Director configurations in nematic droplets with tilted surface anchoring

The polymer dispersed nematic liquid crystal (LC) with the tilted surface anchoring has been studied. The droplet orientational structures with two point surface defects – boojums and the surface ring defect – are formed within the films. The director tilt angle α = 40° ± 4° at the droplet interface and LC surface anchoring strength W_s ~ 10^–6 (J m^?2) have been estimated. The bipolar axes within the studied droplets of oblate ellipsoidal form can be randomly oriented are oriented randomly relatively to the ellipsoid axes as opposed to the droplets with homeotropic and tangential anchoring.     

Electrically controlled local Frédericksz transition in a layer of a nematic liquid crystal

A local Frédericksz transition caused by electrically controlled surface anchoring in a nematic liquid crystal has been implemented owing to the displacement of surface-active ions. In the initial case, the nematic is homeotropically aligned by monomolecular layers of cetyltrimethylammonium bromide cations adsorbed on the surface of substrates. One of the substrates in a static electric field becomes free of a cation layer and specifies planar boundary conditions; as a result, a homeoplanar orientation structure is formed. The features of the dynamics of the optical response of the cell under study that are characteristic of the effect under consideration are discussed.     

Inverse regime of ionic modification of surface anchoring in nematic droplets

JETP Letters - It was found that the effect of the ionic modification of anchoring in liquid-crystal droplets can be implemented in the inverse regime. Droplets of 4-n-pentyl-4′-cyanobiphenyl...     

Orientational structures in nematic droplets with conical boundary conditions

Slightly diluted magnetic systems described by the disordered three-dimensional Potts model with the number of spin states q = 3 are studied in the case of a simple cubic lattice. The position of the tricritical point in the phase diagram is determined using the histogram Monte Carlo technique.     

Orientational transition in the cholesteric layer induced by electrically controlled ionic modification of the surface anchoring

A reorientation of cholesteric liquid crystal with a large helix pitch induced by the electrically controlled ionic modification of the surface anchoring has been studied. In initial state, the cholesteric helix is untwisted completely owing to the normal surface anchoring specified by the cations adsorbed at the substrates. As a result, the homeotropic director configuration is observed within the cell. Under the action of DC electric field, one of the substrates becomes free from the layer of surface active cations, therefore, setting the planar surface anchoring. The latter, in turn, leads to the formation of the hybrid chiral structure. The threshold value and dynamic parameters have been estimated for this process as well as the range of control voltages, which do not allow the electrohydrodynamic instabilities. The twisted hybrid director configuration observed in the experiment has been analysed by means of the simulation of polarisation change of light propagating through the cholesteric layer with asymmetric (planar and homeotropic) surface anchoring on the cell substrates.     

Electrically induced anchoring transition in nematics with small or zero dielectric anisotropy

The orientational transitions induced by electrically controlled ionic modification of surface anchoring in liquid crystal cells based on the nematics with small or zero dielectric anisotropy Δε are considered. The type of director reorientation is shown to be independent of the sign of dielectric anisotropy and can be the same for the nematics with both negative and positive Δε. Besides, the orientational transition and corresponding switchable optical states do not depend on the Δε value and can be effectively realised even for the nematics with zero dielectric anisotropy.     

Electrically induced anchoring transition in cholesteric liquid crystal cells with different confinement ratios

Reorientation of cholesteric liquid crystal induced by the electrically controlled ionic modification of surface anchoring within the cell with confinement ratio exceeding 1 has been studied. The change of homeotropic surface anchoring to the planar one on the electrode-anode substrate under the action of DC voltage causes the formation of the modulated hybrid-aligned cholesteric layer in the cell. Optical texture of the liquid crystal layer with such an orientation structure is the linear periodic stripes. Homogeneity of emerging optical texture depending on the confinement ratio as well as on the prehistory of voltage application has been considered. It has been found that the ionic modification of surface anchoring results in total transformation of the diffraction pattern observed after the laser beam passing through the sample.     

Transformation of cholesteric orientational structures and optical textures induced by the electric field–driven ionic modification of surface anchoring

The reorientation of a cholesteric liquid crystal with a large helical pitch induced by the electric field–driven modification of surface anchoring is investigated. In the initial state, the liquid crystal cell has a homeotropic alignment of the director. An applied dc electric field produced a twisted homeoplanar structure of the cholesteric.     

Liquid crystal materials with ionic-surfactant operation

A conceptually new approach for developing methods of control of liquid crystals is discussed. The approach is based on modifying surface anchoring. A brief review of the authors’ research on developing ionic-surfactant control of liquid crystalline materials is described. This approach is based on the effect of electrically induced modification of the boundary conditions by ions of surface-active compounds.     

Inverse regime of ionic modification of surface anchoring in nematic droplets

It was found that the effect of the ionic modification of anchoring in liquid-crystal droplets can be implemented in the inverse regime. Droplets of 4-n-pentyl-4′-cyanobiphenyl nematic doped with ionic cetyltrimethylammonium bromide surfactant were dispersed in polyvinyl alcohol and investigated. In the initial state, nematic droplets have a radial structure with homogeneous homeotropic anchoring typical of the surfactant used. In the presence of a dc electric field, the boundary conditions become tangential in the surface area left by the cations. That results in the transformation of an orientational structure following by various scenarios. For the new states of nematic droplets, the distribution of the director field was analyzed and the corresponding textural patterns were numerically calculated.