Anchoring energy

When the liquid crystal director n is aligned along the direction specified by the polar angle G and azimuthal angle (p, the anisotropic part of the surface energy - referred to as the anchoring energy function - of the liquid crystal isfs=fs G,(p). When G = Gg and p = (po fs has the minimum value of 0, and thus = and dfs/d(p =0. 

The materials above and below the interface are different, and there is no reflection symmetry about the interface. If 0 0, the anchoring energy does not have azimuthal rotational 

For a homogeneous anchoring, 0 9O°, If we define a = jil2-0, which is the polar angle defined with respect to the cell surface. Equation (1.116) becomes 

Equations (1.116) and (1.117) are valid only for small deviations. For large deviations, quartic terms must be included. The anchoring strengths can be determined experimentally, which will be discussed in Chapter 5. 

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An essential requirement for device applications is that the orientation of the molecules at the cell boundaries be controllable. At present there are many techniques used to control liquid crystal alignment which involve either chemical or mechanical means. However the relative importance of these two is uncertain and the molecular origin of liquid crystal anchoring remains unclear. Phenomenological models invoke a surface anchoring energy which depends on the so-called surface director , fij. In the case where there exists cylindrical symmetry about a preferred direction, hp the potential is usually expressed in the form of Rapini and Popoular [48]... 

Yokoyama, H., and Van Sprang, H. A. A novel method for determining the anchoring energy function at a nematic liquid crystal-wall interface from director distortions at high fields./. Appl. Phys. 57, 4520 (1985). 

Typically, 10" cm. Of course, the complete analysis has to include the surface anchoring energy, latent heat, etc. 

In order to perform the measurements of the anchoring energy coefficients described in the previous subsection, the orientational fluctuation relaxation time needs to be determined as a function of sample thickness. This can be best done using dynamic light scattering. 

Table 4.2. Comparison of anchoring energy coefficients of 4-n-pentyI-4 -cyanobiphenyl (5CB) at T = 32° C and 4-n-octyl-4-cyanobiphenyl (8CB) at T = 37° C on rubbed Nylon and photosensitive poly-(vinyl)-cinnamate. Different anchoring coefficients on rubbed Nylon are achieved by different rubbing strengths.

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. 5.9. Room temperature azimuthal surface anchoring energy coefficient of the 5CB liquid crystal on LPP PVCN substrate as a function of the LP UV irradiation time. The dashed line is guide to the eye.

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