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

TL;DR

This study uses molecular dynamics simulations to analyze how the shape of particles affects the structural relaxation and fragility in dense liquids, revealing that anisotropic particles like ellipses and dimers exhibit similar slow dynamics despite different static packing properties.

Contribution

It introduces a universal scaling framework for the structural relaxation times of anisotropic particles in dense liquids, highlighting the role of aspect ratio in fragility and dynamic behavior.

Findings

Fragility decreases with increasing aspect ratio.

Universal scaling function describes both translational and rotational relaxation.

Similar slow dynamics observed for dimers and ellipses despite different static packings.

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 \phi, temperature T, and aspect ratio \alpha. We are able to collapse the \phi and T-dependent structural relaxation times for disks, and dimers and ellipses over a wide range of \alpha, onto a universal scaling function {\cal F}_{\pm}(|\phi-\phi_0|,T,\alpha), 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 \alpha-dependent scaling exponents \mu and \delta and packing fraction \phi_0(\alpha) 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 \phi_0, m(\phi_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.