Surface anchoring and pitch variation in thin smectic C* layers in an electric field

The variations of the pitch of smectics C* in thin planar layers in an exte rnal electric field and their dependence on the surface anchoring are inves tigated theoretically. The proposed mechanism of the change in the number o f half-turns of the helical structure in a finite-thickness layer upon a ch ange in the applied field is the slip of the director on the surface of the layer through the potential barrier of surface anchoring. The equations de scribing the pitch variation in an external field and, in particular, the h ysteresis in the jumpwise variations of the pitch for opposite directions o f field variation are given and analyzed for arbitrary values of the field. For weak fields, it is found that the pitch variation in the layer is of a universal nature and is determined by only one dimensionless parameter, S- d = K-22/dW, where K-22 is the Frank torsion modulus, W is the surface anch oring potential, and d is the layer thickness. The possibility of direct de termination of the form of the anchoring potential from the results of corr esponding measurements is considered. Numerical calculations for the deviat ion of the director from the direction of alignment on the layer surface an d pitch variations, as well as the points of pitch jumps and hysteresis in the field, are made for the Rapini model anchoring potential for values of the parameters for which the pitch variation weakly depends on the directio n of the field applied in the plane perpendicular to the spiral axis of sme ctics C*. The changes in the pitch variation in stronger fields are discuss ed, and the optimal conditions for observing the discovered effects are for mulated. (C) 2001 MAIK "Nauka/Interperiodica".