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Structure, Response, and Flow of Dense Granular Materials

致密颗粒材料的结构、响应和流动

基本信息

批准号：
1809762
负责人：
Joshua Socolar
金额：
$ 50.78万
依托单位：
Duke University
依托单位国家：
美国
项目类别：
Continuing Grant
财政年份：
2018
资助国家：
美国
起止时间：
2018-07-01 至 2021-06-30
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1809762&HistoricalAwards=false
关键词：
Structure Response Flow Dense Granular

项目摘要

Non-Technical Abstract:This project seeks to understand grain-scale structures that form and evolve in granular materials as they are pushed or pulled by applied strains such as shear or compression. Granular materials are everywhere. They are used in a multitude of applications from handling ores, coal and wheat to pellets used for forming many manufactured plastic products. However, the basic understanding of the way that granular materials respond when they are strained or when they flow is very limited. This lack of knowledge causes significant problems for industry that must handle massive amounts of granular materials. There are two reasons that make it difficult to understand granular materials that are in dense fluid-like and solid-like states. The first is the difficulty of looking inside these materials with tools that can probe key quantities, such as the forces that act between grains. The second is that particles carry forces in very unequal ways. Only a small subset of grains, the so-called force chains, carry the majority of the forces that arise from applied strains or stresses, and the rest are "spectators", bearing almost no force. Understanding the formation and evolution of the force chains is the key challenge for dense granular materials. This project addresses this challenge through experiments using special photoelastic particles that allow precise experimental determination of forces between particles, and hence throughout a granular material. Photoelastic materials, including many plastics, transmit polarized light differently when they are subject to forces. By making grains from photoelastic materials, and placing everything between crossed polarizers, the force chain particles become immediately obvious. With this technique, it is possible to track how the force chains form and change, particularly when they are sheared. Shear occurs when a layer of grains is pushed one way on its top, and in the opposite way on the bottom. Shear is one of the most important ways to change the state of a granular material, and creates long force chains. When shear is continuous in time, the force chains form and break, leading to average forces on the opposite sides of the sheared material. Recent work has shown that the average forces for steady shear depend in a very predictable way on how fast the shearing is carried out. The present project involves experiments to understand how the evolution of the force chains, which change rapidly, lead to the average forces reported earlier. The answers to how force chains form, and how they respond to shear have the potential to answer fundamental questions about the behavior of granular materials. The experimental work involves students at all levels, from high school through Ph.D. candidates. Particular care will be given to involve women and minorities in all aspects. The project will involve outreach to industry, in terms of applications, and to elementary and secondary school students through science demonstrations and visits. The PI is also involved in a number of professional activities that benefit the field, including being editor in chief of the journal Granular Matter, and organizing parts of national and international meetings. All aspects of STEM, science, technology, engineering, and mathematics are present in this project. Technical Abstract:This project addresses two key intertwined questions for dense granular materials through a series of novel experiments. The experiments involve quasi-two dimensional photoelastic particles, enabling powerful methods to measure forces and other quantities at the particle scale. The key questions are: What are the microscopic processes that lead to the evolution of granular force networks? and What are the physical origins of stresses for steadily sheared granular materials, sometimes referred to as rheology? Key question 1 concerns self-organization of grain scale structures in response to applied protocols, for example stresses or strains. Granular materials form complex networks, force chains, whose evolution, starting at the microscale, is basic to the origin of jammed granular states and granular dynamics in flowing systems. Collective granular response is both surprising and poorly understood at microscopic scale. How can a stress-free state become jammed under shear, with no change in area? What microscopic processes enable memory effects under cyclic strains, or the Janssen effect? What determines the lowest packing fraction for a random loose packing? Force networks self-organize to create the observed responses, but little if anything is known about the processes that generate the networks. Key question 2 is: What are the physical origins of rheology? More precisely, what relates the effective friction, which is the ratio of shear stress and pressure, to a dimensionless shear rate, the inertial number, involving the pressure, density, and particle size? Empirical "local" models describe relatively simple flows, but are inadequate for more complex cases. Recent non-local models extend the local ones. But, in the past, there are few experiments that probe inside materials, determining local properties, to definitively test these models. Experiments in this project use high speed imaging of photoelastic particles in several shear-dominated flows to test these models to determine all properties of the shear flow, including the full stress tensor. For both questions 1 and 2, the experiments use a range of different quasi-2D photoelastic particles having different friction, shape and stiffness. This project involves: 1) determining local granular structures that self-organize in response to a range of protocols, and 2) relating the evolution of these structures to large scale response. The interconnection of macroscopic and specific collective microscopic processes in dry granular materials, specifically to understand how force networks form and evolve, has not been achieved previously to the principal investigator's knowledge. This understanding has the potential to explain many complex processes that are usually treated at a macroscopic phenomenological level. For shear flows, this work provides experimental tests and comparisons to recently proposed models of rheology. A key goal is understanding multi-scale dynamical processes that underlie these models. This project involves continued efforts to transfer knowledge derived from DMR- supported research to industrial applications through connections to the International Fine Particle Research Institute (IFPRI). In particular, flows in hoppers are an industrially relevant flow involving rheology. The principal investigator and other project participants regularly involve undergraduates and high school students in the lab. The project involves outreach to students in elementary and secondary schools through science activities, and demonstrations. The project involves lab members at all stages of intellectual development, with special attention to the recruitment of under-represented groups, in particular, women and minorities. The PI is involved in a number of activities including editorship of Granular Matter and conference organization that support research and foster collaboration in the granular and complex fluids communities.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.

