CAREER: Time-dependent Structures of Soft Materials under Flow: A Rheo-Scattering Approach to the Study of Thixotropic Yield Stress Fluids

该奖项支持实验研究和教育，以了解软材料从固体到液体的转变方式。这种“屈服应力材料”有着广泛的用途。其中包括石油化工、光伏、制药、食品和3D打印应用。这些应用都需要一种材料，它可以在某些条件下保持所需的形状，但可以根据需要进行流动。理想情况下，人们可以设计这些材料的特性，使其与相关工艺的具体要求相匹配。然而，没有准确的方法来确定这些材料产生的精确条件。这项研究将开发实验方法，将我们在人体长度尺度上观察到的机械变化与分子水平上发生的变化联系起来。这将使新材料的设计以及更有效的工业过程成为可能。该项目的综合教育和推广部分支持更广泛地向学龄儿童推广，以及研究生和本科生的研究培训和指导。外展工作包括旨在通过针对代表性不足的群体的项目向崭露头角的科学家传授软物质研究的复杂性的项目。通过这些项目，以及与校园中学女生的合作，将向年轻学生介绍软物质和3D打印的概念。通过结合通过这项工作获得的更高层次的材料，本科课程将得到加强。该职业奖将开发工具和基础科学，以理解和预测在触变屈服应力流体中观察到的随时间变化的结构和流变转变。在混合实验中，流变学信息与小角度x射线散射和x射线光子相关光谱相结合，为同时获得必要的长度尺度特征（纳米-微米）和时间尺度（毫秒-小时）提供了理想的工具。已知的触变屈服应力流体的浓缩胶体悬浮液的层次结构复杂性由于物理参数空间广而提出了挑战。使物料远离平衡状态的加工流进一步扩展了该参数空间。我们将建立一个基于流变散射的方法来指导测量和最终设计结构和流变动力学。从流变- xpcs的两时间相关性中获得的瞬态分子水平结构动力学将与PI首创的新型瞬态流变分析方法进行配对和比较。PI将开发新的结构-属性-处理关系，以捕获应用变形下破坏和重构的复杂性。研究人员将设计、开发和实施流变学方案，以梳理触变材料中瞬态可恢复弹性应变的信息。PI先前的工作已经开发出用于非平衡处理的流变分析工具，非常适合于确定触变性材料的瞬态流变性质。该项目将建立在屈服和触变性之间的等效性基础上，揭示在流动中屈服和重组的潜在物理原因，通过结合流变学表征、微观结构探针和模型开发，加快其在真实系统中的受控设计。这些方面中的每一个都将以互利、协同的方式解决基本问题（屈服和重组的流变学、抗屈服的触变性、新模型），为材料的发现和设计提供独特而合理的方法。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。

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.