SHEAR-INDUCED PHASE-SEPARATION IN CATIONIC SURFACTANT SOLUTIONS AROUND A ROTATING SPHERE

The results of an experimental investigation of the flow of a highly d ilute cationic surfactant solution (tetradecyl trimethyl ammonium brom ide with added sodium salicylate as counterion, equimolar 2.4 mM) arou nd a rotating sphere are presented. The flow and the shear-induced pha se transition are visualized by means of a Toepler Schlieren optics. T he buildup of the shear-induced structures occurs only above a critica l shear rate. Once this critical value is exceeded the shear-induced p hase separation starts after a characteristic deformation with the she ar rate reduced by the critical one. Further analysis of the obtained data is performed on basis of an analytical calculation of the flow ar ound a rotating sphere in a second order fluid (Thomas and Waiters, 19 64; Giesekus, 1965). From some characteristic features of the shear-in duced structures as induction time and position the parameter describi ng the elastic properties of the fluid is estimated.