Optical-field-induced Freedericksz transition of a nematic liquid crystal doped with diazophenyl
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Fréedericksz transition
A general formula for calculating the Freedericksz transition of twisted nematic liquid crystal cells has been derived by taking into consideration the following factors: unequal elastic constants and arbitrary dielectric anisotropy for the liquid crystal medium, and a general form of the anisotropic liquid crystal-to-substrate interfacial potential with negligible pretilt angle. The numerical computation of the threshold voltage as a function of the anchoring anisotropy has been demonstrated for two nematic mixtures. Increasing the degree of multiplexing of twisted nematic cells seems feasible using certain nematic mixtures with weak anchoring anisotropy.
Fréedericksz transition
Biaxial nematic
Anchoring
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Abstract The occurence of optically induced Freedericksz transitions (OFT) for twist in homogeneously aligned nematic and/or Smectic-C (Sm-C) liquid crystal film studied. The Euler equation for the molecular director and the equation describing the evolution of the light polarization state along the sample are solved simultaneously. The stabilit y of the undistorted structure is therefore investigated. Multiple threshold intensities for OFT are found. Exact solution for steady-state conditions are discussed: inparticular either intensity-or polarization-driven opticalbistability loops are reported.
Fréedericksz transition
Biaxial nematic
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We propose to measure the ratio κ31 of the bend elastic constant K3 over the splay elastic constant K1 of a nematic phase by using an optical method based on the measurement of the birefringence of homeotropic/planar hybrid samples. This method is compared to the classical measurements using the Freedericksz transition under AC electric field and its advantages and disadvantages are discussed. Both methods are applied to the measurement of K1 and K3 close to the nematic-to-SmA phase transition of the liquid crystal 8CB and the nematic-to-NTB phase transition of the mixture 8CB + 50 wt% CB7CB. The role of flexoelectricity in these measurements is analysed, as well as the role of the pretilt angle and the anchoring energy of the molecules on the plates limiting the samples.
Fréedericksz transition
Homeotropic alignment
Flexoelectricity
Biaxial nematic
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Distortions of nematic liquid crystals are studied based upon the molecular pair potential model which is spatially anisotropic and dependent on elastic constants of liquid crystals. The perfect nematic order is assumed in the theoretical treatment, that means the orientation of the molecular long axis coincides with the director of liquid crystal and the total free energy equals to the total interaction energy. Three kinds of the basic Freedericksz transition are investigated analytically and a new form of potential parameters related to the elastic constants K_i is obtained. The thermodynamic properties of nematics described as pair potential model are studied by Monte Carlo simulations.
Fréedericksz transition
Biaxial nematic
Pair potential
Constant (computer programming)
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Abstract Optically induced molecular reorientation in nematic liquid crystals is an interesting phenomenon, suitable of many practical applications, due to the large electric and optical anisotropy of such materials in the nematic phase. Therefore great importance has the study of the dynamics of the molecular reorientation. In the usual configuration the cell contains a homeotropically aligned nematic liquid crystal and a linearly polarized laser beam impinges on it. However, the presence of a Freedericksz transition requires the use of some experimental ingenious contrivances: such as the application of a magnetic field or performing the experiment only for a large enough incidence angle. We show here that the molecular reorientation dynamics of a nematic liquid crystal can be easily studied without any magnetic field, even at normal incidence, using a hybrid aligned nematic liquid crystal cell: i.e. a cell with homeotropic alignment on one boundary and planar alignment on the other. Key Words: nonlinear opticsliquid crystals
Homeotropic alignment
Fréedericksz transition
Biaxial nematic
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Molecules designed to possess large dipole moments have been synthesized and doped into liquid crystals for the purpose of reducing the threshold voltage associated with the Freedericksz transition in nematics. These molecules were tested in the commercial nematic mixture E7 at varying concentrations and the resultant mixtures were evaluated to determine their nematic-isotropic transition temperature, threshold voltage, dielectric anisotropy, and visco-elastic constant as a function of dopant concentration. We have shown that these materials reduce the threshold voltage by up to 25 % at the 10 wt.% doping level but do so via a reduction of the splay elastic constant, not by the increase of the dielectric anisotropy as was expected. As a consequence of this result these dopants have also shown to increase the response time of the dopant/liquid crystal mixture.
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Abstract A new method for quick and pretty accurate measurements of splay, twist and bend elastic constants of nematic liquid crystals is experimentally verified. The main concept relies on exploiting only the electric field and determining magnitudes of nematic elastic constants from threshold fields for Freedericksz transitions in only one hybrid in-plane-switched cell. In such cell the deformations of an investigated liquid crystal are controlled by three separated pairs of electrodes confining measurement domains. In two of them inter-digital electrodes are mounted on one cell cover. Splay, twist and bend elastic constants can be measured by a proper choice of electrodes’ configuration together with orienting cover coatings (without applying magnetic fields). In this paper, we describe layout of our cells and results of experimental tests by using different liquid crystals: 5CB and 6CHBT (with positive dielectric anisotropy), Demus’ esters (with negative dielectric anisotropy) and new liquid crystals mixtures produced in our university.
Fréedericksz transition
Biaxial nematic
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We theoretically demonstrate that the application of a transverse next-to-static electric field to a planar nematic liquid crystal cell - obtained by strong planar anchoring of the liquid crystal on the first surface and negligible interaction with the second surface - causes an electro-reorientational transition of the Freedericksz kind. We show that, above the transition the cell behaves as a twisted nematic liquid crystal cell with total twist depending on the applied voltage. Preliminary experimental results confirming the predicted effect are presented.
Fréedericksz transition
Biaxial nematic
Anchoring
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Fréedericksz transition
Biaxial nematic
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Abstract In four substances exhibiting reentrant nematic phases the threshold voltage of the Freedericksz transition was measured as function of the frequency in the nematic and reentrant nematic phases. The isotropy frequency and the relaxation frequency were calculated using the experimental data. We found that extrapolated to the same temperature the relaxation frequency of the reentrant nematic phase is larger than that of the nematic phase.
Fréedericksz transition
Reentrancy
Biaxial nematic
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