Dynamics of chiral liquid crystals under applied shear

Abstract

We simulate the alignment dynamics of cholesteric (chiral) rod-like liquid crystals by using a Landau-de Gennes (LdG) expression for microstructure evolution in response to flow. This study is motivated by recent advances in novel cholesteric nanorod dispersions. Prior work on the modelling of cholesterics has suffered from the restriction of helicity to only a single direction, often with a pre-imposed pitch, due to numerical difficulties. This has severely limited cholesteric modelling in regard to both accuracy and experimental relevance. Our simulations avoid this limitation. Relevant forces on rods include solvent-rod drag, nematic alignment, microstructure elasticity and chiral twist. Phase diagrams are developed to demonstrate the response of these systems to variations in chiral and flow forces. Our results indicate that for low shear rates, chiral and elastic forces prevent the rods from moving in response to flow. At high shear rates, the rods tumble and form unique transient structures (combinations of tumbling and cholesteric phases) as flow forces and chiral forces compete. Even if slight alignment is induced at the boundaries, the phase diagram substantially changes, chiefly by constraining the possible chiral phases. This work has immediate relevance to applications which exploit the optical properties of films solidified from cholesteric dispersions.

Current address for Kyle S. Conatser: New Deal HS, New Deal, TX 79350, USA

Keywords:

• cholesteric
• tumbling
• chiral liquid crystals
• finite-element method

Acknowledgements

We thank Virginia Davis and Yueyi Xu for helpful discussions. KSC was supported by a Research Experience for Teachers under NSF through award CBET-1032330. DGV's funding was provided by the TTU Honors Undergraduate Fellows Program and by an NSF REU supplement CBET- 1138790.

Notes

Current address for Kyle S. Conatser: New Deal HS, New Deal, TX 79350, USA