Coupling/decoupling between translational and rotational dynamics in a supercooled molecular liquid

TL;DR

This study uses molecular dynamics simulations to analyze how translational and rotational motions in a supercooled liquid of dumbbells become increasingly coupled or decoupled as temperature decreases, revealing complex dynamical heterogeneities.

Contribution

It provides a detailed analysis of the coupling and decoupling phenomena between translational and rotational dynamics in supercooled liquids, clarifying the roles of relaxation times and diffusion constants.

Findings

Coupling between relaxation times increases with decreasing temperature.

Decoupling between translational and rotational diffusivities occurs as temperature decreases.

Dynamical heterogeneities explain the apparent contradictions in coupling behaviors.

Abstract

We use molecular dynamics computer simulations to investigate the coupling/decoupling between translational and rotational dynamics in a glass-forming liquid of dumbbells. This is done via a careful analysis of the $\alpha$-relaxation time $\tau_{q^{*}}^{\rm C}$ of the incoherent center-of-mass density correlator at the structure factor peak, the $\alpha$-relaxation time $\tau_{2}$ of the reorientational correlator, and the translational ($D_{t}$) and rotational ($D_{r}$) diffusion constants. We find that the coupling between the relaxation times $\tau_{q^{*}}^{\rm C}$ and $\tau_{2}$ increases with decreasing temperature $T$, whereas the coupling decreases between the diffusivities $D_{t}$ and $D_{r}$. In addition, the $T$-dependence of $D_{t}$ decouples from that of $1/\tau_{2}$, which is consistent with previous experiments and has been interpreted as a signature of the "translation-rotation decoupling." We trace back these apparently contradicting observations to the dynamical heterogeneities in the system. We show that the decreasing coupling in the diffusivities $D_{t}$ and $D_{r}$ is only apparent due to the inadequacy of the concept of the rotational diffusion constant for describing the reorientational dynamics in the supercooled state. We also argue that the coupling between $\tau_{q^{*}}^{\rm C}$ and $\tau_{2}$ and the decoupling between $D_{t}$ and $1/\tau_{2}$, both of which strengthen upon cooling, can be consistently understood in terms of the growing dynamic length scale.