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。这些领域包括石化、光伏、制药、食品和3D打印应用。这些应用都需要一种能够在特定条件下保持所需形状但可以按需流动的材料。理想情况下，人们应该根据相关工艺的具体要求来设计这种材料的特性。然而，没有准确的方法来确定这些材料产生屈服的确切条件。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事情联系起来。这将使新材料的设计以及更有效的工业工艺成为可能。该项目的综合教育和外联部分支持对学龄儿童的更广泛的外联，以及研究生和本科生的研究培训和指导。外展工作包括一些项目，旨在通过针对代表性不足的群体的项目，向初出茅庐的科学家传授软物质研究的复杂性。通过这些项目以及与校园女生中学的合作，将向年轻学生介绍软物质和3D打印的概念。本科生课程将通过纳入通过这项工作获得的更高水平的材料而得到加强。这一职业奖项将开发理解和预测触变屈服应力流体中观察到的依赖时间的结构和流变学转变所需的工具和基础科学。将流变信息与小角X射线散射和X射线光子相关光谱相结合的混合实验为同时获取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。由于广泛的物理参数空间，已知的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构成了一个挑战。使材料远离平衡的加工流程进一步扩大了这一参数空间。我们将建立一种基于流变散射的方法来指导测量，并最终指导结构和流变动力学的设计。从Rheo-XPCS的两次关联中获得的瞬时分子级结构动力学将与PI开创的新的瞬时流变学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，以捕捉在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，梳理出关于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了用于非平衡加工的流变学分析工具，这些工具非常适合于确定触变材料的瞬时流变性。该计划将建立在屈服和触变之间已建立的等价性基础上，揭示在流动下屈服和重组的根本物理原因，通过结合流变学特性、微观结构探针和模型开发来加速它们在实际系统中的受控设计。这些方面中的每一个都将以互惠、协同的方式解决基本主题(屈服和重组的流变学、作为反屈服的触变性、新模型)，为材料发现和设计提供独特和合理的方法。该奖项反映了NSF的法定使命，并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，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.