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- Ordering of oblate hard particles between symmetric penetrable wallsPublication . Teixeira, Paulo; Anquetil-Deck, Candy; Cleaver, Douglas J.We find the structure of a model discotic liquid crystal (DLC) confined between symmetric walls of controllable penetrability. The model consists of oblate hard Gaussian overlap (HGO) particles. Particle-substrate interactions are modelled as follows: each substrate sees a particle as a disc of zero thickness and diameterless than or equal to that of the actual particle,, embedded inside the particle and located halfway along, and perpendicular to, its minor axis. This allows us to control the anchoring properties of the substrates, from planar (edge-on) forto homeotropic (face-on) for. This system is investigated using both Monte Carlo simulation and density-functional theory, the latter implemented at the level of Onsager's second-virial approximation with Parsons-Lee rescaling. We find that the agreement between theory and simulation is substantially less good than for prolate HGOs; in particular, the crossover from edge-on to face-on alignment is predicted by theory to occur at, but simulation finds it for. These discrepancies are likely a consequence of the fact that Onsager's theory is less accurate for discs than for rods. We quantify this by computing the bulk isotropic-nematic phase diagram of oblate HGOs.
- Ordering of oblate hard particles between hybrid penetrable wallsPublication . Anquetil-Deck, Candy; Cleaver, Douglas J.; Teixeira, PauloWe report a Monte Carlo (MC) simulation study of a model discotic liquid crystal (DLC) confined between hybrid walls with controllable penetrability. The model consists of oblate hard Gaussian overlap (HGO) particles. Particle-substrate interactions are modeled as follows: each substrate sees a particle as a disc of zero thickness and diameter D less than or equal to that of the actual particle, sigma(0), embedded inside the particle and located halfway along, and perpendicular to, its minor axis. This allows us to control the anchoring properties of the substrates, from planar (edge-on) for D approximate to 0 to homeotropic (face-on) for D approximate to sigma(0), which can be done independently at either substrate. Depending on the values of D-s = D/sigma(0) at the top (D-s(t)) and bottom (D-s(b)) substrates, we find domains in (D-s(b), D-s(t)) space in which particle alignment is uniform planar (UP), is uniform homeotropic (UH), or varies linearly from planar at one substrate to homeotropic at the other (Lin). These domains are separated by regions of bistability (P-Lin and H-Lin), which appear to be wider than for prolate HGOs, and there may be also a small tristable (P-H-Lin) region. Results are compared with the predictions of density functional theory, implemented at the level of Onsager's second-virial approximation with Parsons-Lee rescaling. As in the case of symmetric confinement studied previously, the agreement between theory and simulation is substantially less good than for prolate HGOs: in particular, for the investigated substrate separation L = 6 sigma(0), the Lin configuration is never predicted. These discrepancies are likely a consequence of the fact that Onsager's theory is less accurate for discs than for rods.