Twisted nematic liquid crystal cell

FIGURE 6.4.12 Electro-optic curve for a typical twisted-nematic liquid-crystal cell. 

AN R.F. PLASMA TECHNIQUE FOR PRODUCING TWISTED NEMATIC LIQUID CRYSTAL CELLS... 

S. T. Wuand C. S. Wu, Mixed-mode twisted nematic liquid crystal cells for reflective displays, App/. Phys. 

Fig. 10. Twisted nematic liquid-crystal cell equipped with a polyethylene/silver nanocomposite filter (65).

Polyimide LB films have aligning properties for twisted nematic liquid crystals. This ability was maintained after the pyrolysis. In other words, the pyrolised polyimide LB films can be used as a transparent electrode for liquid crystal cells without rubbing treatments [314,315]. The electrooptical response of twisted nematic liquid crystal cells using pyrolised polyimide LB films was almost the same as those of rubbed polyimide films on ITO (indium-tin oxide) electrodes (Figure 14.43). The mechanism of liquid crystal alignment was studied [316,317]. Later, similar results were obtained using polyimides with slightly different chemical structures. [318,319]. 

Figure 14.43. Electrooptical response of twisted nematic liquid crystal cells using pyrolysed polyimide LB film (a), rubbed polyimide film (b). (Reproduced by permission of The Chemical Society of Japan from ref. 314.)...

Figure 6.8, Tilting and unwinding of the director axis of a 90° twisted nematic liquid crystal cell under the action of an applied field.

US 6,661,489 (US 6,593,989), J. Ha. Kim and J. Hy. Kim, (LG Electronics Inc.), Domain-divided twisted nematic liquid crystal cell and method of fabricating thereof having a differentially removed photosensitive layer, G02F1/1337 (349/124) (December 2003). 

H. Birecki and F. J. Kahn, Effects of Cell and Material Properties on Multiplexing Levels of Twisted Nematic Liquid Crystal Displays, This volume. 

THE TWISTED NEMATIC LIQUID-CRYSTAL DISPLAY CELL... 

EFFECTS OF CELL AND MATERIAL PROPERTIES ON MULTIPLEXING LEVELS OF TWISTED NEMATIC LIQUID CRYSTAL DISPLAYS... 

Sugimura A, Luckhurst GR, On-Yang Z (1995) Director deformation of a twisted chiral nematic liquid crystal cell with weak anchraing boundaries. Phys Rev E 52(l) 681-689 Torrent MC, Sagues F, Arias F, San Miguel M (1988) Freedericksz transition in a periodic magnetic field. Phys Rev A 38(5) 2641-2649... 

Consider the three typical aligned nematic liquid crystal cells as depicted in Figures 6.6a-6.6c corresponding to planar, homeotropic, and twisted NLCs of positive anisotropy. With the applied electric field shown, they correspond to the splay, bend, and twist deformations in nematic liquid crystals. Strictly speaking, it is only in the third case that we have an example of pure twist deformation, so that only one elastic constant K22 enters into the free-energy calculation. In the first and second cases, in general, substantial director axis reorientation involves some combination of splay (S) and bend (B) deformations pure S and B deformations, characterized by elastic constants Kii and respectively, occur only for small reorientation. 

As a simple illustration of the FDTD technique, consider a twisted nematic liquid crystal (E7) cell in one dimension (inz axis). The cell thickness is set equal to 5 pm. The pretilt angles of the top and bottom glasses are assumed to be nearly zero (0.01°) The director of the bottom glass is parallel to the x axis and the total twist angle is set equal to 90°. 

Figure 4.12 Switching behavior of the jr-BTN bistable LC cell with asymmetrical anchoring conditions [46]. Top optical response bottom switching pulse train. Reproduced from F. Yeung and H.-S. Kwok, Truly bistable twisted nematic liquid crystal display using photoalignment technology. Applied Physics Letters 83, 4291 (2003), American Institute of Physics...

Note In the off-state the electro-optical cell contains a thin film of a nematic liquid-crystal with mutually perpendicular directors at the upper and lower glass plates hence to reach the on-state the director performs a 90° twist over the thickness of the liquid crystal film. 

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... 

