Development of a thermodynamically consistent rheological constitutive equation for thixotropic suspensions connecting particle properties to thermodynamics and rheology

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Many materials must have both solid-like and fluid-like properties during their processing and/or use. Examples include construction materials such as cement and asphalt, biological fluids such as blood, consumer products including toothpaste and hand-creams, naturally occurring muds, clays, and sediments, and polymers and pastes used in 3-D printing. To achieve the desired flow behavior, product formulators vary particle properties, such as size, shape, and surface coatings, add flow modifiers and gelling agents, and vary the processing conditions. However, there is no accurate method of predicting the flow behavior. This limits the development of many products and points to a gap in our scientific understanding of these important materials. This research will develop a new theoretical framework, to be validated by innovative experiments, to aid in this endeavor. The work should both improve our scientific understanding of these complex materials and provide engineering guidance for industry on improving processing. In addition, high school students, undergraduates and graduate students will be trained in this multidisciplinary research effort.More technically, this research addresses thixotropy, which is a ubiquitous, and often vexing, property of particle-containing complex fluids, pastes, and soft matter characterized by a complex, time-dependent rheology due to flow-microstructure coupling. Micromechanical approaches are inherently restricted to small scales of length and time, while at the larger scales of importance to engineering applications and processing, only phenomenological models exist. These are often restricted to shear flows, and contain internal parameters with ill-defined physical meaning. The principal investigators plan to remedy this by developing a fundamental and rigorous, multiscale theoretical framework and use this to develop truly predictive constitutive equations, which will be validated against advanced experiments on well-defined, model thixotropic systems. They will develop a new tensorial, multiscale microstructure-based framework that is thermodynamically consistent and generally applicable for all flows (i.e., cast in materially objective fashion). The theory is based on the most modern nonequilibrium thermodynamic (NET) formalism (GENERIC); the investigators at the University of Delaware are among the pioneers of this approach. Coarse graining of a newly derived population balance model based on particle properties for suspensions with yield stress and matching with the continuum model will fix the mesoscale parameters in the NET theory and result in a predictive constitutive model that can be rigorously tested and validated against exact analytical results as well as internationally recognized data on model concentrated suspensions in transient and oscillatory shear flows and non-viscometric flows, and directly against microstructure data obtained using RHEO-optical & RHEO-SANS, leveraging research supported by NIST. The proposed NET multiscale modeling with a simultaneous emphasis on both the rheology and the material's microstructure, combined with tight coordination with the leveraged experimental program sets this work apart from previous efforts. The research will lead to a new capability in the modeling and understanding of complex thixotropic systems that can significantly advance the rational formulation and processing of many materials of industrial and national importance.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.