摘要：该项目旨在了解颗粒材料在受到诸如剪切或压缩等施加应变的推动或拉动时形成和演变的颗粒级结构。颗粒状材料无处不在。它们被用于多种应用，从处理矿石、煤炭和小麦到用于形成许多塑料制品的颗粒。然而，对颗粒材料在拉伸或流动时的反应方式的基本理解非常有限。这种知识的缺乏给必须处理大量颗粒材料的工业造成了重大问题。有两个原因使我们难以理解处于致密流体状和固体状状态的颗粒状物质。首先是很难用工具来探测这些材料内部的关键数量，比如颗粒之间的作用力。第二，粒子以非常不均匀的方式携带力。只有一小部分颗粒，即所谓的力链，承受了由施加的应变或应力产生的大部分力，其余的颗粒是“旁观者”，几乎不承受力。了解力链的形成和演化是致密颗粒材料的关键挑战。该项目通过使用特殊的光弹性粒子进行实验来解决这一挑战，这种实验可以精确地确定粒子之间的力，从而确定整个颗粒材料。光弹性材料，包括许多塑料，当它们受到外力时，会以不同的方式传输偏振光。通过用光弹性材料制造颗粒，并将所有东西放在交叉偏振片之间，力链粒子立即变得明显。利用这种技术，可以跟踪力链的形成和变化，特别是当它们被剪切时。当一层颗粒在顶部被朝一个方向推，在底部被朝相反的方向推时，就会发生剪切。剪切是改变颗粒状材料状态的最重要的方法之一，并产生长力链。当剪切在时间上连续时，力链形成并断裂，导致剪切材料两侧受力平均。最近的研究表明，稳定剪切的平均力以一种非常可预测的方式取决于剪切进行的速度。目前的项目涉及实验，以了解迅速变化的力链的演变如何导致先前报告的平均力。力链是如何形成的，以及它们如何对剪切做出反应，这些问题的答案有可能回答有关颗粒材料行为的基本问题。实验工作涉及各个层次的学生，从高中生到博士生。将特别注意使妇女和少数民族参与所有方面。该项目将涉及在应用方面与工业界的联系，以及通过科学示范和参观向中小学生提供服务。PI还参与了一些对该领域有益的专业活动，包括担任《颗粒物质》杂志的主编，以及组织部分国家和国际会议。STEM、科学、技术、工程和数学的各个方面都出现在这个项目中。技术摘要：本项目通过一系列新颖的实验解决了致密颗粒材料的两个相互交织的关键问题。这些实验涉及到准二维光弹性粒子，这使得在粒子尺度上测量力和其他量成为可能。关键问题是：导致颗粒力网络演化的微观过程是什么？稳定剪切颗粒材料（有时称为流变学）应力的物理来源是什么？关键问题1涉及响应应用协议的晶粒尺度结构的自组织，例如应力或应变。颗粒材料形成复杂的网络，力链，其演化，从微观尺度开始，是堵塞颗粒状态和流动系统中颗粒动力学起源的基础。集体颗粒反应在微观尺度上既令人惊讶又难以理解。无应力状态如何在剪切作用下被卡住，而面积没有变化？在循环应变或杨森效应下，哪些微观过程使记忆效应成为可能？什么决定了随机松散包装的最低包装分数？强迫网络自组织以产生观察到的响应，但对于产生网络的过程知之甚少。关键问题2是：流变学的物理起源是什么？更准确地说，是什么将有效摩擦（剪应力与压力之比）与无量纲剪切速率（惯性数，包括压力、密度和粒径）联系起来？经验“局部”模型描述了相对简单的流程，但不适用于更复杂的情况。最近的非本地模型扩展了本地模型。但是，在过去，很少有实验探测材料内部，确定局部特性，以明确地测试这些模型。本项目的实验使用几个剪切主导流中的光弹性粒子的高速成像来测试这些模型，以确定剪切流的所有特性，包括全应力张量。对于问题1和问题2，实验使用了一系列具有不同摩擦、形状和刚度的准二维光弹性粒子。该项目涉及：1)确定响应一系列协议自组织的局部颗粒结构，以及2)将这些结构的演变与大规模响应联系起来。据首席研究员所知，干燥颗粒材料中宏观和特定集体微观过程的相互联系，特别是为了了解力网络如何形成和演变，以前还没有实现。这种理解有可能解释通常在宏观现象学水平上处理的许多复杂过程。对于剪切流，这项工作提供了实验测试和最近提出的流变模型的比较。一个关键的目标是理解这些模型背后的多尺度动态过程。该项目涉及通过与国际细颗粒研究所（IFPRI）的联系，继续努力将DMR支持的研究所得的知识转移到工业应用中。特别是，料斗中的流动是涉及流变学的工业相关流动。首席研究员和其他项目参与者经常让本科生和高中生进入实验室。该项目包括通过科学活动和示范向中小学生进行宣传。该项目涉及智力发展各个阶段的实验室成员，特别注意招募代表性不足的群体，特别是妇女和少数民族。PI参与了许多活动，包括颗粒物质的编辑和会议组织，支持颗粒和复杂流体社区的研究和促进合作。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。