Disordered packings of binary mixtures of dimer particles

Disordered packings of non-spherical particles and their mixtures are abundant in nature, but have so far attracted only few systematic studies. Previous investigations of binary mixtures of specific convex shapes have established two generic properties: (i) the existence of a unique density maximum when shape or mixture composition of the two species are varied; (ii) the validity of an ideal mixing law indicating that the packing density is independent of the segregation state. These findings were so far only observed for mixtures of convex particles such as spherocylinders, ellipsoids, and spheres. Here, we investigate the packing properties of binary mixtures of frictionless dimer particles simulated by a gravitational pouring protocol in LAMMPS. Our results demonstrate the validity of (i, ii) also for such packings of non-convex particles. Moreover, we investigate the contact statistics of these packings to elucidate the microstructural features that underlie (i, ii). Our results show that the contact number per species also satisfies a simple mixing law and that similar microscopic rearrangements of contacts as in monodisperse dimer packings accompany the formation of the density peak in binary mixtures largely independent of the mixture composition.