Anchoring azimuthal

Spontaneous reflection symmetry breaking in achiral LCs is also well known, driven by specific boundary conditions. A very simple example of this type of chiral domain formation is illustrated in Figure 8.11. Suppose we start with two uniaxial solid substrates, which provide strong azimuthal anchoring ... 

Figure 8.11 Illustration of Mauguin twisted nematic cell, reported in 1911. Substrates are thin mica plates, which are uniaxial with their optic axis parallel to plane of plates. Apparently, uniaxial crystal stmcture of mica produces strong azimuthal anchoring of nematic LCs of Lehmann, such that director is parallel (or perpendicular) to optic axis of mica sheets at both surfaces. Mauguin showed that method of Poincard could be used to explain optics of system if it was assumed that LC sample created layer of material with uniformly rotating optic axis in twisted cells.

A comment should be added on the rotational viscosity of the sample in the experiment. It can easily be shown that in the case of azimuthal anchoring measurements, the viscosity entering the analysis equals the rotational viscosity 71. When measuring the zenithal anchoring coefficient, the effective viscosity has to be corrected according to the scattering geometry. However, in first approximation the pure splay mode viscosity is sufficient. 

Measurements of the LC azimuthal anchoring energy coefficient W(f oi the PVCN substrates (Fig. 5.9) were performed by preparing twist cells with... 

Fig. 4.11. (a) Aging, as observed through the time dependence of the azimuthal anchoring coefficient for 5CB on UV aligned poly-(vinyl-cinnamate) (b) Aging, as observed through the offset parameter h [59]. 

For consistency we go back to the problem of the twisted cell discussed in Section 8.3.2, however, the director angles cp at the boundaries will be not constant but can be changed due to elastic and external torques. Let a nematic layer be confined by two plane surfaces with coordinates zj = —plane through angle cp (there is no tilt, the angle 9 = ti/2 everywhere, and the azimuthal anchoring energy is finite). 

In the Rapini approximation, the zenithal and azimuthal anchoring energies are defined for the director n in terms of angles a and p ... 

Figure 5.10 (a) Schematic diagram of the cell used for azimuthal anchoring measurement, (b) Diagram of the measurement of the twist deviation angle. 

By measuring the twist angle, the azimuthal anchoring strength can be obtained. This method is called the twist angle method (TAM) [15,16]. 

Only few data are available for other liquid crystal phases (apart from nematics). For example, the azimuthal anchoring energy was recently measured in the ferroelectric smectic C phase [75]. 

Such a multistable orientation was observed when a capillary was formed by two mica (sixfold symmetry, three easy directions) or NaCl (fourfold symmetry, two easy directions) plates [80, 81]. The director can be switched from one equilibrium position to the other by an electric field (the azimuthal anchoring energy for 5CB between NaCl substrates was shown to be of the order of 10 erg-cm [80]. 

Fig. 2.3 Receding water contact angles on polyimide and the azimuthal anchoring energy in LC cells as a function of the number of rubbings and the pile impression. Reproduced by permission from [62].

Fig. 2.24 Azimuthal anchoring energy as a function of the groove frequency. Calculated based on the elastic energy.

Neel walls, twist- and tilt-reverses are the three major disclinations in actual LCDs. A Neel wall disclination may be caused by weak azimuthal anchoring. A twist-reverse is a defect in which the twist direction is opposite to the chirality of the added chiral agent. A tilt-reverse is a defect in which the inclination direction induced by applying a voltage is the opposite of the intended direction. These tilt-reverse defects may be caused by the lateral electric field generated by the fringe field of the pixel electrodes. 

Fig. 3.18 Relation between UV dosage and azimuthal anchoring energy.

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