Jamming transitions and kinetic phenomena

*技术摘要*主要目标是通过测量干扰锋面和前线上的颗粒尺度动力学，加深对颗粒系统中干扰动力学的理解。最简单的情况是沉积过程中干扰锋的稳定向上传播，其中颗粒浓度分布将通过医用x射线成像测量，颗粒温度分布(Tg)将通过散斑可见度光谱(SVS)测量。主要的兴趣是偏离阶跃函数分布，Tg的消失，以及类似于动力学非均质性但现在存在梯度和时间演化的动力学的时空相关波动。另一种情况是高漏斗排出的各种谷物的堵塞行为。宏观上，通过平均流量随孔径增大的发散度来研究堵塞转变。微观上，Tg在流体动力时均流场附近的空间相关涨落将被研究；Tg的不稳定性在接近堵塞转变时被假定为增加。这将通过粒子跟踪和通过侧壁的SVS进行研究。计划对一系列钉扎的颗粒内的2D空气流态化颗粒的动力学进行单独的推力。这些项目将把现代物理和测量技术带入研究生和本科生的教育经历。所创造的知识将有助于理解和控制工业和自然环境中的堵塞现象，在工业和自然环境中，堵塞和非堵塞区域之间往往有一个移动的边界。*非技术摘要*除了流体之外，几乎任何东西的流动都有一种令人恼火的倾向，即在施加外力的情况下“堵塞”并停止流动。熟悉的例子包括高速公路上的交通，溜槽里的煤，沙漏里的沙子。这些问题困扰着食品、种子、矿物、矿石、建材和药丸粉末等各种颗粒状材料的加工。在自然界中，泥石流和地震等可怕事件背后隐藏着拥堵现象。在这些情况下，行为都不能得到真正的控制。但由于最近的理论和实验科学，现在对均匀系统的干扰转变有了合理的理解，在均匀系统中，所有的颗粒都受到相同的条件，要么都被干扰，要么都不被干扰。该奖项将支持开创性的研究，即在有界面或“干扰前沿”的情况下，干扰是如何发展的，该界面分隔了干扰和非干扰区域，并且随着时间的推移而变化。想象一下，当沙漏的出口形成堵塞时，或者当颗粒从液体中沉淀出来时会发生什么。尽管材料可能看起来很普通，但科学和实验方法都很复杂。这将是研究生和本科生教育经验的理想选择。将要创造的知识将引起物理学家、工程师和地质学家的兴趣，并将帮助工业避免因堵塞而导致的效率低下和工艺故障。它最终甚至可能通过控制泥石流和侵蚀来帮助拯救生命和财产。

****Technical Abstract****The main goal is to develop an understanding of the kinetics of jamming in granular systems, through measurement of jamming fronts and of grain-scale dynamics across the fronts. The simplest situation is the stationary upward propagation of a jamming front during sedimentary deposition, where the grain concentration profile will be measured by medical x-ray imaging, and the granular temperature profile (Tg) will be measured by Speckle-Visibility Spectroscopy (SVS). Key interest is in the deviations from step-function profiles, the vanishing of Tg, and spatiotemporally correlated fluctuations of dynamics akin to dynamical heterogeneities but now in presence of gradients and time evolution. Another situation is the clogging behavior for various grain types discharged from tall hoppers. Macroscopically, the clogging transition will be studied by the divergence of average discharge mass as hole size increases. Microscopically, it will be studied by spatially-correlated fluctuations of Tg around the hydrodynamic time-average flow field; unsteadiness of Tg is hypothesized to increase on approach to the clogging transition. This will be studied by particle tracking and by SVS through the sidewall. A separate thrust is planned for the dynamics of 2d air-fluidized grains within an array of pinned grains. These projects will bring modern physics and measurement techniques into the educational experience of graduate and undergraduate students. And the knowledge to be created will be helpful for understanding and controlling jamming phenomena in industrial and natural settings, where there is often a moving boundary between jammed and unjammed regions.****Non-Technical Abstract****The flow of almost anything other than a fluid has an annoying tendency to "jam" and come to rest in spite of applied forces. Familiar examples include traffic on a highway, coal in a chute, and sand in an hourglass. Such problems confound the processing of diverse granular materials like foods, seeds, minerals, ores, building materials, and pharmaceutical pills & powders. In the natural world, jamming phenomena underlie horrific events such as mudslides and earthquakes. In none of these cases can the behavior yet be truly controlled. But thanks to recent theoretical and experimental science, there is now a reasonable understanding of the jamming transition for uniform systems, where all the grains are subjected to the same conditions and are all either jammed or unjammed. This award will support groundbreaking research on how jams develop in situations where there is an interface or "jamming front" that separates jammed and unjammed regions and that changes with time. Think of what happens when a clog forms over the outlet of an hourglass, or when grains settle out of from a fluid. Though the materials may seem mundane, the science and the experimental methods are sophisticated. This will be ideal for the educational experience of graduate and undergraduate students. The knowledge to be created will be of interest to physicists, engineers, and geologists, and will help industry avoid inefficiencies and process failures due to jamming. It may ultimately even help save lives and property through control of mudslides and erosion.