Defects and Textures of Liquid Crystals

Lavrentovich, Oleg D.

doi:10.1002/9783527671403.hlc027

Cited by 7 publications

(4 citation statements)

References 171 publications

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“…It can also reduce transmission and visual contrast of some displays with the PDLC. Similar pictures are also obtained for many LC systems with defects …”

Section: Simulation Resultssupporting

confidence: 79%

Optical properties of composite heterophase objects with liquid crystal material for different display applications

et al. 2017

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Liquid crystal (LC) director distribution and optical transmission for different types of heterophase systems with different LC boundary conditions is simulated. The first type is a transparent isotropic material with spherical or cylindrical liquid crystalline objects. There are polymer dispersed liquid crystal, LC fiber, lyotropic LC in polarizing films, LC in microgroove and nanogrooves and pores. The second type is an LC layer incorporating an isotropic transparent or non‐transparent object like microparticles and nanoparticles, spacers, protrusions in multi‐domain vertical alignment LC display et al. The system parameters' influence on LC display performances is discussed.

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“…It can also reduce transmission and visual contrast of some displays with the PDLC. Similar pictures are also obtained for many LC systems with defects …”

Section: Simulation Resultssupporting

confidence: 79%

Optical properties of composite heterophase objects with liquid crystal material for different display applications

et al. 2017

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“…3(d). Note that although λ +1 lines are escaped, the director field is strongly distorted around it, which explains why one sees micronthick lines [28]. This the first experimental evidence of the numerically predicted RSS in cholesterics [1].…”

Section: Bipolar Structurementioning

confidence: 70%

Waltzing route toward double-helix formation in cholesteric shells

Darmon

Benzaquen

Seč

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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Liquid crystals, when confined to a spherical shell, offer fascinating possibilities for producing artificial mesoscopic atoms, which could then self-assemble into materials structured at a nanoscale, such as photonic crystals or metamaterials. The spherical curvature of the shell imposes topological constraints in the molecular ordering of the liquid crystal, resulting in the formation of defects. Controlling the number of defects, that is, the shell valency, and their positions, is a key success factor for the realization of those materials. Liquid crystals with helical cholesteric order offer a promising, yet unexplored way of controlling the shell defect configuration. In this paper, we study cholesteric shells with monovalent and bivalent defect configurations. By bringing together experiments and numerical simulations, we show that the defects appearing in these two configurations have a complex inner structure, as recently reported for simulated droplets. Bivalent shells possess two highly structured defects, which are composed of a number of smaller defect rings that pile up through the shell. Monovalent shells have a single radial defect, which is composed of two nonsingular defect lines that wind around each other in a double-helix structure. The stability of the bivalent configuration against the monovalent one is controlled by c = h/p, where h is the shell thickness and p the cholesteric helical pitch. By playing with the shell geometry, we can trigger the transition between the two configurations. This transition involves a fascinating waltz dynamics, where the two defects come closer while turning around each other.iquid crystals offer fascinating possibilities for producing materials that are organized at a mesoscopic scale, such as photonic crystals or metamaterials (1). In a seminal paper, Nelson (2) proposed to induce valency into simple spherical particles by coating their surfaces with a nematic liquid crystal shell. In this elegant approach, the spherical symmetry of the particle is broken by the presence of topological defects, which stem from frustrations in the liquid crystal orientational order due to the curvature of the particle surface. These defects, once functionalized, could act as sticky surface patches inducing directional particle bonding through patchpatch interactions, which would make possible the fabrication of complex materials by spontaneous self-assembly. The number of surface defects would set the valence of the particle, whereas their position would determine the directionality of the eventual bonds. The type of defect configuration in the shell, and thus its valence, results from a very subtle interplay between topological constraints and elastic free-energy minimization. For a two-dimensional nematic shell, theory predicts a tetravalent configuration where four defects are organized in a tetrahedral fashion (3). These defects have a winding number s i = 1=2, indicating a π-rotation of n, the average molecular orientation, around the defect (4). This is consistent with...

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“…2. 42 In the SE5661 cells, one observes two different textures as the temperature is varied. Above some critical temperature t*, the textures are of the classic N u Schlieren type, Fig.…”

Section: Polarizing Optical Microscopymentioning

confidence: 99%

Domain walls and anchoring transitions mimicking nematic biaxiality in the oxadiazole bent-core liquid crystal C7

et al. 2015

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We investigate the origin of "secondary disclinations" that were recently described as new evidence of a biaxial nematic phase in an oxadiazole bent-core thermotropic liquid crystal C7. Using an assortment of optical techniques such as polarizing optical microscopy, LC PolScope, and fluorescence confocal polarizing microscopy, we demonstrate that the secondary disclinations represent non-singular domain walls formed in a uniaxial nematic phase during the surface anchoring transition, in which surface orientation of the director changes from tangential (parallel to the bounding plates) to tilted. Each domain wall separates two regions with the director tilted in opposite azimuthal directions. At the centre of the wall, the director remains parallel to the bounding plates. The domain walls can be easily removed by applying a moderate electric field. The anchoring transition is explained by the balance of (a) the intrinsic perpendicular surface anchoring produced by the polyimide aligning layer and (b) tangential alignment caused by ionic impurities forming electric double layers. The model is supported by the fact that the temperature of the tangentially tilted anchoring transition decreases as the cell thickness increases and as the concentration of ionic species (added salt) increases. We also demonstrate that the surface alignment is strongly affected by thermal degradation of the samples. This study shows that C7 exhibits only a uniaxial nematic phase and demonstrates yet another mechanism (formation of "secondary disclinations") by which a uniaxial nematic phase can mimic a biaxial nematic behaviour.

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Defects and Textures of Liquid Crystals

Cited by 7 publications

References 171 publications

Optical properties of composite heterophase objects with liquid crystal material for different display applications

Optical properties of composite heterophase objects with liquid crystal material for different display applications

Waltzing route toward double-helix formation in cholesteric shells

Domain walls and anchoring transitions mimicking nematic biaxiality in the oxadiazole bent-core liquid crystal C7

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