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.

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.

Superfluidity inside carbon nanotubes.

Molecular dynamics simulations of equilibrium structures and flows of nonpolar argon atoms confined by single-walled carbon nanotubes (SWCNTs) with circular cross section and rectangular cross section having the same area and the ratio between its sides 1:4 have been performed. It has been shown that, inside these SWCNTs, argon atoms form the spatially ordered structures and, under action of external driving forces they move collectively along SWCNT's axes. It has been also obtained that there are two regimes of such collective movement.

Lifting ordered surfaces: Ellipsoidal nematic shells.

When a material surface is functionalized so as to acquire some type of order, functionalization of which soft condensed matter systems have recently provided many interesting examples, the modeler faces an alternative. Either the order is described on the curved, physical surface where it belongs, or it is described on a flat surface that is unrolled as preimage of the physical surface under a suitable height function. This paper applies a general method that pursues the latter avenue by lifting whatever order tensor is deemed appropriate from a flat to a curved surface.

Dynamics of magnetic (electric) field induced perturbation in homeotropic smectic A structure in liquid crystal cell with substrates having periodic microrelief.

The dynamics of a magnetic (electric) field induced perturbation in a homeotropic smectic A (SmA) structure in liquid crystal cells with substrates having periodic microrelief has been investigated. It has been shown that relaxation times of such perturbation in both switching-on and switching-off regimes are of the order of 10-10s. The results obtained show that previously proposed novel electro (magneto) optic effect in such SmA cell can be utilized in manufacturing of liquid crystal displays with operating times much shorter than those of ordinary modern displays based on the Freedericksz transitions in nematic liquid crystals.

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.

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.

Geodesic defect anchoring on nematic shells.

Nematic shells are colloidal particles coated with nematic liquid crystal molecules, which may freely glide and rotate on the colloid's surface while keeping their long axis on the local tangent plane. Molecular dynamics simulations on a nanoscopic spherical shell indicate that under appropriate adhesion conditions for the molecules on the equator, the equilibrium nematic texture exhibits at each pole a pair of +1/2 defects so close to one another to be treated as one +1 defect. Spirals connect the polar defects, though the continuum limit of the interaction potential would not feature any elastic anisotropy.

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.

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.

Molecular dynamics simulation of a nanoscopic nematic twist cell.

We present molecular dynamics simulations of a nanoscopic nematic twist cell confined within two bounding substrates with conflicting anchoring conditions. The results of our simulations show that the torque transmitted through the cell drops significantly below a certain critical cell's thickness, thus confirming the predictions of the continuum Landau theory extrapolated down to the nanoscopic scale [F. Bisi, E.

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.

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.

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.

Disruption of surface-induced smectic order by periodic surface corrugations.

A Fréedericksz transition measurement is reported for a liquid crystal cell composed of surfactant-coated substrates. One substrate was locally scribed with the stylus of an atomic force microscope to create a nanoscopic grooved structure. The Fréedericksz threshold voltage was found to be smaller in the scribed region than in the unscribed region, indicating that the corrugated surface disrupts surface-induced smectic order, and that the effect grows toward the nematic-smectic-A transition temperature T(NA) in conjunction with the smectic correlation length.

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.

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.

Thermal fluctuations and x-ray scattering from free-standing smectic-A films.

The present paper is devoted to theoretical investigation of thermal fluctuations and correlations between them in free-standing smectic-A films (FSSAF's) formed of liquid crystal compounds with bulk smectic-A-isotropic (SmA-I) and SmA-nematic (SmA-N) phase transitions, as well as small angle x-ray scattering from these FSSAF's. The study took into account the dependence of the bending elastic constant K and the smectic layer compressibility B on the distance from the boundary free surfaces of the films. The results of calculation are compared with those obtained in the framework of the Holyst model [Phys.