Shear thickening in three surfactants of the alkyl family CnTAB: small angle neutron scattering and rheological study.

Three surfactants of the alkyl family only differing by the length of the aliphatic tail and mixed with sodium salicylate are studied in equilibrium and under flow using rheology and small angle neutron scattering. All three undergo the shear thickening transition associated with the emergence and growth of the so-called shear induced phase or SIS. The rheology in light and heavy water are at first compared, and the influence of the deuterated solvent on the transition characteristics is examined. Small-angle neutron scattering (SANS) experiments are performed on nonflowing solutions in order to find the average cross section radius Ri and the local morphology of the micelles. These data are fitted with two models: a first one that is valid for rigid monodisperse cylindrical particles to get Ri, and a second one that is suitable for semiflexible micelles which shall lead to the contour length L of the micelles. Under flow, scattering experiments are performed over a shear rate range covering three flow regimes. In the first one, prior to the shear thickening transition, the patterns are isotropic; during the last two, corresponding to the existence of the SIS, the scattering figures gradually lose their circular symmetry for an elongated elliptic shape characteristic of an anisotropic medium. The average orientation of the micelles is quantified by the anisotropy factor Af which turns out to be of the same order of magnitude for the three surfactants.