Observation and simulation of toron polymorphism: Effects of surface anchoring, elasticity and electric field in cholesterics with smectic-A phase beneath

Abstract

Toron is one of the fundamental chiral topological textures appearing in many spintronic and soft matter systems. Here we report on the observation of torons in cholesteric liquid crystals with positive dielectric anisotropy (Δε > 0). In addition, the materials show smectic-A phase beneath the cholesteric phase, enabling us to see the pretransitional effect on the formation of the torons. In detail, we investigated the relation of toron formability and the external stimuli in terms of temperature and electric field. Importantly, the divergence of the elastic constants (K22 and K33) in the vicinity of the nematic-smectic-A phase transition causes the transition from the toron to the homeotropic alignment. The results reveal that both the homeotropic anchoring strength and the frustrated confinement of the cholesteric helix are important to determine whether the toron texture is selected among other textures like fingers. The confinement ratio, i.e. the ratio of cell thickness to cholesteric pitch, for the incidence of torons was found to be in a wide range of about 0.82 ∼ 1.5. We also show that the torons can be polymer-stabilized so that they retain over a wide temperature window. Even more importantly, the orientational state of the polymer-stabilized toron can be changed by an electric field, realizing fast switching between the dark and bright states. The system may find applications in electric-driven diffraction gratings or privacy protection films with a sub-millisecond response time.