Thinning and thickening effects induced by shearing in lecithin solutions of polymer-like micelles

Abstract

 The effects of shear flow on the lecithin organogels consisting of reverse polymer-like micelles have been investigated by dynamic rheology. It was established that the shear effects depended on the molar ratio of water to lecithin that determined the micellar type in the system. For an organogel with linear flexible polymer-like micelles, thinning was observed. The main features of the rheological behavior bore a resemblance to previously investigated aqueous systems made up of similar but direct polymer-like surfactant micelles. The thinning effects are explicable on the basis of alignment of micellar aggregates along the flow direction and their disentanglement. An organogel with branched micelles did not demonstrate any notable response to the shearing. Unusual behavior was noticed in the case of a jelly-like phase that included the highest amounts of water. The applied steady shear flow induced a thickening effect. This was followed by restructuring of the micellar system at the level of polymer-like micelles and their network. The shearing effects were characterized by slow kinetics. In addition, the system did not revert to the original state after the cessation of steady shear flow even within 8 h. Measurements performed in an oscillation regime on this system showed that shearing should promote a substantial growth of the polymer-like micelles and affect their alignment.