Point and line defects in checkerboard patterned hybrid nematic films: A computer simulation investigation.

We consider a nematic liquid crystal film confined to a flat cell with homeotropic and planar patterned hybrid anchoring and show, using Monte Carlo simulations, the possibility of the system to stabilize line and point defects. The planar anchoring surface is patterned with a chessboardlike grid of squares with alternating random or parallel homogeneous planar anchoring. The simulations show only line defects when the individual domains are small enough, but also point defects when the domain size is significantly larger than the sample thickness.

Elastic constants and the formation of topological defects in hybrid nematic cells: A Monte Carlo study.

We present a Monte Carlo study of the effects of elastic anisotropy on the topological defects which can be formed in nematic films with hybrid boundary conditions. We simulate the polarized microscopy images and analyze their evolution in uniaxial systems for different values of the Frank elastic constants.

From Point to Filament Defects in Hybrid Nematic Films.

We have studied nematic hybrid films with homeotropic alignment at the top surface and various controlled degrees of in plane ordering, going from a random degenerate organization to a completely uniform alignment along one direction, at the bottom one. We show, by Monte Carlo (MC) computer simulations and experiments on photopatterned films with the bottom support surface fabricated with in-plane order similar to the simulated ones, that the point defects observed in the case of random planar orientations at the bottom tend to arrange along a filament as the surface ordering is sufficiently increased. MC simulations complement the polarized microscopy texture observations allowing to inspect the 3D structure of the defects and examine the role of elastic constants.

Influence of boundary conditions on the order and defects of biaxial nematic droplets.

We employ Monte Carlo simulations to study the defects occurring in a nematic droplet formed by biaxial molecules. The simulations are carried out using a lattice model based on a dispersive orientational biaxial potential previously employed to establish the rich phase diagram of the system. The focus of the present investigation is on the molecular organization inside the droplet when bipolar and toroidal anchoring conditions at the surface are considered.

On the Defect Structure of Biaxial Nematic Droplets.

We present a detailed Monte Carlo study of the effects of molecular biaxiality on the defect created at the centre of a nematic droplet with radial anchoring at the surface. We have studied a lattice model based on a dispersive potential for biaxial mesogens [Luckhurst et al., Mol.

Nematic liquid crystals in planar and cylindrical hybrid cells: Role of elastic anisotropy on the director deformations.

Nematic samples filling a flat cell or the annular region between two concentric cylinders with hybrid anchoring conditions at the boundaries are investigated by setting up and minimizing their Frank elastic free energy. The coupling with the surfaces is taken to be strong on one side and weak on the other. The equations are numerically solved and the conditions for which the molecular organization inside the cell becomes uniform are analyzed.

Molecular organization of nematic liquid crystals between concentric cylinders: role of the elastic anisotropy.

The orientational order in a nematic liquid crystal sample confined to an annular region between two concentric cylinders is investigated by means of lattice Monte Carlo simulations. Strong anchoring and homeotropic orientations, parallel to the radial direction, are implemented at the confining surfaces. The elastic anisotropy is taken into account in the bulk interactions by using the pair potential introduced by Gruhn and Hess [T.

Computer simulations of the ordering in a hybrid cylindrical film of nematic liquid crystals.

We present an investigation of the ordering in a nematic liquid-crystal film confined between two cylindrical surfaces with antagonistic (radial and planar) anchoring alignments. A Monte Carlo study of a Lebwohl-Lasher model with suitable boundary conditions has been performed to calculate the ordering and the molecular organization for different film thicknesses. The simulation results are compared with some theoretical predictions obtained with the elastic continuum approach.

Memory effects in nematics with quenched disorder.

We present a combined experimental and Monte Carlo study of a nematic phase in the presence of quenched disorder. The turbidity of a nematic liquid crystal embedded in a porous polymer membrane is measured under different applied field conditions for field-cooled and zero-field-cooled samples. We find that a significant permanent alignment of the nematic can be induced by fields as low as 0.

Nematics with quenched disorder: pinning out the origin of memory.

