Only a few years have passed since the discovery of polar nematics, and now they are becoming the most actively studied liquid-crystal materials. Despite numerous breakthrough findings made recently, a theoretical systematization is still lacking. In the present paper, we take a step toward systematization. The powerful technique of molecular-statistical physics has been applied to an assembly of polar molecules influenced by electric field. Three polar nematic phases were found to be stable at various conditions: the double-splay ferroelectric nematic N_{F}^{2D} (observed in the lower-temperature range in the absence of or at low electric field), the double-splay antiferroelectric nematic N_{AF} (observed at intermediate temperature in the absence of or at low electric field), and the single-splay ferroelectric nematic N_{F}^{1D} (observed at moderate electric field at any temperature below transition into paraelectric nematic N and in the higher-temperature range (also below N) at low electric field or without it. A paradoxical transition from N_{F}^{1D} to N induced by application of higher electric field has been found and explained. A transformation of the structure of polar nematic phases at the application of electric field has also been investigated by Monte Carlo simulations and experimentally by observation of polarizing optical microscope images. In particular, it has been realized that, at planar anchoring, N_{AF} in the presence of a moderate out-of-plane electric field exhibits twofold splay modulation: antiferroelectric in the plane of the substrate and ferroelectric in the plane normal to the substrate. Several additional subtransitions related to fitting the confined geometry of the cell by the structure of polar phases were detected.

Microparticles exhibit light-driven trapping, oscillation, rotation, and complex motions in free-surface liquid crystal films due to Marangoni convection and related director deformations.

The possibility of the existence of discotic nematic phase in a system of rod-like molecules with negative dielectric anisotropy is proved by the methods of molecular-statistical physics.The discotic nematic state is realized instead of isotropic liquid in the presence of electric field.The possibility of existence of discotic nematic state instead of calamitic nematic one in the presence of nanoparticles impurity is also shown.Temperature-induced phase transitions between calamitic and discotic phases at fixed value of electric field are described.It is shown that for any electric field value there is certain temperature range in which calamic phase is observed near nanoparticles and discotic phase is observed far away from nanoparticles.The spatial distributions of orientational order parameters are obtained.It is shown that the distance over which the liquid crystal configuration induced by inhomogeneity source can propagate is proportional to the square root of the difference between the bend and splay elastic constants.

Light-induced director field deformation of a nematic liquid crystal in the field of an obliquely incident laser beam is experimentally studied with aberrational self-action and polarization microscopy. Each of the methods has features associated with the geometry of the light interaction with the director. The combination of methods significantly expands the possibilities of reconstructing the light-induced nonlinear phase shift profile and the director field deformation.

Within the framework of phenomenological approach, a generalized concept of a non-local order parameter, that have the form of a traceless correlation tensor function or a tensor integral operator, is introduced. The main relationships, which determine the equilibrium orientational states and the phase transitions of a deformed nematic liquid crystal, are described. The resulting expression for the free energy is quadratic with respect to the gradients of the order parameter tensor. The limiting transition of the non-local order parameter to the local one leads to an expression that complements the classical Oseen–Frank theory of elasticity. The considered model includes eight independent elastic constants. Three of them relate to "bulk" constants analogous to Frank constants, another three ones relate to "surface" constants, and the remaining two constants determine the anisotropic effect of the gradient of the scalar order parameter on the director field. The constants are necessary for the consideration of a liquid crystal state near defects, as well as for the description of orientation effects caused by the inhomogeneity of order parameter. It is shown that in the free energy expression, the "surface" terms contribute to the director orientation in the case of a non-constant scalar order parameter, even in the approximation of rigid boundary conditions.

Liquid crystals are of great interest in the field of nonlinear optics due to their efficient response to low intensity light fields. Here we present a new, to the best of our knowledge, mechanism of a nonlinear optical response which is observed for a dye-doped dual-frequency nematic liquid crystal. The local increase in temperature caused by the absorption of light beam in the liquid crystal medium leads to spatial variation and inversion of the sign of the dielectric anisotropy. When an alternating current electric field with a frequency close to the cross-over frequency is applied to the liquid crystal cell, the planar orientation sustains at the beam periphery, but elastic deformation occurs in the irradiation region. In the case of a dye dopant with negative absorption dichroism, a first-order orientational transition with large bistability region is obtained.

