Filament Dynamics in Salt-free Viscoelastic Surfactant Solutions

In this work, we study the dynamics of a model wormlike micellar solutions via CaBER, shear rheology and TEM imaging. Wormlike micellar solutions contain cetyltrimethylammonium tosylate (CTAT) in deionized water. At low concentrations (0.7 wt% < c < 1.1 wt%,), solutions exhibit shear thickening and elongational flow shows formation of elastic filaments possibly due to formation of elongation induced structures. In this range of concentrations, the extensional relaxation time is fairly constant around 0.03 s. At higher concentrations 1.1 wt% < c < 2-3 wt%, a significant increase in shear relaxation time is observed. However, beyond 2 wt%, the shear relaxation time decreases and the zero shear viscosity approaches an asymptotic value. TEM images confirm no sign of branched networks beyond the concentration of 2 wt%. It is shown that the maximum Trouton ratio measured from transient rheology experiments decreases as concentration increases and finally reaches a constant value around Tr ≈ 3 for c > 2 wt%.