Effect of Stress−Structure Coupling on the Rheology of Complex Fluids: Poor Polymer Solutions

When sheared, near-θ polymer solutions undergo a dramatic enhancement of concentration fluctuations due to a coupling between polymer stress and concentration. This phenomenon is easily observed as an increase in turbidity. The flow-induced mesoscopic structure is, however, anisotropic and results in an extra “anomalous” stress in the fluid. The dynamics and rheology of such a system is discussed and a phenomenological description for stress evolution as a result of concentration fluctuation enhancement is developed. The rheological properties are shown to be quite unusual and include a shear thickening regime and the appearance of a second overshoot on flow startup. The methodology developed in this paper can be applied to a large number of problems involving flow-modified structure in complex fluids. An approximate stress−optic rule for the anomalous stress is developed and predictions for simple shear flow are compared with experimental results.