TRANSIENT SMALL-ANGLE NEUTRON-SCATTERING EXPERIMENTS ON MICELLAR SOLUTIONS WITH A SHEAR-INDUCED STRUCTURAL TRANSITION

Transient small-angle neutron scattering (SANS) experiments on a shear ed 5 mM aqueous surfactant solution of trimethyltetradecylammonium sal icylate (TTMA-Sal) are reported. The radially symmetric scattering pat tern without shear shows a weak ringlike correlation ''peak''. For ste ady shear rates above a threshold value of GAMMA(th) = 50 s-1, the sca ttering pattern becomes rapidly anisotropic and the ring is compressed into two sharp, symmetric peaks. These changes are related to a shear -induced structural transition. The analysis of the data reveals the e xistence of two different types of micelles: Short rodlike micelles (c alled type I) are always present, and they are only weakly aligned. Ve ry long rodlike micelles (named type II) can be observed only above th e threshold value of GAMMA, and they are strongly aligned. Both types of micelles are assumed to be of cylindrical shape, with a radius R = 1.94 +/- 0.07 nm. We report the results on three time-dependent SANS e xperiments, where the shear rate was raised stepwise from zero to a va lue of GAMMA(ex) = 100, 200, and 400 s-1, respectively. It is found th at the long micelles of type II, which are not present at time t = 0, begin to grow a short time after switching on the shear. The number of surfactant molecules bound in type I micelles decreases, whereas the number of molecules bound in micelles of type II increases. The sum of both remains constant. The growth of the large micelles proceeds acco rding to the Avrami law, which was originally designed to describe nuc leation and growth reactions in metals and alloys.