Insight in shear banding under transient flow

The rheo-optical behavior of a viscoelastic solution of a surfactant subjected to transient shear flows is reported. A steplike shear rate is suddenly imposed and we record the transient physical characteristics of the liquid, i.e., the shear stress sigma, the transmitted intensity I(t), and the scattering pattern. At the inception of the flow the shear stress shows an important overshoot followed by damped oscillations which also appear in the intensity profile. Then a sigmoïdal relaxation process brings the liquid in its final steady state. During this first phase, the diffusion pattern due to enhanced concentration fluctuations can also be observed. A fine anisotropic layer appears near the moving wall shortly after the inception of the flow, but its width starts to increase significantly well after the end of the relaxation process. When the laser beam travels through this band a diffraction pattern can be observed indicating that it is formed by small subbands, the characteristic width of which is in the order of 100 micro m.