing density in binary mixtures with an increasing ratio between small and large particles’ diameters. In packings with the particle size ratio not larger than 0.7, the solid fraction reached maximum when the volume fraction of small spheres was 0.6, which was not observed in samples with higher degree of particle size homogeneity. The average coordination number and packing density followed the same paths with the increasing contribution of small particles in mixtures, indicating a strong relationship between parameters. Detailed analysis of the coordination numbers for contacts between different types of particles showed that, average coordination number in binary mixtures was determined mainly by contacts between large and small particles. The composition of bidisperse samples was also found to strongly affect their spatial structure described in this study by means of the radial distribution function.