Nematic liquid crystals physical properties

Khoo IC (1981) Optically induced molecular reorientation and third-order nonlinear optical processes in nematic liquid crystals. Phys Rev A 23(4) 2077-2081 Khoo IC (1982a) Nonlinear light scattering by laser- and dc-field-induced molecular reorientations in nematic-liquid-crystal films. Phys Rev A 25(2) 1040-1048 Khoo IC (1982b) Theory of optically induced molecular reorientations and quantitative experiments on wave mixing and the self-focusing of light. Phys Rev A 25(3) 1636-1644 Khoo IC (1995) Liquid crystals physical properties and nonlinear optical phenomena. Wiley, New York... 

In order to understand the basic principles of operation of the many different kinds of LCDs being developed and/or manufactured at the present time, it is necessary to briefly describe the liquid crystalline state and then define the physical properties of direct relevance to LCDs. First, the nematic, smectic and columnar liquid crystalline states will be described briefly. However, the rest of the monograph dealing with liquid crystals will concentrate on nematic liquid crystals and their physical properties, since the vast majority of LCDs manufactured operate using mixtures of thermotropic, non-amphiphilic rodlike organic compounds in the nematic state. 

Table 4.3 Relationship between the physical properties of nematic liquid crystals and the corresponding application-relevant display properties.

L. Kramer and W. Pesch, Electrohydrodynamics in nematics. In eds. D.A. Dnmmnr, A. Fnknda and G.R. Luckhurst, Physical Properties of Nematic Liquid Crystals, Inspec, London, 2001. pp. 441-454. 

The static continuum theory of elasticity for nematic liquid crystals has been developed by Oseen, Ericksen, Frank and others [4]. It was Oseen who introduced the concept of the vector field of the director into the physics of liquid crystals and found that a nematic is completely described by four moduli of elasticity Kn, K22, K33, and K24 [4,5] that will be discussed below. Ericksen was the first who understood the importance of asymmetry of the stress tensor for the hydrostatics of nematic liquid crystals [6] and developed the theoretical basis for the general continuum theory of liquid crystals based on conservation equations for mass, linear and angular momentum. Later the dynamic approach was further developed by Leslie (Chapter 9) and nowadays the continuum theory of liquid crystal is called Ericksen-Leslie theory. As to Frank, he presented a very clear description of the hydrostatic part of the problem and made a great contribution to the theory of defects. In this Chapter we shall discuss elastic properties of nematics based on the most popular version of Frank [7]. 

Leslie, F.M. Introduction to nematodynamics. In Dunmur, D., Fukuda, A., Luckhurst, G., INSPEC (eds.) Physical Properties of Liquid crystals Nematics, pp. 377-386, London (2001). Parodi, O. Stress tensor for nematic liquid crystals. J. Phys. (Paris) 31, 581-584 (1970) Miesowicz, M. The three coefficients of viscosity of anisotropic liquids. Nature 158, 27 (1946) Influence of the magnetic field on the viscosity of para-azoxyanisole. Nature 136, 261 (1936). 

In 1963, Richard Williams observed the formation of very regular patterns or domains in a nematic liquid crystal when the material was subjected to an electric field. This report marked the beginning of a new era in research on the electro-optic properties of liquid crystals, a field which had laid dormant for nearly 30 years. During the remaining years of the 1960 s and the early 70 s, numerous studies of electro-optic effects in liquid crystals were performed, and at the same time, investigations into the synthetic and physical chemistry of these materials were conducted. As a result of these efforts, a whole new display industry evolved. 

The synthesis of a great number of materials that exhibit the nematic phase has achieved many different goals. Firstly, much knowledge has been acquired of the effect of stmctural features and various combinations of stmctiual features on melting points, mesophase morphology, and stability. Secondly, many physical properties have been evaluated for a great niunber of nematic liquid crystals and the resrrlts have been linked to the stmcture. Thirdly, mixtures of nematic materials have been formulated that have been... 

Polymer networks which can memorize the orientational order of the nematic liquid crystal environment where they are assembled [71], [72], [73], [74] are particularly attractive because of their potential for a variety of electrooptic technologies. We postpone this subject to the last section and here concentrate our attention on the ordering and structures of these composite materials. These systems have many physical properties analogous to liquid crystals confined to different submicrometer-sized cavities [75], [76] and random porous matrices [77], [78], Large surface-to-volume ratios enable a strong influence of the polymer network on nematic ordering in the liquid crystalline solvent and thus govern optical properties of the composites. 

The existence or nonexistence of mirror symmetry plays an important role in nature. The lack of mirror symmetry, called chirality, can be found in systems of all length scales, from elementary particles to macroscopic systems. Due to the collective behavior of the molecules in liquid crystals, molecular chirality has a particularly remarkable influence on the macroscopic physical properties of these systems. Probably, even the flrst observations of thermotropic liquid crystals by Planer (1861) and Reinitzer (1888) were due to the conspicuous selective reflection of the helical structure that occurs in chiral liquid crystals. Many physical properties of liquid crystals depend on chirality, e.g., certain linear and nonlinear optical properties, the occurrence of ferro-, ferri-, antiferro- and piezo-electric behavior, the electroclinic effect, and even the appearance of new phases. In addition, the majority of optical applications of liquid crystals is due to chiral structures, namely the ther-mochromic effect of cholesteric liquid crystals, the rotation of the plane of polarization in twisted nematic liquid crystal displays, and the ferroelectric and antiferroelectric switching of smectic liquid crystals. 

Physical Properties and Theories of Nematic Liquid Crystals... 

When an electric or magnetic field is applied to a liquid crystal cell, a texture transition occurs to minimize the free energy of the system. These texture changes in cholesteric liquid crystals are physically similar to the Frederiks transition in a nematic liquid crystal and result in a significant change in the optical properties of the layer. Texture transitions have been reviewed previously [8, 9] with allowance made for the sign of the dielectric or diamagnetic anisotropy, the initial texture, and the direction of the applied field. Here, we consider only the instability of the planar cholesteric texture, which has been widely discussed in recent literature. 

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