Thermal property and shear wave speed indicators of phase transitions in a micellar fluid

High concentration (>100 mM) wormlike micellar (WM) fluids are non-Newtonian with micelle lengths in the tens of nanometers. The viscoelastic properties of the fluid are affected by the structure and entanglement of the micelles and thus structural phase transitions can be indirectly studied using mechanical shear waves. Although these structural phase transitions have been extensively studied as a function of concentration, comparably less work is available on the temperature dependence. In this study, shear wave speeds (SWS) were studied as a function of temperature in a cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal)-based wormlike micellar fluid as an indicator of micellar structural changes. The heat capacity and thermal conductivity were also measured as these can be expected to change with structural phase transitions. Discontinuities in SWS were observed between 12 °C and 14 °C indicating the existence of a possible structural phase transition at this temperature. Gradual variation of the thermal properties was observed during controlled heating and cooling, while during autonomous heating from crystallization to fluid, a dramatic increase in both thermal properties peaking near 13.5 °C was observed.