Formation of topological defects in nematic shells with a dumbbell-like shape.

The present study investigates dumbbell-shaped nematic liquid crystal shells. Using molecular dynamics (MD) simulations, we consider the effects of an external electric field on nematic ordering by computing the average molecular alignment's time evolution and equilibrium configuration. We show that the number and location of topological defects are strongly affected by the external field, with the orientational ordering's equilibrium configuration depending on field direction about the shell's long axis. For a transverse external field, it is verified that the defect rearrangement presents a non-linear dynamics, with a field independent characteristic time scale delimiting the short and long time regimes. Effects associated with varying the shell's Gaussian curvature are also analyzed.

Behavior of chiral active nematics confined to nanoscopic circular region.

We performed molecular dynamic simulations of a model active nematic confined to a two-dimensional nanoscopic circular region under both tangential and radial anchoring boundary conditions. This active material is assumed to be composed of elongated chiral particles which interact with each other by means of isotropic Lennard-Jones and anisotropic Maier-Saupe-like potentials. These particles have the lateral appendage emitting a jet of some substance generated by a certain internal chemical reaction. As a result, such elongated particles are exposed to both the reactive self-propelled force and the torque that provide an additional translational movement of particles and a self-rotation with respect to their geometric centers. It has been found that the chiral active nematics under consideration form time-dependent vortex-like structures with two +1/2 topological defects which are similar to experimentally observed structures in active materials.

Tunable topological valence in nematic shells on spherocylindrical colloidal particles.

We perform molecular dynamics simulations of the orientational ordering on nematic shells delimited by spherocylindrical nanoscopic colloidal particles. We show that under conditions of degenerate planar anchoring, the equilibrium director field structure in these shells exhibits pairs of +1/2 topological defects at the poles of spherical cups in the absence of an external electric field. In addition, a certain number of pairs of ±1/2 defects occurs on the spherical cups far from the poles, thus resulting in a total of eight valence spots. A strong field applied along the main spherocylindrical axis removes the ±1/2 defect pairs while it coalesces the polar ones into a single +1 topological defect. A strong transverse field destroys all defects on the spherical cups but generates four +1/2 defects in the cylindrical part. Therefore, an external field can be used to control the number of valence centers in spherocylindrical nematic shells, thus unveiling their capability of acting as multivalent building blocks for nanophotonic devices.

Dynamics of Helfrich-Hurault deformations in smectic-A liquid crystals.

A dynamics of the Helfrich-Hurault deformations in the SmA sample in the case of a rigid homeotropic anchoring has been theoretically investigated. It has been found that, unlike the Freedericksz transition, in the nematic liquid crystals, both the switching-on and switching-off characteristic times for this phenomenon are linearly proportional to the SmA sample thickness, and the switching-off process is much faster than the switching-on one.

A molecular dynamics study of ferroelectric nanoparticles immersed in a nematic liquid crystal.

A large number of interesting phenomena related to the insertion of colloidal particles in liquid crystals (LC) have recently been reported. Here, we investigate effects caused by the addition of spherically shaped ferroelectric nanoparticles to a nematic liquid crystal. Using molecular dynamics (MD) simulations, the density of LC molecules, the orientational order parameter, and the polar and azimuthal angle profiles are calculated as functions of the distance to the center of the immersed nanoparticle for different temperatures of the system. We observe that the assembly of ferroelectric nanoparticles enhances the nematic order in the LC medium changing many properties of its host above the nematic-isotropic transition temperature T (*) (NI) .

Comparison between experiment and theory in the temperature variation of film tension above the bulk isotropic transition in free-standing liquid-crystal films.

Using differential scanning calorimetry, the transition enthalpies and temperatures for the bulk smectic-isotropic phase transition have been measured for a series of liquid-crystal compounds. For five compounds, those values were used as parameters in a microscopic mean-field model to predict the temperature dependence of the difference in free-energy density between a sample of material in a free-standing smectic film and that in the bulk. The model predicts a weak temperature dependence below the bulk clearing point and a pronounced monotonic increase with temperature above the transition temperature. The compounds used in this study were chosen specifically because they were also the subject of a previous independent experimental study [M. Veum, Phys. Rev. E 74, 011703 (2006)] that demonstrated a sudden monotonic increase in the free-standing film tension with temperature, which is qualitatively consistent with the predictions of the above-mentioned mean-field model. This study presents a direct and quantitative comparison between the predictions of the mean-field model and the results from previous tension experiments.

