Defect structures nematic liquid crystals

This simple treatment of liquid crystalline defects is only applicable to nematics, and the detailed appearance of disclination lines will differ from the simple structures described above because of differences between the elastic constants for splay, twist and bend. In smectic phases, defects associated with positional disorder of layers will also be important, and some smectic phase defects such as edge dislocations have topologies similar to those described for crystals. The defect structures of liquid crystals contribute to the characteristic optical tex-... 

A. Fernandez-Nieves, V. Vitelli, A. S. Utada, D. R. Link, M. Marquez, D. R. Nelson, D. A. Weitz, Novel defect structures in nematic liquid crystal shells, Phys. Rev. Lstt. 2007, 99, 157801. 

I. Vilfan, M. Vilfan, and S. Zumer, Defect structures of nematic liquid crystals in cylindrical cavities, Phys. Rev. A, 43, 6873 (1991). 

The FLCs or AFLCs mentioned above are liquid crystalline materials showing a chiral smectic C phase or related phases. Although nematic liquid crystal possesses only directional order, smectic liquid crystals shows layer structures or periodic order of the liquid crystalline molecular centres. In this respect, smectic liquid crystalline materials have more in common with crystalline materials than with nematic liquid crystalline materials. As a result, the alignment of smectic liquid crystal is quite different from that of nematic liquid crystals. Smectic liquid crystals show a large variety of defects because they possesses highly ordered structiu-es. Moreover, the layer structures are irreversibly destroyed by applying stress. This phenomenon poses a major problem for display applications. Therefore, several techniques to prevent the application of force to the liquid crystalline materials of FLC or AFLC devices have been proposed. 

R. Ribotta, A. Joets Defects and interactions with the structure in ehd convection in nematic liquid crystals, in J. E. Wesfreid, S. Zaleski (eds) Cellular Structure in Instabilities, Springer, Berlin, p. 249 (1984)... 

Ferroelectric liquid crystal devices have suffered from a number of characteristic alignment defects which are absent from nematic liquid crystal devices. The most important of these, termed zig-zag defects (from their characteristic shape) result from the chevron structure (Fig. 23). The chevron can form with the layer tilt toward either of the directions perpendicular to the layer planes if there is also a surface pretilt, its direction and that of the layer tilt may... 

Miroshnichenko AE, Pinkevych I, Kivshar YS (2006) Tunable all-optical switching in periodic structures with liquid-crystal defects. Opt Express 14(7) 2839-2844 Ong HL (1983) Optically induced Freedericksz transition and bistability in a nematic liquid crystal. Phys Rev A 28(4) 2393-2407... 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

In general, chiral nematic polymer liquid crystals (LCP) cannot form monodomains in which the rodlike polymers have a spatially uniform orientation within the sample. Typically, because of the high density of orientational defects, the LCPs are textured, with a distribution of polymer orientation. Microscopically, the polymer chains have a preferred orientation with a relatively narrow distribution around the average orientation. Macroscopically, the variation in space of the orientation results in a domain structure. Defects and orientational variations give rise to the polydomain texture and the overall LCP sample may be randomly ordered (Fig. 3). 

We now consider defect structures in the cholesteric liquid crystal. Treating the cholesteric as a spontaneously twisted nematic,... 

The concept of defects came about from crystallography. Defects are dismptions of ideal crystal lattice such as vacancies (point defects) or dislocations (linear defects). In numerous liquid crystalline phases, there is variety of defects and many of them are not observed in the solid crystals. A study of defects in liquid crystals is very important from both the academic and practical points of view [7,8]. Defects in liquid crystals are very useful for (i) identification of different phases by microscopic observation of the characteristic defects (ii) study of the elastic properties by observation of defect interactions (iii) understanding of the three-dimensional periodic structures (e.g., the blue phase in cholesterics) using a new concept of lattices of defects (iv) modelling of fundamental physical phenomena such as magnetic monopoles, interaction of quarks, etc. In the optical technology, defects usually play the detrimental role examples are defect walls in the twist nematic cells, shock instability in ferroelectric smectics, Grandjean disclinations in cholesteric cells used in dye microlasers, etc. However, more recently, defect structures find their applications in three-dimensional photonic crystals (e.g. blue phases), the bistable displays and smart memory cards. 

Due to the layer structure of a smectic liquid crystal, some types of deformation commonly found in the nematic phase are prohibited. Consider a defect-free SmA... 

A useful structural concept introduced by Kleman and FriedeF postulates a quasi-layered structure and explicitly takes into account the natural twist of the system. Concerning defects, we may think of cholesteric liquid crystals as a smectic with an in-plane nematic behavior, similar to the smectic C phase. Instead of using tire concept of a layered structure to account for the twist, we may also consider tire field of twist axis t in addition to the director field n. The two concepts are essentially equivalent, with the twist field being identical with the layer normal. The twist field accordingly suffices the condition t curti = 0, which means that in this twist field no fwist deformation is allowed. The concept of "layers" or twist-field is an approximation, which may not be valid in the core of the defects. We assume that the core structures of cholesterics (especially those with weak chirality) are similar to that of nematics. 

Comparable to the nematic (N) phase of rod-like compounds the least orda ed (usually highest temperature) mesophase exhibited by disc-like molecules is also the nematic (discotic. No) phase the index D simply refers to their molecular shape. Both nematic phases are of the same symmetry and identical types of defects are seen in both cases [5] they exhibit similar fluid Schlieren textures [3,6]. However, the nematic phases of these two low-molar-mass liquid crystals are not miscible [3,6] and phase separation occurs due to fundamental differences in their molecular structures. 

---