Chiral: axis textures

Liquid crystalline (LC) solutions of cellulose derivatives form chiral nematic (cholesteric) phases. Chiral nematic phases are formed when optically active molecules are incorporated into the nematic state. A fingerprint texture is generally observed under crossed polarizers for chiral nematic liquid crystals when the axis of the helicoidal structure is perpendicular to the incident light (Fig. 2). 

Cholesteric liquid crystal (ChLC)-driven electronic paper has been investigated mainly by Kent Display [2], Since ChLCs are chiral molecules, the particular color of light depends on the pitch denoted as P, where the pitch is the distance along the helical axis for ChLC to twist 360 and is determined by the amotmt and type of chiral additive within the liquid crystal mixture. ChLCDs are driven by switching the different textures of the ChLC electrically [1, 3], as shown in Fig. 4. 

Apart from the triphenylene mentioned in Section 11.2, one other case has been reported where a chiral columnar phase structure manifests itself in an asymmetric texture 2,3,9,10,16,17,23,24-octakis-(5-3,7-dimethyloctyloxy)-phthalocyanine exhibits two columnar phases above room temperature [8], [14]. The one at the higher temperature has a rectangular column lattice, the other a hexagonal one. When cooled quickly from the isotropic liquid, left-handed spirals appear in the flower-like texture of the highly ordered room temperature phase (Figure 11.8). X-ray and circular dichroism measurements indicate a helical superstructure with a pitch of 55 A (a 16 molecules). It seems very probable that in this case the molecules are weakly tilted and that the tilt direction spirals around the column axis, especially because the phase at higher temperatures is rectangular and therefore most probably tilted as well. 

A magnetic field can be used to enforce fingerprint textures also in rod-like nematics, if the sign of the anisometry of the diamagnetic susceptibility of the chiral phase is positive. Then the alignment of the helix axis parallel to... 

The observed optical properties of chiral nematic films depend critically on the direction of the director at the surface interface and on how this propagates to the bulk material. If the director is oriented along the surface of the cell using suitable alignment agents, such as rubbed polyimide, PVA, or PTFE, then the helix axis direction (see Fig. 1) is perpendicular to the substrates, as shown in Fig. 2 a. In this case, an optically active transparent planar texture is obtained. It is this texture that is normally used to observe the bright iridescent reflection colors initially observed by Reinitzer and Leh-... 

There is a further geometry of practical interest for light incident on chiral nematic films, related to the pitch of the helix in which we consider light propagating in a direction normal to the helix axis, i.e., as in the fingerprint texture, but with a pitchp less than A. In this short pitch chiral nematic case, the chiral optical tensor is averaged in space and the macroscopic optic axis is col-linear with the helix axis [27]. The macro-... 

As described earlier, the spectacular optical properties of chiral nematics are determined by the helicoidal pitch, the birefringence (and refractive index), the direction and polarization of the incident light, and the arrangement of the helix axis. For normally incident light the direction of the helix axis gives rise to the three classical textures depicted in Fig. 2, and typical photomicrographs taken with crossed polarizers are shown in Fig. 6 these are ... 

These optical properties of chiral nematic materials have all been observed experimentally. There have been quite extensive theoretical studies carried out by Mauguin [60], Oseen [61], and de Vries [26] to explain how these properties arise from the he-licoidal structure. Kats [62] and Nityanan-da [63] have derived exact wave equations to explain the propagation of light along the optic axis and Friedel [32] has reviewed the main textures observed with chiral nematics. We will outline the important elements of these studies in the next sections (Secs. 2.2.1.1 -2.2.1.3). In Sec. 2.2.1.4 we will consider how the helicoidal pitch, and... 

As a result of the layered nature of the chiral nematic structure, like the smectic A, it can also exhibit focal-conic textures [79] and both phases exhibit screw and edge dislocations. A dislocation corresponds to a displacement of the layered structure in a plane orthogonal to the layer and may be formed by the pairing of two disclinations of opposite sign. A screw dislocation has a singular line along the screw axis and is equivalent to a f-screw disclination in a chiral nematic. An edge dislocation corre-... 

In the White-Taylor device the chiral nematic (p A) is doped with an anisotropic dichroic dye. With homeotropic boundary conditions and low voltages, the focal-conic texture becomes axially aligned in the plane of the device. The dye spirals with the director and the random directions of the helix axis in the plane of the device ensure that unpolarized light is absorbed uniformly in this state. Application of a high field (see... 

Figure 38. Schematic operation of the White-Taylor dye guest-host chiral nematic electrooptic cell. In (a) for zero applied field the axis of each focal-conic domain is random in the x, y plane, as therefore is the dye, using homeotropic surface alignment. In (b) the texture is planar for the zero field state and therefore the dye spirals around the z direction. In (c) the focal conic (a) or planar (b) transition to homeotropic nematic has taken place above the threshold voltage V,], (WT). The black ellipses represent the dyes in the chiral nematic matrix.

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