Assessing flow alignment of nematic liquid crystals through linear viscoelasticity

Area of Science:

• Nematodynamics
• Rheology of Liquid Crystals
• Materials Science

Background:

• Shear alignment in rodlike nematic liquid crystals is governed by the reactive parameter (lambda), where lambda > 1 indicates alignment.
• Traditional measurements of lambda often involve complex experimental setups.
• Understanding flow alignment is critical for predicting liquid crystal behavior under shear stress.

Purpose of the Study:

• To present a novel, economical method for assessing flow alignment in nematic liquid crystals.
• To utilize small amplitude oscillatory flow and linear viscoelasticity to determine the reactive parameter (lambda).
• To demonstrate a reentrant viscoelastic transition linked to the alignment-nonalignment transition of the director.

Main Methods:

• Application of nematodynamic theory by Leslie and Ericksen.
• Analysis of small amplitude oscillatory Poiseuille flow.
• Measurement of storage modulus (G') and loss angle (delta) in linear viscoelasticity.
• Analytical and scaling methods applied to the Leslie-Ericksen equations for 8CB liquid crystals.

Main Results:

• The storage modulus (G') is directly proportional to (lambda - 1) when the director is aligned with the flow.
• A reentrant viscoelastic transition (viscoelastic -> purely viscous -> viscoelastic) is observed with increasing lambda.
• The proposed method provides a quantifiable link between viscoelastic properties and shear alignment.
• Methodology validated using 4-n-octyl-4'-cyanobiphenyl (8CB) liquid crystals.

Conclusions:

• Small amplitude oscillatory flow offers an accessible and economical tool for measuring the reactive parameter (lambda) in nematic liquid crystals.
• This rheological approach simplifies the study of shear alignment and director transitions.
• The findings provide a valuable addition to the experimental toolkit for nematodynamicists.