Applicability evaluation of the stress-optic law in Newtonian fluids toward stress field measurements

Abstract

A novel rheoscopic fluid displaying coloration due to flow-induced birefringence is investigated through laboratory experiments to ascertain if it satisfies the stress-optic law in fluids in direct stress field measurements of Newtonian fluids. To validate this experimentally, two types of spin-up experiments ensuring two-dimensional flows were performed: color visualization using the rheoscopic fluid and velocity field measurement by particle image velocimetry. Using a wave-number analysis, the orientation angle of the birefringence was determined from a single color snapshot in axisymmetric and unidirectional flows. Direct comparisons between color and shear stress values made it possible to establish a unique relation between the two, verifying that the stress-optic law can be applied in the fluid. The results of the present study suggest a promising avenue toward enabling temporally and spatially resolved stress field measurements in Newtonian fluids.