RHEOLOGICAL AND FLOW BIREFRINGENCE STUDIES OF A SHEAR-THICKENING COMPLEX FLUID - A SURFACTANT MODEL SYSTEM

A common surfactant solution system is investigated to demonstrate the characteristic and generic features observed in shear-thickening self-assembling fluids. Rheological and flow birefringence measurements on the aqueous CTAB/sodium-salicylate solutions disclose a number of unusual flow behaviors under different flow conditions. All solutions with concentrations ranging from 0.5 to 5 mM exhibit shear thickening above certain critical shear rates, accompanied by an increase in flow birefringence over a range of temperature from T = 14 to 36-degrees-C. An extraordinarily long induction period, which is found to be inversely proportional to the applied shear rate, precedes the establishment of the shear-thickening state. This dynamic state is highly flow birefringent, strongly nonlinear viscoelastic and possesses long relaxation times. We deduce that shear-induced coagulation results in self-assembling of surfactant molecules to form new micelles which are of exceedingly large sizes and which can survive against thermal disturbance for a long time.