A challenging scientific and engineering problem is accurately predicting the flow of complex fluids containing solid particles, such as nanoparticles, colloids, crystals and larger particulates. Such suspensions are encountered in a wide array of consumer products (e.g. paints, cosmetics, and foods), in nature (e.g. sediments, concentrated protein solutions and blood), as well as multiphase fluids of significant national concern (e.g. transuranic tank sludge), and as critical components in emerging devices for energy storage (flow batteries). The flow of such suspensions are often thixotropic, whereby the material's rheological properties are time and flow history dependent. The proposed research addresses this challenge though a transformative approach, namely developing a multiscale microstructure-based framework that will be thermodynamically consistent and generally applicable for all flows. A key component will be to identify the internal variables that properly define the microstructure. Validation of the theory and modeling will be accomplished by leveraging a complementary development of new experimental techniques to simultaneous measure the rheology and microstructure of such systems. Thus, a systematic, combined theoretical/experimental investigation on well-defined, model thixotropic systems is an important component of the research plan. The intellectual merits of the research include the development of a robust, theoretically rigorous framework for modeling the general, time dependent flow of concentrated suspensions. This systematic and integrated effort will lead to a new capability in the modeling and understanding of complex thixotropic systems that can significantly aid in the rational engineering of many materials of industrial and national importance. The broader impacts of this work include educating PhDs and BCHEs in suspension rheology necessary for careers in industry and academic research. Modules are to be developed for K-12 education by building on and improving current laboratory demonstrations. The models and knowledge developed in this research have potential to improve many industries as the processing and flow of thixotropic suspensions is critical to chemicals, materials, and pharmaceutical manufacturing and handling. As an application of particular importance, the PIs will be working with partners at Sandia National Laboratory to address the national challenge of processing suspensions of radioactive tank sludge at Hanford and other critical issues concerning nuclear waste management.
