Inhomogeneous structure formation and shear-thickening in worm-like micellar solutions

The time evolution of flow-induced structures in low-concentration worm-like micellar solutions is studied using direct visualization and rheological methods. The visualization measurements show growth of a new viscous shear-induced phase (SIP) starting on the inner wall of a Couette shear cell at a time t₁ after the commencement of shear flow. The SIP continues to grow across the shear cell until it fills the entire gap at a time t₂. Rheological measurements show that the time t₁ corresponds to the induction time for the increase in stress and that t₂ corresponds to the plateau time for the saturation of stress. These measurements provide convincing evidence that the increase of stress with time results from the inhomogeneous growth of a new shear-induced phase in the flow cell. The growth of the new phase across the cell is approximately linear in time. (*) Permanent address: Laboratoire d'Ultrasons et de Dynamique des Fluides Complexes, URA au CNRS no. 851, Université Louis Pasteur, 4 rue Blaise Pascal, 67070 Strasbourg, France.