Study of flow-induced structures in a shear thickening micellar solution by simultaneous rheometrical and electric current measurements

Flow-induced structures in a shear thickening equimolar aqueous solution of 40 mM cetylpyridinium, chloride and sodium salicylate were studied in this work by simultaneous rheometrical and electric current measurements. For this purpose, a constant electric field was applied in the direction of the velocity gradient in a Couette cell. It was found that the measured electric current increased under stable flow conditions, and is scaled linearly with the logarithm of the shear rate. This behavior is explained on the basis of a theoretical analysis. However, the conductivity of the solution was drastically modified in the presence of shear-induced structures. To account for this behavior it is suggested free ions dominate conduction. The presence of shear-induced structures parallel to the flow direction reduced the mobility of the free ions in the direction of the velocity gradient, and therefore, the measured electric current. Unstable flow conditions led to breakage of the shear-induced structures and allowed ions to escape, producing a sharp increase in the measured electric current. Finally, the findings in this work provide a simple and inexpensive way to detect the orientation and to follow the dynamics of structured phases in this type of solutions. (c) 2007 Elsevier B.V. All rights reserved.