Phase equilibria, fluid structure, and diffusivity of a discotic liquid crystal

Molecular Dynamics simulations were performed for the Gay-Berne discotic fluid parameterized by GB(0.345, 0.2, 1.0, 2.0). The volumetric phase diagram exhibits isotropic (I-L), nematic (N-D), and two columnar phases characterized by radial distribution functions: the transversal fluid structure varies between a hexagonal columnar (C-D) phase (at higher temperatures and pressures) and a rectangular columnar (C-O) phase (at lower temperatures and pressures). The slab-wise analysis of fluid dynamics suggests the formation of grain-boundary defects in the C-O phase. Longitudinal fluid structure is highly periodic with narrow peaks for the C-O phase, suggestive of a near-crystalline (yet diffusive) system, but is only short-ranged for the C-D phase. The I-L phase does not exhibit anisotropic diffusion. Transversal diffusion is more favorable in the N-D phase at all times, but only favorable at short times for the columnar phases. In the columnar phases, a crossover occurs where longitudinal diffusion is favored over transversal diffusion at intermediate-to-long timescales. The anomalous diffusivity is pronounced in both columnar phases, with three identifiable contributions: (a) the rattling of discogens within a transient "interdigitation" cage, (b) the hopping of discogens across columns, and (c) the drifting motion of discogens along the orientation of the director.