Rods are less fragile than spheres: Structural relaxation in dense liquids composed of anisotropic particles

Abstract

We perform extensive molecular dynamics simulations of dense liquids composed of bidisperse dimer- and ellipse-shaped particles in 2D that interact via repulsive contact forces. We measure the structural relaxation times obtained from the long-time decay of the self-part of the intermediate scattering function for the translational and rotational degrees of freedom (DOF) as a function of packing fraction ϕ, temperature T, and aspect ratio α. We are able to collapse the ϕand T-dependent structural relaxation times for disks, and dimers and ellipses over a wide range of α, onto a universal scaling function {\cal F}_{\pm}(|ϕ-ϕ_0|,T,α), which is similar to that employed in previous studies of dense liquids composed of purely repulsive spherical particles in 3D. {\cal F_{\pm}} for both the translational and rotational DOF are characterized by the α-dependent scaling exponents μand δand packing fraction ϕ_0(α) that signals the crossover in the scaling form {\cal F}_{\pm} from hard-particle dynamics to super-Arrhenius behavior for each aspect ratio. We find that the fragility at ϕ_0, m(ϕ_0), decreases monotonically with increasing aspect ratio for both ellipses and dimers. Moreover, the results for the slow dynamics of dense liquids composed of dimer- and ellipse-shaped particles are qualitatively the same, despite the fact that zero-temperature static packings of dimers are isostatic, while static packings of ellipses are hypostatic.