Memory effects and glassy behavior have been repeatedly observed in disordered nematic liquid crystals but the connection between these effects and the system topology remained unrevealed. We present an analysis of the local and global topology of the nematic ordering in the presence of quenched disorder and we show that nematics with quenched disorder can be mapped into a system of pinned defect lines and that the memory of the system stems from the pinning of these strings.

Structures and transitions in thin hybrid nematic films: a Monte Carlo study.

We confirm by Monte Carlo simulations of a Lebwohl-Lasher lattice spin model the existence of a biaxially ordered nonbent structure in a liquid-crystalline cell subject to opposing boundary conditions. We report on the observation of the bending transition from the biaxial to the bent-director structure when the temperature of the system is lowered. The structural transition is monitored both by the change of the order parameters and by heat capacity.

Nematics with dispersed polymer fibrils: a Monte Carlo study of the external-field-induced switching.

We present a Monte Carlo study of molecular ordering in nematics with dispersed regular and random arrays of straight and distorted polymer fibrils. We focus on the collective molecular reorientation--the switching--resulting from the competing aligning effects of fibrils and of a progressively applied transversal external field, and for straight fibrils identify structural Fréedericksz and saturation transitions. The role of fibril topography in the switching is monitored by simulating electric capacitance Slightly distorted fibrils are shown to give a sharper switching at a lower threshold.

Topological defects in schlieren textures of biaxial and uniaxial nematics.

Monte Carlo and theoretical studies of thin 3D films of biaxial and uniaxial nematics with tangential boundary conditions show distinct differences in structure and evolution of topological defects. In the uniaxial films, defects of strength k=+/-1 are point defects that bear no bulk singularity and disappear by annihilation with each other. In the biaxial films, k=+/-1 defects are true singular bulk disclinations that split into pairs of k=+/-1/2 lines; the latter disappear by annihilation processes of the type +1/2-1/2=0.

Polymer network-induced ordering in a nematogenic liquid: a Monte Carlo study.

In this Monte Carlo study we investigate molecular ordering in a nematogenic liquid with dispersed polymer networks. The polymer network fibers are assumed to have rough surface morphology resulting in a partial randomness in anchoring conditions, while the fiber direction is assumed to be well defined. In particular, we focus on the loss of long-range aligning capability of the network when the degree of disorder in anchoring is increased.

Nematics with quenched disorder: how long will it take to heal?

Nematics with quenched disorder have been repeatedly predicted to form glass phases. Here we present turbidity experiments and computer simulations aimed at studying glass key features such as dynamics and history dependence in randomly perturbed nematics. Electric field-cooling alignment has been employed to prepare samples in suitably oriented starting states.

NMR spectra from Monte Carlo simulations of polymer dispersed liquid crystals.

We present the calculation of NMR line shapes, including dynamical effects, of polymer dispersed liquid crystals starting from the Monte Carlo configurations simulated for a lattice spin model. We consider droplets with radial, bipolar, and random boundary conditions and examine to what extent their predicted deuterium NMR spectra differ in the presence of molecular motion.

Dynamical and field effects in polymer-dispersed liquid crystals: monte carlo simulations of NMR spectra.

We analyze the dynamical aspects of molecular ordering in nematic droplets with radial and bipolar boundary conditions-as encountered in polymer-dispersed liquid crystals-by calculating and interpreting the corresponding 2H NMR spectra. In particular, we focus on effects of molecular motion such as fluctuations of molecular long axes and translational diffusion, and on external field ordering effects. As in our previous paper [Phys.

Nematics with quenched disorder: what is left when long range order Is disrupted?

It is now generally accepted that even low amounts of quenched disorder disrupt long-range order in anisotropic systems with continuous symmetry. However, very little is known on the key item of the nature of the residual order, if any, and particularly if this has quasi-long-range or truly-short-range character. Here we address this problem both experimentally for the nematic 6CB in dilute aerosils and with computer simulations.

Acute hepatitis B in infants born to HBsAG asymptomatic carrier mothers.

Within an eight-month period, five cases of acute hepatitis B were observed in children born to asymptomatic HBsAg carrier mothers. At delivery, four mothers were HBeAg-positive and one was anti-HBe-positive. At an average age of 17+/-3 weeks the infants showed high transaminases levels and appearance of markers of HBV infection.