Experimetal measurement of the director orientation at the free surface of a nematic liquid crystal
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Biaxial nematic
Thermotropic crystal
Biaxial nematic
Mesogen
Lyotropic
Columnar phase
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The affine transformation model---formulated for ellipsoidal particles with axis ratio Q and previously used for the calculation of the viscous properties of nematic liquid crystals with perfect orientation order---is generalized to realistic uniaxial phases with imperfect order (Maier-Saupe order parmeter S1). For the nematic liquid crystals N-(4'-methoxybenzylidene)-4-(n-butyl)aniline (MBBA) and p-methoxy-p'-n-butylazoxybenezene (N4) and some cyanobiphenyls (kCB and kOCB with k=5,8) a comparison of the theoretically obtained Miesowicz viscosities with experimental data is made.
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MBBA
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The formation of biaxial nematic phases in mixtures of surfactants and water is discussed. Emphasis is given to experimental data that confirm the existence of a biaxial nematic phase. Some new microscopic observations and birefringence measurements are reported. Biaxial nematic phases were observed in two systems that have, in addition, two uniaxial nematic phases. The general features of the phase transitions involving these nematic phases are theoretically analyzed on the basis of a Landau expansion. Specific relations for the order parameters are derived that compare favorably with the experimental results.
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Abstract The field effects in nematic liquid crystal layers are reanalysed using catastrophe theory. The layer with pretilted director orientation and obliquely applied magnetic field, the hybrid aligned nematic cell and twisted nematic structures are considered. The stable solutions are identified and transitions between them are specified. The results are in essential agreement with previous work. Some details concerning the behaviour near the threshold are revealed.
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Catastrophe Theory
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In this work experimental and theoretical results will be used to show that under convenient conditions; appropriated geometry, well chosen boundary conditions, and suitable external fields, the elastic and dynamic behavior of a sample of a biaxial nematic liquid crystal can become similar to the one observed on usual uniaxial nematic liquid crystals. It is shown that this result can be used to determine the values of some combinations of the rheologic parameters of the biaxial phase.
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We describe new topologies in the phase diagrams involving biaxial nematic liquid crystals. We find that a direct isotropic-biaxial nematic phase transition is possible. Our results show that three different biaxial nematic phases can occur. We outline how the novel phase diagrams could be detected experimentally.
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Abstract Abstract In his classic paper on the elastic behaviour of nematic liquid crystals published in 1958, Sir Charles Frank hints at the existence of a nematic phase with biaxial symmetry in addition to the conventional nematic phase with its uniaxial symmetry. Little notice appears to have been taken of this intriguing suggestion until 1970 when Freiser presented a molecular theory of nematics formed from biaxial molecules. This predicted that such materials should exhibit isotropic, uniaxial nematic and biaxial nematic phases although for a certain critical biaxiality the system should pass directly from the isotropic phase to a biaxial nematic. Despite these clear predictions it was not until the late 1980s that the claims to have prepared compounds exhibiting a thermotropic biaxial nematic phase began to appear; these originated from the laboratories of Jaques Malthtte, S. Chandrasekhar and Klaus Praefcke. However, subsequent studies of some of the materials suggested that the nematic phases claimed to be biaxial were, in fact, uniaxial. Although the formation of real biaxial nematics is in doubt, computer simulators have produced biaxial nematic phases in keeping with the theoretical predictions. In addition, about 17 years ago Alfred Saupe had discovered a biaxial nematic phase formed by a lyotropic system.
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The Ericksen-Leslie equations, describing the dynamic behavior of nematic liquid crystals, have been applied to a twisted nematic layer and have been numerically solved for a number of cases. The results account well for the observed dynamic behavior of displays using a twisted nematic layer. In particular, an explanation is given for the ’’bounce’’ in the optical transmission of a twisted nematic cell between polarizers after switching off the applied field. Materials having a frequency dependence of the sign of the dielectric anisotropy show a reversal of twist after the frequency of the applied field has been switched. This can also be explained. It is shown that fluid motion is essential to the occurrence of these phenomena.
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Polarizer
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Analytic expressions are obtained using the generalized geometrical-optics approximation for the field-off state optical field and transmission of the general double layer twisted nematic and supertwisted nematic liquid crystal displays with arbitrary display parameters. A variety of display characteristics can be obtained from such structures. In particular, the strong interference colors in the field-off state single layer supertwisted nematic displays can be completely removed in a double layer supertwisted nematic display; thus, a black or white off state is possible. For the double layer 90° twisted nematic, besides the complete black or white field-off state, it can also exhibit off state display performance similar to that for either the single layer 180° supertwisted nematic or the optical mode interference display.
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Double layer (biology)
Dark state
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