Twist disclinations perpendicular

In this case the singular line is perpendicular to the twist axis. On going round this line, one gains or loses an integral number of half-pitches. The director pattern around the -edge disclination was first worked out by de Gennes who proposed a nematic twist disclination type of solution ... 

Perpendicular disclinations, sometimes called twist disclinations, occur in nematics and have been discussed by de Gennes [105]. In a perpendicular disclination it can... 

The Volterra process for creating these disclinations is the same as for nematic disclinations. For the screw disclination the plane of cut is parallel to the cholesteric twist axis while for the edge disclination it is perpendicular to it. 

As the incidence angle is made smaller, the irregular shape of the disclination lines changes. At 45° beam incidence (Fig. 5) all disclinations run in the beam direction. Again, turning the cell between crossed polarizers changes the transmission only slightly. It appears that two preferred orientations are established. The director is either in the beam direction or perpendicular to it. The parallel disclinations separate areas of reverse twist. 

Not only are disclination lines aligned along the field direction, but also wall defects are anisotropically distributed in the aligned sample. This is best understood by first defining a rotation axis for a wall defect. A rotation plus a translation is required to map the lamellar pattern on one side of a wall defect to the other. I refer to this rotation axis as the rotation axis for the wall. If the wall contains its rotation axis, it is a bend wall, and if the axis is perpendicular to the wall, it is of twist character. The wall is of mixed character if the rotation axis is in between. In the field-aligned sample, the rotation axes of the wall defects are aligned predominantly along the direction of the applied field, e.. Thus, walls with normals parallel to S. have primarily twist character, and walls with normals nearly perpendicular to S. have primarily bend character. Examples of bend walls are indicated in Fig. 28a. 

There are other dischnations besides axial disclinations that form in nematic liquid crystals. In axial dischnations, the rotation axis of the director in traversing a loop aroimd the disclination is parallel to the disclination. In a twist dischnation, the rotation axis is perpendicular to the disclination. Figure 2.15 shows +1/2 and +1 strength twist dischnations in which the rotation axis for the director is along the y-axis and the dischnation points along the z-axis Due to the fact that the director twists, an entirely new class of dischnations form in chiral nematic liquid crystals. Likewise, the spatial periodicity of both chiral nematic and smectic hquid crystals ahows for defects in the perio(hc stmcture in addition to defects in the director configuration. These additional defects are quite different and resemble dislocations in solids. 

The situation is simpler if the column lattice is helically distorted perpendicular to the column axes blocks of parallel columns are stacked on top of each other with a finite angle between the columns of adjacent blocks (Figure 11.10) [18], The blocks are thus separated by planar tilt grain boundaries, and parallel linear screw disclinations lie within these boundary planes (Figure 11.11). This structure is perfectly analogous to the twist grain boundary phases of chiral smectic liquid crystals. 

As for achiral disclinations, the Volterra process may be used to create screw or edge disclinations by cutting parallel or perpendicular, respectively, to the chiral nematic twist axis. 

---