| Summary: | Non-linear viscoelastic behavior of a wormlike micellar solution of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) in the large amplitude oscillatory shear (LAOS) is examined. A transient response of the shear stress after applying a large amplitude sinusoidal strain is measured by the stress control rheometer. The start-up wave form of the shear stress is divided by four patterns, such as (a) no-stress overshoot, (b) linear shear hardening, (c) extreme shear hardening related with SIS and (d) stress fluctuation, by increasing the strain amplitude. After three cycles, the stress wave becomes a periodic steady pattern. The obvious non-linear behavior, the pattern (c) and (d), is observed at the strain amplitude γ0 = 3 or higher. The periodic stress signal in the periodic steady state is evaluated by both the geometrical interpretation and the LAOS analysis of the new frameworks. As a result of separating the viscous and the elastic ingredient stress from the measuring stress, the rheology property changes suddenly around γ0 = 20. These characteristics correspond closely to specific flow behavior of a start-up shear flow in the same shear strain. The micelles structure is changed over a certain shear rate and strain, as known by SIS, shear-induced structure, and generate the opaque in the solution. In this case, the solution begins to be opaque at shear strain γ = 3 and becomes most opaque around γ = 20. Moreover, the onset condition coincides with the appearance of the higher odd harmonics in the periodic stress signal analyzed by FFT method.
|
|---|