Visual observation of dispirations in liquid crystals

In this work, wedge-screw and twist-edge dispirations in liquid crystals were visually observed. In smectic phases, such as the antiferroelectric Sm${\mathit{C}}_{\mathit{A}}$ and the dimeric Sm${\mathit{C}}_2$, where the molecules or the mesogenic groups in the adjacent smectic layers tilt in opposite senses, we found the schlieren texture with the strength of m=±1/2 which should not be seen in the normal SmC phase. The observation indicates the existence of a wedge-screw dispiration; i.e., a π-wedge disclination (m=1/2) accompanied by a screw dislocation that is characterized by the Burgers vector b whose magnitude is equal to the layer thickness d. A threadlike defect line with ±π-wedge disclinations at the ends observed in freely suspended films manifests a twist-edge dispiration; i.e., a linked structure of a π-twist disclination and a ‖b‖=d edge dislocation.