Phases of liquid crystals

Above liquid ciystals lose their directional order and behave in many respects like liquids. All bulk physical parameters also assume an isotropic form although the molecules are anisotropic. 

The isotropic phase is, nevertheless, a very interesting and important phase for both fundamental and applied studies. It is fundamentally interesting because of the existence of short-range order, which gives rise to the critical temperature dependence of various physical parameters just above the phase transition temperature. These critical behaviors provide a good testing ground for the liquid crystal physics. 

On the other hand, recent studies have also shown that isotropic liquid crystals may be superior in many ways for constracting practical nonlinear optical devices (see Section 12.6), in comparison to the other liquid crystalline phases (see Chapter 8). In general, the scattering loss is less and thus allows longer interaction lengths, and relaxation times are on a much faster scale. These properties easily make up for the smaller optical nonlinearity for practical applications. 

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McMillan W L 1971 Simple molecular model for the smectic A phase of liquid crystals Phys.Rev A 4 1238-46... 

Fig. 7. Snapshots from a smectic-A phase of liquid crystal dimer molecules composed of two Gay-Berne mesogens linked by a flexible alkyl chain. Data from [112] supplied by Dr M R Wilson, University of Durham...

Another mechanism of chiral amplification that extends over an even larger scale has been reported by Huck et al. [119] The molecule 12-(9 H-thioxantbene-9 -yli-dene-12H-benzo[a]xanthene (Fig. 11.6), which has no chiral center, nevertheless exists, like the helicenes, in two chiral forms defined by their enantiomeric configurations. Consistent with the discussion in Section 11.2.3, a small net handedness (ca. 0.7 %) could be induced in racemic solutions of this molecule by use of ultraviolet CPL. However, introducing 20 wt% of this molecule, which contained a 1.5% chiral excess of one roto-enantiomer, into a nematic phase of liquid crystals produced macroscopic (100 pm) regions of a chiral cholesteric liquid crystal phase. The... 

Smectic Phases of Liquid Crystals with One Aromatic Ring and One RP-Chain... 

Miyachi K, Matsushima J, Ishikawa K, Takezoe H, Fukuda A (1995) Spontaneous polarization parallel to the tilt plane in the antiferroelectric chiral smectic-CA phase of liquid-crystals as observed by polarized infrared-spectroscopy. Phys Rev E 52 R2153-R2156... 

Diehl, P. and C.L. Khetrapal. 1969. NMR studies of molecules oriented in the nematic phase of liquid crystals. Springer-Verlag, Berlin. 

The sterically overcrowded stilbene derivatives were also useful dopants which caused photoinduced transition between induced cholesteric and nematic phases of liquid crystals [45, 46]. By alternate irradiation with light of different wavelengths, or left or right circularly polarized light, the phase transitions were successfully induced. 

A14N NMR study of order fluctuations in the isotropic phase of liquid crystals has been reported. (209) The experimental data for the isotropic phases of -azoxyanisole and of diethylazoxy benzoate are accounted for in terms of short range order fluctuations of the nematic and of the smectic types respectively. 

Figure 1.10. The TGB phase of liquid crystals. (From Goodby et al., 1993. Reproduced by permission of Taylor Francis, )...

C. Similarity in Orientational Dynamics Between the Isotropic Phase of Liquid Crystals and Supercooled Liquids... 

In general, the term liquid crystal is used to describe an intermediate phase between liquid and solid occurring in some organic compounds. The phase of liquid crystal can be divided into two mesophases smectic and nematic. Nematic liquid crystals can be further divided as chiral nematic or archiral nematic. In chiral nematic liquid crystals, sterol-related compounds are called cholesteric, and non-sterol-based compounds are termed chiral nematic. For heat transfer applications, encapsulated forms of chiral nematic [71] or the composite liquid crystal sheets of the cholesteric type [72] are commonly used. Recently, the application of micro-encapsulated liquid crystals has become more popular in heat transfer measurements because of the fast response and easy paintbrush or spray application to the test surface. 

