Simultaneous measurement of extensional stress and flow birefringence field for uniaxially extending worm-like micellar solutions

TL;DR

This study introduces a novel rheo-optical method combining high-speed polarization imaging and liquid dripping to simultaneously measure extensional stress and birefringence in worm-like micellar solutions, revealing microstructural deformation.

Contribution

The paper presents a new technique that enables simultaneous visualization and measurement of stress and birefringence during uniaxial extension of complex fluids, linking microscale structure to macroscale rheology.

Findings

Successful visualization of birefringence evolution in micellar solutions.

Confirmation of stress-optical rule applicability under extension.

Comparable stress-optical coefficients in extension and shear flows.

Abstract

The present study proposes a novel and simple rheo-optical technique to investigate the relation between the rheology of complex fluids and their internal structural deformation under uniaxial extensional flow. The macroscale results of viscoelasticity from rheological measurements and microscale results of birefringence from optical measurements are combined to evaluate the microstructural deformation and orientation state inside the fluids under extensional stress. The proposed technique combines a liquid dripping method with a high-speed polarization camera to measure the extensional stress and flow-induced birefringence field simultaneously. In the liquid dripping method, temporal evolution images of the liquid filament diameter for fluids dripping from a nozzle are measured to obtain the extensional stress loading on the liquid filament. These images are captured with a high-speed polarization camera connected to a micro polarization element alley, enabling high-speed imaging of the birefringent field. Worm-like micellar solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) with varying concentrations of CTAB and NaSal are employed as the measurement targets. Consequently, we successfully visualized temporally developing images of the birefringence field of uniaxially extending worm-like micellar solutions induced by the orientation of micelles toward the extensional direction. Furthermore, the proposed technique supports investigating the conditions for establishing the stress-optical rule, which is the linear relation between stress and birefringence for complex fluids. The stress-optical coefficient, a proportionality constant indicating the sensitivity of birefringence to stress, is analyzed from these measurements. The stress-optical coefficient under uniaxial extensional flow is confirmed to be comparable to that under shear flow.