Description of short-range order in glass formers

Unlike crystals, liquids and amorphous solids do not possess a long-range order. This does not mean that a local arrangement of molecules is completely random. MD simulation of several molecular systems reveals existence of preferred shapes of clusters formed by neighboring molecules. The distribution of shapes of angstrom size clusters exhibits strong dependence on temperature as the glass transition is approached. The temperature dependence vanishes for clusters of the size of nanometer and larger, where the average shape of the cluster becomes approximately spherical. It is observed that the cluster shapes distribution differs in the metastable vs equilibrated state at the same temperature. The systems investigated include: thermal – Lennard-Jones dumbbells and model coarse-grained ortho-terphenyl and athermal – polydisperse hard spheres. Implications for the search of the order parameter controlling the glass transition will be discussed.