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Uniaxial nematic liquid crystals

In a uniaxial nematic liquid crystal, the spatial orientation of the optical axis is determined by the orientation of the director. Due to thermally excited orientational director fluctuations, the spatial direction of the optical axis is not constant in time. As a result, any light illuminating the sample is... [Pg.55]

From the microscopic point of view the macroscopic polarization in a uniaxial nematic liquid crystal medium is given by the following expansion ... [Pg.12]

When the temperature is decreased, the material transforms into the nematic phase, which is the most common and simplest liquid crystal phase, where the molecules have orientational order but still no positional order. The molecules can still diffuse around, and the translational viscosity does not change much from that of the isotropic liquid state. The long axis of the molecules has a preferred direction. Although the molecules still swivel due to thermal motion, the time-averaged direction of the long axis of a molecule is well defined and is the same for all the molecules at macroscopic scale. The average direction of the long molecular axis is denoted by n which is a unit vector called the hquid crystal director. The short axes of the molecules have no orientational order in a uniaxial nematic liquid crystal. [Pg.3]

Uniformly oriented uniaxial nematic liquid crystals have non-polar cylindrical symmetry in the absence of an external electric field. If the liquid crystal molecules have a permanent dipole along the long molecular axis, the potential for the orientation of the dipole has reflection symmetry about the plane perpendicular to the director n, and the dipole has the same probability to be parallel and anti-parallel to the director, as shown in Figure 4.3(a). If the permanent dipole is perpendicular to the long molecular axis, the potential for the orientation of the dipole is... [Pg.132]

In order to model the liquid crystal director configuration, we must first know how Uquid crystals interact with externally applied electric fields. Many liquid crystal devices make use of uniaxial nematic liquid crystals which are dielectrics. We consider the electric energy of nematic liquid crystals in externally applied electric fields through dielectric interaction. A typical liquid crystal device cell is shown in Figure 7.1, where the liquid crystal is sandwiched between two parallel substrates with transparent electrodes. The electric energy of the liquid crystal is given by [11-13]. [Pg.213]

We now consider how the properties of LCEs affect light. Light propagation in typical uniaxial nematic liquid crystals is the same as in uniaxial solid crystals two eigenmodes are allowed, the extraordinary mode with polarization parallel, and the ordinary mode, with polarization perpendicular to the plane defined by the wave vector k and the optic axis - the nematic director - n. Although they are soft and... [Pg.99]

A complete assessment of the literature on the hydrodynamics of liquid crystals is beyond the scope of this handbook. Therefore, only the most important fundamentals for the description of flow phenomena will be discussed. Further details can be found in the references cited at the end of each section and in the review articles and books of Porter and Johnson [1], de Gennes [2], Stephen and Straley [3], Jenkins [4], de Jeu [5], Vertogen and de Jeu [6], and de Gennes and Prost [7]. Two review articles of Leslie [8, 9] are exclusively devoted to this subject. Experimental results for uniaxial nematic liquid crystals are described in Chap. Ill, 2.6 of Vol. 2A of this Handbook. [Pg.487]

The hydrodynamic theory for uniaxial nematic liquid crystals was developed around 1968 by Leslie [10, 11] and Ericksen [12, 13] (Leslie-Ericksen theory, LE theory). An introduction into this theory is presented by F. M. Leslie (see Chap. Ill, Sec. 1 of this Volume). In 1970 Parodi [14] showed that there are only five independent coefficients among the six coefficients of the original LE theory. This LEP theory has been tested in numerous experiments and has been proved to be valid between the same limits as the Navier-Stokes theory. An alternative derivation of the stress tensor was given by Vertogen [15]. [Pg.487]

Most of the hydrodynamic effects observed in nematic liquid crystals can be explained by the LEP theory. Its generality is sufficient for the following discussion. According to this theory the viscous part of the stress tensor a j for an incompressible uniaxial nematic liquid crystal is... [Pg.488]

