Chevrons textures

Fig. 13. Chevron texture with typical transition bars observed upon cooling from the nematic state for polyester prepared from di-n-propyl-p-terphenyl-, carboxylate and tetrame-thylene glycol. Crossed polarizers. From ref. 20.

Figure 35. A model of chevron textures (the chevrons are viewed from the top) showing alternating angular and rounded contours, at both the slide and the coverslip level. Prismatic domains d and d are filled with antiparallel conical sheets, in continuity along the generators r, s, t and u. Parabolic contours M, N and P join and form angles along the lines L at the coverslip level C, and similarly arcs M, N and P join and form angles along the lines L at the slide level S.

A planar wall has been described in smectic C phases, giving rise to a particular chevron texture. This has been shown... 

In case of strong planar and azimuthal anchoring when the director is fixed at the surfaces, typically a so-called chevron texture forms, where the layers have a kink in the middle of cells. The formation of the chevron layer structure leads to defects, where regions of opposite kink directions meet. 

Typical chevron texture with the widely accepted layer and director structures are illustrated in Figure 6.13. 

Transitions with the participation of liquid crystals sometimes show characteristic phenomena. If a nematic modification turns to a smectic A or smectic C phase, transient stripes in the form of a myelinic texture (also called chevron texture or striated texture) are often visible. Typically for the polyester prepared from di-w-propyl-/ -terphenyl-4,4" carboxylate and tetramethylene glycol, the nematic phase separates from the isotropic liquid on cooling in droplets which coalesce and form large domains. Cooling of the threaded-schlieren texture produces a transition to the smectic A phase this change is characterized by transition phenomena, mostly stripes, which broaden into larger areas ( transition bars ). 

The chevron structure and zig-zag defects were shown to disappear if a low-frequency electric field of sufficient amplitude is applied to an FLC cell [152-154]. The voltage required for the reorientation decreases with increasing spontaneous polarization. The texture appearing resembles the book-shelf geometry of smectic layers perpendicular to the substrates. It has minimum response time, maximum transmission difference between both stable states, and a relatively uniform viewing area. Therefore this texture, referred to as quasi-bookshelf geometry [152], seems to be very promising for display applications. 

Fig. 6.1.4 Changes in texture induced by AC-field treatment, (a) Texture I. Initial virgin texture with chevron layer structure and zigzag defects, (b) Textures II. A rooftop texture is observed after the application of an AC field of medium strength, (c) Texture HI. A quasi-bookshelf layer structure with stripe domains is observed after the apphcation of a strong AC field.

The stripe-shaped texture parallel to the rubbing direction is observed due to the formation of periodic undulation of the layer structure (Fig. 6.3.19). In the case that a strong AC voltage is applied, the chevron structure is deformed into the bookshelf structure and the strain occurs. The strain is relaxed by the rotation of the layer direction [55] which forms the periodic layer undulation. 

In Fig. 6.3.19, the structure of the stripe-shaped texture is shown. The width of the stripe becomes equal to the cell gap and the layer rotation angle becomes equal to chevron angle [68]. In certain conditions the tilted bookshelf structure can also be realized in this the width of the stripe becomes twice that of the cell gap [54]. 

On the other hand, in the cell with a stripe-shaped texture formed by applying a 15 V im , 10 Hz AC voltage at the transition from SmA phase to SmC phase, the realization of excellent bistability was confirmed even with a short pitch length and a large spontaneous polarization. For this kind of cell, the structure of horizontal chevrons as shown in Fig. 6.3.21 is proposed. 

Fig. 6.3.21 Horizontal chevron structure of stripe-shaped texture.

Scheme 11.3 Definition of the azimuthal angle x and scheme of the perpendicular chain axis orientation (a, a ) and the chevron-Uke texture (b, b ) in the reciprocal (a, b) and direct space (a, b )...

Fig. 11.9 Bi-dimensional X-ray diffraction patterns recorded at small angle (SAXS) of the samples iPP4 with [rr] = 5.9 mol% (a-d) and iPP5 with [rr] = 11.0 mol% (a ). Data are collected in situ, at the beam-line 16.1 of the Synchrotron Radiatirai Source in Daresbury (Cheshire, UK), 2.36 mm/min deformation rate, incident wavelength i = 1.4 A, 20 s/frame. The stretching direction is vertical. Arrows in (c) and (b ) and (c ) indicate the polarization of intensity off the equator and meridian due to the chevron-like texture (Scheme 11.3b )...

The textures described above belong to the same phase and are not separated by sharp interfaces. The frontiers are fuzzy and are often the sites of hybrid, but interesting, textures. Among these, at the limit between polygons and fans, one finds occasional chevrons, the organization of which is shown in Fig. 35 [93]. 

Figure 31. Optical patterns accompanying different EHD processes, (a) Electrolytic mode for the homeo-tropic orientation of a nematic liquid crystal, (b) Ka-pustin-Williams domains (KWD) in homogeneously oriented nematic, (c) Anisotropic EHD mode for the planar texture of a cholesteric, (d) A chevron structure due to interference of two instabilities (KWD and inertial mode).

Figure 97. The fact that even after switching the polarization P is not entirely in the direction of the applied field will tend to raise the chevron structure into a more upright position, so decreasing the effective d but breaking up the layers in a perpendicular direction. This gives a characteristic striped texture from the newly created, locked-in defect network.

Figure 98. Amplitude-controlled gray scale in SSFLC. The chevron structure is transformed to a quasi-bookshelf (QBS) structure by external field treatment. In addition to giving gray shades, the QBS structure increases the brightness and the viewing angle. This method of producing gray levels was developed by the Philips (Eindhoven) group, who called it the texture method .

Typical texture of srtrface stabilized SmC filtn with chevron domains (from http //www.kth.se/ fakulteter/tfy/kmf/lcd/lcd l.htm). Bar length 25 pm. Illustration of the layer and director structure. (Illustration is adapted from the Ph.D. dissertation of Chenghui Wang, Kent, 2003.)... 

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