Techniques for Measuring Anchoring Energies

There are various experimental techniques for measuring the anchoring energy which may be divided into two different groups according to whether an external perturbating field is applied (fleld-on techniques) or not (fleld-off techniques). In any case, the kind of distortion of the director fleld in a surface layer is of great importance. In different experiments for the determination of W the parameters varied were  

FIGURE 3.9. Definition of the anchoring extrapolation length. Strong anchoring, W = ooj b = 0 (curve 1) finite anchoring 0 VF oo, 60 0, b 0. 

Some literature values for W are collected in Table 3.1. It is seen that the data obtained with different techniques are scattered over a wide range, from 10 to 1 erg-cm .  

In this section, we would like to discuss only the principal approaches to measurements of the anchoring energy. The best results have been obtained with field-on techniques. However, their consideration requires the knowledge of the electrooptical behavior of hquid crystals which is to be discussed in the following chapters. Here we will only list the most important approaches and show some interesting examples. 

To calculate higher-order terms in the expansion (3.35) we should analyze the dependence of the profile of the director orientation in a hybrid cell on 

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