Modelling extensional flow properties of solutions of polymers and thread-like micelles

Just as spiders and silkworms make materials by spinning solutions of polymer into long threads, humans spin such materials into filaments for textiles, filters, and many other applications. The same kind of flow, namely “extensional flow,” occurs in combination with shear flow when such materials are drawn into fibers, forced into bottles, or when consumers apply shampoos and soaps to their hair or bodies. Thus, the behavior of long polymers and long thread-like aggregates of surfactant (i.e., “micelles”) in extensional flow is basic to the design of many products. Yet their behavior in solution is still poorly understood. This is due to the difficulty of controlling extensional flows of these materials in the lab, and of modelling the complex interactions of their fluid mechanical and molecular behavior. Therefore, a new method of analysis will be used that combines a molecular simulation technique (“Brownian dynamics”), a method of accounting for the entanglements of the polymers or micelles (“slip-link simulations”) and proper treatment of the fluid mechanics of filaments. This combination will finally allow experimental data to be interpreted properly. The result will be that extensional flows of these materials can be better predicted, and the materials can be better designed for their intended applications. The importance of the area will be illustrated to K-12 students through the example of how an inability to clear sticky polymeric mucus from lungs is responsible for the severity of disease in cystic fibrosis. The “slip-spring” and “pointer” simulation methods developed in our group will be used to model extensional flow of polymeric solutions and threadlike micellar solutions measured in “capillary break-up” (CaBER) and “drop-on-substrate” (DoS) rheometers. These rheometers are the common ones used for extensional measurements of mobile polymer and surfactant solutions. The modeling will combine proper description of the flow kinematics in these measurements with advanced simulations of ensembles of polymers and micelles. This will help disentangle uncertainties created by the confounding effects of the non-ideal flow kinematics in these devices from the complex rheology of these fluids, leading to a breakthrough in understanding and more accurate modelling and design of materials. The motivations and methods will inform a summer outreach program to K-12 students at the University of Michigan and in Detroit-area schools, using hands-on polymeric materials in extensional flows. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.