Reversible and irreversible flow-induced phase transitions in micellar solutions

It is well known that rodlike/wormlike micelles can self-organize under flow to form viscoelastic gel phases. Flow-induced structure (FIS) formation is typically accompanied by an enhancement in the shear viscosity. While configurational dynamics of and collisions between micelles in flow and electrostatic inter-micelle interactions are recognized as the key factors that influence such phase transitions, there are no universally applicable criteria for the onset strain rate as function of salt/surfactant concentration. Further, FIS formation is generally considered reversible, i.e., the structure disintegrates quickly upon flow cessation. In this work, first, we examine the effect of salt concentration on the critical strain rate for CTAB/NaSal solutions of rodlike micelles and show that a "self-similar" phase transition regime, characterized by a constant critical strain, exists. Second, we show that under strong (elongational) flow conditions, the phase transition is irreversible, leading to the formation of permanent nanogels that are stable long after the flow is stopped. Atomic force microscopy shows that the gel phase obtained under strong flow conditions consists of highly aligned micelles.