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

We demonstrate a unique shear-induced crystallization phenomenon above the equilibrium freezing temperature (T(K)°) in weakly swollen isotropic (Li) and lamellar (La) 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 Lc melts back to the equilibrium mesophase). This is different from the shear-driven crystallization below T(K)°, which is irreversible. Rheological optical observations show that the growth of the crystalline phase occurs through a preordering of the Li phase to an La phase induced by shear flow, before the nucleation of the Lc phase. Shear diagram of the Li phase constructed in the parameter space of shear rate (γ) vs. temperature exhibits Li → Lc and Li → La transitions above the equilibrium crystallization temperature T(K)°, in addition to the irreversible shear-driven nucleation of Lc in the Li phase below T(K)°. 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.