Thermal and spectral behavior of ultrasonically generated shear waves in a viscoelastic micellar fluid

Abstract

Wormlike micellar (WM) fluids, which flow when subjected to long term stresses, are mechanically viscoelastic over shorter durations. These fluids are birefringent under shear, allowing the study of shear wave propagation using both optical and acoustic modalities. In this work, the temperature and frequency dependent behavior of ultrasonically generated shear waves in a WM fluid is studied. A fluid consisting of hexadecyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSAL) combined in a 5:3 ratio is used in a 200 mM concentration. A high intensity focused ultrasound (HIFU) beam generates radiation pressure in the fluid and can induce shear waves of sufficient amplitude to be visualized optically when the beam is modulated. By pulsing the HIFU beam, a train of shear waves are generated which propagate laterally from the focal region. The temperature and frequency dependent behavior of the HIFU generated shear waves are correlated with the rheological and microstructural properties of the fluid. Discontinuities in slope in the temperature dependence of the shear modulus and viscosity provide evidence to suggest that there are phase transitions of the 200 mM CTAB/NaSal WM fluid at about 28°C and 38°C.