Viscosity Of Liquid Crystals Research Articles

Effect of a bounding surface on the rotational viscosity of liquid crystals

The effect of a surface, bounding a nematic-liquid phase, on the rotational viscosity γ1 is investigated on the basis of a statistical approach employing direct correlation functions. Specific calculations are performed for a model system consisting of ellipsoidal molecules interacting via a Gay-Berne potential near the bounding surface.

Microscopic description of nematic liquid crystal viscosity.

The problem of nematic viscosity is shown to be successfully solved within the framework of a microscopic description. The Kuzuu-Doi approach to hydrodynamics [N. Kuzuu and M. Doi, J. Phys. Soc. Jpn. 52, 3486 (1983)] for a system consisting of anisotropic molecules is reviewed. It has been shown that the incorrect form of the microscopic stress tensor that is used by Doi [M. Doi, J. Polym. Sci. 19, 229 (1981)] does not influence final expressions for the viscosity coefficients due to the fact that the external magnetic field is taken into account. Molecular expressions for the Leslie viscosity coefficients are calculated by use of the theory of Osipov and Terentjev [Z. Naturforsch. Teil A 44, 785 (1989); Phys. Lett. A 134, 301 (1989)]. A systematic solution of the differential kinetic equation is presented. The nonequilibrium distribution function obtained from the kinetic equation allows us to transform Osipov-like expressions for the viscosity coefficients exactly into those obtained by Kuzuu and Doi. The above-mentioned approaches are proved to be equivalent. A realistic mean potential is applied to the expressions obtained for the Leslie coefficients for the case of 4-methoxybenzylidene-4\ensuremath{'}-n-butylaniline. A comparison of theoretical results and appropriate experimental data is shown to be very good.

Nonequilibrium molecular dynamics of liquid crystals

During the last 15 years, noneyuilibrium molecular dynamics (NEMD) has been successfully applied to study transport phenomena in fluids that are isotropic at equilibrium. A natural extension is therefore to study liquid crystals, which are anisotropic al equilibrium. The lower symmetry of these systems means that the linear transport coefficients are considerably more complicated than in an isotropic system. Part of the reason for this is that there are crosscouplings between tensors of different rank and parity. Such couplings arc symmetry-forbidden in isotropic phases. In this paper. we review some of fundamental theoretical results we have derived concerning the rheology of liquid crystals. report NEMD simulations of thermal conductivity and shear viscosity of liquid crystals, and present NEMD simulations of shear cessation phenomena. All of the NEMD results are presented for a model liquid crystal fluid which is a modification of the Gay-Borne fluid. The results obtained are in qualitative agreement with experimental measurements on liquid crystal systems.

Electroviscous effect of nematic liquid crystals

Liquid crystals are characterized by the orientation of their molecules, the major axis of whose direction can be controlled by an applied electric or magnetic field. In the present paper the viscosity variation of three thermotropic liquid crystals in the nematic phase having different dielectric constants is studied at varying electric field strength, shear field strength and temperature. The results show that the viscosity of liquid crystals can be varied by the applied electric field strength and that a lower temperature reduces the threshold electric field strength for the electroviscous effect to appear as well as decreasing the attainable values at higher electric field strength. In addition, the frequency of the applied electric field has little effect on electroviscosity.

Frame Sequential Color Thin Film Transistor/Liquid Crystal Displays: Display Characteristics and Grayscale Consideration

We report a theoretical study on the optical characteristics of a frame sequential color thin film transistor/liquid crystal display (TFT/LCD) operating with the TN LCD. Important liquid crystal (LC) and display parameters (cell gap, LC surface pretilt angle, applied voltage, LC viscosity, frame rate, and operation sequence) are considered and optimized to improve the display performances. Both the modeling and experimental results for a TN LCD using an EM Merck ZLI-3651 LC material are presented. We also present detailed theoretical results for a TFT/LCD operating at frame rates of 30 Hz and 71 Hz. Sufficiently fast switching-on and switching-off dynamic responses can be obtained by raising the temperature to 40-60°C to reduce the LC rotational viscosity. A display operating at the 71 Hz frame rate, can obtain a good bi-level display, but it is difficult to obtain a good display with 512 colors.

Notizen: Comment on the Osipov-Terentjev Calculation of the Viscosity of Liquid Crystals

AbstractThe calculation of the viscosity of liquid crystals based on the rotational diffusion approximation recently reported by Osipov and Terentjev [1 a] is critically examined. It is shown that the main result of their treatment - the evaluation of the activational exponent - arises from the neglect of the second derivative of the distribution function with respect to the azimuthal angle in the Fokker-Planck equation. The correct asymptotic formula for the rotational viscosity coefficient is derived.

Developments of reflection mode liquid crystal light valves using Bi12 SiO20 as the photoconductor

This paper describes the development of liquid crystal light valves (LCLVs) using bismuth silicon oxide (BSO) as the photoconductor at the Sowerby Research Centre of British Aerospace. The results of two LCLVs are presented and compared, both of which use a 250 micrometre thick slice of BSO. The variation in time response with liquid crystal viscosity is investigated. A resolution of 27 line pairs/mm (50% MTF point) was obtained for the first device. However, refinements in fabrication and measurement techniques yield a resolution of 31 line pairs/mm for the second light valve.

Complex Shear Viscosity of Liquid Crystals in the Isotropic Phase of Nematics

Abstract By use of the torsional quartz crystal method, the real and imaginary parts of the complex shear mechanical impedance of several nematogenic materials were measured as functions of the temperature at frequencies ranging from 50 to 150 kHz. From the impedance, we have determined the real and imaginary parts of the complex shear viscosity, in which a remarkable pretransitional anomaly is observed in the isotropic phase near the isotropic-nematic phase transition temperature, T c. In the nematic phase and the isotropic phase except for the vicinity of T c, the Newtonian behaviour is observed. The behaviour of the complex shear viscosity in the isotropic phase has been analyzed by the theory of de Gennes, and it is shown that the observed behaviour can be well described by the theory.

Influence of solutes on material constants of liquid crystals and on electro-optical properties of twisted nematic displays

Measurements are reported on the changes in properties of nematic liquid crystals with strong positive dielectric anisotropy which are induced by doping the liquid crystals with nonmesomorphic solutes. The dopands decrease the dielectric anisotropy and the viscosity of liquid crystals, as well as influencing the elastic constants k11 and k33 and the ratio k33/k11. The consequent changes of the static and dynamic electro-optical properties of twisted nematic displays when using doped liquid crystals are due to changes of the material constants. The threshold voltage of TN displays decrease by as much as 40% when using a solute concentation of 8% whereas the on-transient times on TN displays decrease by a factor of 4.

Non-Newtonian Viscosity of Liquid Crystals Formed by Cholesteryl Oleyl Carbonate

The steady flow viscosity of cholesteryl oleyl carbonate was determined in a range of shear rate from 0.1 to 70 sec1 and over a temperature range from 15 to 45°C. A viscosity maximum associated with a marked non-Newtonian behaviour was observed at a temperature range around 31°C of the cholesteric-isotropic transition. The linearity of the flow curves in the cholestric mesophase suggested that the structure of the liquid crystal was easily broken by a fairly low shear stress. The measurements of the temperature dependence of the d.c. electrical conductivity, the optical transmittivity, and the dynamic rigidity were also carried out. According to Denny-Brodkey's theory, the equilibrium constants between broken and unbroken structures were estimated near the transition temperature.

A remark concerning the viscosity of liquid crystals

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