Surface anchoring energy

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]... 

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

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.

Fig. 7.12. Azimuthal surface anchoring energy density as a function of separation between neighburing AFM lines.

The theory of the local Prederiks transition [85, 86] considers the competition of two terms, the surface anchoring energy due to the short range forces W responsible for one type of orientation (homeotropic in the previous example), and the long-range Van der Waals forces U z) integrated over their penetration radius... 

The main defect in this kind of theory is the assumption that the orientational order is nematic like in the interfacial region, which is generally not true (see Section 10.3.1). Another problem is linked to the fact that the surface anchoring energy is unknown. This surface anchoring energy is difficult to measure directly and the result of any indirect measurement depends on how the response of the system to a disorientation is modeled. One way out is to use the orientational distribution of the molecules in the surface layer as a boundary condition of the nematic order [47] (see also Section 10.3.2]. 

Finally, we will discuss some consequences of the 13 term on elastic deformations in nematic phases. First, if is of the same order as other elastic constants, it will be effective in much more situations than will K24. In fact, in most cases where K24 is effective, will have to be considered as well. This has been pointed out for stripe domains in hybrid aligned films [190], surface transitions in nematics on solid surfaces [224], nematic droplets [47] and cylindri-cally confined nematics [214]. Many authors have emphasized the modification of the apparent surface anchoring energy by the term (see above). 

D MEASUREMENT TECHNIQUES OF THE SURFACE ANCHORING ENERGY D1 External Field OffMethod... 

D3 Other Data on the Surface Anchoring Energy Measured for Nematics... 

In conclusion, many of the available data and theoretical investigations are related so far to two liquid crystals only, MBBA and 5CB. These are not sufficient to understand fully the sur ce-induced effects produced by the anisotropic surface anchoring energy. 

The above analysis assumes that the undulations of the layers vanish at the boundaries. Ishikawa and Lavrentovich [131] have modelled a lamellar system of cholesteric liquid crystal which allows layer undulations near the boundaries. This was achieved by adding a finite surface anchoring energy to a bulk energy that is essentially of the same form as that stated above for SmA. This idea could perhaps also be modified to model general SmA or SmC liquid crystals. The results in [131] seem to indicate that the incorporation of finite surface anchoring leads to a decrease in the theoretical threshold for the onset of the Helfrich-Hurault transition. 

The azimuthal LC surface anchoring energy was more than 10 " J/m, which is comparable with rubbed PI films. 

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