Disclinations in nematics

The properties of disclinations in nematics bear some striking similarities with screw disclinations in crystals and vortex hlaments in superfluids, but at the same time there are important differences that cannot be overlooked while drawing detailed analogies. ... 

R.B. Meyer, On the Existence of Even Indexed Disclinations in Nematic Liquid Crystals, Phil. Mag., 27, 405-424 (1973). 

Disclinations in tire nematic phase produce tire characteristic Schlieren texture, observed under tire microscope using crossed polars for samples between glass plates when tire director takes nonunifonn orientations parallel to tire plates. In thicker films of nematics, textures of dark flexible filaments are observed, whetlier in polarized light or not. This texture, in fact, gave rise to tire tenn nematic (from tire Greek for tliread ) [40]. The director fields... 

Figure 3. a) Disclinations in an otherwise smooth nematic texture of PBT in MSA b) Ring images observed on warming a nematic PBT solution see text. The large grid spacing is 80/im. 

In his classic work on liquid crystals, Friedel (26) summarized his observations of disclination reactions in nematic liquid crystals... 

Table I. Disclination Interactions in Nematic Liquid Crystals...

Figure 10.21 Classes of disclinations in a nematic, with the disclination line perpendicular to the plane of the page. [From Larson 1988 (adapted from Chandrasekhar 1992), with permission from Cambridge University Press.]...

Figure 10.22 A sequence of images of disclinations in 5CB at various times t after a pressure jump Ap = 4.7 MPa, sufficient to induce a transition from an isotropic state at 3.6 MPa and 33°C to a nematic state. The field of view is 360 pm. (Reprinted with permission from Chuang et al. 1991 Copyright 1991. American Association for the Advancement of Science. ...

There is only the orientational order in nematics so that only disclinations may appear with no dislocations. In the other kinds of liquid crystals,... 

As remarked in chapter 1, the nematic state is named for the threads that can be seen within the fluid under a microscope (fig. 1.1.6(a)). In thin films sandwiched between glass plates these threads can be seen end on. A typical example of the texture in a plane film of thickness about 10 /tm between crossed polarizers - the structures a noyaux or schlieren textures - is given in fig. 1.1.6(6). The black brushes originating from the points are due to line singularities perpendicular to the layer. In analogy with dislocations in crystals, Frank proposed the term disinclinations , which has since been modified to disclinations in current usage. 

Fig. 3.5.4. Brushes connecting a pair of disclinations of equal and opposite strengths, s = 1 and — 1, in nematic MBBA. Crossed polarizers rotated clockwise by 22.5° in each successive photograph. In (d) the directions of extinction are parallel to the edges of the picture. (Nehring and Saupe. )...

Fig. 3.5.5. (a) 5 = +1 disclinations and (fc)s = disclination in a nematic film (left) between crossed linear polarizers (right) between crossed circular polarizers. 

A stability analysis has shown that twist disclinations are less favourable than wedge disclinations in elastically anisotropic media. This may explain why the former are so rarely seen in ordinary nematics. 

As already indicated briefly in 3.5.8 the effect of elastic anisotropy has some interesting implications for cholesterics, especially for long-pitched structures. We have seen that disclination pairs in nematics have angular forces in the presence of elastic anisotropy. For all practical purposes, the solutions that were obtained for nematics will hold good for each nematiclike cholesteric layer, except that the layers now twist continuously in the... 

The effect of structural disclinations upon texture and the origins of these in local deformations of or discontinuities in the arrangement of the molecules have been studied only in the case of nematic MCLCPs. The primacy aim of this chapter is therefore to present to the reader defects and textures in nematic MCLCPs. Some of consequences of constructing a nematic phase from long... 

Point defects are another class of defects in nematics. When a line, such as the wedge disclination of Fig. 3 a, with 5=1, escapes... 

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. 

Fig. 8.12 Voltena process in nematic liquid crystal placed between two glasses with side view (a) and top view (b). The part of the nematic shown by arrows directed from the right to the left was initially removed (virtually), turned by angle ti about the 0-axis and put back into the empty cavity. A plane wall S and a linear disclination loop L are formed...

Consider a disclination with its ends fixed at the opposite plates of a planar nematic cell. Such a disclination connects the two glass plates as in Fig. 8.13a. If we are looking at it from the top along the z-direction we can see the director distribution n (x, y) in the Ay-plane around the disclination. In a polarization microscope, in the same cell, we can see different n(jc, y) patterns corresponding to disclinations shown in Fig. 8.14. A point in the middle of each sketch shows the disclination under discussion that has its own strength s. 

Intensity fluctuations are also clearly visible. However, the practiced eye will notice that only disclinations of whole number rank are present in the preparation. Focal conic zones remain unchanged, up to some small details. But where only one colour was observed previously, we now often find two colours separated by a thin wall. All these observations are compatible with the simple idea that molecules are tilted relative to the plane of the layer, as shown in Fig. 9.11a. When there is no external field, the tilt direction remains indeterminate, just as we found for the directions of optical axes in nematic phases, which gave rise to their threadlike texture. However, the absence of disclinations of whole number rank is characteristic of a layered structure. The two colours can be understood as being due to occurrence of positive and negative tilts in thin preparations (see Fig. 9.11b). These arguments are corroborated by crystallographic studies. We have thus discovered a second type of layered liquid crystal, called the smectic C phase, or Sc- Note that this tilting does not preclude a liquid-type order within layers (a kind of 2-dimensional nematic phase). 

As mentioned earlier, most studies of field interactions with liquid crystals are done using thin films with a well-defined initial state, usually a monodomain or a thin film with a simple distortion induced by incommensurate surface anchoring. These conditions simplify observation and theoretical analysis. However, most liquid crystal materials that are not specially prepared contain topological defects that are very important to their response to external fields. One class of defect commonly observed in nematics is the disclinalion line. At a disclination line the director field is ill defined. The director field turns around the disclination line a multiple of half-integer times. Several disclination lines are shown in Fig. 8. 

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