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

许多材料在加工和/或使用过程中必须同时具有固体和流体性质。例如，水泥和沥青等建筑材料、血液等生物流体、牙膏和护手霜等消费品、天然泥浆、粘土和沉积物，以及3d打印中使用的聚合物和糊状物。为了达到理想的流动性能，产品配方师改变颗粒特性，如大小、形状和表面涂层，添加流动调节剂和胶凝剂，并改变加工条件。然而，目前还没有准确的方法来预测其流动特性。这限制了许多产品的发展，并指出我们对这些重要材料的科学理解存在差距。这项研究将发展一个新的理论框架，通过创新的实验来验证，以帮助这一努力。这项工作既可以提高我们对这些复杂材料的科学认识，也可以为工业改进加工提供工程指导。此外，高中生、本科生和研究生将在这项多学科研究工作中接受培训。从技术上讲，本研究解决了触变性问题，触变性是一种普遍存在的、经常令人烦恼的、含颗粒的复杂流体、膏体和软物质的特性，其特征是由于流动-微观结构耦合而具有复杂的、随时间变化的流变性。微力学方法固有地局限于长度和时间的小尺度，而在对工程应用和处理具有重要意义的大尺度上，只有现象学模型存在。这些通常仅限于剪切流动，并且包含物理意义不明确的内部参数。主要研究人员计划通过开发一个基本的、严格的、多尺度的理论框架来解决这个问题，并利用它来开发真正的预测本构方程，这些方程将在定义良好的、模型触变系统的高级实验中得到验证。他们将开发一种新的张量，多尺度微结构为基础的框架，该框架在热力学上是一致的，并且通常适用于所有流动（即以材料客观的方式铸造）。该理论是基于最现代的非平衡热力学（NET）形式主义（一般）；特拉华大学的研究人员是这种方法的先驱之一。基于具有屈服应力的悬浮液颗粒特性的新导出的种群平衡模型的粗粒化，将固定NET理论中的中尺度参数，并产生一个预测本构模型，该模型可以根据精确的分析结果以及国际公认的瞬态和振荡剪切流和非粘性流中模型集中悬浮液的数据进行严格的测试和验证。利用NIST支持的研究，直接针对使用RHEO-optical & RHEO-SANS获得的微观结构数据。提出的NET多尺度建模同时强调流变学和材料的微观结构，结合与杠杆实验程序的紧密协调，使这项工作有别于以往的努力。这项研究将为复杂触变体系的建模和理解提供一种新的能力，这将极大地促进许多具有工业和国家重要性的材料的合理配方和加工。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.