Dynamics in a supercooled liquid of symmetric dumbbells: Reorientational hopping for small molecular elongations

TL;DR

This study uses molecular dynamics simulations to explore how symmetric dumbbells in a supercooled liquid exhibit different freezing behaviors in translational and rotational motions depending on molecular elongation, revealing complex glass transition dynamics.

Contribution

It demonstrates the decoupling of odd and even rotational degrees of freedom and provides evidence for a distinct type-A transition for odd rotations, expanding understanding of glass transition mechanisms.

Findings

Large elongations lead to simultaneous freezing of translation and rotation.

Small elongations show decoupled odd rotational motion remaining ergodic.

Evidence suggests a separate type-A transition for odd rotational degrees.

Abstract

We present extensive molecular dynamics simulations of a liquid of symmetric dumbbells, for constant packing fraction, as a function of temperature and molecular elongation. For large elongations, translational and rotational degrees of freedom freeze at the same temperature. For small elongations only the even rotational degrees of freedom remain coupled to translational motions and arrest at a finite common temperature. The odd rotational degrees of freedom remain ergodic at all investigated temperature and the temperature dependence of the corresponding characteristic time is well described by an Arrhenius law. Finally, we discuss the evidence in favor of the presence of a type-A transition temperature for the odd rotational degrees of freedom, distinct from the type-B transition associated with the arrest of the translational and even rotational ones, as predicted by the mode-coupling theory for the glass transition.