Anchoring Transitions

Orienting action of solid surfaces on nematic liquid crystals is known as the anchoring. An Anchoring Transition consists in a singular change in the anchoring direction due to a continuous variation of some thermodynamic and structural parameters characterizing a solid/nematic interface. The structure of the interface is in fact sensitive, for example, to variations of chemical potentials of substances composing the nematic phase and to structural modifications of the solid substrate (due, for example, to surface strains). An anchoring transition can take place by a jump from one orientation to another. Such a transition has all static and dynamic characteristics of a phase transition of first-order (nucleation and growth of domains surrounded by walls etc.). An anchoring transition can also have the critical character of a second-order phase transition (occurrence of fluctuations of orientation, divergent susceptibility to external fields). Experimental examples of anchoring transitions occurring at the E9/crystal interfaces will be presented and discussed in more details. A mean field theory can be used for construction of generic anchoring diagrams in Fourier space; the interfacial grand canonical potential can be decomposed into a limited Fourier series and the Fourier coefficients can be treated as independent variables in the search for the minima. The correspondence with experimental anchoring diagrams is established by introducing dependence of Fourier coefficients on the thermodynamic and structural parameters.