Collaborative Research: RUI: Jammed granular matter within networks of pins: Structure, elasticity, plasticity and rheology under shear

非技术摘要：颗粒材料在日常生活中随处可见。所谓“颗粒状”，不仅指盐、大米或沙子等硬物，也指气泡等软物，以及细胞和行人等活体。致密的颗粒状物质可以通过堵塞、崩塌或突然凝固成无序的“堵塞”结构来表现出戏剧性的行为。这个研究项目提出了这样一个问题：当颗粒与固定的骨架接触时，这些现象是如何表现出来的？在这项研究中，框架是一个由微小障碍组成的格子，或称“大头针”。(在两个维度上，人们可能会想到弹珠游戏中的大头针。)销钉既影响颗粒填料发生突然转变的时间，也影响所形成的堵塞固体的结构和动力学。除了理论上的兴趣，这个项目还有一些应用，比如利用障碍物来防止粒子或人的堵塞；以及制造更具弹性和更小体积的新堵塞材料。通过实验和计算机模拟研究了销钉对材料结构和动力学的影响。主要研究人员是四名教职员工，在两所只提供物理学本科学位的机构中，每一所都有一名实验物理学家和一名计算物理学家。他们的学生直接与教师合作，在一个理论丰富且高度实用的领域体验计算和/或实验研究。技术摘要：颗粒状材料的包装，例如在工业供应链或生物体中，表现出结构或流动行为的显著变化，如突然重排、坍塌和突然堵塞通道。这样的变化是由于几何上的挫折，因为颗粒经历了堵塞和堵塞的过渡。变形和流动在很大程度上是由跨系统的力网络决定的，它在外力和扭矩、约束和内部颗粒相互作用之间达成了妥协。虽然已经有许多关于致密颗粒物质的研究，但关于颗粒系统的流变学的研究很少，其中包括一种令人联想到限制的效应：系统内部局部钉扎位置形式的冻结自由度。这个项目解决了关于应力和流场的空间相关性的关键的、开放的问题，是对这样的销子的存在如何影响结构和流动的系统研究，无论是以晶格的形式还是以无序阵列的形式(因此，淬火的有序或无序)。主要研究人员的活动包括：确定这种新的有序/无序转变的相图；计算弹性模数、局部应力场和表征钉扎几何形状影响的力网络的参数；使用粒子尺度跟踪来描述剪切下微观重排导致的堆积结构和运动学；以及将这些见解扩展到活性颗粒物质的第一步。这项工作既需要数值模拟，也需要通过两个主要是本科生机构的四名研究人员的合作进行实验。实验涉及平面、简单和Couette剪切，使用光弹性颗粒的二维组件。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。

Nontechnical Abstract:Granular Materials are ubiquitous in daily life. By “granular” one means not only hard objects like salt, rice, or sand; but also soft objects like bubbles, and living entities like cells and pedestrians. Dense granular materials can behave dramatically by clogging, avalanching, or suddenly solidifying into a disordered “jammed” structure. This research project asks the question: How do these phenomena manifest themselves when the grains are in contact with a fixed framework? In this research, the framework is a lattice of diminutive obstacles, or “pins”. (In two dimensions, one might think of the pins in a Pachinko game.) Pins influence both when a sudden transition occurs in a granular packing, and the structure and dynamics of the jammed solid which forms. Beyond its theoretical interest, this project has applications like utilizing obstacles for the prevention of jamming of particles or people; and making new jammed materials which are more elastic and less bulky. The influence of pins on the material’s structure and dynamics is studied both with experiments as well as computer simulations. The principal investigators are four faculty members, with an experimental and a computational physicist at each one of two institutions which offer only undergraduate degrees in physics. Their students work directly with faculty to experience computational and/or experimental research in a field that is both theory-rich and highly practical. Technical Abstract:Packings of granular materials, as for example in industrial supply lines or in living organisms, exhibit striking changes in structural or flowing behavior such as abrupt rearrangements, collapses and sudden blockading of channels. Such changes are due to geometrical frustration, as grains experience clogging and jamming transitions. Deformation and flow are largely determined by system-spanning force networks, which broker a compromise between external forces and torques, confinement, and internal granular interactions. While there have been many studies of dense granular matter, there have been few on the rheology of granular systems which include an effect reminiscent of confinement: frozen degrees of freedom in the form of localized pinning sites internal to the system. This project, which addresses critical, open questions on spatial correlations of stress and flow fields, is a systematic study of how the presence of such pins, either in the form of a lattice or a disordered array (hence, quenched order or disorder) influences structure and flow. Activities of the principal investigators include determining the phase diagram for this novel order/disorder transition; calculating elastic moduli, local stress fields and parameters characterizing the force network as influenced by pinning geometry; use of particle-scale tracking to describe packing structure and kinematics due to microscopic rearrangements under shear; and first steps toward extending these insights to active granular matter. The work entails both numerical simulation and experiments through a collaboration between four investigators at two primarily undergraduate institutions. Experiments involve planar, simple, and Couette shear using two-dimensional assemblies of photoelastic grains.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.