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 $G^{\prime }$ 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 $G^{\prime }$ and in the “loss angle” $\delta ={\tan }^{-1}(G^{″}∕G^{\prime })$. The methodology is demonstrated by analyzing the Leslie-Ericksen equations for small-amplitude oscillatory Poiseuille flow of $\left(4\text{−}\mathrm{n}\text{−}\mathrm{\text{octyl}}\text{−}{4}^{\prime }\text{−}\mathrm{\text{cyanobiphenyl}}\right)$ (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.

• Received 29 October 2003

DOI:https://doi.org/10.1103/PhysRevE.70.011701