Granular Fluctuation and Dissipation

这项实验研究是针对非粘性颗粒材料对外力的响应。强烈的强迫可以产生类似流体力学的平稳流动，但伴随着微小的波动和限制流动速度的非弹性颗粒-颗粒碰撞。微弱的强迫可能导致间歇性的类似雪崩的流动，或者根本没有流动。类似的行为也出现在其他物质中，如泡沫、稠密胶体或过冷液体；以堵塞的名义，详细的现象学和类比的范围处于凝聚态物理学的前沿。这里将用高速摄像机和一种新的准弹性激光光散射方法来测量颗粒尺度的波动，我们称之为“散斑可见度光谱学”。其一是探索微观动力学在多大程度上定量地类似于热平衡系统中的波动。另一个重点是将波动与消散联系起来，特别是在撞击陨石坑的背景下。参与这项研究的研究生和本科生将接受尖端实验技术方面的培训；这将为他们在学术界、工业或政府的一系列职业生涯做好准备。%本研究旨在解决沙子等颗粒系统如何应对外力的基本难题。现代社会依赖于处理各种颗粒状材料，从食品到建筑用品，再到药品等等。然而，令人惊讶的是，实际设备很容易突然出现故障，比如堵塞、堵塞或分离。归根结底，这是因为我们仍然不知道施加在大系统上的力是如何导致相邻颗粒之间的相对运动的，这就是能量是如何消散的。在这里，拟议的工作将通过将高速视频与一种新发明的激光散射方法相结合而取得新的突破。这将允许在足够短的尺度上研究颗粒运动和能量耗散，以捕捉关键物理，这是以前的手段所不可能做到的。这个项目的学生接受涉及现代电子和光学物理的实验方法的严格培训，并可以在学术、工业科学或工程领域寻求职业生涯。*

This experimental research is directed at the response of noncohesive granular materials to applied forces. Strong forcing can create smooth hydrodynamic-like flow, but with microscopic fluctuations and inelastic grain-grain collisions that limit the speed of flow. Weak forcing can lead to intermittent avalanche-like flow or no flow at all. Similar behavior occurs in other materials, like foams or dense colloids or supercooled liquids; the detailed phenomenology and extent of analogy are at the forefront of condensed matter physics, under the name of jamming. Here the grain-scale fluctuations will be measured by high-speed video and by a new quasielastic laser light scattering method we call "speckle-visibility spectroscopy". One thust is to explore the extent to which the microscopic dynamics are quantitatively analogous to fluctuations in a thermal equilibrium system. Another thrust is to relate the fluctuations to dissipation, particularly in the context of impact cratering. The graduate and undergraduate students involved in this research will be trained in cutting-edge experimental technology; this will prepare them for a range of careers in academia, industry or government.%%%This research is aimed at fundamental puzzles in how granular systems, like sand, respond to applied forces. Modern society relies on handling all sorts of granular materials, ranging from foods to building supplies to pharmaceuticals and more. Yet actual equipment is surprisingly prone to sudden failure, like clogging or jamming or demixing. Ultimately, this is because we still don't know how applied forces on large systems cause relative motion between neighboring grains, which is how energy is dissipated. Here the proposed work will break new ground by combining high-speed video with a newly invented laser light scattering method. This will allow study of grain motion and energy dissipation at short enough scales to capture the crucial physics, which was not possible with prior means. Students in this program receive rigorous training in experimental methods involving modern electronic and optical physics, and can pursue careers in academic or industrial science or engineering.***