Light Scattering and Rheology of Complex Fluids Driven far from Equilibrium

In these lectures, we explore two examples of systems driven far from equilibrium by the application of shear. With these two examples, we investigate different experimental strategies which are designed to directly probe the connection between macroscopic non-linear rheology and the microscopic structure and dynamics of a broad range of soft materials. The study of these systems illustrate the importance of performing simultaneous measurements of the microscopic structure, flow, and rheological properties of soft materials when such systems are driven far from equilibrium by shear flows. There are several reasons for this. First, the flows are frequently inhomogeneous. Such inhomogeneities can arise from various mechanisms; the two most frequently observed and discussed are hydrodynamic instabilities and flow-induced phase transitions. Other difficulties can also arise for the case of virtually any flow that is not a pure shear flow. In pure extensional flows, for example, the nonlinear rheological properties of the fluid under study can modify the flow field in ways that are extremely difficult to predict. Thus, without a detailed knowledge of the flow field, it is virtually impossible to develop a meaningful theory. Second, systems do not always tend towards a steady state. And even when they do, the steady state is not necessarily characterized by any general principle of detailed balance to constrain the theory which one can construct. Furthermore, the structures that develop under shear often do not resemble the structures found in the same system in equilibrium. That is, the nonequilibrium structures frequently cannot be described as perturbations of the equilibrium structures. Therefore, as important as microscopic structural measurements are for understanding and developing theories for systems in equilibrium, they become even more important when systems are driven far from equilibrium.