Abstract
We study a system of uniaxial hard ellipsoids by molecular dynamics simulations, changing both the aspect-ratio X 0 (X 0 = a/b, where a is the length of the revolution axis and b is the length of the two other axes) and the packing fraction φ. We calculate the translational ⟨r 2(t)⟩ and rotational ⟨Φ2(t)⟩ mean squared displacements, the translational D trans and the rotational D rot diffusion coefficients and the associated isodiffusivity lines in the φ − X 0 plane. For the first time, we characterize the cage effect through the logarithmic time derivative of log⟨r 2(t)⟩ and log⟨Φ2(t)⟩. These quantities exhibit a minimum if the system is supercooled and we show that, consistently with our previous findings, for large and small X 0 values, rotations are supercooled, contrary to translations. In agreement with this scenario, while the self-intermediate scattering function exhibits stretched relaxation (i.e. glassy dynamics) only for large φ and X 0 ≈ 1, the second order orientational correlator C 2(t) show stretching only for large and small X 0 values. As further evidence of this pre-nematic order driven glass transition, we observe a decoupling of the translational and rotational dynamics, which generates an almost perpendicular crossing of the D trans and D rot isodiffusivity lines.
Acknowledgement
We acknowledge support from MIUR-PRIN.