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转变为液体的方式。这种“屈服应力材料”有着广泛的用途。这些应用包括石化、光伏、制药、食品和3D打印应用。这些应用都需要一种材料，该材料可以在某些条件下保持所需的形状，但可以根据需要流动。理想情况下，人们会设计这些材料的特性，以匹配相关过程的具体要求。然而，没有准确的方法来确定这些材料屈服的精确条件。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流程成为可能。该项目的综合教育和外联部分支持对学龄儿童进行更广泛的外联，沿着研究生和本科生研究培训和辅导。外展工作包括旨在通过针对代表性不足的群体的计划来教授崭露头角的科学家关于软物质研究的复杂性的计划。通过这些项目的推广，以及校园中学的女孩将向年轻学生介绍软物质和3D打印的概念。 本科课程将通过纳入通过这项工作获得的更高水平的材料来加强。这个CAREER奖将开发理解和预测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所需的工具和基础科学。流变学信息与小角X射线散射和X射线光子相关光谱相结合的混合实验提供了理想的工具，可以同时访问必要的长度尺度特征（nm -微米）和时间尺度（ms -小时）范围。已知为触变性屈服应力流体的浓缩胶体悬浮液的分级结构复杂性由于宽的物理参数空间而构成挑战。使材料远离平衡的加工流进一步扩展了这个参数空间。我们将建立一个基于流变散射的方法来指导测量和最终的结构和流变动力学设计。瞬态分子水平的结构动力学从两个时间的相关性从rheo-XPCS将配对，并与PI开创的新的瞬态流变分析方法进行比较。PI将开发新的结构-性能-加工关系，以捕捉应用变形下破坏和重组的复杂性。研究人员将设计，开发和实施流变学协议，梳理出有关触变材料中瞬时可恢复弹性应变的信息。PI之前的工作已经开发出了用于非平衡加工的流变分析工具，这些工具非常适合确定触变材料的瞬态流变性质。该计划将建立在屈服和触变性之间的已建立的等效性的基础上，阐明在流动下屈服和重构的潜在物理原因，通过结合流变学表征、微观结构探针和模型开发，加快它们在真实的系统中的受控设计。这些方面的每一个都将以一种互利、协同的方式解决基本问题（屈服和重组的流变学、作为抗屈服的触变性、新模型），为材料的发现和设计提供一种独特而合理的方法。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

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.