Chiral ferronematic liquid crystals: a physico-chemical analysis of phase transitions and induced helical twisting

A possibility was assessed to modify physico-chemical properties of ferronematic mixtures by introducing additional mesogenic and non-mesogenic components. The mixture, consisting of RM734 and DIO at the 70:30 weight ratio of the components, was doped with nematic 5CB and its chiral isomer CB15. This allowed a substantial lowering of the ferronematic temperature range, as well as induction of helical twisting in both nematic (N) and ferronematic (N-F) phases. The effects of 5CB and CB15 on the N-F to N phase transition were nearly identical, but the isotropic transition temperature decreased more strongly upon addition of CB15. Optical transmission vs. temperature measurements showed lower transmittance in the N-F phase, probably due to scattering on the domain boundaries. In the vicinity of the N-F -> N transition, a noticeable range of anomalously low transmission was noted both on heating and cooling, suggesting a complex character of molecular reorientation features. In the chiral N-F (N*(F)) system, helical twisting was recorded using Grandjean-Cano wedge, with the helical pitch decreasing upon cooling in both N* and N*(F) phases. In the cooling mode, transient undulations emerged in the N*(F) phase. The complex picture of the ferronematic transition is supported by POM images taken at different temperatures.