Nematic liquid crystals dielectric anisotropy

Note 1 The nematic liquid crystal must have a negative dielectric anisotropy (Af < 0), and a positive anisotropy (Aa > 0). The optical texture corresponding to the flow pattern consists of a set of regularly spaced, black and white stripes perpendicular to the initial direction of the director. These stripes are caused by the periodicity of the change in the refractive index for the extraordinary ray due to variations in the director orientation. 

Table 3.1 Transition temperatures CC), dielectric anisotropy (As) and conductivity (Or cm ) for nematic liquid crystals (1-11) and a commercial mixture (Merck) designed for DSM-LCDs...

A nematic liquid crystal of negative dielectric anisotropy is aligned with the director aligned orthogonal to the cell walls by means of a surfactant orientation layer, see Figure 3.4. One or two linear, elliptical or circular polarisers are... 

Table 3.3 Molecular structures, transition temperatures (°C) and dielectric anisotropy (Ae measured at 20°C) for nematic liquid crystals (15-27)...

A standard TN-LCD consists of a nematic liquid crystal mixture of positive dielectric anisotropy contained in a cell with an alignment layer on both substrate surfaces, usually rubbed polyimide, crossed polarisers and a cell gap of 5- 0fim, see Figure 3.7. The nematic director is aligned parallel to the direction of rubbing in the azimuthal plane of the device. The alignment layer induces a small pretilt angle (6 1-3°) of the director in the zenithal plane. The... 

The nematic liquid crystal mixture containing the pleochroic dye is of positive dielectric anisotropy and is aligned parallel with the director parallel to the substrate surfaces. Therefore, plane polarised light is absorbed by the dye molecules in non-addressed areas of the display and they appear coloured. 

The application of an electric field above the threshold value results in a reorientation of the nematic liquid crystal mixture, if the nematic phase is of negative dielectric anisotropy. The optically active dopant then applies a torque to the nematic phase and causes a helical structure to be formed in the plane of the display. The guest dye molecules are also reoriented and, therefore, the display appears coloured in the activated pixels. Thus, a positive contrast display is produced of coloured information against a white background. The threshold voltage is dependent upon the elastic constants, the magnitude of the dielectric anisotropy, and the ratio of the cell gap to the chiral nematic pitch ... 

Nematic Liquid Crystal Hosts of Positive Dielectric Anisotropy... 

Nematic liquid crystals of negative dielectric anisotropy with a low rotational viscosity and a high value of the twist elastic constant k22 are required, see Equations 38-40. Furthermore, a high value for the resistivity and short... 

Perfluoroalloxy liquid crystals were prepared by the Williamson ether synthesis using perfluoro iodopropene. These materials can be mixed with nematic liquid crystal materials to provide liquid crystal compositions having low viscosity, low refractive index anisotropy, high dielectric anisotropy, and broad nematic phase ranges. 

Abstract A systematic overview of various electric-field induced pattern forming instabilities in nematic liquid crystals is given. Particular emphasis is laid on the characterization of the threshold voltage and the critical wavenumber of the resulting patterns. The standard hydrodynamic description of nematics predicts the occurrence of striped patterns (rolls) in five different wavenumber ranges, which depend on the anisotropies of the dielectric permittivity and of the electrical conductivity as well as on the initial director orientation (planar or homeotropic). Experiments have revealed two additional pattern types which are not captured by the standard model of electroconvection and which still need a theoretical explanation. 

In equations (5)-(8), i is the molecule s moment of Inertia, v the flow velocity, K is the appropriate elastic constant, e the dielectric anisotropy, 8 is the angle between the optical field and the nematic liquid crystal director axis y the viscosity coefficient, the tensorial order parameter (for isotropic phase), the optical electric field, T the nematic-isotropic phase transition temperature, S the order parameter (for liquid-crystal phase), the thermal conductivity, a the absorption constant, pj the density, C the specific heat, B the bulk modulus, v, the velocity of sound, y the electrostrictive coefficient. Table 1 summarizes these optical nonlinearities, their magnitudes and typical relaxation time constants. Also included in Table 1 is the extraordinary large optical nonlinearity we recently observed in excited dye-molecules doped liquid... 

Kang SW, Sprunt S, Chien LC. 2001. Switchable diffraction gratings based on inversion of the dielectric anisotropy in nematic liquid crystals. Appl Phys Lett 78 3782 3784. 

H.P. Hinov and L.K. Vistin, Parallel and cross-like domains due to d.c. and low frequency (< 2 Hz) electric fields in nematic liquid crystal layers with negative dielectric anisotropy, J. Phys. France 40(3), 269-292, (1979). 

A. Buka, B. Dressel, W. Otowski, K. Camara, T. Toth-Katona, L. Kramer, J. Lindau, G. Pelzl and W. Pesch, Electroconvection in nematic liquid crystals with positive dielectric and negative conductivity anisotropy, Phys. Rev. E 66(5), 051713/1-8, (2002). doi 10.1103/PhysRevE.66.051713... 

E.I. Rjumtsev and S.G. Polushin, Electrohydrodynamic instabilities in nematic liquid crystals with large positive dielectric anisotropy, Liq. Cryst. 13(5), 623-628, (1993). doi 10.1080/02678299308026335... 

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