Macroscopic and microscopic behaviors of binary mixtures of different particle shapes and particle sizes

The packing density and initial modulus of binary mixtures were studied using the discrete element method (DEM). In addition, the behavior of particle contacts (microscopic behavior) was examined. The binary mixture contains two similar ellipsoids of different sizes. The particle size ratio (r), the ratio of particle sizes between a small particle and a large particle is either 0.1 or 0.2. The particle shape of these binary mixtures is described by the ratio between the major length and the minor length of an ellipsoid (AR = 1.2, 1.5, or 1.7). Very dense samples of different AR and fines contents were generated. In general, the study shows that r affects the initial modulus but not the relationship between void ratio and fines content. The behaviors of samples (AR = 1.5 and 1.7) are similar but different from those of AR = 1.2. The DEM results were fitted by three empirical models for binary mixture. These three models described the relationship between void ratio and fines contents very well. Degree of fitness is related to the number of parameters in the model. Although packing density is not affected by sample shape (cube versus cuboid), initial modulus is affected by sample shape. Microscopic investigation on particle contacts indicated that AR affects the number of contacts but not the distributions of contacts among large-to-large, small-to-small, and large-to-small particles. Greater initial modulus was found for samples of r = 0.1. When less than 20% of small particles were introduced to the large particles, initial modulus decreases significantly from the initial modulus of monodisperse samples.