Assessing flow alignment of nematic liquid crystals through linear viscoelasticity

Shear alignment of rodlike nematic liquid crystals is found when the reactive parameter lambda>1. Measurements of lambda usually require complex experiments. This paper presents a method based on the nematodynamic theory of Leslie and Ericksen that assesses flow alignment through small amplitude oscillatory flow. The method is based on the fact that the effect of lambda on the storage modulus G' of linear viscoelasticity, when the director is along the flow direction, is directly proportional to lambda-1. Thus the alignment-nonalignment transition for increasing lambda is a reentrant viscoelastic transition: viscoelastic (lambda<1) -->purely viscous (lambda=0) -->viscoelastic (lambda>1) that is reflected in the storage modulus G' and in the "loss angle" delta= tan(-1) (G"/G'). The methodology is demonstrated by analyzing the Leslie-Ericksen equations for small-amplitude oscillatory Poiseuille flow of (4-n-octyl- 4' -cyanobiphenyl) (8CB) using analytical and scaling methods. Since linear viscoelastic moduli are easily accessible, the proposed methodology is an additional useful and economical tool for nematodynamicists.