A viscoelastic measurement with visualization of shear waves in a wormlike micellar fluid

Abstract

Rheometers have been used extensively for evaluating dynamic viscoelasticity of viscoelastic fluids by measuring the stress response to the sinusoidal strain applied to the fluids. It was reported that dynamic viscoelasticity was also derived based on the propagation velocity and the damping ratio of the shear waves propagating in the fluids. In this study, a polarization imaging camera and a particle image velocimetly (PIV) are utilized to visualize shear waves generated by an oscillating plate in a viscoelastic wormlike micellar fluid in order to derive dynamic viscoelasticity. Entangled wormlike micellar fluid of 3.0×10^-3 mol/l cetyltrimethylammonium bromide (CTAB) water solution with additive of sodium salicylate (NaSal) as counter ions at a molar ratio of S =4 was used in this study for a test fluid. Propagation velocities were obtained from a sequence of visualized images of propagating waves, which show good agreement to the velocities calculated from the products of wave frequencies and wavelengths. Damping wave profiles were derived from the retardation distributions which were calculated from the captured birefringence images. The dynamic viscoelasticity data derived from the propagation velocities and the damping ratios were in close agreement to the measured data using a rheometer, which shows validity of this method at relatively high frequencies. When the ratio of the amplitude to the wavelength exceeds a critical value, onsets of equilibrium points rising are observed in the damping retardation profiles depending on the amplitude, which attributes to the orientation of micelles on the plate.