Frustration of nanoconfined liquid crystals due to hybrid substrate anchoring

Abstract

We present Monte Carlo simulations of liquid-crystalline material confined to a nanoscopic slit-pore. The simulations are carried out under isothermal conditions in a specialized isostress ensemble in which N fluid molecules are exposed to a compressional stress τ_‖ acting on the fluid in directions parallel with the substrate planes. Fluid-fluid and fluid-substrate interactions are modelled as in our previous work (M. Greschek et al., Soft Matter, 2010, 6, 1898). We study several anchoring mechanisms at the solid substrate by introducing an anchoring function 0 ≤ g(û) ≤ 1 that depends on the orientation û of a fluid molecule relative to the substrate plane; g(û) “switches” the fluid-substrate attraction on or off. Here we focus on various heterogeneous (i.e., hybrid) anchoring scenarios imposing different anchoring functions at the opposite substrates. As in our previous study we compute the isostress heat capacity which allows us to identify states at which the confined fluid undergoes a structural transformation. The isotropic-nematic transformation turns out to be nearly independent of the specific anchoring scenario. This is because the director in the nematic phase assumes a direction that is a compromise between the directions enforced by the competing anchoring scenarios at either substrate. On the contrary, at lower compressional stresses molecules prealign near the slit-pore's midplane in specific directions that depend on details of the anchoring scenario.