Liquid crystals twisted nematic

The structure of liquid crystals can broadly be classified as nematic, cholesteric and smectic, see Fig. 1. None of them have full three-dimensional (3-D) positional order, but some degree of orientational order. Most often the constituent molecules are elongated, as indicated in Fig. 1, but distinctly flat molecules make up the socalled discotic liquid crystals. The nematic phase has only orientational ordering of the molecules. The collection of molecules have one symmetry axis called the director n. The cholesteric phase has only orientational order, formed by the constituent chiral molecules. The director twists with a pitch comparable to the wavelength of light. 

Figure C2.2.11. (a) Splay, (b) twist and (c) bend defonnations in a nematic liquid crystal. The director is indicated by a dot, when nonnal to the page. The corresponding Frank elastic constants are indicated (equation(C2.2.9)).

The first stable commercial liquid crystal display (LCD) device was the twisted nematic (TN) [110], still widely... 

Schadt M and Flelfrich W 1971 Voltage-dependent optical activity of a twisted nematic liquid crystal Appl. Phys. Lett. 18 127-8... 

Other more exotic types of calamitic liquid crystal molecules include those having chiral components. This molecular modification leads to the formation of chiral nematic phases in which the director adopts a natural helical twist which may range from sub-micron to macroscopic length scales. Chirality coupled with smectic ordering may also lead to the formation of ferroelectric phases [20]. 

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

A physicist would say the liquid crystal adopted a twisted nematic structure. 

Supertwisted nematic display, 15 114 Super twisted nematic liquid crystal display (STN-LCD), 9 340 Super ultra-low emissions vehicle (SULEV), 13 855... 

Fig. 30. Schematic representation of the twist deformation (a) changes in the components of the director w, defining the orientation change (b) twist deformation of an oriented layer of a nematic liquid crystal.

Twisted nematic liquid crystal sandwiched between two glass plates, with the director aligned parallel to the plates, with one of the plates turned in its own plane about an axis normal to it. 

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. 

Colour Displays from Twisted Nematic Liquid Crystals... 

It can be safely predicted that applications of liquid crystals will expand in the future to more and more sophisticated areas of electronics. Potential applications of ferroelectric liquid crystals (e.g. fast shutters, complex multiplexed displays) are particularly exciting. The only LC that can show ferroelectric property is the chiral smectic C. Viable ferroelectric displays have however not yet materialized. Antifer-roelectric phases may also have good potential in display applications. Supertwisted nematic displays of twist artgles of around 240° and materials with low viscosity which respond relatively fast, have found considerable application. Another development is the polymer dispersed liquid crystal display in which small nematic droplets ( 2 gm in diameter) are formed in a polymer matrix. Liquid crystalline elastomers with novel physical properties would have many applications. 

Afterwards there appeared what has become the main application liquid crystal displays (LCDs) based on the twisted nematic (TN) mode. These are commonly used for flat panel displays (e.g., desk calculators). Thin film transistor (TFT) LCDs enabled a large number of segments (e.g., 640 x 1024) to be used and they had advantages like... 

The directors (long molecular axes) of the constituent molecules in nematic phases are parallel to one another on average. This is the only order present in nematic liquid crystals, which are the most fluid type of liquid-crystalline phase. Molecules that form cholesteric phases must be optically active or contain an optically active dopant. As the phase name implies, the constituent molecules are frequently steroids and most commonly are cholesteric esters or halides. A conceptual model of the cholesteric phase includes layers of molecules in nematic-like positions, each layer being twisted slightly with respect to the ones above and below it. When the phase consists only of optically active molecules, the angle of twist between layers is typically less than one degree. Several subclasses of discotic phases exist. In all, the molecular planes of the constituent molecules are parallel. However, the discs can pack in nematic-like arrangements (ND) or in columns that are internally ordered (D ) or disordered (Dd) and may be stacked vertically,... 

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...

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