Textures in smectics

Fig. 1.1.7. Focal conic textures in smectic A. (a) The polygonal texture. Crossed polarizers. (Friedel. ) (6) Simple fan-shaped texture. Crossed polarizers. (Sack-...

S. Shojaei-Zadeh, S.L. Anna, Role of surface anchoring and geometric confinement on focal conic textures in smectic-A liquid crystals. Langmuir 22, 9986-9993 (2006)... 

Layers that are mainly vertical and the presence of disclinations (see Fig. 32 h) are the two very common characteristics of fan textures in smectic and cholesteric phases. The name fan texture comes from the circular contours that are often visible around disclinations in, for example, cholesterics, and from the frequent radial decorations (see... 

Statistical properties of focal conic textures in smectic liquid crystals (with R. Bidaux, M. Boccara, G. Sarma, L. de S ze and 0. Parodl), J. Phvs,. (Paris) 34, 661 (1973). 

Coates, D., Crossland, W. A., Morrissey, J. H., and Needham, B., Electrically induced scattering textures in smectic A phases and their electrical reversal, J. Phys. D AppL Phys., 11, 2025, 1978. 

Compared to the corresponding textures in smectic A phases, they are less regular and disturbed by additional disclinations. These C structures can be twisted by the addition of optically active compounds. 

Smectic C modifications exhibit two microscopic textures the Schlieren texture and the focal-conic fan texture. As previously discussed, the Schlieren texture of the smectic C phase can be distinguished from that of the nematic phase by the fact that it exhibits only singularities with S = 1. Compared to the corresponding texture in smectic A, the broken focal-conic fan texture of a smectic C is less regular and disturbed by additional disclinations. If the C phase is formed on cooling a smectic A phase, then the Schlieren texture will be obtained from a homeotropic A texture and the broken fan texture will be obtained from the simple focal-conic texture of the preceding A phase. 

It was first reported in the early 1970s that these melt processible polymers could best be described as thermotropic systems which usually display an nematic texture in the melt phase [5]. Subsequently, a number of additional phases have been reported ranging from discotic structures to highly ordered smectic E G systems with three dimensional order. In the last several years an IUPAC sponsored study on nomenclature on thermotropic LPCs has been underway. A more complete set of definitions will be available shortly as a result of Recommendation No. 199 IUPAC [6]. 

Evans et al. also showed that the 1 1 mixture of BAN and (3, y-distearoyl-phos-photidylcholine (DSPC) gives a smectic A texture in the temperature range of 57.3 to 100°C [21]. This is the first notice of lyotropic lamellar liquid crystals formed in the ionic medium. Additionally, Seddon et al. [28] and Neve et al. [29] have described the long-chained A-alkylpyridinium or l-methyl-3-alkylimidazolium ions to display smectic liquid-crystalline phases above their melting points, when Cl or tetrachloro-metal anions like CoCl " and CuCl " are used as the counter ions. Lin et al. have also noted the liquid crystal behavior of 1-alkylimidazolium salts and the effect on the stereoselectivity of Diels-Alder reactions [30]. However, liquid crystals are classified as ionic liquid crystals (ILCs), and they are distinguished from liquid crystals that are dispersed in ionic liquids. Although the formation of micelles and liquid crystal phases in ionic liquids have been thus reported, there has been no mention of the self-assembly of developed nano-assemblies that are stably dispersed in ionic liquids. In the next section the formation of bilayer membranes and vesicles in ionic liquids is discussed. 

The radius a of the onions in the intermediate shear-rate regime of lyotropic smectics depends on shear rate, scaling roughly as a A similar texture size scaling rule is found in nematics (see Section 10.2.7) there it reflects a balance of shear stress r y against Frank elastic stress. In smectics, the two important elastic constants B and Ki have differing... 

