Disclination

As in crystals, defects in liquid crystals can be classified as point, line or wall defects. Dislocations are a feature of liquid crystal phases where tliere is translational order, since tliese are line defects in tliis lattice order. Unlike crystals, tliere is a type of line defect unique to liquid crystals tenned disclination [39]. A disclination is a discontinuity of orientation of tire director field. 

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... 

Bibo A M and Peterson I R 1989 Disclination recombination kinetics in water-surface monolayers of 22-tricosenoic acid Thin Solid Films 178 81-92... 

Inelastic deformation of any solid material is heterogeneous. That is, it always involves the propagation of localized (inhomogeneous) shear. The elements of this localized shear do not occur at random places but are correlated in a solid. This means that the shears are associated with lines rather than points. The lines may delineate linear shear (dislocation lines), or they may delineate rotational shear (disclination lines). The existence of correlation means that when shear occurs between a pair of atoms, the probability is high that an additional shear event will occur adjacent to the initial pair because stress concentrations will lie adjacent to it. This is not the case in a liquid where the two shear events are likely to be uncorrelated. 

Disclinations can be expected when shear occurs between two parallel polymer chains (Gilman, 1973). This has been postulated to account for anelas-tic relaxation in some polymers at low temperatures. A general discussion of disclinations in polymers has been given by Li and Gilman (1970). 

J. J. Gilman, Plastic Relaxation via Twist Disclination Motion in Polymers, Jour. Appl. Phys., 44, 2233 (1973). 

From the diameter of the disclinations, the pitch can be measured30 furthermore, clockwise rotation of the lens generates a shrinking or an expansion of the circles, according to the cholesteric handedness from this, the helical handedness can be deduced.31 It should be stressed that this is not a spectroscopic method and gives results completely independent from those obtained with CD or optical rotation. 

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. 

The smooth texture could be disrupted by causing flow in cells of type B by rotating a partially filled cell about its axis, with the latter in a horizontal plane. Flow resulted in the appearance of disclinations and increase in both Rhv(q) Rw(q) until these... 

The dry nematic solution exhibits a smooth texture after being annealed, with a field of disclinations at any glass-solution interface. A nematic phase produced by cooling the isotropic phase will exhibit a complex, mottled texture that slowly anneals to the smooth texture. The ratio Rvv(q)/ Hv(q) (for e = 20 deg.) for either morphology indicates appreciable orientational averaging of the orientation fluctuation. Photon correlation scattering on the... 

Note 1 Diselinations are responsible for some optical textures seen with a polarizing microscope, such as the schlieren texture formed by disclination lines in nearly vertical orientations, whose projections are seen as dark points with two or four emerging dark stripes or brushes (see Definition 4.9.2). 

Note A nucleus can indicate either the end of a disclination line terminating at the surface of a sample or an isolated defect. 

The product s0r yields the angle by which the director turns on a closed curve around the disclination center. If a complete circuit is made around the center of an 5 = I/2 disclination, the director rotates by 71. For 5 = 1 a similar circuit yields a total director rotation of 2ti. So, 5 = I/2 defines a 7r-line disclination and 5 = 1 defines a 27r-Iine disclination. 

Fig. 22. (a) Identification of the angles and 6 used to describe a disclination. (b) Director arrangement of an 5 = I/2 singularity line. The end of the line attached to the sample surface appears as the point s = + V2 (points P). The director alignment or field does not change along the z direction. The director field has been drawn in the upper and the lower surfaces only. 

Fig. 23. Schematic representation of the director alignments at disclinations with different values of s and (J)o,s 72 correspond to two-brush defects and s = 1 to four-brush...

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