The Structure Factor in Flowing Wormlike Micellar Solutions

In this work we present theoretical and experimental studies on the structure factor in micellar systems. Cetyl trimethyl ammonium tosilate (CTAT) solutions in absence of salts exhibit a flow behavior similar to polymer solutions, while upon an increase in the ionic strength with NaCl, their behavior changes to that of a kinetic-controlled breakage-reformation process. The relaxation mechanism thus changes from that of a spectrum of relaxation times into an averaged dominant relaxation mode, representative of reversible kinetics. Micellar solutions exhibit mechanical instabilities under flow, which may be increased by concentration fluctuations. Elastic stresses increase concentration fluctuations, which are experimentally observed by rheooptical techniques, such as Rheo-SALS, depicting characteristic "butterfly" patterns. Scattering patterns provide information on the structure factor, arising from concentration fluctuations of the flowing solution. Upon addition of salt to a CTAT solution, we observe agreement with the Cox-Merz rule, revealing that the solution is in the fast-breaking regime approaching a single relaxation time kinetics. This behavior cannot be predicted by polymer solution theories which do not incorporate the breakage-reformation kinetics. Here, we propose a kinetic model by which the structure factor can be calculated for these micellar solutions. A shift in the maximum of the scattering patterns as the shear rate is increased is predicted by the new theory, which can be explained by considering flow-induced changes in the structure of the solutions with the underlying kinetics of breakagereformation. Agreement with experiments is exhibited.