Microscopic and macroscopic description of anchoring at the nematic-solid substrate interface.

A simple microscopic mean-field model for a flat nematic liquid crystal (NLC) sample in a contact to the solid substrate surface is offered. An interaction between NLC molecules is simulated by the well known McMillan model potential, and an orienting action of the solid substrate surface on NLC molecules is modeled by a short-range external field which acts directly only on molecules within the first molecular layer of the nematic sample adjacent to the substrate surface. For an undistorted NLC sample, the model allows the calculation of local order parameter profiles for different values of strength of this external orienting field and temperature of the sample. These profiles are used in a description of a director field distortion caused by a certain external action and in the calculation of the anchoring energy coefficient used in a macroscopic description of the anchoring at the nematic-solid substrate interface. Dependence of this coefficient on the strength of the short-range orienting field is obtained, and an unequivocal relation between the magnitude of and the orientational order parameter profile near the substrate surface is established. The temperature dependence of the coefficient W calculated from the offered microscopic model is in good agreement with the experimental data on NLC MBBA.

Temperature dependence of the Casimir-like force in free-standing smectic films.

The thermal Casimir-like force in free-standing liquid crystal films close to the smectic-A-nematic transition temperature is computed using a quadratic functional approach. In the framework of a microscopic mean-field model of free-standing smectic-A films, the temperature dependence of the order parameter profiles is computed and later used to estimate the elastic coupling variability in the vicinity of first- and second-order bulk smectic-A-nematic phase transitions. The strong nonuniformity of the coupling constant profiles promotes a significant increase of the fluctuation-induced force over three orders of magnitude, especially in thin films. This result reinforces the possible predominance of the thermal Casimir force as compared to the standard van der Waals interaction in thin smectic-A liquid crystal films.

Smectic layer displacement fluctuations in solid substrate supported smectic-A films.

In this paper we present results of calculations of static and dynamic characteristics of smectic layer displacement fluctuations in solid substrate supported smectic-A films with due regard for asymmetric profiles of the bending elastic constant K and the smectic layer compressibility B. We also take into account difference in properties of boundary surfaces of the film, namely, the surface tension of the free surface is taken to be finite whereas that of a film-substrate interface is assumed to be infinitely large. Profiles of the smectic layer displacement fluctuations and correlations between them are calculated for films formed of liquid crystalline compounds with the bulk smectic-A to nematic phase transition. The calculations are performed at temperatures much lower than that of the bulk phase transition and at maximum temperatures of existence of films of given thickness. The time dependent displacement-displacement correlation functions for thermal smectic layer displacement fluctuations and time dependent intensity-intensity correlation functions for diffuse x-ray scattering from the films are also calculated. It is shown that unlike free-standing smectic-A films, an effect of the temperature on dynamics of smectic layer displacement fluctuations in solid substrate supported smectic-A films can be observed in experiments on coherent x-ray dynamic scattering from not very thick films (N approximately 20) and at significantly smaller wave vector transfer component in the film plane.

Disjoining pressure in free-standing smectic-A films and its effect on their reflectivity.

The results of calculations of a disjoining pressure and smectic layer spacings in free-standing smectic-A films (FSSAF's) heated above the temperature of disappearance of the smectic order in bulk liquid crystal samples are presented. An effect of the disjoining pressure on the optical reflectivity of FSSAF's having a different number of the smectic layers is investigated. The results of calculations are in agreement with results of experimental optical-reflectivity investigations of FSSAF's.

Influence of substrate microrelief on the Fréedericksz transition in a thin nematic cell.

An effect of substrate surface regular wavy microrelief on the Freedericksz transition in a thin homeotropically aligned nematic cell near the smectic-A-nematic second-order transition temperature is theoretically investigated. It is shown that because of the suppression of an interfacial smectic-A structure, the Fréedericksz critical field for the cell with substrates having a sufficiently sharp microrelief should be significantly lower than that for the cell of the same thickness, but with perfectly flat substrates.