Flow regimes of dilute surfactant solutions

The shear-thickening of dilute surfactant solutions is investigated using w ater/cethyltrimethylammonium tosylate (CTAT) systems. We show that shear in duced structures (SIS) are obtained in Couette and parallel-plate flows whe n the shear stress is higher than a critical shear stress, independently of the type of shear cell. Three different regimes are observed in Couette fl ow, confirming the results of Hu and co-workers on TTAA/NaSal/water systems . Flow regimes in parallel-plate flow geometries are studied with the aid o f particle image velocimetry (PIV). Particular attention is paid to the cal ibration of the experimental set-up using pure water in order to distinguis h between deviations of the rheometric velocity profile due to the onset of purely hydrodynamic instabilities from those due to the occurrence of SIS. It is noteworthy that spatial and temporal fluctuations in the local veloc ity fields are observed when SIS exists, even when rheometric measurements show an apparently steady-state situation. When no SIS is present we found that the surfactant solutions behave like N ewtonian fluids. Laminar secondary flows are present for both cells under t he same conditions as those found for classical hydrodynamics. (C) 2001 Els evier Science B.V. All rights reserved.