Tunable backflow in chiral nematic liquid crystals via twist-bend nematogens and surface-localised in-situ polymer protrusions

ABSTRACT

Dynamic electro-optic response of the liquid crystal (LC) director shows a backflow effect that is manifested as an optical bounce in chiral nematic LCs (N*LC) during field-induced homeotropic-twisted transition. The bend elastic constant (K₃₃) strongly influences the dynamics of backflow at the N*LC in homeotropic-twisted transition. The cyanobiphenyl LC dimers – CB7CB, CB9CB and CB11CB – possess a unique characteristic of inherent bend molecular configuration that lowers K₃₃. With the modulation of the effective K₃₃ in dimer-doped N*LCs, we report the tunability of the optical bounce that decreases with the increase in the length of flexible spacers in LC dimers. The doped LC dimers with short spacer lengths not only generate a strong backflow with an enhanced twist degeneracy of the LC director across the cell, but also prolong the time of disappearance of the optical bounce. Furthermore, we demonstrate the suppression of the optical bounce with surface localised polymer protrusions having 50–100 nm diameters, which allow faster dynamic relaxation process and reduced backflow. We envision a novel design of a tunable microfluidic device for precise flow control of organic or inorganic matter in LC medium that exploits the tunable backflow in LC dimer-doped N*LCs.

Graphical Abstract

KEYWORDS:

• Backflow
• optical bounce
• liquid crystal dimers
• topological defects
• polymer stabilisation

Acknowledgements

This work was supported by the Ohio Third Frontier (OTR) Venture Startup Fund from Ohio Development Services Agency (ODSA) under Grant No. TECG 2015–0128. The SEM images were obtained at the characterisation facility of the Liquid Crystal Institute, Kent State University, supported by the Ohio Research Scholars Program Research Cluster on Surfaces in Advanced Materials. The authors gracefully acknowledge the technical support provided by Dr Min Gao and Dr Lu Zou for microscopy experiments. The authors thank Kai-Han Chang, Sahil Gandhi and Andrii Varanytsia for their technical support during electro-optical experiments. L.-C.C. initiated and supervised the research. D.A.P., J.M.D.S. and C.T.I synthesised the LC dimers CB7CB, CB9CB and CB11CB. L.-C.C. conceived the idea of OB modulation in bimesogen-mediated long-pitch cholesteric liquid crystals and polymer stabilisation for suppression of OB. V.J. produced the electro-optical cells and carried out electro-optic measurements, POM and SEM. V.J. wrote the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Ohio Third Frontier (OTR) Venture Startup Fund from Ohio Development Services Agency (ODSA) under Grant No. TECG 2015–0128.