Exploring sensitivity of the extensional flow to wormlike micellar structure

Abstract The non-linear dynamics of a series of salt-free surfactant solutions is investigated via a combination of TEM imaging, shear rheology and capillary breakup extensional rheometery. Surfactant solutions contain Hexadecyltrimethylammonium p-toluenesulfonate (CTAT) in de-ionized water. At low surfactant concentrations, 0.7 wt% λ E  ≈  0.03 s. At higher concentrations, 1.1 wt%  ≤  c Tr  ≈  3, for higher concentrations. This finding is in-line with the structural parameters inferred from the shear rheology and also the TEM images. Therefore, we confirm that the extensional rheology is sensitive to structural transitions near the critical concentration. Finally, we show that the ratio of the extensional relaxation time to the shear relaxation time, λ E / λ , is always less than unity regardless of whether relaxation is dominated by reptation or breakage of the wormlike micelles.