This award supports experimental research and education to understand the ways that soft materials transition from acting as solids to acting as liquids. Such "yield-stress materials" have a wide variety of uses. These include petrochemical, photovoltaic, pharmaceutical, food, and 3D printing applications. These applications share the need for a material which can retain a desired shape under certain conditions but can be made to flow on demand. Ideally, one would design the characteristics of such materials to match the specific requirements of the relevant process. However, there is no accurate way of determining the precise conditions under which these materials yield. This research will develop experimental methods that link the mechanical changes we observe on human length scales to what happens at a molecular level. This will enable the design of new materials as well as more efficient industrial processes. The integrated education and outreach component of this project supports broader outreach to school-age children, along with graduate and undergraduate research training and mentorship. Outreach efforts consist of programs designed to teach budding scientists about the complexity of soft matter research through programs that target underrepresented groups. Outreach through these programs, and with Campus Middle School for girls will introduce young students to the concepts of soft matter and 3D printing. Undergraduate classes will be enhanced by incorporation of higher-level material obtained through this work.
This CAREER award will develop the tools and fundamental science needed to understand and predict the time-dependent structural and rheological transitions observed in thixotropic yield stress fluids. Hybrid experiments in which rheological information is paired with Small-Angle X-ray Scattering and X-ray Photon Correlation Spectroscopy provide ideal tools to simultaneously access the necessary range of length scale features (nm - micron) and time scales (ms - hours). The hierarchical structural complexity of concentrated colloidal suspensions known to be thixotropic yield stress fluids poses a challenge due to a wide physical parameter space. Processing flows that take the materials far from equilibrium expand this parameter space further. We will establish a rheo-scattering-based methodology to guide measurements and ultimately design of structural and rheological dynamics. Transient molecular-level structural dynamics obtained from two-time correlations from rheo-XPCS will be paired and compared with novel transient rheological analysis methods pioneered by the PI. The PI will develop new structure-property-processing relationships to capture the complexity of destruction and restructuring under applied deformation. The researchers will design, develop, and implement rheological protocols that tease out information regarding the transient recoverable elastic strain in thixotropic materials. Prior work from the PI has developed rheological analysis tools for out-of-equilibrium processing that are perfectly suited to determining the transient rheological nature of thixotropic materials. This program will build on the established equivalence between yielding and thixotropy, shedding light on the underlying physical causes of yielding and restructuring under flow, expediting their controlled design in real systems by combining rheological characterization, microscopic structural probes, and model development. Each of these aspects will address fundamental topics (the rheology of yielding and restructuring, thixotropy as anti-yielding, new models) in a mutually beneficial, synergistic fashion that provides a unique and rational approach to materials discovery and design.
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.
