Examination of flow birefringence induced by the shear components along the optical axis using a parallel-plate-type rheometer

TL;DR

This paper investigates flow birefringence caused by shear components along the optical axis using a rheometer and polarization camera, revealing a power-law relationship and expanding the understanding of stress-optic behavior in 3D flows.

Contribution

It introduces a novel rheo-optic measurement approach for shear flows along the optical axis and establishes a universal power-law characterization of flow birefringence.

Findings

Flow birefringence follows a power-law with the second invariant of the deformation-rate tensor.

Flow birefringence can be characterized by coordinate-independent invariants.

The stress-optic law can be expanded for 3D stress fields along the optical axis.

Abstract

In this study, the concept of rheo-optics is applied that explores the flow birefringence caused by stress components along the optical axis of the camera since it is often overlooked in the traditional theories of photoelastic flow measurement. A novel aspect of this research is that it involved conducting polarization measurements on simple shear flows, specifically from a perspective in which a shear-velocity gradient exists along the camera's optical axis. A parallel-plate-type rheometer and a polarization camera are employed for these systematic measurements. The experimental findings for dilute aqueous cellulose nanocrystal suspensions demonstrates that the flow birefringence can be expressed as a power law based on the power of the second invariant of the deformation-rate tensor. This suggests that flow birefringence can be universally characterized by the coordinate-independent invariants and a pre-factor determined by the direction of polarization measurement. By adjusting the nonlinear term in the stress-optic law, its applicability could be expanded to include three-dimensional fluid stress fields in which the stress is distributed along the camera's optical axis.