Because of the additional translational order, the dislocations can exist in the cholesteric and smectic liquid crystals, which makes the texture of these liquid crystals even more complicated. Each liquid crystal phase shows characteristic textures and thus the optical texture becomes an important means to differentiate the phase of the liquid crystals. Liquid crystalline polymers have the same topologically stable defects as small molecular mass liquid crystals do, but the textures may be different due to the difference in the energetic stability of the same topological defects in both low molecular mass and polymeric liquid crystals (Kleman, 1991). In Chapter 3 we will discuss the textures in detail. 

Nevertheless in polymeric liquid crystals the same types of orientational defects and thus the same types of textures as present in the low mass counterparts have been observed. The textures often formed by polymers are the threaded texture, the schlieren texture and the focal conic texture of smectics. As is for low mass liquid crystals, the texture is a consequence of defects (disclinations and dislocations, refer to Chapter 1) present in the liquid crystal and is characteristic of a specific type of the phase. The texture examination has become a very useful tool in the determination of the type and nature of the polymeric liquid crystals. 

Disclination in smectic phases may also show up in the form of schlieren textures. It was believed that only the whole-numbered singularities are present in these phases. However more recent work has shown that certain smectic Ca phases can also give schlieren textures with half-numbered singularities (Watanabe et al, 1989 1992 Niori et al., 1995). Other methods such as X-ray scattering may be needed for an unambiguous characterization of the phase. 

The formation of the TGBA phase from the isotropic liquid can also be observed in free standing films. The defects formed in this case tend to be filamentary in nature, see Plate 8. Such a texture is not normally seen in free standing films of other liquid crystals, and therefore the presence of this defect texture is diagnostic for the characterization of TGB phases. In addition to the observation of such filamentary textures in free standing films, filaments were also observed embedded in homeotropically aligned smectic A phases, see Plate 9. 

Because of the layered structure, defects in the cholesteric can be likened in many respects to those in smectic A. Both of them exhibit focal conic textures and both allow for the existence of screw and edge dislocations. To discuss these similarities we employ a coarse-grained approximation in which the cholesteric distortions are considered to be small and to vary slowly over a pitch. In this approximation the free energy of distortion may be expressed in terms of layer displacement u parallel to the twist axis ... 

Fig. 2.9 A POM images of 8CB in PEI coated (Aa) rectangular microchannel, (Ab) V-shaped microchannel, and (Ac) trapezoidal microchannel (scale bar 50 mm) [48]. Schematic illustrations of 8CB textures in three types of microchannels in (Aa-Ac) [48]. The yellow rods represent each 8CB molecules. Reproduced with permission [48]. Copyright 2011, Royal Society of Chemistry. B Optical (first column) and POM images (second and third columns) of LC droplet according to the aspect ratio of confined PDMS in smectic-A phase (Ba), nematic phase (Bb) and isotropic phase (Be) [49]. Reproduced with permission [49]. Copyright 2012, American Physical Society...

J.B. Fournier, I. Dozov, G. Durand, Surface frustration and texture instability in smectic-A liquid crystals. Phys. Rev. A 41, 2252-2255 (1990)... 

Figure 4 Schlieren textures in calamitic thermotropic liquid crystals (A) two brushed singularities seen exclusively in the nematic phase, and (B) four bmshed singularities seen for both smectic and nematic phases. The dark lines show the orientation of the director. (Data from Gray and Goodby, 1984.)...

Smectic A and C phases are the most common smectic phases. They exist in several texture variants. Similar to the nematic phase, smectic A phase can exhibit the homogenous texture. The corresponding texture of the smectic C is the schlieren texture, In this texture, the layers are parallel to the sample surface. The schlieren indicates strong distortions of the director field [1]. 

The optical polarizing microscopy analysis of these copolymers showed in all cases schlieren nematic textures, in contrast to the corresponding polyethers which present smectic type textures. All these copolyethers are soluble in aromatic and halogenated solvents. 

FIGURE 12 Mechanisms for inducing transitions between scattering (focal conic) and clear (homeotropic) textures in a smectic A device. The material is assumed to have positive dielectric anisotropy. 

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