Reversible shear-induced crystallization above equilibrium freezing temperature in a lyotropic surfactant system

Abstract

We demonstrate a unique shear-induced crystallization phenomenon above the equilibrium freezing temperature Graphic in weakly swollen isotropic Graphic and lamellar Graphic mesophases with bilayers formed in a cationic-anionic mixed surfactant system. Synchrotron rheological X-ray diffraction study reveals the crystallization transition to be reversible under shear (i.e., on stopping the shear, the nonequilibrium crystalline phase Graphic melts back to the equilibrium mesophase). This is different from the shear-driven crystallization below Graphic, which is irreversible. Rheological optical observations show that the growth of the crystalline phase occurs through a preordering of the Graphic phase to an Graphic phase induced by shear flow, before the nucleation of the Graphic phase. Shear diagram of the Graphic phase constructed in the parameter space of shear rate Graphic vs. temperature exhibits Graphic and Graphic transitions above the equilibrium crystallization temperature Graphic, in addition to the irreversible shear-driven nucleation of Graphic in the Graphic phase below Graphic. In addition to revealing a unique class of nonequilibrium phase transition, the present study urges a unique approach toward understanding shear-induced phenomena in concentrated mesophases of mixed amphiphilic systems.