We have elaborated a theoretical approach for the description of polar nematic phases observed by Nishikawa et al. [Adv. Mater. 29, 1702354 (2017)0935-964810.1002/adma.201702354], their structures, and transitions between them. Specific symmetry contributions to the pair molecular potentials provide the molecular mechanisms responsible for the formation of proper and improper polarity on the macroscopic level. An improper antiferroelectric nematic M2 phase can arise between paraelectric nematic M1 and proper ferroelectric nematic MP in the temperature scale. The local polarization in M2 arises mostly due to the local splay deformation. The director distribution in M2 represents the conjugation of cylindrical waves with opposite splay and polarization signs. The director and polarization are parallel to the cylindrical domain axes in the middle of each cylinder but exhibit considerable (mostly radial) deformation on the periphery of each cylinder. Polarization vectors are mostly stacked antiparallel on the borders between the domains without the director disruption. The domain size decreases with the decreasing temperature, the percentage of the antiferroelectric decouplings increases, and M2 exhibits the first-order phase transition into proper ferroelectric MP. With the increasing temperature the domain size in the M2 phase increases, the domination of particular polar orientation of molecules reduces, and finally, the domain size diverges at particular temperature corresponding to the second-order phase transition from M2 to paraelectric M1. Variations of the polar and nonpolar orientational order parameters are estimated within each phase and between the phases. Our experimental and computer simulation results (also presented in the paper) fully support our theoretical findings.

The processes of light-induced orientation at oblique incidence of a light beam on a dye-doped nematic liquid crystal (NLC) are experimentally studied with the simultaneous use of the methods of aberrational light beam self-action and polarization microscopy. Each method has features associated with the geometry of light propagation in an NLC, and their combination significantly expands the possibilities of reconstructing the light-induced profile of the nonlinear phase shift and director deformation field.

Local illumination with a light beam leads to thermo-orientational processes in a frustrated chiral nematic film with a free surface. Light-induced hydrodynamic flow and orientational structure create an adaptive platform for the collection, translation and rotation of suspended spherical microparticles. The demonstrated approach has potential applications in soft robotics, micro-object delivery systems, and other micro- and nanotechnologies.

We report the light beam action on the nematic liquid crystal film with a free surface. It was found that a weak light absorption by the liquid crystal substrate dramatically changes the orienting properties of the light beam; in particular, a thermal gradient field induces an umbilical defect formation.

A comparative study of optical orientation in nematic liquid crystals (NLCs) doped with a low-molar-mass bis-azobenzene dye (monomer) and a comb-shaped polymer with side fragments similar in structure to the monomer has been carried out. Both types of bis-azobenzene dopants induced a signalternating nonlinearity in the NLC: the positive when the angle Psi between the light field and the director is less than a certain critical value Psi(c) and the negative in the opposite case of Psi(c) < Psi <= 90 degrees. The transition from the monomer to polymer led to a decrease in the critical angle Psi(c), i.e., to an expansion of the region of negative nonlinearity. At the same concentration of chromophores, an increase in both negative and positive nonlinearities occurs. The magnitudes of the optical-torque enhancement factors due to trans- and cis-isomers (eta(T) and eta(C)), and the ratios eta(T)/eta(C) for the low- and high-molar-mass dopants are compared. The results obtained show the possibilities of increasing the orientational optical nonlinearity when passing from low-molar bis-azobenzene dye dopants to the corresponding polymers and are important for elucidating the optimal architecture of absorbing additives. (C) 2021 Elsevier B.V. All rights reserved.

Liquid crystals (LCs) are distinctive materials that are applicable to a wide range of disciplines, such as continuum mechanics, optics of anisotropic media, statistical physics, and crystallography. The diverse structures and eye-catching topological defects of LCs in confined geometries are affected by nematic elasticity, chirality, and surface anchoring. Herein, we report the formation and detailed configuration of cholesteric LC (CLC) microdroplets with different pitches pierced by electrospun poly(methyl methacrylate) microfibers. Two kinds of surface anchoring, namely, homeotropic anchoring at the air–CLC droplet interface and planar anchoring at the fiber–CLC droplet interface, coexist in this system. By controlling temperature and light irradiation, the system exhibits thermal- and photo-dependent LC morphological and topological evolutions. The observed structures are complemented by numerical simulations of possible director fields decorated by defects. The externally controllable CLC necklaces constitute extraordinary systems for exploring the morphology and topological defects and open a route for applications in topological remote control, nanoscience, biomedical research, and the development of devices based on topologically structured soft media.

The effect of nonlinear light action on a thin (∼10 µm) films of the nematic liquid crystal deposited onto the absorbing substrate is experimentally investigated. The dynamics of the orientational and thermocapillary effects is directly studied. The two types of orientational processes were found out. The first one appears for several hundreds of milliseconds when the light beam irradiation is turned on or off. The second one develops much slowly and does not relax during the light beam irradiation.

