Disclinations boundary

The two structures so formed are analogous to a particle and its antiparticle. They can annihilate one another with release of energy but otherwise can only be destroyed at the boundary of the medium. In a similar manner, a suitable injection of energy can create a disclination pair in the body of the material. 

Figure 13 depicts a coalescence event that took place in 7 sec. Like other coalescence events in which new disclinations are produced, a 2tt disclination appeared at the position of the former boundary between the mesophase spherules. Frenkel (27) has pointed out that the time constant t for such coalescence phenomena is given approximately by... 

Xj < 2A0Z3 and attractive for x > 2AoZ,. Thus, as in the case of smectic A (see fig. S.4.7), there can result a clustering of like edge disclinations to form a grain boundary . Such clustering is often observed in fingerprint textures (fig. 4.2.7). 

The different kinds of incoherent twin boundaries that have been observed can be related to the very different roles they play isolated wedge microtwins are equivalent to disclinations. They are probably stabilized by the A or B steps visible on the images, while repeated wedge microtwins which do not present any step are equivalent to superdislocations. They allow the junction between different regions and are very stable. The incoherent boundaries are as different as the role that the microtwins play. 

Distortions and defects can be interpreted in terms of the continuum theory through equations derived from the expressions of the elastic energy and the imposed boundary conditions. Solutions are known in certain simple situations. Oseen [35] has found configurations, named disinclinations by Frank [33], or disclinations today, which are solutions of this problem for planar samples in which the director n is confined to... 

The study of defects in liquid crystal systems is rooted in the understanding of defects in the solid state. For instance, crystals are rarely perfect and usually contain a variety of defects, e.g., point defects, line defects, or dislocations, and planar defects such as grain boundaries. In addition to these typical imperfections of the solid state, liquid crystals can also exhibit defects known as disclinations. These defects are not usually found in solids and result from the fact that mesophases have liquid-like structures that can give rise to continuous but sharp changes in the orientations of the molecules, i.e., sharp changes in orientation occur in the director field. 

Fig. 8.18 Boodjooms. Structure of the director with two boodjooms in a nematic drop with tangential alignment of molecules at the surfaces (a), linear disclination with a point defect at the boundary of a nematic layer (b), and the same point defect (boodjoom) after aimihilation of the linear disclination (c)...

The cholesteric texture occurring with perpendicular boundary orientation, on the other hand, is not a uniform texture. Under the microscope the entire field of view is filled with right-and left-handed spirals. This cholesteric texture is known as the scroll texture and appears very much like an end-on view of a bundle of rolled-up scrolls. Capacitance measurements show that the cholesteric helical axis in this texture is still predominantly perpendicular to the plane of the layer. An exact analysis of the structure of this texture, even for the case of equal elastic constants, seems difficult to achieve. Important from the applications point of view is that the scroll texture is adopted without disclinations immediately after a display element is turned off and that this structure is essentially nonscattering. Homeotropic boundary orientation therefore solves the problem of display after-images. 

A special case was considered in [21]. The dielectric instability was investigated in the hybrid [4] or so-called corkscrew texture. A wedge-form cell was prepared with the planar and homeotropic orientation of a cholesteric mixture at opposite boundaries. It was shown that the Cano-Grandjean disclinations are not observed in this case and the electrooptical response... 

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