Shear-induced structure change and flow-instability in start-up Couette flow of aqueous, wormlike micelle solution

摘要

The occurence of shear-induced structures (SIS) of a wormlike micellar solution is investigated using an aqueous solution of cethyltrimetylammonium bromide (CTAB)/aqueous sodium salicylate (NaSal) with composition CTAB (0.03 mol/L)/NaSal (0.23 mol/L). A concentric cylinder Couette cell made of transparent glass is used to generate stepwise start-up shear flow. The shear stress and the brightness, which is an index of the transparency of the fluid, are measured simultaneously and the fluid motion is observed by adding tracer particles to the fluid. Measurements of flow birefringence and light intensity of a transmitted laser beam passing parallel to the vorticity axis are also carried out to monitor flow-induced microstructural changes. The transient behavior of the macroscopic properties, i.e., the shear stress and the brightness, are consistent with each other and with the optical microstructural measurements, confirming that the occurrence of the transparent-to-opaque change in the wormlike micelle solution is caused by a change in its structure. The transient behavior of SIS with the time is correlated to the shear strain and divided into four regions depending on the shear rate. At the highest shear-rate range, a strong instability occurs immediately following a maximum in the shear stress. Upon cessation of the shear stress, elastic recoil is observed along with a sudden recovery to the original transparent state of the solution. (c) 2006 The Society of Rheology.