Wormlike micellar fluids form by the self-aggregation of surfactant molecules in aqueous solution. These non-Newtonian fluids have been well studied and are used in the oil industry, hydraulics, and medical research. However, little is known regarding the structure of the three-dimensional networks in which the “worms” become entangled and possibly branched, especially at high concentrations. What is known is that this composition results in two distinctive fluid characteristics: viscoelasticity and strain-birefringence. The latter is exploited in this work in order to study the shear wave speed and attenuation in 500/300mM CTAB/NaSal fluid. Three different experiments were conducted using either a laser/diode system or a camera/backlight system. The average speed of a shear wave in wormlike micellar fluid was determined to be 63.47cm/s for this concentration at room temperature with no consistent effect from aging. Temperature, however, had a significant impact. Around 35oC, there was a dramatic drop in shear speed. The steady linear decrease in micelle length with increasing temperature does not account for this steep decline. A possible explanation is that the micelle conformation changes at this point—a topological phase transition. It is recommended that the rheology around this temperature be examined
