Investigation of the structuration of micellar solutions by using rheometry

and electric current measurements

Rodrigo Sanchez, Jose Perez-Gonzalez, Lourdes de Vargas

Laboratorio de Reologia, Departamento de Fisica, Escuela Superior de Fisica y Matematicas, Instituto Politecnico Nacional,

Apdo. Postal 118-209, C. P. 07051, Mexico D. F. MEXICO

Surfactants in solution are able to form molecular aggregates known as micelles, which can take different shapes depending on the concentration and ionic strength of the solution. In the semidilute regime cylindrical wormlike micelles are able to entangle and exhibit a highly viscoelastic behavior that sometimes resembles that observed in high molecular weight polymers. However, in contrast to polymers in which the contour length of the molecule is fixed, micelles in solutions can break and recombine, thus permitting modes of stress relaxation that are absent in polymeric systems. This characteristic along with variations in surfactant concentration and ionic strength of the solutions give rise to a wealth of rheological behaviors, including flow instabilities such as shear banding and spurt, flow induced phase transitions, and shear thickening resulting from shear induced structures. In this work, the structuration of shear banding and shear thickening aqueous micellar solutions formed by cetylpyridinium chloride and sodium salicylate is studied by a combined method of rheometry and electric current measurements. It is shown that the flow field induces changes in the electric resistance of the solutions, which are related to the dynamics of shear induced structures. In fact, the electric current as a function of the shear stress faithfully mimics the flow curves for the studied solutions, thus providing further evidence of changes in the fluid structure.