The dynamics and rheology of shear -banding wormlike micelles and other non -Newtonian fluids
Non-Newtonian fluids play an important role in our daily world. While the nomenclature may be unfamiliar to most people, these fluids, also referred to as “viscoelastic”, comprise a class of materials found in a wide variety of items ranging from food, to cosmetic products, to the plastic containers they are all contained within. The design and tunability of non-Newtonian fluids is only possible through an understanding of their complex dynamics and rheology.
Industrial plastics are formed into a final product through a host of fabrication techniques in which the material quality and finish is important. Sharkskin, observed in the extrusion of commercial polymer melts, is an instability that has adverse effects on surface finish. We have performed experiments towards the goal of not simply eliminating, but controlling the instability. By focusing on the rheology of polymer melts, our results have shown that the instability is caused purely by specific thermal conditions within the extrusion die, which can be both characterized and precisely controlled.
Another important class of viscoelastic fluids is solutions of surfactants. Their unique molecular amphiphilic chemistry allows them to form long wormlike micellar structures, which behave like a “living polymer”. These fluids are known to exhibit a flow phenomenon called “shear-banding”; above a critical stress, they enter a non-linear regime characterized by a stress plateau in which the fluid forms distinct bands of varying shear-rate. By measuring velocity profiles using particle-image velocimetry (PIV) and stress-fields using a flow-induced birefringence (FIB) method, both at very high resolution, we have uncovered some interesting rheological behavior underlying the shear-banding kinematics. We have also performed extensional rheology experiments to supplement the shear results. Our study focused on obtaining detailed experimental data to provide insight into complex flow behavior and drive the development of constitutive models to predict shear-banding and other phenomena. This experimental methodology and the resultant models provide a highly detailed framework with which it is possible to design viscoelastic fluids having specific rheological properties for applications as intricate as tissue-engineered biomaterials.