Viscoelastic nanocomposites based on a matrix of semidilute aqueous solutions of entangled wormlike (polymer-like) micelles of a cationic surfactant and aluminosilicate nanoplates of montmorillonite clay, efficiently acting as crosslinks, were prepared and studied. The produced materials represent a new type of a self-assembling nanocomposite system. The results of changes in viscoelastic properties of nanocomposites when adding clay nanoplates are well described by a proposed interaction model which implies adsorption of wormlike micelles by end-caps to the surfactant layer formed on the nanoplates surface.

In this study, we first demonstrate the synthesis of recyclable polymer beads for Cu 2+ sensing based on radially aligned liquid crystal (LC) assistance. To detect metal ions in water, sensing probe monomers and polymers derived from rhodamine B were synthesized. Recyclable polymeric LC beads were prepared from LC monomers RM257 and RM105, a nonreactive mesogen of 5CB and a rhodamine B-derived monomer. Due to the assistance of radially aligned LCs, highly sterically hindered spirocyclic terminal groups of rhodamine B-derived monomers were aligned and fixed at the outer surface of liquid crystal beads. The color of the polymeric LC beads changed from light pink to deep pink after the beads were dropped into an aqueous Cu 2+ solution. The results were ascribed to the spiro ring-opening mechanism. The addition of a nonreactive mesogen resulted in the porous structure of the polymeric LC beads. The high sensitivity of the Cu 2+ solution using polymeric LC beads was confirmed. The fabricated polymeric LC beads were recycled by putting the polymeric LC beads into aqueous ammonia. The removal of Cu 2+ from polymeric LC beads was due to the formation of [Cu(NH 3 ) 4 ] 2+ . This recyclable LC bead sensor is an easy method for the detection of metal ions in aqueous solutions.

Dye-doped nematic side-chain liquid-crystalline polymers possess extraordinary large optical nonlinearity and ability to store the induced orientational deformations in a glassy state, which makes them a very promising material for photonic applications. In this study, the phase structures were generated and recorded in the bulk of a 50-μm layer of a nematic liquid-crystalline side-chain polymer, containing polyacrylate backbone, spacer having five methylene groups, and phenyl benzoate mesogenic fragment. The polymer was doped with KD-1 azodye. The director field deformations induced by the light beam close to the TEM01 mode were studied for different geometries of light–polymer interaction. The phase modulation depth of 2π was obtained for the 18-μm spacing between intensity peaks. The experimental data were analyzed based on the elastic continuum theory of nematics. The possibility to induce and record positive and negative microlenses in the polymer bulk was shown experimentally.

Orientational structures in spherical-cap droplets of chiral nematic liquid crystal are investigated experimentally and numerically. The droplets under the study have homeotropic anchoring with a flat rigid substrate and either degenerated planar or homeotropic anchoring with spherical interface between the liquid crystal and the isotropic liquid. The axially symmetric skyrmion-like structures arise when the droplet size is large enough with respect to the cholesteric pitch. The influence of electric field on the orientational structures is studied.

— The properties of the aberrational pattern resulting from the self-action of the light radiation mode TEM 01 in liquid-crystal systems are studied. It is found that the interference of light rays corresponding to two peaks of intensity and nonlinear phase shift leads to the formation of a system of fringes in the Fraunhofer region; their angular period is controlled by the distance between peaks. These fringes fill the entire aberrational pattern (concentric ring pattern) or only its part, depending on the type of the profile of the light-induced phase shift. The possibilities of using the properties of the aberrational pattern in the TEM 01 mode for studying the light-induced reorientation of the nematic liquid crystal director are shown.

In this study, the beam mode conversion in a nematic liquid crystal film deposited on the end-face of a single-mode optical fibre is considered. The infrared laser radiation delivered by this fibre induces a defect structure in the initially homeotropic liquid crystal. This leads to the formation of an output light beam with the annular intensity distribution in its cross-section. The obtained beam mode maintains its transverse profile in free space and can be interpreted as an optical vortex.

We propose a simple and effective method of liquid crystal photoalignment that does not require any preliminary treatment of the cell substrates. To this aim, a small amount (0.1 wt %) of azobenzene carboxylic acid is added into the nematic liquid crystal. After filling the liquid crystal cell, a part of the dopant molecules is spontaneously adsorbed and attached to the glass surface by hydrogen bonds. This allows one to switch the boundary conditions of liquid crystal between homeotropic and planar due to the reversible trans-cis photoizomerization. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement