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 tl after the commencement of shear flow. The SIP continues to grow across the shear cell until it fills the entire gap at a time ts. Rheological measurements show that the time t(1) corresponds to the induction time for the increase in stress and that t(2) 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.