Structural relaxation in dense liquids composed of anisotropic particles

We perform extensive molecular dynamics simulations of dense liquids composed of bidisperse dimer- and ellipse-shaped particles in two dimensions that interact via purely repulsive contact forces. We measure the structural relaxation times obtained from the long-time a 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 phi, temperature T, and aspect ratio alpha. We are able to collapse the packing-fraction and temperature-dependent structural relaxation times for disks, and dimers and ellipses over a wide range of a, onto a universal scaling function F-+/-(vertical bar phi-phi(0)vertical bar, T, alpha), which is similar to that employed in previous studies of dense liquids composed of purely repulsive spherical particles in three dimensions. F-+/- for both the translational and rotational DOF are characterized by the alpha-dependent scaling exponents mu and delta and packing fraction phi(0)(alpha) that signals the crossover in the scaling form F-+/- from hard-particle dynamics to super-Arrhenius behavior for each aspect ratio. We find that the fragility of structural relaxation at phi(0), m(phi(0)), decreases monotonically with increasing aspect ratio for both ellipses and dimers. For alpha > alpha(p), where alpha(p) is the location of the peak in the packing fraction phi(J) at jamming onset, the rotational DOF are strongly coupled to the translational DOF, and the dynamic scaling exponents and phi(0) are similar for the rotational and translational DOF. For 1 < alpha < alpha(p), the translational DOF become frozen at higher temperatures than the rotational DOF, and phi(0) for the rotational degrees of freedom increases above phi(J). 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. Thus, zero-temperature contact counting arguments do not apply to structural relaxation of dense liquids of anisotropic particles near the glass transition.