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

TL;DR

This study investigates how adding chiral and non-chiral dopants to ferronematic liquid crystals affects phase transitions, inducing helical twisting and altering temperature ranges, with detailed optical and structural analyses.

Contribution

It provides new insights into the physico-chemical modifications of ferronematic mixtures through doping, revealing effects on phase behavior and helical structures.

Findings

Doping lowers the ferronematic temperature range.

Helical twisting is induced in both nematic and ferronematic phases.

Optical transmittance decreases in the ferronematic phase, indicating scattering phenomena.

Abstract

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 standard 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 (NF) phases. The effects of 5CB and CB15 on the NF 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 NF phase, probably due to scattering on the domain boundaries. In the vicinity of the NF - 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 NF (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.