SHEAR THICKENING IN LOW-CONCENTRATION SOLUTIONS OF WORMLIKE MICELLES - I - DIRECT VISUALIZATION OF TRANSIENT-BEHAVIOR AND PHASE-TRANSITIONS

Shear thickening of low-concentration solutions of wormlike micelles i s investigated using simultaneous rheological and visualization measur ements. Shear-induced. structures (SISs) are directly visualized in tr ansparent Couette cells using a laser light scattering technique simil ar to dark-field microscopy. From these measurements, four different r egimes of behavior are identified. In regime I, which occurs below a c ritical shear stress alpha(c), the shear rate increases monotonically with stress and no shear thickening or SISs are observed. In regime II , which occurs for stresses greater than sigma(c) but less than alpha( s), SISs nucleate inhomogeneously and grow from the inner cylinder of the Couette cell. In this regime, the steady state shear rate initiall y decreases with increasing stress and then increases again as the str ess is raised. The steady state in regime II is characterized by two c oexisting states separated by a cylindrical interface ( concentric wit h the Couette cylinders). Near the inner cylinder, viscous SISs are ob served, while near the outer cylinder, a much less viscous fluid simil ar to the original micellar solution is observed. The steady state in regime II is observed only under conditions of controlled stress. In r egime III, which occurs for stresses above sigma(s) but below sigma(f) , SISs nucleate homogeneously throughout the shear cell and appear to fill the gap. Regime IV is characterized by the fracture of SIS and is observed at sigma(f). The shear-thickening transition bears many simi larities to a phase transition as opposed to a simple hydrodynamic ins tability. (C) 1998 The Society of Rheology.