Simulation of the dynamics of hard ellipsoids

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.

Keywords:

• computer simulation
• glass transition
• hard ellipsoids
• mode coupling theory
• nematic order

Acknowledgement

We acknowledge support from MIUR-PRIN.