Reorientation of Surface Quasi-Nematic Layers

Let us go back to the discussion of the Frederiks transition in a homogeneously oriented nematic with positive dielectric anisotropy (splay distortion). A conventional sandwich cell is used which is very convenient in this case, because the Kerr effect is not observed when the light wave vector coincides with the field direction. Let us imagine that we are measuring the temperature dependence of the anchoring energy of the nematic using the saturation field for the complete director reorientation. For 5CB we have the left part of Fig. 4.39 [226]. 

FIGURE 4.38. Relaxation times associated with nematic order fluctuations n and with molecular relaxation (t2) as functions of temperature (Kerr effect in 5CB) [225]. 

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 

The field response of the isotropic phase has also been discussed in terms of the Landau model for the N-I phase transition [227]. Apart from the conventional quadratic and cubic terms the gradient of the order parameter was taken into account. The theory predicted the temperature dependence of the saturation field in the form 

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