Nematic liquid crystals viscosity

The formation of ECC is not only an extension of a portion of the macromolecule but also a mutual orientational ordering of these portions belonging to different molecules (intermolecular crystallization), as a result of which the structure of ECC is similar to that of a nematic liquid crystal. After the melt is supercooled below the melting temperature, the processes of mutual orientation related to the displacement of molecules virtually cannot occur because the viscosity of the system drastically increases and the chain mobility decreases. Hence, the state of one-dimensional orientational order should be already attained in the melt. During crystallization this ordering ensures the aggregation of extended portions to crystals of the ECC type fixed by intermolecular interactons on cooling. 

Six viscosity coefficients required for a description of the dynamics of an incompressible, nematic liquid crystal. 

Polarized light is the must powerful tool for investigating liquid crystals, all of which exhibit characteristic optical properties. A smectic liquid crystal transmits light more slow ly perpendicular to the layers than parallel to them. Such substances are said to be optically positive. Nematic liquid crystals are also optically positive, bui their action is less definite than that of smectic liquid crystals. However, the application of a magnetic field to nematic liquid crystals lines up their molecules, changing their optical properties and even their viscosity. 

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. 

The next major improvement in sample quality was the result of developments in direct synthesis. Oriented samples were obtained by the polymerisation of acetylene with the conventional titanium catalyst on a single crystal substrate and in nematic liquid crystals, see Tsukamoto (1992). Heat-treating the catalyst was found to produce polymer that could be stretched to give oriented samples. This resulted in a higher electrical conductivity along the orientation direction, up to 2xl05 fl-lm-1 being obtained. Naarman and co-workers heat treated the catalyst at 393 K and carried out the reaction in silicone oil at room temperature (Theophilou et al., 1986). Silicone oil was chosen as it has the same viscosity at room temperature as the usual solvents... 

Hgurc 4.14 (a) A typical phase diagram for a poly (p-phenylene terephthala-mide)-sulfuric acid system, a solution that is used to make Kevlar fiber (after Kikuchi, 1982). (b) Viscosity versus polymer concentration in PPTA/HjSO solution. Note the sharp drop in viscosity at about 20% which corresponds to a transition between isotropic and nematic liquid crystal phase. 

The theory for various molecular dynamics simulation algorithms for the calculation of transport coefficients of liquid crystals is presented. We show in particular how the thermal conductivity and the viscosity are obtained. The viscosity of a nematic liquid crystal has seven independent components because of the lower symmetry. We present numerical results for various phases of the Gay-Berne fluid even though the theory is completely general and applicable to more realistic model systems. 

Comparison of the Miesowicz viscosities of prolate (p) and oblate (o) nematic liquid crystals. The entries for zero field have been obtained by using the Green-Kulw relation (4.4)-(4.6). The entries for finite field have been obtained by applying the SLLOD equations (3.9). Note that the EMD GK estimates and the NEMD estimates agree within the statistical error. 

The fluctuation relations for the viscosities have also been generalised to biaxial nematic liquid crystals. They have been evaluated numerically for a biaxial version of the Gay-Beme fluid consisting of a linear string of oblate Gay-Beme ellipsoids, the axes of which point in the same direction. The flow... 

We have presented EMD and NEMD simulation algorithms for the study of transport properties of liquid crystals. Their transport properties are richer than those of isotropic fluids. For example, in a uniaxially symmetric nematic liquid crystal the thermal conductivity has two independent components and the viscosity has seven. So far the different algorithms have been applied to various variants of the Gay-Beme fluid. This is a very simple model but the qualitative features resembles those of real liquid crystals and it is useful for the development of molecular dynamics algorithms for transport coefficients. These algorithms are completely general and can be applied to more realistic model systems. If the speed of electronic computers continues to increase at the present rate it will become possible to study such systems and to obtain agreement with experimental measurements in the near future. 

D. Baalss and S. Hess, Nonequilibrium Molecular Dynamics Studies on the Anisotropic Viscosity of Perfectly Aligned Nematic Liquid Crystals,... 

S. Sarman, Green-Kubo Relations for the Viscosity of Biaxial Nematic Liquid Crystals, J. Chem. Phys. 105 (1996) 4211 ... 

Carlsson, T. Theoretical investigation of the shear flow of nematic liquid crystals with the Leslie viscosity as > 0 hydrodynamic analogue of first order phase transition. Mol. Cryst. Liq. Cryst. 1984, 104, 307-334. 

In nematic liquid crystals, the viscosity depends on the relative orientation between the shear gradient and the orientation of the nematic phase. Close to a surface, the orientation is usually governed by surface orientational anchoring [77]. Anchoring transitions, for instance induced by the adsorption of an analyte molecule to the surface [78], can therefore be easily detected with the QCM [79,80]. This reorientation induced by adsorption amounts to an amplification scheme the expected shift in the resonance frequency and bandwidth... 

Consequently only five independent coefficients actually exist in the nematic liquid crystals. The viscosity tensor is no longer symmetrical and hence a viscous moment appears... 

In the case of the nematic liquid crystal MBBA, the viscosity in the nematic phase is lower than in the isotropic phase. At 50 C the nematic phase reappears under 0.2 MPa of pressure then, the viscosity decreases and rises again exponentially as pressure increases. At 90°C the nematic state reappears at 1,2 MPa. 

Flow and viscosity studies in nematic liquid crystals... 

The yus represent the six coefficients of viscosity of a nematic liquid crystal. However, the number of independent coefficients reduces to five if we assume Onsager s reciprocal relations. 

We shall now discuss the application of the Ericksen-Leslie theory to some practical problems in viscometry. Probably the first precise determination of the anisotropic viscosity of a nematic liquid crystal was by Miesowicz. He oriented the sample by applying a strong magnetic field and measured the viscosity coefficients in the following three geometries using an oscillating plate viscometer ... 

The viscosity coefficients may also be determined by studying the reflexion of ultrasonic shear waves at a solid-nematic interface. The technique was developed by Martinoty and Candau. A thin film of a nematic liquid crystal is taken on the surface of a fused quartz rod with obliquely cut ends (fig. 3.7.1). A quartz crystal bonded to one of the ends generates a transverse wave. At the solid-nematic interface there is a transmitted wave, which is rapidly attenuated, and a reflected wave which is received at the other end by a second quartz crystal. The reflexion coefficient, obtained by measuring the amplitudes of reflexion with and without the nematic sample, directly yields the effective coefficient of viscosity. 

The twist viscosity can be determined from a measurement of Typically, t(0) for a film of 25 um is about 10" s. This gives an idea of the order of magnitude of the relaxation time for most nematic liquid crystal devices. 

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... 

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