Granular Fluctuation and Dissipation

本实验研究针对非粘性颗粒材料在外力作用下的响应。强大的作用力可以产生光滑的流体动力学样的流动，但微观波动和非弹性颗粒-颗粒碰撞会限制流动速度。弱作用力可导致间歇性雪崩状流动或根本没有流动。类似的行为也发生在其他材料中，如泡沫或致密胶体或过冷液体；详细的现象学和类比的程度是凝聚态物理的前沿，以干扰的名义。在这里，将通过高速视频和一种新的准弹性激光散射方法来测量粒度波动，我们称之为“斑点可见光谱”。一个重点是探索微观动力学在多大程度上在数量上类似于热平衡系统中的波动。另一个重点是将波动与耗散联系起来，特别是在撞击坑的情况下。参与本研究的研究生和本科生将接受前沿实验技术的培训；这将为他们在学术界、工业界或政府的一系列职业生涯做好准备。这项研究的目的是解决像沙子这样的颗粒系统如何对施加的力作出反应的基本难题。现代社会依赖于处理各种颗粒状材料，从食品到建筑材料到药品等等。然而，实际的设备出人意料地容易出现突然故障，比如堵塞、干扰或脱混。最终，这是因为我们仍然不知道施加在大系统上的力是如何引起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，提出的工作将通过将高速视频与新发明的激光散射方法相结合，开辟新的领域。这将允许在足够短的尺度上研究颗粒运动和能量耗散，以捕捉关键的物理现象，这是以前的手段不可能做到的。该专业的学生在现代电子和光学物理的实验方法方面接受严格的训练，可以从事学术或工业科学或工程方面的职业

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.***