CAREER: Oscillatory shear of entangled polymers: characterization and stability analysis

In everyday life, the properties of materials are often judged by how they react to simple actions like poking, prodding, stretching, and squeezing. Similarly, in scientific and industrial applications – from polymer recycling to 3D printing – measuring a material’s response to repeated pushing and pulling is a good way to predict its performance. Despite the usefulness of these tests, however, there is no universal way of to interpret their results. This project will show that systematically tweaking the deformation cycle and analyzing the resulting changes can lead to new insights into a material’s behavior in a broad range of processes. The scientific tools developed for this project will have implications in other engineering disciplines where analogous “push-and-pull” tests are applied, including signal processing, process controls, and electrical circuits. Finally, this award will support outreach efforts in STEM education for high school students. In partnership with programs at UCLA, a high school students from the Los Angeles and Inglewood Unified School districts will receive training in computer literacy and scientific computing, including applications related to the project's research. Large Amplitude Oscillatory Shear (LAOS) flows are widely used by industry for material characterization of complex fluids, from commodity polyolefins to next-generation 3D printing gels. In principle, LAOS data contains valuable information about the structure and composition of a fluid, but the quantitative interpretation of experimental LAOS measurements is still an active area of research. This project will generalize the concept of parallel superposition rheology to LAOS flows, allowing a complete characterization of the material’s linear response function for perturbations about a base LAOS deformation cycle. This linear response function can be formulated as a harmonic transfer matrix (HTM), and preliminary data suggests that the HTM can be useful for both fundamental material characterization and prediction of shear banding flow instabilities. Overall, our proposed research will deploy a combination of theoretical, computational, and experimental tools to formalize and validate this new approach to LAOS banding stability analysis, develop interpretive frameworks for quantitative material characterization, and explain distinctions in the underlying bifurcation structure of steady and oscillatory shear banding flow instabilities. 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.