The polymer dispersed nematic liquid crystal 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 = 40 4 at the droplet interface and LC surface anchoring strength Ws ~ 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.
This study is concerned with the optical response of an electrooptical material consisting of nematic liquid crystal as well as ionic surfactant as a dopant. The dopant is a key component to carry out the working of the resulting device through configuration switching. The operational principle is based on the surface anchoring transition induced by a steady electric field. The dynamic characteristics of the electro-optical cell can be considerably improved when the nematic layer is reoriented from the initially homeoplanar director configuration into the twisted state. Besides, a method to shorten the relaxation time is demonstrated by using a controlling pulse with a bipolar waveform.
Orientational structures formed in nematic and cholesteric layers with tangential-conical boundary conditions have been investigated. LC cells with one substrate specifying the conical surface anchoring and another substrate specifying the tangential one have been considered. The director configurations and topological defects have been identified analyzing the texture patterns obtained by polarizing microscope in comparison with the structures and optical textures calculated by free energy minimization procedure of director field and finite-difference time-domain method, respectively. The domains, periodic structures and two-dimensional defects proper to the LC cells with tangential-conical anchoring have been studied depending on the layer thickness and cholesteric pitch.
В данной работе рассмотрены капли нематика сплюснутой формы, капсулированные в полимере, задающем конические граничные условия. Расчеты по методу расширенного эластического континуума Франка показали возможность образования ряда различных структур в таких каплях при варьировании размера капель. Экспериментальные исследования методом поляризационной оптической микроскопии образцов композитных пленок подтверждают результаты расчетов, демонстрируя образование в рассматриваемой системе следующих ориентационных структур: радиал-биполярной структуры с закрученным дефектом-ежом и двумя гиперболическими буджумами, аксиал-биполярной структуры с кольцевой дисклинацией и двумя радиальными буджумами и структуры с дефектом-ежом, гиперболическим буджумом и радиальным буджумом. Такое разнообразие возможных топологий капель обусловлено сложным балансом между энергиями упругости поля директора, дисклинаций и сцепления с поверхностью, обеспеченным коническими граничными условиями.
Polymer dispersed nematic liquid crystal (PDNLC) films with conical boundary conditions at the LC-polymer interface are considered. The conical surface anchoring with tilt angle 40 • initiates forming the axial-bipolar director configuration inside nematic droplets. This droplet structure exhibits the strong scattering of light polarized parallel to the bipolar axis. In the initial state, the bipolar axes in all droplets are oriented randomly, and therefore PDNLC film scatters a light of any polarization. Electric field applied along the film plane orients the bipolar axes unidirectionally in the whole droplet ensemble, that results in its high polarization-dependent transmittance. Such PDNLC films can be used in the electrically controllable linear polarizers characterized by 89% value of the transmittance for the perpendicular polarized light and high extinction ratio 590:1 at the electric field 0.34 V/µm.
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 analyzed by means of the simulation of polarization 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 realized even for the nematics with zero dielectric anisotropy.
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