Conventional AMLCDs use nematic liquid crystals in a twisted configuration between two sheets of glass or plastic, one of which is the backplane containing the TFTs. The space between the two sheets is called the cell gap. Display operation relies on some subtle features of the electro-optic behavior of nematic liquid crystals... 

H. Seiberle and M. Schadt, LC-conductivity and cell parameters their influence on twisted nematic and supertwisted nematic liquid crystal displays. Mol. Cryst. Liq. Cryst. 239(1), 229-244, (1994). 

Fig. 5. At the left, molecules of normal hemoglobin or of oxygenated sickle-cell-anemia hemoglobin are shown, with random orientations, and at about the average distance apart characteristic of red-cell contents. At the right long strings of molecules of deoxygenated sickle-cell-anemia hemoglobin are shown, assuming the parallel orientation characteristic of the nematic liquid crystals that presumably form within the red cells in the venous blood of patients with sickle-cell anemia, and twist the red cells into the abnormal shape characteristic of the disease.

Room temperature nematic liquid crystals have been developed for electro-optical applications [13-15]. In particular, twisted nematic (TN) liquid crystal displays have been widely used for practical display devices [ 13-15,38). In the TN cells, nematic liquid crystals form twisted alignment due to the influence of rubbed aligmnent polymer layers coated on the substrates (Fig. 7a). The TN cells are placed between two crossed polarizers. Without electric fields, the twisted LC aligmnent induces optical rotation of incident polarized... 

Fig. 12.16 Bistable twist cell. Right- and left-handed twist-structures of a nematic liquid crystal with the same elastic energy (a) and the angular dependence of total free energy (b)...

In an extensive work on polysiloxane polymer liquid crystals, Finkelmann et al. examined their performance in a twisted nematic electro-optical cell. In parallel with the polymer measurements, analogous studies were carried out on low molar mass nematic compounds of equivalent dielectric anisotropy (Ae). Both the low molar mass nematic materials and the polymer side-group moieties were mixtures of methoxy- (or alkyloxy-) and chlorine-terminated benzoate esters. Cells were prepared using rubbed polyimides and in the polymer case the samples were annealed for several hours to obtain defect-free aligned textures. Measurement of the threshold voltages, for both the low molar mass compounds and the polymer, showed ... 

FIGURE 4.12. Variation with voltage of the optical characteristics of twist cells between parallel polaroids d = 30 /xm, a mixtiure of nematic liquid crystal with Ae = +0.22, T = 23 C) [66]. (1) Transmission with a cell conventionally arranged to the polarization of the incident ray (2) transmission when the cell is rotated around the normal through 45 from the original position (3) phase lag calculated from curve 2. Axes to the right and bottom refer to (1) and (2) and axes to the left and top refer to (3). 

If the electrodes of a cell which have been pretreated by rubbing are further coated with a layer of surfactant (e.g., lecithin), the resulting orientation of the molecules of the nematic liquid crystal will be homeotropic. A cell formed in such a way that the directions of initial rubbing are perpendicular to each other transmits light under parallel polars. In an electric field a nematic liquid crystal with negative dielectric anisotropy (e.g., MBBA) must reorient itself into a twist structure, and transmission through the cell decreases to zero (reverse twist effect) [51]. 

This effect has been observed experimentally in comparatively thick cells (d 50 /xm) [113]. In cells with d 20 /xm, the final twisted state (in the field) proves to be insufficiently stable and the nematic liquid crystal layer is gradually transformed into a planar structure. The addition of small quantities of cholesteric liquid crystals to the initial nematic mixture enables a stable twisted structure to be achieved with the application of a field and improves the electrooptical characteristics of the device. The electrooptical response of electrically induced twist nematic cells includes intensity oscillations observed both in the switching on and switching off regimes [114]. These oscillations take place due to the variation of birefringence, which are not important in the usual twist effect. 

Domain patterns arise in sufficiently strong fields with both a homogeneous [80, 81] and a homeotropic [82] initial orientation of the nematic liquid crystal. The period of these domains is always of the order of the thickness of the layer (but not of the separation between electrodes). The Kapustin-Williams domains also occur in twist cells in this case, the strips are oriented at an angle of 45° to the direction of rubbing of the electrodes [87]. 

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