|the wagner group|
Concentrated surfactant solutions can self-assemble into various structures, ranging in complexity from simple spherical micelles to highly entangled micellar networks. Applications range from consumer products and drug delivery devices to manufacturing aids for advanced materials. In many products, the rheological behavior is closely linked to consumer acceptance. For example, many surfactant systems lose thermodynamic stability either at rest or under shear, which is highly undesirable for many applications. Such phenomena make it important to have knowledge of the microstructure and flow behavior of surfactant solutions.
Above the critical aggregation concentration, amphiphilic molecules such as surfactants self-assemble into micelles. In dilute solutions, the most preferable micellar structure is a sphere, depicted below. However, rodlike micelles can form at high salt concentrations in ionic surfactant solutions and in mixed surfactant solutions with and without salt. As the concentration of surfactant or salt increases, it becomes energetically ˇ°expensiveˇ± to create new rods due to high curvature at the rod endcaps, and axial growth of existing rods is favored. As the micelles further extend at higher surfactant concentrations, the rodlike micelles become flexible, or "wormlike". Above an overlap concentration of surfactant, c*, solutions enter the semi-dilute regime where the wormlike micelles are long enough to entangle and form a network, similar to polymer solutions.
M.E. Helgeson, P.A. Vasquez, N.J. Wagner, and E.W. Kaler. "Rheology and spatially resolved structure of cetyltrimethylammonium bromide wormlike micelles through the shear banding transition", Journal of Rheology 2009 53(3): 727-756.   [doi]