Transient Shear Stress Response After Flow Start-up and Flow Reversal in Low Molecular Weight Nematic Liquid Crystals

Abstract

ABSTRACT Within the framework of the Leslie–Ericksen theory we develop a unified model for the interpretation the complex shear stress response of tumbling nematics after flow start-up and flow reversal. The model accounts quantitatively for: (i) the damping of the oscillations, the vanishing of the doublet structure and eventually a pronounced decay after flow start-up, and (ii) the progressive reappearance of the oscillations after flow reversal (echo phenomenon). Within this model the loss of director coherence, induced partially by noise and partially by a distribution of velocity gradients, is a significant additional cause of damping. A primary cause (but not sufficient) is the director drift towards the vorticity axis as pointed out by other authors. On the other hand, the reappearance of the oscillation is related to reversibility dictated by the mechanical Leslie–Ericksen equations combined with the velocity gradient distribution.