Uncovering the microscopic origins of nonlinear rheology in glassy nanocolloidal suspensions

软固体，如凝胶、糊状物和乳状物，当受到足够的压力时，会改变其形状和流动。了解和控制这一行为对于这些材料在从陶瓷和药品到食品和个人护理产品的应用中的加工至关重要。在微观层面上，这种变形和流动涉及成分的重新排列，这些成分往往难以识别和表征。此外，一旦流动停止，保持软固体新形状所需的力可能会随着时间的推移呈现出漫长而缓慢的演变，这取决于先前流动的细节。这一现象为调整软固体的性质提供了一种潜在的机制；然而，人们对支撑这种缓慢演变的材料内部的微观运动知之甚少。该奖项将支持一项新颖的实验研究，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特性联系起来。一个核心特征是一种使用X射线来提供前所未有的分辨率的材料内部运动的技术，其长度小到一个分子。这些用于研究变形和流动的软固体的X射线方法的发展将产生远远超出计划的实验的影响。这一新工具具有跨越物理、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动美国研发所需的人才库做出贡献。此外，该奖项将通过与当地一所磁铁科学高中的合作，为巴尔的摩市公立学校的学生提供在调查人员的实验室进行研究实习的机会。软质无序固体可以具有特征的力学特性，如屈服应力的出现、流动时的剪切变薄和触变性。这种变形和流动行为是这些材料在一系列技术中应用的核心，并提供了与堵塞和软玻璃流变学等理论概念的接触点。非线性流变学的根源是剪切引起的软固体内部微观结构和微观结构动力学的变化。了解这些微观结构动力学和宏观流变性之间的联系仍然是力学和胶体科学领域的中心目标。这一努力具有挑战性，因为强烈的空间和时间波动可以表征对应力的微观反应，以及许多软无序固体的非平衡性质，这使得它们的性质不仅取决于热力学条件，还取决于它们的历史。这项拟议的研究将利用X射线光子相关光谱(XPCS)的独特优势来阐明软材料中显著的纳米级结构动力学，为这一问题带来有洞察力的前景。具体地说，这项研究将利用和扩展新开发的实验能力，以新颖的方式将XPCS与流变学相结合，以揭示宏观变形和流动起源的颗粒尺度和介观动力学。通过三个专注于形成软玻璃和浓缩凝胶相的纳米胶体悬浮液的具体和相互关联的项目，这些实验将阐明导致应力松弛、屈服和老化的微观过程，从而能够测试关于软无序固体的机械特性的主流理论观点。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.