Fréedericksz states in elastically varying nematic liquid crystals in the pi-cell geometry.

Abstract

Nematic liquid crystals confined to pi cells are known to exhibit a multistable electric behavior. As previous studies show, under an increasing electric field, nematics of rodlike molecules with their elastic constants related as K_{22} (twist) <K_{11} (splay) <K_{33} (bend), undergo a continuous primary bifurcation at a critical voltage U_{c} from the bulk-planar state H to the bulk-inclined state I, followed by a discontinuous transition into the bulk-vertical state V above U_{c}. On turning the field off, the V state continuously transforms into a metastable π-twisted state T that relaxes discontinuously to the H. The present study examines the evolution and relaxation behavior of Fréedericksz's states in calamitic and bent-core nematics that differ significantly in their elastic anisotropy. Indeed, nematics with bent-shaped molecules present an anomalously low bend elastic constant, K_{33}<K_{22}<K_{11}. Our findings are the following: (a) The field-on sequence H→I→V, as also the field-off sequence V→T→H, remain common to both nematic types. (b) The two systems, however, differ in terms of the stability of the V state in a reducing field; in calamitics, the V geometry transforms discontinuously to the I above U_{c}, and a second-order I→H change follows below U_{c}; by contrast, in bent-core nematics, whether the core is flexible or rigid, the V state once formed remains stable at all voltages above U_{c}, with the V→T→H transition taking place only below U_{c}. The studies also include anisotropic propagation of fronts associated with the T→H transformation, pseudobiaxial conoscopic features of the reoriented states, periodic electric instabilities that differentiate the T and H geometries, and a comprison of computed and experimental results on cell capacitance and birefringence.