Using particle shape to induce tilted and bistable liquid crystal anchoring

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=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.

Ultraconfined oblate hard particles between hybrid penetrable walls

We have investigated, by Monte Carlo simulation, the orientational structure of very thin films of a discotic liquid crystal (DLC) confined between hybrid walls of controllable penetrability, as a function of wall separation L_{z}. Our purpose was to clarify whether, as predicted by continuum theory, the preferred orientation of the DLC is uniform, changes linearly, or changes discontinuously, when L_{z} and the anchoring strengths at either wall are changed. The model consists of oblate hard Gaussian overlap (HGO) particles: each wall sees a particle as a disk of zero thickness and diameter D less than or equal to that of the actual particle σ_{0}, embedded inside the particle and located halfway along, and perpendicular to, its minor axis. This provides a particle-level mechanism to control the anchoring properties of the walls, from planar (edge-on) for D∼0 to homeotropic (face-on) for D∼σ_{0}, which can be done independently at either wall. As in our earlier work [C. Anquetil-Deck et al., J. Phys. Chem. B 124, 7709 (2020)1520-610610.1021/acs.jpcb.0c05027], which was restricted to L_{z}=6σ_{0}, depending on the values of D_{s}≡D/σ_{0} at the top (D_{s}^{t}) and bottom (D_{s}^{b}) walls, we find domains in (D_{s}^{b},D_{s}^{t}) space in which particle alignment is uniform planar (UP), uniform homeotropic (UH), or varies linearly from planar at one wall to homeotropic at the other (L), but no bistable or tristable regions are identified between these domains. Most importantly, there appears never to occur an abrupt change of the LC orientation when the walls strongly favor different anchorings, in general agreement with the scenario proposed by Velasco and co-workers [D. de las Heras et al., Phys. Rev. E 79, 011712 (2009)1539-375510.1103/PhysRevE.79.011712], but in contrast to the behavior of equivalent calamitic systems [F. Barmes et al., Phys. Rev. E 69, 061705 (2004)1539-375510.1103/PhysRevE.69.061705; Phys. Rev. E, 71, 021705 (2005)1539-375510.1103/PhysRevE.71.021705; C. Anquetil-Deck et al., Phys. Rev. E 86, 041707 (2012)1539-375510.1103/PhysRevE.86.041707]. However, for the thinnest films investigated (L_{z}=2σ_{0}), the system is unable to accommodate a rotation of the preferred particle orientation from one wall to the other and adopts instead a tilted configuration, similar to that reported earlier for Gay-Berne films in symmetric confinement [T. Gruhn et al., Thin Solid Films 330, 46 (1998)0040-609010.1016/S0040-6090(98)00799-8; Mol. Phys. 93, 681 (1998)10.1080/002689798169014] but which, as far as we know, has been missed in most earlier work.

Pressure-tensor method evaluation of the interfacial tension between Gay-Berne isotropic fluid and a smooth repulsive wall

The interfacial properties of a confined thermotropic liquid crystalline material are investigated using a molecular dynamics simulation technique. The pairwise interaction among the soft ellipsoidal particles is modeled by the Gay-Berne (GB) potential. The GB ellipsoids are confined by two soft, smooth, repulsive walls defined by the Weeks-Chandler-Andersen (WCA) potential. The aperiodic confinement due to walls makes the system mechanically anisotropic. Hence using the pressure-tensor method, the interfacial tension of an interface between the bulk isotropic (I) phase and WCA wall at various number densities (ρ) is calculated. From the pressure tensor and orientational order profiles, the arrangement of ellipsoids in the bulk and the vicinity of the wall is determined. The effect of system size and the wall-particle interaction strength (ε_W) on is also analyzed by varying the system size and ε_W.

Structure and Interfacial Tension of a Hard-Rod Fluid in Planar Confinement

The structural properties and interfacial tension of a fluid of rodlike hard-spherocylinder particles in contact with hard structureless flat walls are studied by means of Monte Carlo simulation. The calculated surface tension between the rod fluid and the substrate is characterized by a nonmonotonic trend as a function of the bulk concentration (density) over the range of isotropic bulk concentrations. As suggested by earlier theoretical studies, a surface-ordering scenario is confirmed by our simulations: the local orientational order close to the wall changes from uniaxial to biaxial nematic when the bulk concentration reaches about 85% of the value at the onset of the isotropic-nematic phase transition. The surface ordering coincides with a wetting transition whereby the hard wall is wetted by a nematic film. Accurate values of the fluid-solid surface tension, the adsorption, and the average particle-wall contact distance are reported (over a broad range of densities into the dense nematic region for the first time), which can serve as a useful benchmark for future theoretical and experimental studies on confined rod fluids. The simulation data are supplemented with predictions from second-virial density functional theory, which are in good qualitative agreement with the simulation results.

