Director twist angle

The transmitted light intensity was calculated by the Berreman 4x4 matrix method [22]. The director tensor is determined by the director tilt angle 7 and the director twist angle (f), which are expressed as... 

In Fig. 5.1.25, the director twist angle 4> is plotted as a function of the cell thickness direction Y at various surface pretilt angles, where represents the director twist angle at the chevron interface and is expressed as... 

It is shown that memory angle 0m is approximately equal to half of the total director twist angle in the cell and determined by the following equation,... 

Pig. 5.1.21 Coordinate systems used for simulations. Y and Z represent the cell thickness direction and the smectic layer normal, respectively, and z is the perpendicular line of the cone, (a) Chevron layer system, (b) Cone system, (c) Director tilt angle 7 and director twist angle . (d) Definition of the smface... 

Fig. 21. Schematic representation of the cholesteric phase 9 = twist angle, ra b = distance of molecules perpendicular to the director, h = pitch axis...

The major difference between the configuration of the OMI sandwich cell and other STN-LCDs is that the optical path difference (5 = And 1 pm) is much lower. There is no requirement for a significant pretilt (0 < 0 < 5°), the twist angle of the chiral nematic layer is lower (180°), the front polariser is parallel to the nematic director (a = 0°) and the polariser and analyser are crossed (P = 90°). The 180° twist gives rise to strong interference between the two elliptically polarised rays. If the optical path difference is small, e.g. 0.4 m, a bright, white, non-dispersive off-state is produced. The chiral nematic mixture should be of positive dielectric anisotropy, low birefringence and exhibit a low cell gap to pitch ratio dip 0.3). 

Table 3.18 Transition temperatures (°C), elastic constants (ku, k22, kjj, 10 N), dielectric anisotropy ( e), dielectric constant measured perpendicular to the director (b2.)> birefringence ( n), refractive index measured parallel and perpendicular to the director ( ng and xto), viscosity (yi, mm s ), threshold voltage (V), rate of change of voltage with temperature (%°C ) and twist angle (°) measured at 20° C unless otherwise stated for three typical nematic mixtures for STN-LCDs ...

Finkelmann and Stegemeyer have analyzed the composition dependence of the twisting power in binary mixtures of thermotropic liquid crystals, using the Goossens theory. The twisting angle

nematic layers is given as follows for a mixture of molecules of type 1 and 2,... 

Fig. 3.4.8. Computed relative capacitance change AC/Q and optical transmission r between parallel polarizers (both parallel to the director at one of the boundaries) of a twisted nematic film as functions of H/H. The total twist angle is n/2. Film thicknesses 13 and 54 The threshold for optical transmission increases with the thickness of the film. (After Van Doom. )...

Fig. 3.4.9. Computed values of the tilt angle in the midplane of the sample versus voltage for various twist angles for a standard TN mixture. Cell thickness/pitch = /2n in all cases and the director tilt at the surface 0 = 1°.

The theory of periodic distortions has been discussed thoroughly. " Eecause of thermal fluctuations, the director orientation is perturbed and one may therefore write n = (1,, 0), where 0 and are, respectively, the splay and twist angles, which are assumed to be small. Eearing in mind the experimental result, one may assume that 0 and are functions of y and z. Retaining terms to the first order in 0, and their derivatives, one obtains the following equations ... 

Flexodomains have also been observed and analysed in twisted geometry (with a twist angle 7t/2) in BMAOB (4-n-butyl-4 -methoxyazoxybenzene) in DC electric fields.The flexodomains are oriented almost parallel to the initial director orientation in the midplane of the cell (i.e. at an angle... 

Another example is a twist nematic cell with a planar orientation of the director at both boundaries 9 = ti/2 differing by their azimuth, cp = 0 and Jt/2. In such cells, the areas with the director twist in the bulk by angle -rjt/2 and —Jt/2 have the same... 

Finally, we find the dependence of the twist angle on the z-coordinate in a twist cell with soft director anchoring at one boundary ... 

When a nematic is put in contact with a crystalline substrate, tbe surface of wbicb possesses the A-fold rotational symmetry (e.g., IV = 6 for mica, A7 = 4 for NaCl), the director is free to choose any of those N easy axes. In experiments, the orientation depends on the pre-history of the sample. A director field n(r) in a nematic drop put on the surface of a crystal acquires the same N-fold symmetry. In a sandwich cell, when crystalline axes of the opposite interfaces coincide, different domains are observed, with uniform structure or twisted through an angle 2k/N [20]. Using a properly oriented external in-plane field one can switch domains from one of the possible N orientations to another. Thus we have multistable alignment. When the crystalline axes of the opposite interfaces do not coincide, many domains with different twist angles are possible. 

Fig. 6. Computed director tilt angle at midplane (O ) and at surface (0 j), and transmittance for 90° twisted cholesteric cell with surface energy function Ug. Surface tilt at zero field Is 86°. (Listed kg and risO...

Here p is the angle between the polarization axis and the front LC director, (p is the twist angle,... 

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