Simulation of random packing of binary sphere mixtures by mechanical contraction

The mechanical contraction simulation for random dense packings is extended to binary mixture of spheres. The volume packing density as a function of sphere composition follows a characteristic triangular shape and resembles previous experiments on length scales from colloidal particles to metal shots. An excluded volume argument, which qualitatively explains trends in random packing densities of monodisperse particles, is insufficient to account for this triangular shape. The coordination number, or the average number of contacts on a sphere, shows a remarkable dip from 6 to 4 at the crossover from many small spheres to many large spheres, which has not been reported earlier. An explanation is given in terms of caging effects.