Birefringence Patterns of Nematic Droplets
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Abstract:
Birefringence patterns of nematic liquid crystalline droplets are investigated both theoretically and experimentally. Gridding method is developed to calculate optical birefringence patterns of nematic droplets with different director configurations. Series of birefringence patterns of the four typical nematic droplets (radial, bipolar, axial and equatorial) with different orientations of droplets' axes are obtained. By comparing experimental polarizing photographs with theoretical birefringence patterns, the authors find out that only bipolar configuration represents the self-organized structure of nematic droplets in our epoxy-based polymer dispersed liquid crystal (PDLC). It is also pointed out that the very gridding method can be easily extended to deal with other complicated anisotropic entities.Keywords:
Biaxial nematic
An analytic expression for the optical field propagating normally in a general twisted nematic liquid-crystal structure is obtained for the first time. Based upon this, analytic expressions are obtained for the optical transmissions of the general twisted nematic liquid-crystal displays (including standard twisted nematic, supertwisted, supertwisted birefringence effect, lower twisted, and optical mode interference) with arbitrary polarizer orientations, liquid-crystal indices of refraction, twist angle, surface pretilt angle, and cell thickness. Routes for improving the display performances are discussed.
Polarizer
Biaxial nematic
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Electronegativity
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In this work, experimental birefringence data of several nematic liquid crystals, which share the presence of a nematic–isotropic and a nematic–crystalline phase transition, are used to show that the birefringence presents a universal behaviour which encompasses the entire range of the nematic phase. The meaning and consequences of such behaviour are discussed; as the order parameter is proportional to the birefringence, this result implies that the order parameter has an extended universal profile, similar to a corresponding state law, which is independent of the liquid crystal compound and becomes determined once the temperature interval of the existence of the nematic compounds is obtained.
Biaxial nematic
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Abstract We report a simple interference method to determine the dispersion of the extraordinary refractive index and birefringence of highly conjugated and coloured nematic liquid crystals used as light‐emitting materials in organic electroluminescent devices. The measurements are made in the nematic glass phase at room temperature. The birefringence is highly dispersive and values up to 1.1 are obtained. Chiral groups are incorporated into the end chains giving a chiral nematic liquid crystal with a very wide stopband in the visible region. The Berreman matrix method is used to simulate transmission through the chiral nematic liquid crystal cell using the refractive index parameters obtained experimentally. Excellent agreement between theory and experiment is found.
Biaxial nematic
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Birefringence patterns of nematic liquid crystalline droplets are investigated both theoretically and experimentally. Gridding method is developed to calculate optical birefringence patterns of nematic droplets with different director configurations. Series of birefringence patterns of the four typical nematic droplets (radial, bipolar, axial and equatorial) with different orientations of droplets' axes are obtained. By comparing experimental polarizing photographs with theoretical birefringence patterns, the authors find out that only bipolar configuration represents the self-organized structure of nematic droplets in our epoxy-based polymer dispersed liquid crystal (PDLC). It is also pointed out that the very gridding method can be easily extended to deal with other complicated anisotropic entities.
Biaxial nematic
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A linear relation is found between the cube of the optical birefringence and the temperature range of the biaxial nematic phase in the lyotropic mixtures of potassium laurate, l-decanol, D2O. Such a relation which is consistent with the Landau model of the uniaxial to biaxial nematic phase transitions may be used as a criterion for preparing samples of the desired temperature range of the biaxial nematic phase.
Lyotropic
Biaxial nematic
Atmospheric temperature range
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We report the measurements of birefringence as a function of temperature of a binary system 4-cyanophenyl [4'(4''-n-heptylphenyl)]benzoate (7CPB) + 4-cyanophenyl 4-nonylbenzoate (9.CN) showing a nematic–smectic Ad-re-entrant nematic phase sequence by means of the optical transmission method. The temperature dependence of the birefringence has been determined from the transmitted intensity data and the orientational order parameters have been calculated. These observations indicate that re-entrant nematic to induced smectic Ad and induced smectic Ad to nematic phase transitions for all the mixtures are of second order. There is a continuous change in the Δn values at the nematic–smectic Ad and smectic Ad-re-entrant nematic phase transitions. However, for some mixtures a slight increase in birefringence on cooling in the vicinity of the smectic Ad-re-entrant nematic transition has been observed. We have also fitted our experimental results with those calculated from the modified McMillan theory as proposed by Luckhurst and Timimi.
Biaxial nematic
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Biaxial nematic
Thermotropic crystal
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We report on the experimental and numerical results of the nonlinear light beam propagation in planar samples of chiral nematic liquid crystals, for both high- and low-birefringence chiral nematic liquid crystals. We investigate the properties and stability of obtained nematicons. We found that reorientational spatial solitons require larger excitation powers in low-birefringence chiral nematics than in high-birefringence but remain more stable and self-focused.
Biaxial nematic
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Abstract Novel liquid crystal compounds with extraordinary large birefringence have been studided from the viewpoint of optical anisotropy. A large birefrinence was obtained in the compounds compared with that in 5CB, while the orientational order parameters determined by FT-IR measurement are almost the same. From the calculated results of molecular polarizability, it is concluded that the large optical anisotropy is mainly caused by the electric anisotropy.
Optical anisotropy
Acetylene
Mesogen
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