Homogeneous alignments

FIGURE 5.30. Phase contrast 6 of a homogeneously oriented hquid crystal dming one period of the controUing harmonic potential o, parameter p = Kz e /KxyEyy, 

172] there was considered the case of a small Ae/e ratio (5.77), when the field and director profile could be calculated independently. The optical response of the cell was evaluated by the phase contrast (5.87), where the corresponding substitution of the refractive indices must be made n n . If the potential tpoix) is harmonic. Table 5.2(1), the phase contrast 6 is also a periodic function of x with the period Ts = 27r/o s (Fig. 5.29(a)). During one spatial period 5(x) has two maxima corresponding to maximum values of dtl ofdx and two minima where d pol x = 0. The amplitude of the phase contrast increases for smaller parameters / = Ksse /K e (Fig. 5.30). 

Let us note that in Fig. 5.29(a) the director reorientation takes place at any small field, while in Fig. 5.29(b) there exists a threshold for the director reorientation. 

A variation of phase contrast,, firom 6 = to 6 = 0, was numerically calculated for the potential of type 2, Table 5.2, induced by a semi-infinite flat capacitor edge and was compared with the experimental one for two different layer thicknesses, namely, d = 20 /xm and d = 44 fim [162] (Fig. 5.31). According to (5.88), in this case the transmitted intensity between the crossed polaroids I = sin nS varies from / = 1 to / = 0, i.e., the phase contrast variation directly corresponds to the intensity variation. As seen firom Fig. 5.31 the transient region is of the order of layer thickness d, and remains larger in the case when the director is located in the plane of a nonuniform field (L ii x. Fig. 5.29(a)). 

The latter result proved to be confirmed in [158-160] where both types of homogeneous orientations were considered. On the basis of (5.79) and the 

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K.S. Whitehead, M. Grell, D.D.C. Bradley, M. Jandke, and P. Strohriegl, Highly polarized blue electroluminescence from homogeneously aligned films of poly(9,9-dioctylfluorene), Appl. Phys. Lett., 76 2946-2948, 2000. 

Note 2 Sometimes a uniform planar alignment is called a uniform homogeneous alignment . The latter term is not recommended. 

The cell contains a nematic mixture with a twist of 270° and homogeneous alignment with a high pretilt angle (0), see Figure 3.16. The nematic mixture is composed of one or several dichroic dyes, a chiral dopant and a nematic host of low birefringence. 

The homogeneously aligned nematic liquid crystal mixture does not rotate the plane polarised light generated by the first polariser. Therefore, the light is absorbed by the analyser. Very efficient absorption of light takes place. 

Y. Kawanishi, T. Tamaki, M. Sakuragi, T. Seki, Y. Suzuki, and K. Ichimura, Photochemical induction and modulation of nematic homogeneous alignment by the polarization photochro-mism of surface azobenzenes, Langmuir 8, 2601-2604 (1992). 

A glass slide (ca. 1cm X 1 cm) is spin-coated with a solution of Probimide 32 in dimethylformamide. The coated slide is annealed at high temperatures (ca. 200°C), followed by unidirectional rubbing with a soft cloth, to yield a hard, inert, rubbed polymer film, suitable for enticing homogeneous alignment of a mesophase. 

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... 

AJV = Wj - JVjj is the net anisotropic anchoring constant. NPs tend to orient parallel to n for AW>0, and recent experiments suggest this condition. Furthermore, taking into account Eq. (16) the distribution probability function of the NPs within a homogeneously aligned nematic LC phase... 

The TN cell is schematically shown in Figure 1.19. The liquid crystal sandwiched between two indium-tin-oxide (ITO) coated glass plates is about a few microns in thickness. The liquid crystal molecules on the top and bottom plates are homogeneously aligned and twisted by an angle of 7t/2. The liquid crystal molecules have a positive dielectric anisotropy sa > 0 so that an electric field will rotate the liquid crystal director up. The threshold voltage is given by... 

The Cano wedge can be used to measure the cholesteric pitch. Two substrates are wedged by a small angle 2y. The cholesteric liquid crystal between the substrates is homogeneously aligned. Under polarized microscopy, the equally spaced disclinations appear at the middle of the wedge. The separation of the lines 2d is associated with the pitch P... 

A sheet of cholesteric liquid crystals is sandwiched between two glass plates separated by a gap of tens microns. The cholesteric liquid crystals on two glass plates are homogeneously aligned to form the planar texture. The cell displays a bright color. The color varies according to the view angle and temperature. This is an important characteristic of cholesteric liquid crystals—selective reflection. 

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