Twist nematic lines

When the distance between the AFM lines is increased, it is possible to observe under the polarizing microscope single twist nematic lines, originating from a single AFM rubbing line. An example is shown in Fig. 7.11, where one can clearly observe a very thin twisted nematic region above a single AFM line [33]. 

The first TN-LCD were simple, directly addressed segment displays as still used, e. g., for wrist watches. When attempts were made to increase the information content of the displays by time-sequential addressing in rows and lines (multiplexing) the limits of the TN cell were soon met. At higher multiplex ratios [14] contrast loss occurred, because of ever shorter addressing times. The development of the super-twisted nematic (STN) cell in 1984 [15] pushed the practicable limit to higher multiplex ratios, but it did not lead to a general solution of the problem. 

We now consider a twist disclination loop in a twisted nematic. The nematic is supposed to have a planar structure with the director parallel to the xy plane and an imposed twist of q per unit length about the z axis, and the disclination loop of radius R is supposed to be in the xy plane. The director distortions are planar, = cos = sin = 0. On going once round the disclination line at any point on the loop, the director orientation changes by 2tis, the sign of which may be either the same as that of q or opposite. 

Fig. 2.4. A transverse electric field, indicated by the arrow pointed towards the right at the top of the figiu-e, tilts the apolar director as shown by double headed arrows in a specific direction due to the flexoelectric effect on a 90° twisted nematic cell. The tilting direction reverses if the field direction is reversed. The transmitted intensity mesisured with a polarized light beam traversing the cell vertically as indicated by the dashed line will be identical in the two cases. On the other hand, with an oblique beam, the transmitted intensities for the two tilted director structures will be different, and can be used to me siu-e the flexocoefficient (adapted from Kischfai et cU. ).

The influence of the variation of several parameters on the maximum number of scanned lines of a reflective of transmissive twisted nematic liquid-crystal matrix display has been investigated by numerical calculation. 

At the present time, as shown by the pocket calculator, materials are available with adequate electro-optic performance where the duty cycle is 1 in 2, 3 or 4. Where temperature compensation is used, 1 in 7 duty cycle is possible O with mixtures of cyanobiphenyls and benzoate esters. For a duty cycle of 1 in 10, temperature compensation is a must and the electro-optic performance is determined principally by the threshold sharpness. By way of example. Fig. 36 shows a viewing cone plot for ZLl 1253 (dotted line curves) compared with a sharper threshold material (1.65) (solid line curves) in an 8y, low tilt cell. By single frequency addressing, a 1 in 10 duty cycle is approaching the maximum for an acceptable performance in a twisted nematic display. 

FIGURE 4 Variation with rms voltage of transmission with crossed polarizers for three angles of incidence (solid lines) and cell capacitance (broken line) for a twisted nematic device. 

It is reasonable to assume that the hne shapes shown in Fig. 13 reflect the presence of helical structures in cholesteric LCs even in the strong magnetic field, and it can safely be assumed that the helical structure directions were perpendicular to the magnetic field, as proposed by Meyer [14]. At first, Eq. (18), in which uniaxial rotation is assumed, was used to simulate the hne shapes in Fig. 13. According to the field effect on cholesteric LCs, in which, macroscopically, cholesteric LCs are spontaneously twisted nematics, Eq. (18) was integrated in terms of / l, which should have provided the pseudo-line shapes of helical structures in the magnetic field. However, it was very difficult or nearly impossible to reasonably reproduce the experimental line shape with our analysis. Therefore, the concept of biaxiality for... 

The core structure of cholesteric discli-nations was interpreted by K16man and Frie-del [2, 3]. The rotation vector considered in the Volterra process is normal to the cholesteric axis and is either parallel to the molecules or normal to them, this resulting in a core structure that is either continuous, with a longitudinal nematic alignment of directors in the core (A disclinations), or discontinuous (t disclinations), with a singular line of the type encountered in non-twisted nematic liquids. 

In the 1980s, the research and development of AM LCDs were very active, but it had not yet opened great application. On the other hand, STN-LCD (super twisted nematic LCDs) for PCs and Japanese word processors were realized in the 1980s [7], and also colored STN-LCDs for PCs were developed. The electro-optical response increased steeply with increasing applied voltage for those cells and enough contrast was provided even if the number of scanning lines increased, and... 

This set-up allows a pixel to be addressed at each intersection of a row and a column. This works line for nematic LCs in modest sized displays, i.e. up to 120 000 pixels, but beyond this size there is an increase in switching times and cross-talk between adjacent pixel elements leading to a loss in contrast. This problem can be overcome by using STN LCs, which are materials where the hehcal twist is increased to between 180° and 270°. These super twist LCs give a much sharper image than the 90° materials. This system is ideal for monochrome displays but even with these materials the response times start to get very slow with the several million pixels that are required for high contrast, full-colour displays. 

Fig. 17a-c. Elastic constants for a splay b twist c bend deformations of a nematic phase. The full lines represent the director... 

Experiments demonstrate that at even higher Er, the rolls become unstable and irregular. Ultimately, defect lines called disclinations form in the flow direction. As the linear analysis concerns the behavior of infinitesimal disturbances, the growth of the instability and further bifurcations are inaccessible to such analyses. This motivated Feng, Tao, and Leal to carry out a direct numerical simulation of a sheared nematic. Using the LE theory, with the one-constant approximation, they predicted a cascade of instabilities illustrated in Fig. 3. Steady state rolls first appear at Er = 2368. The director twists toward the flow (z) direction at the center of the cells. With increasing Er, the secondary flow and the director twisting intensify. 

In a liquid crystal cell based on the TN mode, a homogeneously aligned layer of a nematic liquid crystalline material with positive dielectric anisotropy (Ac), helically twisted by 90°, is placed in an ITO-lined glass cell between crossed polarizers... 

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