Twisted-nematic geometry

Fig. 23 Controlled chirality by twisted nematic geometry, (a) Chemical structure of the molecule used for the experiment. The molecule shows N and B4-like phases, (b) CD spectra for cells with various twisting angles [22]...

R.A. Ewings, C. Kischka, L.A. Parry-Jones and S.J. Elston, Measurement of the difference in flexoelectric coefficients of nematic liquid crystals using a twisted nematic geometry, Phys. Rev. E 73(1), 011713/1-6, (2006). 

Ferroelectric liquid crystals (FLC) are of great interest due to their fast electro-optical response which is about 1,000 times faster than conventional twisted nematic cells [131]. The geometry used is called a surface stabilized FLC cell which utilizes a very thin gap (=2 pm) to unwind the FLC supramolecular pitch (=1-2 pm) since the bulk FLC materials do not show macroscopic polarization. This very thin gap, however, leads to difficulties in manufacturing large panels and very poor shock resistance. Researchers have proposed the concept of microphase stabilized FLC [79,109, 130] using FLC-coil diblock copolymers for electro-optical applications as shown in Fig. 15. This concept takes advantage of ferroelectric liquid crystallinity and block copolymer microphase separation since the block... 

In experiments with cholesteric liquid crystals (geometry III), extraordinary high viscosity rim is observed, few orders of magnitude higher than the viscosity of the isotropic phase or a non-twisted nematic. It seems surprising because the local structure of nematics and cholesterics is the same. In addition such a flow is strongly non-Newtonian with increasing shear rate (s) rim decreases, as schematically shown in Fig. 9.9. In the case of the Poiseuille flow, the viscosity depends also on the radius of a capillary. 

When a bistable TN display is optimized, the transmittance of one of the stable states should be 0 and the transmittance of the other stable state should be 1. The parameters of the display are the twist angles (f). In + (p) of the stable states, the angle a,- of the entrance polarizer, the angle of the exit polarizer, and the retardation F of the liquid crystal. As discussed in Chapter 3, the transmittance of a uniformly twisted nematic display in the geometry shown in Figure 3.3 is... 

There are also electro-optic effects using either a different geometry of surface stabilization or a completely different mechanism In the twisted ferroelectric smectic-C cell [54] the moleeules form in the zero field state a quarter helix which is removed when a dc field of either polarity is applied the optical effect is achieved in the same way as in a twisted nematic cell. Compounds with a short chiral smectic-C pitch in a thick cell are used for the distorted helix ferroelectric (DHF) device [55] this effect uses the optical difference between the zero-field state eharacterized by a fully developed short-pitch helix, and structures with a distorted or almost unwound helix in the presence of an applied field optically addressed spatial light modulators can take advantage of the DHF effect [56]. Further applications of ferroelectric liquid crystals are switchable diffraction gratings [57]. 

Figure 62. Comparison between twisted nematic and FLC switching geometry.

FIGURE 2.32 The geometry of a twisted nematic liquid crystal device. Alignment layers are designed such that the molecules adjacent to one glass plate are twisted 90° to those at the other glass plate. 

Figure 12.3. Experimental set up for optical limiting action using laser induced nematic liquid crystal axis realignment effect. Upper diagram shows the geometry of the twisted nematic cell. The focused spot diameter of the Argon laser on the film is 150 microns. Sample thickness 25 microns. Dye concentration 0.5 %.

Analysis of the optical properties of liquid crystals in more complex geometries, such as the twisted nematic state, can also be used to determine the refractive indices of nematics. Generalised transmission ellipsometry [13] has been used to measure the principal refractive indices and then-dependence on wavelength over the spectral range 350 to 1700 nm for 5CB and mixtures of 5CB with its chiral isomer CB15 (4-cyano-4 -(2methyI)-butylbiphenyl). 

It is not proposed to derive the switch-on time for a twisted nematic device in this text. However, it should be mentioned that in more complex geometries, such... 

Fig. 24. Three principal types of orientational effects induced by electric (E) and magnetic (H) fields in nematic low molecular liquid crystals. At the top of the figure the initial geometries of molecules are shown. Below the different variants of the Frederiks transition — splay-, bend- and twist-effects are represented...

Here nd are elastic constants. The first, is associated with a splay deformation, K2 is associated with a twist deformation and with bend (figure C2.2.11). These three elastic constants are termed the Frank elastic constants of a nematic phase. Since they control the variation of the director orientation, they influence the scattering of light by a nematic and so can be determined from light-scattering experiments. Other techniques exploit electric or magnetic field-induced transitions in well-defined geometries (Freedericksz transitions, see section (C2.2.4.1I [20, M]. 

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