Als-Nielsen J, Litster J D, Birgeneau R J, Kaplan M and Safinya C R 1980 Lower marginal dimensionality. X-ray scattering from the smectic-A phase of liquid crystals Ordering in Strongly Fluctuating Condensed Matter Systems ed T Riste (New York Plenum)... 

Here, 6 represents the angle between an individual molecule and the director, and this is summed over all molecules. Typically, for a nematic phase, 0.7 >S> 0.4 note that the transition from the isottopic liquid to the nematic phase is discontinuous and so at the transition, S will jump from a value of zero to about 0.3-0.4. While beyond the scope of this chapter, it is important to note that it is the nematic phase of liquid crystals that is the basis for almost all display applications. 

Fig. 15. A twist grain boundary in the Abrikosov phase of liquid crystals...

These rates are attainable for molecules dissolved in the nematic phases of liquid crystals such spectra give information about molecular geometry, in... 

Owing to the various mechanisms for optical nonlinearities present in the ordered as well as isotropic phases of liquid crystals, almost all nonlinear optical phenomena have been observed. Some of these phenomena were studied for their novelty, others have been developed into diagnostic tools or practical devices. In accordance with the basic mechanism involved, these observed effects are grouped together under the following... 

P. A. Madden, F. C. Saunders and A. M. Scott, "Degenerate Four-Wave Mixing in the Isotropic Phase of Liquid Crystals The Influence of Molecular Structure," IEEE J. Quantum Electronics, OE-22. p. 1287 (1986). 

Figure 16 Variation of physical parameters in the nematic phase of liquid crystals.

L.M. Blinov, M.I. Barnik, H. Ohoka, M. Ozaki and K. Yoshino, Surface and flexoelectric polarisation in nematic and smectic A phases of liquid crystal 4-octyloxy-4 cyanobiphenyl, Jpn. J. Appl. Phys. 40(8), 5011-5018, (2001). 

Fig. 11.26 A scheme of a hybrid cell that supports the splay-bend distortion and manifests the flexoelectric polarization (a) and an experimental temperature dependence of the sum of flexoelectric coefficients in the nematic phase of liquid crystal 5CB (b)...

V. A. Belyakov, E. I. Demikhov, V. E. Dmitrienko, and V. K. Dolganov, Optical activity, transmission spectra, and structure of blue phases of liquid crystals, Sov. Phys. JETP, 62, 1173 (1985). 

Diehl, P., Khetrapal, C.L. NMR Studies of Molecules Oriented in the Nematic Phase of Liquid Crystals. 1, 1-96 (1969). 

In terms of elastic or electromagnetic properties, if two of the three directions in a material are equivalent, the material is said to be uniaxial. The nematic and smectic A phases of liquid crystals are uniaxial, since all directions perpendicular to the director are equivalent and different from the direction of preferred orientational order. Solids with hexagonal, tetragonal, and trigonal symmetry are also uniaxial. If all three directions in a material are inequivalent, then the material is biaxial. The liquid crystalline smectic C phase is biaxial because one direction perpendictrlar to the director is in the plane of the layers while the other direction perpendictrlar to the director makes an angle equal to the tilt angle with the layers. Solids of orthorhombic, monoclinic, and triclinic symmetry are also biaxial. 

Fig. 4. Schematic of the setup for optical wavefront conjugation using a liquid-ciystal (LC) film. Ex, object beam, 2- reference beam, A aber-rator, M mirror, mirror to be used for self-oscillation effect, VBS variable beam splitter to control the intensity of E, BS beam splitter, R total reflector. For an example of wavefront conjunction result, see Fekete et al. [20] (isotropic phase of liquid crystal) or [20] (nematic phase).

I. C. Khoo and R. Normandin, Nanosecond laser induced optical wave mixing and ultrasonic wave generation in the nematic phase of liquid crystals, Opt. Lett., vol. 9, pp. 285-287, 1984 see also I. G. Khoo and R. Normandin, Nanosecond laser induced ultrasonic waves and erasable permanent gratings in smectic liquid crystal, J. Appl. Phys., vol. 55, pp. 1416-1418, 1984. 

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