Clustering and Nucleation in Metastable Fluids of Hard Polyhedra.

Abstract

Crystallization on the colloidal scale is influenced by numerous factors. I have taken advantage of recent advances in computational techniques and resources to focus on systems of hard polyhedra and investigate the differences in crystallization processes, nucleation, clustering, and local order in metastable fluids due to particle shape.

The crystallization behavior of hard particle systems is influenced by the strength of local orientational preferences, and the affinity of particles for aligning face-to-face, leading to bias toward specific local packing arrangements such as the locally dense icosahedral or hexagonally close-packed configurations. We report the driving force needed for crystallization of a selection of polyhedra known to assemble into the face centered cubic crystal: Disdyakis Triacontahedron, Trigyrate Rhombicosadodecahedron, Truncated Icosahedron, Rhombic Triacontahedron, and Rhombic Dodecahedron. Shapes that promote local environments (arrangements of neighbors in the fluid) that are incompatible with the local environment in the crystal require a similar driving force to spheres to observe crystallization directly. For these shapes the crystallization probability increase relatively slowly with the increasing driving force, so the fluid remains metastable to a higher driving force. Polyhedra that promote local environments in the fluid consistent with the local environment of a particle in the crystal, or that had low energetic barriers to rearrangement of neighbors relative to particle orientation, are observed to crystallize more easily than spheres. Local order and emergent entropic preference for face-to-face contact play roles in clustering and crystallization of metastable hard polyhedra fluids. In order to better understand the transition from fluid to solid, I have extended non-biasing forward flux-type rare event sampling methods, and generated ensembles of non-equilibrium states intermediate between fluid and solid.