Force Maps, Aging, and Elasticity in Random, Non-Equilibrium Solids

技术摘要：光学显微镜将用于三维测量无序固体内部的结构、拓扑、力和弹性常数。这些材料的巨大科学挑战是它们都是随机的，远离热平衡，因此不容易用热力学来描述。该项目的基本方法是测量二维或三维粒子的位置，以及它们的热波动或接触力对它们形状的扭曲。使用的颗粒范围从微米大小的中性浮力胶体球（其运动是热的）到20微米大小的重液滴（其形成零有效温度的静态堆）。在小颗粒之间诱导的吸引相互作用导致凝胶化，这一过程也将反过来研究。这些数据允许对力、弹性常数和材料结构进行异常明确的比较，这对开发和测试理论模型至关重要。高中生、本科生和研究生参与研究。该项目还为软物质物理课程的开发提供了信息，该课程将与其他机构的教师共享。摘要：无序固体在日常生活中非常常见，例如窗户玻璃、聚合物凝胶、颗粒凝胶和砂堆。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沙粒——但越来越多的证据表明，它们是一种更普遍的现象——“堵塞”的例子。该项目的目标是使用先进的光学显微镜技术来测量这些材料的结构、拓扑、力和弹性常数。一组测量集中在浓缩乳液中液滴之间的力，这可以作为没有静摩擦的沙堆的有用模型。一组平行的测量集中在随机粒子聚集体（凝胶）的形成和融化上。结果将有助于深入和广泛适用的理解弹性和老化在无序固体。该项目的一个关键组成部分是高中生、本科生和研究生的参与，他们将接受软物质物理方面的培训，适合在工业、国家实验室或学术界工作。

Technical Abstract:Optical microscopy will be used to measure structure, topology, forces and elastic constants inside disordered solids in three dimensions. The great scientific challenge of these materials is that they are both random and far from thermal equilibrium and thus not readily described by thermodynamics. The basic approach of this project is to measure the positions of particles in two or three dimensions, along with their thermal fluctuations or the distortions of their shape by contact forces. The particles used will range from micron-size neutrally buoyant colloidal spheres -- whose motions are thermal -- to 20-micrometers heavy droplets, which form static piles with zero effective temperature. Attractive interactions induced among the small particles lead to gelation and this process will also be studied in reverse. The data allow unusually explicit comparisons of forces, elastic constants, and material structure, which are essential to develop and test theoretical models. High-school, undergraduate, and graduate students participate in the research. The project also informs development of a course in soft matter physics, which will be shared with instructors at other institutions.Non-technical Abstract: Disordered solids are very common in everyday life, with examples including window glass, polymer gels, particle gels, and sand piles. Although the microscopic details of these examples differ tremendously -- molecules in one case and millimeter-sized sand grains in another -- there is increasing evidence that they are examples of a more general phenomenon known as "jamming". The goal of this project is to use advanced techniques of optical microscopy to measure structure, topology, forces, and elastic constants throughout these materials. One set of measurements focuses on the forces among droplets inside a concentrated emulsion, which serves as a useful model of a sandpile without static friction. A parallel set of measurements focuses on the formation and melting of random particle aggregates (gels). The results will contribute to a deep and broadly applicable understanding of elasticity and aging in disordered solids. A key component of this project is the participation of high-school, undergraduate, and graduate students, who will receive training in soft matter physics suitable for careers in industry, national labs, or academia.