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 diff erent surface treatments giving monostable planar and homeotropic boundary conditions. The textures are discussed in the light of the helical structur e of the TGBA* phase. Depending on the compound, the underlying phase is ei ther SmA* or SmC*, whereas the adjacent phase at higher temperature is chol esteric (N*). Sample preparations in wedge-shaped cells subjected to a slig ht temperature gradient exhibit TGBA* textures much more typical for the ch olesteric than for the ordinary SmA* phase. For instance, Grandjean steps a nd fingerprint textures are observed for planar and homeotropic boundary co nditions, 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 conventiona l bulk SmA* phase is observed. Under certain conditions, a cholesteric phas e in the vicinity of a twist inversion point may exhibit very similar textu res to the TGBA* phase near the transition to the SmA* phase. On exemplifie d textures similarities are discussed and differences pointed out.