A. M. Parshin

Friedericksz Threshold Field in Bipolar Nematic Droplets with Strong Surface Anchoring

A numerical method has been developed for calculating the director configuration in ellipsoidal droplets of a nematic liquid crystal with strong tangential anchoring in a uniform magnetic field of an arbitrary orientation. A relation has been obtained for determining the Friedericksz threshold corresponding to the beginning of the reorientation of the central region of a droplet when the field is orthogonal to the biopolar axis. The effect of the breaking of the orthogonal condition on the threshold character of the orientation process is considered. The reorientation of the ensemble of bipolar droplets of the 5CB nematic liquid crystal dispersed in polyvinyl butyral has been studied by the magneto-optical method. Comparative analysis of calculation data and measured values of the threshold field has been performed.

Magnetic-field control of the transmission of a photonic crystal with a liquid-crystal defect

The method of modulation of transmittance of a multilayer photonic crystal with a nematic liquid-crystal defect upon the orientational transition from the homeotropic to the planar state is investigated. The orientation of the director of the nematic is controlled by a magnetic field in the B-effect mode. The method of recurrent relations is used for numerical simulation of transmission spectra of the photonic crystal structure under investigation.

Magnetic-field-assisted formation of alignment polymer coatings in liquid crystal cells

We describe a new method for obtaining liquid crystal (LC) layers with planar orientation in plane-parallel cells, which is based on the technology of LC-polymer interface formation in solution under the action of an applied magnetic field. The azimuthal anchoring energy of LC at the polymer surface has been determined by measuring the angle of orientation of the nematic LC director on the substrate surface as a function of the magnetic field. The LC orientation provided by the proposed method is stable, and the anisotropy of LC anchoring is comparable with that achieved using well-known methods of alignment polymer film preparation by rubbing.

Magnetic-Field-Induced Structural Transition in Polymer-Dispersed Liquid Crystals

The structural transition occurring in droplets of nematic liquid crystals formed in a polymer in magnetic field is studied. The transition is caused by field-driven weakening of tangential surface anchoring and develops following two scenarios: bipolar structure/homogeneous structure or bipolar structure/radial structure. The first scenario suggests a cyclic transition occurring upon thermal fluctuations without field. The structural transition is considered within a model of the biaxial liquid crystal/polymer interface. The model describes tendency of the nematic director to align tangentially on a droplets surface, normally under the action of flexoelectric polarization, and axially along the magnetic-field-imposed easy orientation axis.

The director field distribution with the strongly pinned alignment in nematic structures at the polymer surface

We investigate orientational nematic liquid crystal structures that form at the polymer surface due to adsorption, their stability against the schlieren texture and features of point and linear singularities in them. We demonstrate that surface disclination lines, which also occur due to molecular adsorption, can lead to azimuthal anchoring of a bulk layer and make the singularities with the charge m = ±1 stable. We consider the director field distribution in a nematic layer as a result of the combination of two aligning factors, which are the linear disclination that tend to impose uniform planar ordering and point defects that impart the radial configuration to the director. With the use of the calculation, we analyse the energies of elastic distortions of the director field for the radial–planar, radial–homeotropic and radial–radial configurations, depending on the nematic layer thickness.

Non‐monotonic temperature dependence of the alignment of a nematic mixture on a ferroelectric substrate

Polarization optic techniques have been applied to study specific features of the anisotropic interaction between a dye‐doped eutectic mixture of nematic liquid crystals p‐methoxybenzylidene‐p‐n‐butylaniline and p‐ethoxybenzylidene‐p‐n‐butylaniline and a polar surface of a ferroelectric triglycine sulphate crystal over the temperature range including the substrate Curie point T c. It has been found that the temperature‐induced structural changes in the nematic layer in the vicinity of T c are related to the changes in the orientational part of the tensor order parameter Qik . The temperature dependence of the director angle θ¯, averaged over the nematic layer, has been obtained from the effective dichroism values of solute absorption. The experimental data were interpreted using the model, in which the anisotropic part of the surface energy has two terms with orthogonal easy axes.

Use of Catastrophe Theory to the Study of Large-Scale Fluctuations in Nematic Droplets Prepared Under Magnetic Field

В рамках теории катастроф изучены крупномасштабные флуктуации в каплях нематика 5ЦБ, изготовленных по растворной технологии в присутствии магнитного поля. Из анализа потенциальных кривых установлено, что флуктуации обусловлены вариацией директора нематика между потенциальными минимумами, связанными с энергией сцепления жидкий кристалл-полимер и магнитным полем. Определена связь управляющих параметров катастрофы сборки с параметрами сцепления. Рассмотрено влияние магнитного поля на управляющие параметры, ведущее к структурному переходу в капле жидкого кристалла, который соответствует трансформации директора из биполярной структуры с расширенными полюсами в радиальную конфигурацию. На основании проведенного анализа оценены значения энергий сцепления жидкого кристалла с полимерной поверхностью. Ключевые слова: капсулированный полимером жидкий кристалл, энергия сцепления, катастрофа сборки, управляющие параметры, структурный переход.

Light transmission of liquid crystal domains formed by polycarbonate surface

We investigate the optical transmission of a monochromatic laser beam passing through an individual domain of a nematic liquid crystal formed by the polycarbonate surface. The domain has the radial orientation structure with a disclination line on the polymer surface, which gradually transforms to “pseudoplanar” alignment at the coherence distance ξ from the surface. The dependence of the optical transmittance on an electric field applied to the domain are different at various light polarization directions relative to the disclination lines and, in the absence of an analyzer, are accompanied by interference oscillations. To explain the results obtained, a domain is considered to be a gradient lens with the refractive index variable in the plane perpendicular to the laser beam and the resulting deflecting effect that was collected at the beam path through the liquid crystal layer.

Electro- and Magnetooptic Effects in Liquid Crystal Domains Formed by Polymer Surface

С использованием электрического и магнитного полей исследовано прохождение лазерного луча через отдельный домен, а также через ансамбль доменов, сформированных поверхностью поликарбоната. Обнаружены интерференционные эффекты в зависимости от направления поляризации света относительно линии дисклинации отдельного домена. В скрещенных электрическом и магнитном полях, приложенных к ансамблю доменов, наблюдалось исчезновение интерференционных экстремумов при увеличении магнитного поля. Полученные результаты объяснены в рамках градиентной оптики. Проведено сравнение магнитной и электрической длин когерентности c длиной когерентности домена. Ключевые слова: жидкий кристалл, полимер, доменная структура, показатель преломления, интерференция, электрическое поле, магнитное поле.

Orientational changes in the nematic liquid crystal structure on a polymer surface induced by phase separation in a magnetic field

The change in the domain texture of the 5CB nematic liquid crystal on the surface of a polyvinyl butyral gelatinous polymer solution cured in a magnetic field is studied optically. The alignment of the nematic is described by the effective order parameter S* of the polydomain cells. The anchoring energy of a liquid crystal with gelatinous polymer Wϕ = 1 × 10−3 erg/cm2 is determined.