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

TL;DR

This study measures flow birefringence caused by shear components along the optical axis using a parallel-plate rheometer, revealing a power-law relationship with shear rate crucial for 3D flow polarization analysis.

Contribution

It introduces a method to measure birefringence induced by shear components along the optical axis, which was previously neglected in conventional photoelastic theories.

Findings

Birefringence increases monotonically with stress components along the optical axis.

Birefringence follows a power law with respect to shear rate.

The results are relevant for polarization measurements in three-dimensional flows.

Abstract

The present study investigated the flow birefringence induced by shear components along a camera's optical axis, which has been neglected in conventional theories of photoelastic measurements. Measurements were conducted for a wide range of shear rates from a direction perpendicular to the shear using a high-speed polarization camera and a parallel-plate-type rheometer. The measurement results obtained from a fluid with low viscoelasticity, specifically a dilute suspension of cellulose nanocrystals, showed that the birefringence increases monotonically as the stress components along the camera's optical axis increase. It was also found that the birefringence showed a power law with respect to the shear rate. This letter reports a key fact required for polarization measurements of shear rate (shear stress) in three-dimensional flows.