Texture Transitions

FIGURE 6.6. Behavior of a planar texture in fields of different directions 

Several cases are possible here, depending on which initial texture we are dealing with. 

A planar texture in an electric field E applied parallel to the axis h of the helix. 

This is a stable configuration and is shown in Fig. 6.6(a) where the cholesteric planes are denoted by solid lines. In this instance the field only induces stabilization of the fluctuations of the director [18]. As a consequence, the order parameter is increased and displacement of the selective reflection maximum in the longwave spectral region is observed. 

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Han, W.H. Rey, A.D. Dynamic simulations of shear-flow-induced chirality and twisted-texture transitions of a liquid-crystalline polymer. Phys. Rev. E. 1994, 49, 597-613. 

Since the early days of texture research the fact that fee metals and alloys develop two different types of rolling texture, the copper type and the brass type, has been a mystery to the texture community. One may say that it is the classical texture problem. As mentioned in section 1 it was already in the nineteensixties suggested, on the basis of circumstantial evidence, that the texture transition is governed by cross slip. In 1968 one of the present... 

Thus, the atomic-scale modeling provides a convincing support for the idea that the texture transition is governed by cross slip as suggested on the basis of circumstantial evidence. As opposed to the original suggestion [5,6] we suggest an indirect ("catalytic") effect of cross slip [22,25],... 

G. A. Held, L. L. Kosbar, I. Dierking, et al., Confocal microscopy study of texture transitions in a polymer stabilized cholesteric liquid crystal, Phys. Rev. Lett., 79, 3443 (1997). 

Electric fields have been used to switch a cholesteric texture between the planar, the focal conic, and the homeotropic state. The planar and focal conic states persist after the field is turned off, the choice determined by the amplitude and frequency of the applied field and the rate at which it is turned off. These textural transitions were discovered by Heilmeier and Goldmacher [32] and are used today for color display panels that exhibit gray-scale memory [33,34]. 

The optical properties of cholesteric liquid crystals are very specific and are determined by the pitch and arrangement of the axis of the helix and the polarization of the incident light. In an external field changes occur both in the direction of the axis of the helix (texture transitions) and in its pitch (untwisting of the helix). Before considering these field variations let us give a brief account of the optical properties of cholesteric liquid crystals in the absence of a field. Comprehensive reviews of the topics have been given recently in [1, 2]. 

Thus, even in this favorable case of the allowed splay-distortion, the Prederiks transition is, in fact, unobservable (ghost-transition). Instead, we observe a texture transition accompanied by the appearance of a number of defects. 

The texture transitions have been observed in smectics A many years ago [104], however, they are especially pronounced in materials with large dielectric anisotropy. A high-quality planar texture goes to a quasi-homeo-tropic optically transparent texture via intermediate structural defects (80CB, Ae = 8.2, d = 50 /xm, and voltage 30 V) [105]. Other substances were studied in [106]. A homeotropic texture can also be obtained by applying the field to the less-ordered focal-conic texture if Ae > 0 [107] (erasing a defect structure produced either by the thermal action of a laser or by electrohydrodynamic instability [108]). 

Only one of the alternatives studied by Parodi produces realistic values of the threshold field for a texture transition. The structure of the transition layers for this case is shown in Fig. 6.30(b). The periodic distribution of... 

FIGURE 6.30. Homeotropic texture of a smectic A in an electric field Ae > 0. (a) Initial geometry (b) Parodi texture transition [109] (c) wave-like instability [7], experimentally observed patterns corresponding to a Parodi transition [109] (d) from a homeotropic orientation and (e) from a planar orientation. 

The texture transition can also be observed for smectic A liquid crystals with negative dielectric anisotropy [112]. In that case, the transition from a homeotropic into a planar texture occurs. The threshold of this, dielectric transition, can be modified (lowered) at the low frequencies of an applied field by the anisotropy of the electrical conductivity of a substance. 

Fig. 1.38. Experimentally observed texture following grain growth induced by a temperature change AT from the deposition temperature in Ag thin films, up to 500 nm in thickness. The films were deposited on MgO(lOO) substrates coated with an amorphous silicon dioxide layer. The solid line corresponds to the texture transition prediction of (1.32). Adapted from Thompson and Carel (1995).

Thompson and Carol (1995) have experimentally studied texture evolution in Ag thin films and have shown that the competition between strain energy minimization and surface energy minimization leads to film textures that are consistent with the transition predicted by (1.32). Figure 1.38 shows their experimental results on texture evolution in Ag polycrystalline thin films of different thicknesses as a function of temperature excursion AT from the deposition temperature. Here, the Ag films were deposited on MgO(lOO) substrates coated with amorphous silicon oxide. The solid line in this figure denotes the texture transition condition predicted by (1.32). 

This texture transition from focal-conic to nematic is irreversible, provided there is perpendicular alignment at the cell boundaries. The helix structure forms differently with a decreasing electric field than with an increasing field, as shown in Fig. 45d. Also the critical field strength F l... 

When an electric or magnetic field is applied to a liquid crystal cell, a texture transition occurs to minimize the free energy of the system. These texture changes in cholesteric liquid crystals are physically similar to the Frederiks transition in a nematic liquid crystal and result in a significant change in the optical properties of the layer. Texture transitions have been reviewed previously [8, 9] with allowance made for the sign of the dielectric or diamagnetic anisotropy, the initial texture, and the direction of the applied field. Here, we consider only the instability of the planar cholesteric texture, which has been widely discussed in recent literature. 

Untwisting of a helix is observed only with positive dielectric or diamagnetic anisotropy and only in a field perpendicular to the helical axis h. Thus, in some cases, the director reorientation (or a texture transition) has to occur before untwisting is possible. 

Texture transitions are particularly pronounced when an electric field is applied to materials having a large dielectric anisotropy. A planar texture undergoes transition to a quasihomeotropic optically transparent texture via intermediate structural defects [150,151]. The threshold voltage observed experimentally for a transition from a planar to a homeotropic texture depends on the layer thickness according to f/oc (for the Frederiks transition the threshold voltage is independent of thickness). A model that accounts for the experimental data (at least partly) has been developed by Parodi [152] who assumed the formation of transition layers between the surface and the bulk of a sample. A discrepancy between the calculated and observed periods of the texture instability may be due to a nonuniform... 

A further interesting use of the focal-conic to homeotropic texture transition is in infrared modulation [272]. Here it was found possible to modulate infrared light at A=8-12 pm with a maximum transmission of 87%, a contrast of 93%, and turn on and off times of 1 ms and 125 ms, respectively. A further window examined was 3-5 pm, and this work suggests that other chiral nematic electrooptic effects could be exploited in the near infrared. In communications technology a 2x2 optical switch for fiber-optics has been developed [273] using a chiral nematic film and two switchable nematic waveplates. It has been demonstrated that this is suitable for LED or laser sources. The device worked at 1.318 pm and had switching times of 40 ms with -26 dB crosstalk between unselected fibers. There will clearly be further advances in this use of the unique optical properites of chiral nematics. 

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