A phenomenological model for the undulating twist grain boundary-C* phase

We propose a simple phenomenological model which is able to account for the various twist grain boundary (TGB) phases, including the recently discover ed undulating twist grain boundary-C* (UTGB(C)*) phase. In the UTGB(C)* pha se, the smectic C* (SmC*)-like blocks and the grain boundaries separating t hem undulate to form a two-dimensional square lattice perpendicular to the TGB helix axis. We treat the grain boundaries separating adjacent smectic b locks as interfaces with an anisotropic interfacial tension. At moderate ch iral strengths we find a TGB(A)-TGB(C)-SmC* sequence. As the chiral strengt h is increased this goes to the sequence TGB(A)-UTGB(C)*-SmC*. Such sequenc es have been observed experimentally.