A review of textures of the TGBA* phase under different anchoring geometries

Polarizing microscope textures of the twist grain boundary A* (TGBA*) phase are reviewed for two different compounds in different geometries with different surface treatments giving monostable planar and homeotropic boundary conditions. The textures are discussed in the light of the helical structure of the TGBA* phase. Depending on the compound, the underlying phase is either SmA* or SmC*, whereas the adjacent phase at higher temperature is cholesteric (N*). Sample preparations in wedge-shaped cells subjected to a slight temperature gradient exhibit TGBA* textures much more typical for the cholesteric than for the ordinary SmA* phase. For instance, Grandjean steps and fingerprint textures are observed for planar and homeotropic boundary conditions, respectively. Preparation of smectic droplets clearly reveals the helical axis of the TGBA* phase to be perpendicular to the helical axis of the helielectric SmC* phase. For thin samples, a suppression of the TGBA* helix leading to a surface-induced structure corresponding to a conventional bulk SmA* phase is observed. Under certain conditions, a cholesteric phase in the vicinity of a twist inversion point may exhibit very similar textures to the TGBA* phase near the transition to the SmA* phase. On exemplified textures similarities are discussed and differences pointed out.