The anchoring of rodlike liquid-crystalline molecules at solid surfaces plays an important role in the design of novel nanotechnological devices such as, for example, biosensors. In this work we investigate the impact of various anchoring scenarios on the isotropic-nematic phase transition of weakly anisometric, prolate mesogens confined to a mesoscopic slit-pore. We employ isostress ensemble Monte Carlo simulations in which the isotropic-nematic transition is driven by applying an external stress in the direction parallel to the substrate plane. The fluid-fluid interaction is described by a Lennard-Jones potential with an orientation-dependent attractive term. Our simulations show that different anchoring scenarios may shift the isotropic-nematic phase transition considerably. We locate this transition through response functions such as an isostress heat capacity, isothermal compressibility, and the Maier-Saupe order parameter. Our results suggest that for the present model system the isotropicnematic transition is likely to be continuous. For the planar anchoring scenario we observe the formation of a new layer of fluid molecules during a structural transformation preceding the isotropicnematic transition.

We present a molecular dynamics study of reentrant nematic phases using the Gay-Berne-Kihara model of a liquid crystal in nanoconfinement. At densities above those characteristic of smectic A phases, reentrant nematic phases form that are characterized by a large value of the nematic order parameter S≃1. Along the nematic director these "supernematic" phases exhibit a remarkably high self-diffusivity, which exceeds that for ordinary, lower-density nematic phases by an order of magnitude. Enhancement of self-diffusivity is attributed to a decrease of rotational configurational entropy in confinement. Recent developments in the pulsed field gradient NMR technique are shown to provide favorable conditions for an experimental confirmation of our simulations.

By means of Monte Carlo simulations in the isothermal-isobaric ensemble, we perform a finite-size scaling analysis of the isotropic-nematic (IN) phase transition. Our model consists of egg-shaped anisometric Lennard-Jones molecules. We employ the cumulant intersection method to locate the pressure P* at which the IN phase transition occurs at a given temperature T. In particular, we focus on second-order cumulants of the largest and middle eigenvalues of the alignment tensor. At fixed T, cumulants for various system sizes intersect at a unique pressure P*. Various known scaling relations for these cumulants are verified numerically. At P*, the isobaric heat capacity passes through a maximum value c(P)(m), which depends on the number of molecules N. This dependency can accurately be described by a power law such that lim(N→∞)c(P)(m)(N)→∞. For sufficiently large N, the pressure at which c(P)(m) is located shifts only very slightly in agreement with the apparent insensitivity of the cumulant intersection to N. In addition, we analyze our data in terms of Landau's theory of phase transitions. Our results are consistent with a weakly discontinuous entropy-driven phase transition.

We present Monte Carlo simulations of liquid-crystalline material confined to a nanoscopic slitpore. 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, DOI:10.1039/B924417D). We study several anchoring mechanisms at the solid substrate by introducing an anchoring function 0 ≤ g ( u) ≤ 1 that depends on the orientation u of a fluid molecule relative to the substrate plane; g ( u) "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 in specific directions that depend on details of the anchoring scenario.

Using a combination of isothermal-isobaric Monte Carlo and microcanonical molecular dynamics we investigate the relation between structure and self-diffusion in various phases of a model liquid crystal using the Gay-Berne-Kihara potential. These molecules are confined to a mesoscopic slit pore with atomically smooth substrate surfaces. As reported recently [seeM. G. Mazza et al., Phys. Rev. Lett. 105, 227802 (2010)], a reentrant nematic (RN) phase may form at sufficiently high pressures and densities. This phase is characterized by a high degree of nematic order and a substantially enhanced self-diffusivity in the direction of the director n that exceeds that of the lower-density nematic and an intermittent smectic-A phase by about an order of magnitude. Here we demonstrate that the unique transport behavior in the RN phase may be linked to a confinement-induced packing effect that causes the formation of supramolecular, stringlike conformations. The strings consist of several molecules traveling in the direction of n as individual "trains" consisting of chains of molecular "cars."