Use of particle-tracking velocimetry and flow birefringence to study nonlinear flow behavior of entangled wormlike micellar solution: From wall slip, bulk disentanglement to chain scission

In this work, we have investigated nonlinear flow behavior of an entangled living polymer solution made of wormlike micelles of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal), using a combination of rheometric, particle tracking velocimetric (PTV), and flow birefringence (FB) techniques. The responses of the entangled micellar solution to various modes of shear including startup, creep, large-amplitude oscillatory shear (LAOS), and step shear are analyzed with PTV and FB. Apart from the usual nonlinear behavior analogous to that observed in entangled polymer solutions, i.e., shear inhomogeneity in the form of either interfacial slip or nonlinear velocity profile in the bulk that is plausibly due to deformation-induced chain disentanglement, the entangled micellar solution shows a sign of molecular rupture (i.e., chain scission) when it is subjected to a high rate of shear beyond a critical strain level of 4.0. LAOS at amplitude of 6.0 produces a similar rupture in the sample interior. On the other hand, creep flow can avoid such rupture, allow development of less sharp boundaries among the different layers of varying local shear rates, and still reach high apparent rates. Finally, application of 1713 observations not only confirms the PTV measurements of shear inhomogeneity but also reveals structural nonuniformity in the flow-vorticity plane both during a startup shear and in relaxation after a large step shear.