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

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 = Ds sigma 0, where sigma 0 = 2b. We used NVT Monte Carlo simulations and showed that for small and large Ds, planar (edge-on arrangement) and homeotropic (face-on arrangement) anchoring are stable. The molecular volume absorbed by the substrates for each Ds is calculated analytically and the critical values of the transition parameter DTs 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 Ds approximate to DTs and checked the influence of the packing fraction on the anchoring competition. The system in two cases, maximally penetrable and impenetrable substrates with Ds = 0 and Ds = 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,Ds = 0, and 1.0, the isotropic-nematic transition packing fraction of confined HGO particles and OHE particles were calculated and compared.