Anchoring director-field interactions

As mentioned earlier, most studies of field interactions with liquid crystals are done using thin films with a well-defined initial state, usually a monodomain or a thin film with a simple distortion induced by incommensurate surface anchoring. These conditions simplify observation and theoretical analysis. However, most liquid crystal materials that are not specially prepared contain topological defects that are very important to their response to external fields. One class of defect commonly observed in nematics is the disclinalion line. At a disclination line the director field is ill defined. The director field turns around the disclination line a multiple of half-integer times. Several disclination lines are shown in Fig. 8. 

Then, for temperatures above the phase transition point, we will still have an optical response to an applied field because the field interacts with the director of quasi-nematic surface layers adjacent to electrodes which are responsible for the birefringence shown earlier in Fig. 3.3. With increasing field the saturation of the optical response is observed (at Esat) due to the complete reorientation of the director in surface layers. Comparing the electrostatic energy accumulated in a surface layer of thickness (T) = o T — (a correlation length discussed earlier) with the anchoring... 

However, it is popular to use the director as far as possible to visualize liquid crystal structures. The director field shows elastic behavior, and can be influenced by external electric and magnetic fields. At interfaces, the orientation of n is usually anchored due to interactions between the liquid crystal... 

When a nematic liquid crystal is confined, such as when sandwiched between two parallel substrates with alignment layers, in the absence of external fields, the orientation of the liquid crystal director is determined by the anchoring condition. When an external electric field is applied to the liquid crystal, it will reorient because of the dielectric interaction between the liquid crystal and the applied field. If the dielectric anisotropy is positive (Ae > 0), the hquid crystal... 

Before outlining the effect of fields on the orientation of the director, it must be emphasized that surface interactions and boundary conditions are usually of importance. The models applied here assume that there is a well-defined director distribution in the absence of any external field, and in practice this can only be provided by suitable treatment of boundary surfaces. The strength of surface interactions must also be considered as this will influence the equilibrium director configuration in the presence of a field. For the simplest description of field effects in liquid crystals it is usual to assume an infinite anchoring energy for... 

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