Dynamic behavior of twisted nematic liquid-crystal layers in switched fields
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Abstract:
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.Keywords:
Biaxial nematic
Polarizer
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Abstract The response of a twisted nematic liquid crystal trapped between flat and parallel aligning layers is analysed. Using a recently developed form for the relevant integrals it has been possible to evaluate the twist and tilt of the director for any applied voltage for twists up to a maximum of 90°. It is shown that the equations describing the cell simplify in the high voltage limit and allow an analytic solution. This leads to considerable improvement in the computational procedures and also enables the twist profile to be expressed in a particularly simple algebraic form.
Biaxial nematic
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Twisted nematic liquid (TN) crystal structures have been widely employed to rotate the input polarization of the light by 90 degrees. For optimum performance, materials optimization is necessary for exact linear polarization rotation. By using Stokes parameter formalism, we derive the generalized relationship, which allows the calculations of the parameters that are necessary for arbitrary rotation of the linearly polarized light for twisted structures with arbitrary amounts of twist. We show that 'Gooch and Tarry' relation is a specialized case of our generalized results.
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Stokes parameters
Biaxial nematic
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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.
Biaxial nematic
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Mesogen
Biaxial nematic
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Abstract A novel technique has recently been developed for the determination of the twist elastic constant K22 in nematic liquid crystals. The method is based on the measurement of the threshold voltages of the unwound and π-twisted regions in a wedge cell geometry. It has the advantage that the chiral nematic pitch and the cell spacing do not need to be accurately known. In this paper the value of K22 is determined using this method for the material MLC6424. This is compared with the value derived from measurements in the conventional twist geometry in a cell with uniform spacing. Keywords: Chiral NematicLiquid CrystalNematicTwist Elastic ConstantWedge Cell
Constant (computer programming)
Wedge (geometry)
Biaxial nematic
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Abstract An approximate solution is derived for the structure of a twist wall between two regions of opposite 180° twist in a nematic liquid crystal film. The result is used to show that a twist wall is unstable unless the elastic constants of the nematic are such that K22≤ ½(K11 + K33). Thus the observation of such walls in nematic liquid crystals is evidence of this elastic anisotropy. Experimental evidence is given for the validity of the approximate solution obtained.
Biaxial nematic
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In order to determine 2-dimensional (2D) cell thickness and twist angle distributions in twisted nematic (TN) liquid crystal (LC) devices, we propose a 2D Stokes parameter method (2DSPM). A 2DSPM system is set up using a polarizing microscope, in which some preliminary experiments are conducted to demonstrate the validity of this 2DSPM. Experimental results of 2D distributions of cell thickness and twist angle in TN LC sample cells are given for the first time. The measurement accuracy of the 2DSPM system is analyzed.
Stokes parameters
Biaxial nematic
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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.
Biaxial nematic
Double layer (biology)
Dark state
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