The orientation-dependent part of the magnetic free energy density of a uniaxial nematic liquid crystal is found from Eqs. (1) and (2) ... [Pg.1096]

For amorphous systems with cylindrical symmetry, such as aligned uniaxial nematic liquid crystals or oriented amorphous polymers, the equivalent of the radial distribution function is the cylindrical distribution function (CDF). This function can be written as [2]... [Pg.129]

For uniaxial nematic liquid crystals, the z axis for diffusion is defined to be the symmetry axis of the phase, i.e. the director. The number of independent components is now reduced to two Du = Dzz and Dj. = Dxx = Dyy, and the difiiision equation becomes... [Pg.273]

For uniaxial nematic liquid crystals, the two components are obtained by solving EQN (3). If Du and Di are independent of time or coordinates, the solutions give... [Pg.274]

Such polymers adopt, when affected by a mechanical field, an optically uniaxial homeotropic structure polymers B.1.2, B.1.7, B.1.8 (Table 8) have positive birefringence polymers B.1.1, B.1.8. (Table 9) have negative birefringence, which has not been reported to our knowledge, for low-molecular nematic liquid crystals. Although the authors do not comment on the cause for the observed phenomenon, the fact in itself is sufficiently uncommon. [Pg.210]

The first nematic guest-host prototype nematic guest-host display device contained a nematic liquid crystal (4-butoxybenzoic acid) and a pleochroic dye (methyl red or indophenol blue) sandwiched between two (Nesa) electrodes dTn 12/im) rubbed uniaxially, but with no additional orientation layer, see Figure 3.14. One polariser was fixed to the front substrate surface with its direction of maximum absorption parallel to the rubbing direction and, therefore, the nematic director. [Pg.110]

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. [Pg.354]

Keresztury and coworkers have recently measured the IR LD spectra of phenol aligned in a uniaxially oriented liquid crystal nematic phase, thereby providing new... [Pg.372]

This can be also illustrated as follows a fatty man can be surrounded by 4 - 5 closely-packed fat men. But the same man can be surrounded even by 10 slim men. The latter can parallel the increasing dimensionality of space for the fat man. Consequently, the elongation of fluctuations due to the action of the strong electric field in the homogeneous phase of binary mixtures of limited miscibility is equivalent to increased dimensionality d = A. The uniaxial symmetry is natural also for the isotropic phase nematic liquid crystals. It may be considered for supercooled nitrobenzene due to intermolecular interactions (Fig. 5). ... [Pg.175]

Liquid crystals are optically anisotropic media with the exception of the cubic phases, such as the D-phase and blue phase. The refractive index or dielectric constant of liquid crystals varies according to the orientation of the molecular axes (or the optical axes). For example, the nematic liquid crystals are optically uniaxial and exhibit remarkable birefringence, that... [Pg.4]

If the liquid crystalline molecules are not cylindrically symmetrical, S must be expressed by a tensor (traceless tensor). The nematic liquid crystal state (a uniaxial symmetrical system) is the main concern of the book. For such a phase, the order parameter can be denoted by the scalar in Equation 1.11. [Pg.28]

In bulk nematic liquid crystals, the average equilibrium order is uniaxial with the order parameter Qo = aoTo and oq = S. Thus, the correlation lengths of the 5 independent degrees of freedom of the nematic order read... [Pg.272]

For the purpose of this article, we focus our attention on the nematic mesophase smectic orders are more crystal-like and thus are beyond our scope. Typical nematic liquid crystals are characterized by a uniaxial order, though imperfect, along the preferred axis of the domain. No such long-range order exists in directions transverse to the domain axis. In most examples, low molar mass (monomer) liquid crystals carry flexible tails. Conformational ordering of these tails in the mesophase has been extensively studied in relation to the odd-even character of the phase behavior with the number of constituent atoms of the pendant chain. Various statistical models and theories have been presented [52-57]. In most cases, however, the ordering of the tail is relatively weak [58,59]. [Pg.131]


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