Uncovering the microscopic origins of nonlinear rheology in glassy nanocolloidal suspensions

当受到足够的力时，软固体如凝胶、糊状物和乳液将改变它们的形状和流动。 了解和控制这种行为是这些材料在从陶瓷和药品到食品和个人护理产品等应用中的加工的核心。 在微观层面上，这种变形和流动涉及到通常难以识别和表征的成分的重新排列。 此外，一旦流动停止，维持软固体新形状所需的力可以随着时间显示出长期缓慢的演变，这取决于先前流动的细节。 这种现象提供了一个潜在的机制来定制软固体的性质;然而，在这种缓慢的演变背后的材料内的微观运动知之甚少。 该奖项将支持一项新的实验研究，以确定与软固体中流动的开始和停止相关的关键微观运动，并将这些运动与固体的宏观性质联系起来。 一个核心特征是一种技术，它使用x射线提供前所未有的分辨率，可以在分子大小的材料中运动。 这些研究变形和流动下软固体的X射线方法的发展将远远超出计划的实验。 这种新工具具有跨越物理学、化学工程和生物科学的潜在应用。 将接受先进X射线方法培训的研究生和本科生将进一步为推进美国研发所需的人才库做出贡献。 此外，该奖项将提供巴尔的摩市公立学校的学生有机会在研究者的实验室，通过与当地的磁铁科学高中的合作伙伴关系的研究实习。软无序固体可以拥有特有的机械性能，如屈服应力的出现，剪切变稀后流动，和触变性。 这种变形和流动行为对于这些材料在一系列技术中的实用性至关重要，并提供了与理论概念（如堵塞和软玻璃流变学）的接触点。 在非线性流变学的根源是剪切引起的内部微观结构和微观结构动力学的软固体的变化。 理解这些微观结构动力学和宏观流变学性质之间的联系仍然是力学和胶体科学领域的中心目标。 这一努力是具有挑战性的强烈的空间和时间波动，可以表征微观响应应力和许多软无序固体的不平衡性质，这使得它们的性质不仅取决于热力学条件，而且还取决于它们的历史。 拟议的研究将利用x射线光子相关光谱（XPCS）的独特优势，用于阐明软材料中突出的纳米尺度结构动力学，为这个问题带来深刻的见解。 具体而言，该研究将利用和扩展新开发的实验能力，将联合收割机XPCS与流变仪以新的方式结合起来，以揭示宏观变形和流动起源处的颗粒尺度和中尺度动力学。 通过三个具体的和相互关联的项目集中在纳米胶体悬浮液，形成软玻璃和集中凝胶相，实验将阐明微观过程导致应力松弛，屈服，该奖项反映了NSF的法定使命，并被认为是值得通过使用基金会的学术价值和更广泛的影响审查标准。

Soft solids such as gels, pastes, and emulsions will change their shape and flow when subjected to sufficient forcing. Understanding and controlling this behavior is central to the processing of these materials in applications ranging from ceramics and pharmaceuticals to food and personal care products. At the microscopic level, this deformation and flow involves rearrangements of the constituents that are often difficult to identify and characterize. Furthermore, once flow stops, the force needed to maintain a soft solid's new shape can display a long, slow evolution with time that depends on details of the preceding flow. This phenomenon provides a potential mechanism for tailoring the properties of the soft solids; however, the microscopic motions within the material that underlie this slow evolution are poorly understood. This award will support a novel experimental investigation to identify the key microscopic motions associated with the onset and cessation of flow in soft solids and to connect these motions with the solids' macroscopic properties. A central feature is a technique that uses x-rays to provide unprecedented resolution of the motions within materials at lengths as small as a molecule. The development of these x-ray methods for investigating soft solids under deformation and flow will have impact far beyond the planned experiments. This new tool has potential applications that span physics, chemical engineering, and the biosciences. Graduate and undergraduate students who will be trained in advanced x-ray methods will further contribute to the talent pool needed to advance research and development in the United States. In addition, the award will provide Baltimore City public school students with opportunities for research internships in the Investigator's laboratory through partnership with a local magnet science high school.Soft disordered solids can possess characteristic mechanical properties, such as the emergence of a yield stress, shear thinning upon flow, and thixotropy. This deformation and flow behavior is central to the utility of these materials in a range of technologies and provides a point of contact with theoretical concepts such as jamming and soft glass rheology. At the root of the nonlinear rheology are shear-induced changes to the internal microstructure and microstructural dynamics of the soft solids. Understanding the connections between these microscopic structural dynamics and macroscopic rheological properties remains a central goal for the fields of mechanics and colloid science. This effort is made challenging by the strong spatial and temporal fluctuations that can characterize the microscopic response to stress and by the out-of-equilibrium nature of many soft disordered solids, which causes their properties to depend not only on thermodynamic conditions but also on their history. The proposed research will take advantage of the unique strengths of x-ray photon correlation spectroscopy (XPCS) for illuminating the salient nanometer-scale structural dynamics in soft materials to bring insightful perspectives to this problem. Specifically, the research will leverage and expand newly developed experimental capabilities that combine XPCS with rheometry in novel ways to reveal the particle-scale and mesoscale dynamics at the origins of macroscopic deformation and flow. Through three specific and inter-related projects focused on nanocolloidal suspensions that form soft-glass and concentrated-gel phases, the experiments will elucidate the microscopic processes leading to stress relaxation, yielding, and aging in ways that enable tests of prevailing theoretical ideas regarding the mechanical properties of soft disordered solids.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.