Liquid Dynamics And Crystal Melting Of A Two Dimensional Molecule

Abstract

Despite extensive study of molecular materials, their dynamic behaviour as fragile liquids and their phenomenally slow crystal growth rates, are both observations we don't yet fully understand. This thesis introduces a 2D molecular liquid based on ortho-terphenyl, used to investigate the behaviour of the liquid state through molecular dynamics simulations. Using newly developed methods for describing the dynamic behaviour of individual molecules, we explain the onset of dynamic heterogeneities resulting from jump dynamics, where motions occur through large reorganisations. We also observe a slowdown of dynamics within fragile liquids at temperatures above the melting point, providing evidence these are properties of the glass transition rather than the supercooled liquid as they are normally described. With an understanding of the impact of length scales for describing dynamics, we can attribute the observed decoupling of rotational and translational diffusion to the coupling of rotations and translations within each molecule. In our study of crystal growth, we find the rigid structure of the molecular liquid and crystal inhibits the transition from a crystalline to liquid-like state, drastically slowing the melting of the crystal phase. These transitions are made even slower by the suppression of rotational motion at the interface of the crystal. The results found in this thesis using a 2D molecule provide a framework for investigating these effects in 3D simulations and experimental systems.