Disentangling the dynamics of shear banding in entangled polymer solutions

Plastics are ubiquitous to modern society, used in food packing, electronic devices, transportation, sports equipment, and medical devices. The precursor to all formed plastics is a polymer fluid. Polymer fluids are a unique type of fluid, as the chain-like polymer molecules act like springs, and within a collection of fluid molecules, the polymer springs become entangled. Unlike water and many other fluids, this complex fluid does not respond to the forces of stress and shear in a manner that is proportional to the applied force. This non-proportional response is classified as Non-Newtonian. Non-Newtonian fluids have not been adequately described by general engineering models of fluid flow. Without general models, prediction of flow in industrial processing is limited to trial-and-error. Under some circumstances, such as slow flow or low concentration, many of these complexities can be ignored. However, these are not the most industrially relevant conditions, where high concentration and fast flow are prevalent. This project supports a fundamental experimental study on the fast flow of concentrated polymer fluids, and uses state-of-the-art imaging techniques to directly image single polymer molecules to reveal the microscopic dynamics of complex fluid flow.

The challenge in understanding the dynamics of concentrated polymer fluids is the nonlinear viscoelasticity of entangled polymer solutions, particularly the formation of shear-banding in concentrated polymer solutions at high shear rates. These phenomenon are difficult to probe experimentally. This research project combines high-speed confocal microscopy with a custom shear cell to investigate the origin of shear-banding flows of a model system, namely, concentrated DNA solutions. The applications of these unique experimental tools to the problem will resolve a controversy regarding on shear-banding in polymer fluids. The phase diagram of the shear-induced dynamics of concentrated DNA solutions is being mapped. The project aims to establish a direct link between the microscopic dynamics of DNA molecules and the macroscopic flow behavior of polymer fluids. The project is elucidating the dynamics of single polymer molecules in concentrated solutions under fast shear and experimentally validating competing theories on shear banding of polymer fluids. These results are uncovering the missing theoretical understanding of concentrated polymer fluids under fast flows and, ultimately, practical engineering models for polymer fluids under the most industrially relevant conditions. Increased control and prediction of polymer flow will increase throughput and efficiency, and therefore greatly impact the U.S. manufacturing sector. This research project also involves K-12 outreach programs with local high school students, especially those underrepresented in science and engineering.