Mapping Forces and Elasticity in Random Solids

这个软凝聚态物理项目涉及无序固体，包括玻璃状材料、聚合物凝胶、颗粒凝胶和砂堆。尽管微观细节差别很大，但越来越多的证据表明，这些材料是一种更普遍的被称为“堵塞固体”的材料。从一个基本的观点来看，这些材料的巨大挑战是它们远离热平衡，连续弹性可能不适用于微观或介观长度尺度。该项目将使用光学方法获得力的直接和定量成像，它们的空间相关性，以及三维样品中粒子间连接的拓扑结构。结果将与最近在表面或二维材料上的理论、模拟和测量结果进行比较。这个重点是对单分散液滴荧光表面的共聚焦光学显微镜，一个模型系统，允许在三维样品内部的力成像。通过改变所使用的液滴的大小，所研究的系统将在那些由有吸引力的粒子间力固化并受到强烈热波动的系统，以及那些在有效的零温度下由重力应力固化的系统之间变化。该项目还研究了这些力图是如何被外部应力或材料内部施加的点力改变的。该项目为学生在应用或基础项目的研究提供了深入的培训。无序固体在日常生活中非常常见，例如窗户玻璃、酸奶、煤烟和沙堆。尽管这些例子的微观细节差别很大——一种是分子，另一种是毫米大小的粗糙沙粒——但有令人信服的证据表明，在一个理论框架内，它们可能是可以理解的。从一个基本的观点来看，这些材料的巨大挑战是它们永远不会接近热平衡，也不会形成有序的（晶体）结构。此外，连续介质弹性理论在相当于基本粒子数倍大小的长度尺度上可能是不正确的。这项工作将提供这些材料内部的力、结构和连通性的第一张实验图。主要的实验挑战是如何观察沙堆内部并测量相邻颗粒之间的力。在这里，这是用液滴（而不是沙粒）来完成的，它的变形是用光学显微镜在三维中量化的。除了研究一些新问题外，这些测量还将首次对理论预测和计算机模拟进行测试，并将与之前在二维材料或三维材料表面进行的实验联系起来。该项目还为各级学生提供软凝聚态物理和适合学术或工业研究职位的先进研究技术方面的培训。

This soft condensed matter physics project deals with disordered solids, which include glassy materials, polymer gels, particle gels, and sand piles. Although the microscopic details differ tremendously, there is increasing evidence that these materials are examples of a more general class known as jammed solids. From a fundamental point of view, the great challenge of these materials is that they are far from thermal equilibrium and continuum elasticity might not apply at microscopic or mesoscopic length scales. This project will use optical methods to obtain direct and quantitative imaging of forces, their spatial correlations, and the topology of inter-particle connections in three dimensional samples. Results are to be compared to recent theories, simulations and measurements on surfaces or on two-dimensional materials. This focus is on confocal optical microscopy of monodisperse liquid droplets with fluorescent surfaces, a model system that allows imaging of forces in the interior of three-dimensional samples. By varying the sizes of the droplets used, the systems studied will be varied between those solidified by attractive inter-particle forces and subject to strong thermal fluctuations, to those solidified by gravitational stresses at, effectively, zero temperature. The project also investigates how these force maps are changed by external stresses or by point forces applied inside the material. The project provides in-depth training of students for research in applied or fundamental programs.Disordered solids are very common in everyday life, with examples including window glass, yogurt, soot, and sand piles. Although the microscopic details of these examples differ tremendously - molecules in one case and millimeter-sized rough sand grains in another - there is compelling evidence that they may be understandable within a single theoretical framework. From a fundamental point of view, the great challenge of these materials is that they never approach thermal equilibrium and do not form ordered (crystalline) structures. Moreover, continuum elasticity theory might be incorrect at length scales comparable to several times the size of the basic particle. This work will provide the first experimental maps of forces, structure, and connectivity inside these materials. The major experimental challenge is, somehow, to peer inside a sand pile and measure forces between adjacent particles. Here this is accomplished using liquid droplets (instead of sand grains) whose deformation is quantified in three dimensions using optical microscopy. In addition to allowing investigation of a number of new questions, the measurements will provide the first tests of theoretical predictions and computer simulations, and will connect with earlier experiments on two-dimensional materials or on the surface of three-dimensional materials. The project also provides training for students at all levels in soft condensed matter physics and advanced research techniques suitable for academic or industrial research positions.