Density functional theory of liquid crystals and surface anchoring: hard Gaussian overlap-sphere and hard Gaussian overlap-surface potentials

This article applies the density functional theory to confined liquid crystals, comprised of ellipsoidal shaped particles interacting through the hard Gaussian overlap (HGO) potential. The extended restricted orientation model proposed by Moradi and co-workers [J. Phys.: Condens. Matter 17, 5625 (2005)] is used to study the surface anchoring. The excess free energy is calculated as a functional expansion of density around a reference homogeneous fluid. The pair direct correlation function (DCF) of a homogeneous HGO fluid is approximated, based on the optimized sum of Percus-Yevick and Roth DCF for hard spheres; the anisotropy introduced by means of the closest approach parameter, the expression proposed by Marko [Physica B 392, 242 (2007)] for DCF of HGO, and hard ellipsoids were used. In this study we extend an our previous work [Phys. Rev. E 72, 061706 (2005)] on the anchoring behavior of hard particle liquid crystal model, by studying the effect of changing the particle-substrate contact function instead of hard needle-wall potentials. We use the two particle-surface potentials: the HGO-sphere and the HGO-surface potentials. The average number density and order parameter profiles of a confined HGO fluid are obtained using the two particle-wall potentials. For bulk isotropic liquid, the results are in agreement with the Monte Carlo simulation of Barmes and Cleaver [Phys. Rev. E 71, 021705 (2005)]. Also, for the bulk nematic phase, the theory gives the correct density profile and order parameter between the walls.

Interfacial anisotropy in the transport of liquid crystals confined between flat, structureless walls: a molecular dynamics simulation approach

Molecular dynamics simulations of uniaxial Gay-Berne ellipsoids as prolate liquid crystal molecules confined between two flat, structureless walls have been carried out in order to investigate anisotropy in their dynamic properties. Several physical quantities are profiled as a function of distance from a wall. The walls stimulate ellipsoids into different behaviors from those of the bulk system. The profiles of self-diffusion coefficients, which are distinguished in each direction of a director-based coordinate system, show that the ellipsoids are more diffusive parallel to the walls and less diffusive perpendicular to the walls with decreasing distance from the walls. According to the self-rotation coefficient and rotational viscosity profiles, ellipsoids are easy to rotate parallel to the walls and hard to rotate in the plane perpendicular to the walls. The analyses of velocity autocorrelation functions, angular velocity autocorrelation functions, director angular velocity autocorrelation functions, and their spectra are useful for the investigation of anisotropy near the walls. We conclude that the flat, structureless wall not only prevents ellipsoids from diffusing and rotating in the plane perpendicular to the walls, but also stimulates them to diffuse and rotate in the plane parallel to the walls.

Stability of Nematic and Smectic Phases in Rod-Like Mesogens with Orientation−Dependent Attractive Interactions

The stability of isotropic (I), nematic (N), smectic A (Sm A), and hexatic (Hex) liquid crystalline phases is studied for a fluid of molecules with a rod-like shape and dispersive interactions dependent on orientation. The fluid is modeled with the spherocylindrical Gay-Berne-Kihara interaction potential proposed in a recent work, with parameters favoring parallel pair orientations. The liquid crystal phase diagram is characterized for different molecular aspect ratios by means of Monte Carlo simulations in the isobaric-isothermal ensemble. Three types of triple points are observed, namely, I-Sm A-Hex, I-N-Sm A, and N-Sm A-Hex, leading to island-shape domains for the smectic A phase. The resulting phase diagrams are compared with those derived previously for prolate fluids of ellipsoidal and spherocylindrical symmetry. It is concluded that the stability of the layered phases with respect to the nematic phase is enhanced in the spherocylindrical fluids due to geometrical constraints. Furthermore, the anisotropy of the dispersive interactions induces a stronger dependence of the overall phase diagram on temperature and aids in the energetic stabilization of the hexatic crystalline phase with respect to the fluid smectic A phase.