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无处不在。“颗粒状”不仅指盐、大米或沙子等硬物;还指气泡等软物以及细胞和行人等生物体。致密的颗粒状材料可以通过堵塞、雪崩或突然凝固成无序的“堵塞”结构而表现出戏剧性的行为。 这个研究项目提出了一个问题：当颗粒与固定的框架接触时，这些现象是如何表现出来的？在这项研究中，框架是一个小的障碍，或“针”格。 (In两个维度，人们可能会想到弹球盘游戏中的大头针。当颗粒填料中发生突然转变时，销钉会影响颗粒填料的结构和动力学。除了理论上的兴趣之外，该项目还可以应用于利用障碍物来防止颗粒或人的堵塞;以及制造更具弹性和体积较小的新堵塞材料。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员，两个只提供物理学本科学位的机构各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，在理论丰富和高度实用的领域进行计算和/或实验研究。技术摘要：颗粒状材料的填料，例如在工业供应线或生物体中，在结构或流动行为方面表现出惊人的变化，例如突然重排，坍塌和突然堵塞通道。这种变化是由于几何挫折，因为颗粒经历堵塞和堵塞过渡。变形和流动在很大程度上取决于系统跨越力网络，它在外力和扭矩、约束和内部颗粒相互作用之间进行折衷。虽然已经有许多研究致密颗粒物质，有一些颗粒系统的流变学，其中包括一个效果，让人想起禁闭：冻结自由度的形式，本地化的钉扎网站内部的系统。该项目解决了应力场和流场空间相关性的关键性开放性问题，系统研究了这种钉的存在如何以晶格或无序阵列的形式（因此，淬火有序或无序）影响结构和流动。主要研究人员的活动包括确定这种新的有序/无序过渡的相图;计算弹性模量，局部应力场和表征力网络的参数，受钉扎几何形状的影响;使用粒子尺度跟踪来描述剪切下微观重排引起的包装结构和运动学;以及将这些见解扩展到活性颗粒物质的第一步。 这项工作需要通过两个主要本科院校的四名研究人员之间的合作进行数值模拟和实验。实验涉及平面，简单，和库埃特剪切使用二维组件的光弹性grains.This奖项反映了NSF的法定使命，并已被认为是值得通过评估使用基金会的智力价值和更广泛的影响审查标准的支持。

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.