Computer simulation study of confined oblate hard ellipsoid liquid crystals: Hard-disk-wall interaction.

Abstract

In this study, instead of an approximate hard Gaussian overlap model, the effects of confinement on a system of oblate hard ellipsoid (OHE) particles interacting with planar substrates through the hard-disk-wall potential (HDW) were studied via computer simulation. In HDW, the thick oblate molecule with elongation k=a/b<1 is replaced by a thin disk with a diameter D=D_{s}σ_{0}, where σ_{0}=2b. We used NVT Monte Carlo simulations and showed that for small and large D_{s}, planar (edge-on arrangement) and homeotropic (face-on arrangement) anchoring are stable. The molecular volume absorbed by the substrates for each D_{s} is calculated analytically and the critical values of the transition parameter D_{s}^{T} were predicted from planar to homeotropic anchoring. Also, the transition parameters for two particles' elongations, k=0.2 and 0.345, are achieved via simulation. The results are approximately in agreement with the predicted values. Our results for the OHE particles with k=0.345 correspond to the hard Gaussian overlap results of Teixeira et al., qualitatively. We used an NPT Monte Carlo simulation to study the system in the region of D_{s}≈D_{s}^{T} and checked the influence of the packing fraction on the anchoring competition. The system in two cases, maximally penetrable and impenetrable substrates with D_{s}=0 and D_{s}=1.0, are investigated via NPT Monte Carlo simulations, and the isotropic-nematic transition packing fraction was compared. In addition, the orientational structure of k=0.2 and 0.345 OHEs confined between thin symmetry walls was studied as a function of wall separation. In addition, for k=0.2,D_{s}=0, and 1.0, the isotropic-nematic transition packing fraction of confined HGO particles and OHE particles were calculated and compared.