SHEAR THICKENING IN LOW-CONCENTRATION SOLUTIONS OF WORMLIKE MICELLES - II - SLIP, FRACTURE, AND STABILITY OF THE SHEAR-INDUCED PHASE

The rheology of the shear-thickened state is investigated in low-conce ntration solutions of wormlike micellar solutions using mechanical, op tical, and velocity profile measurements. The zero-shear-rate viscosit y of the solutions increases by more than a factor of 1000 as the conc entration of surfactant is increased from 1 to 10 mM. By contrast, the apparent viscosity of the shear-thickened state of these same solutio ns is observed to be remarkably independent of concentration over a wi de range of shear rates. This is shown to be a consequence of the deve lopment of slip layers between the very viscous gellike shear-induced structures (SISs) which form in the bulk of the surfactant solution an d on the walls of the Couette devices in which the measurements ar mad e. As the applied shear stress is increased even further, there is evi dence that the SIS fractures give rise to shear-rate-independent stres s and an apparent viscosity, which decreases with increasing shear rat e. After the SIS fractures, large fluctuations in the shear rate are o bserved to result from an SIS which alternately fractures, slips, heal s, and refractures. The fracture stress is proportional to the concent ration and inversely proportional to the cell radius. (C) 1998 The Soc iety of Rheology.