A comparison of jamming behavior in systems composed of dimer- and ellipse-shaped particles†

We compare the structural and mechanical properties of static packings composed of frictionless convex (ellipses) and concave (rigid dimers) particles in two dimensions. We employ numerical simulations to generate static packings and measure the shear stress in response to applied simple shear strain as a function of the aspect ratio and amount of compression. We find that the behavior near jamming is significantly different for ellipses and dimers even though both shapes are roughly characterized by the aspect ratio and possess the same number of translational and rotational degrees of freedom per particle. For example, we find that ellipse packings are hypostatic (not isostatic as found for dimers), display novel power-law scaling of the static linear shear modulus and contact number with the amount of compression, and possess stress-strain relations that are qualitatively different from that for dimers. Thus, we observe that important macroscopic properties of static packings of anisotropic particles can depend on the microscale geometrical features of individual particles.