Shear-Thickening in Salt-Free Aqueous Solutions of a Gemini Cationic Surfactant: A Study by Small Angle Light Scattering

Abstract

The shear-thickening regime of dilute cationic surfactant solutions is studied by means of small-angle light scattering. The 2-dimensional intensity pattern measured in the flow velocity−vorticity plane exhibits an intense streak pattern perpendicular to the flow direction. The main features of this streak pattern are as follows: (i) a periodic modulation of the intensity in the vorticity direction, (ii) fluctuations of the intensity and orientation as a function of scattering volume, (iii) a strong correlation of the average intensity with the apparent shear viscosity, and (iv) an average shape independent of the applied shear rate. The corresponding anisotropic optical contrast profile with a characteristic modulation length about 33 μm in the vorticity direction cannot be interpreted in terms of the alignment of large anisotropic colloidal objects in the flow but, instead, is consistent with the existence of an aligned heterogeneous gellike layer in the gap.