CAREER: Nonlinear Waves and Fluctuations in Jammed Systems

非技术：作为一个社会，我们非常擅长把东西整理好：汽车在车库里按指定的顺序停放，橘子在当地杂货店里整齐地堆放，文件在办公桌上整齐地堆放。我们这样做，也许是无意识的，因为有序的安排往往节省最多的空间。同样，在所有的固体形式的物质中，物理学家可能最了解晶体材料；结构顺序使它们更容易概念化和数学定义。然而，许多固体材料可以是刚性的，但没有明确的秩序。这些材料在受到外界刺激时往往表现出特殊的行为。想想沙漏底部的一堆沙子。看起来足够稳定的东西可能会在增加一些额外的颗粒后突然崩溃。甚至是混乱的交通堵塞：是什么决定了流动状态和僵化状态之间的界限？我们的世界充满了类似的例子，即系统存在于接近边缘稳定的区域。这个项目旨在了解一个被卡住的无定形固体是如何振动和波动的。通过使用“模型”系统，如悬浮在气态等离子体中的胶体尘埃，首席研究员和研究小组可以可视化粒子的单个运动，这些运动构成了非晶系统中几乎毫不费力的结构重排。这些研究活动不仅揭示了非晶态固体（如玻璃）中常见的单个分子运动，而且还在广泛的物理系统中提供了动力学和无序之间的联系。在研究工作的同时，该项目还包括在Dekalb县的一所当地小学创建一个课后科学俱乐部，Dekalb县拥有佐治亚州第三大学校系统，也是最多样化的县。技术：本项目重点研究非晶固体中机械刚性的性质和起源，这是一个渗透到凝聚态物理和材料科学许多领域的主题。实验和模拟都用于研究低频振动模式的动力学以及在堵塞和玻璃系统中几何约束的作用，在这些系统中，温度或密度的微小变化会导致运动的动力学时间尺度的巨大增加。该项目涉及两个主要主题，第一个是无序固体中低频振动模式的起源。40年来，人们已经知道分子玻璃在低频处具有多余的振动模式。研究这种现象的理想模型系统是悬浮在等离子体中的胶体粒子的欠阻尼集合，通常被称为“尘埃等离子体”。该系统用于直接可视化和表征T = 0振动模式，这在分子玻璃中是不可能的。第二个项目解决了堵塞系统中波动引起的力。干扰相变与二阶相变有许多相似之处，如临界点附近的长度尺度发散。通过实施几何约束来控制可能存在于干扰附近的力链频谱，类似于热系统中的临界卡西米尔力，研究团队可以量化这种长度。实验使用软滑的聚合物水凝胶颗粒，其大小可以随盐浓度的变化而变化，作为“沙盒”来研究没有颗粒摩擦时的干扰过渡。在这两项研究工作的同时，该项目还包括在Dekalb县的一所当地小学建立一个课后科学俱乐部，Dekalb县拥有格鲁吉亚第三大的学校系统，也是格鲁吉亚最多样化的县。

Nontechnical:As a society, we are very good at putting things in order: cars parked in designated rows in a garage, oranges piled neatly at the local grocer, or papers stacked in perfect bundles on an office desk. We do this, perhaps unconsciously, because ordered arrangement often saves the most space. Similarly, out of all of the solid forms of matter, physicists perhaps know the most about crystalline materials; the structural order makes them easier to conceptualize and to define mathematically. However, many solid materials can be rigid, yet have no well-defined order. These materials often behave in peculiar ways in response to external stimuli. Consider a pile of sand at the bottom of an hourglass. What seems stable enough can suddenly avalanche upon the addition of a few extra grains. Or even a snarled traffic jam: what determines the boundary between a flowing state and a rigid one? Our world is full of similar examples where systems exist in a region near marginal stability. This project aims to understand the ways in which a jammed, amorphous solid can vibrate and fluctuate. By using "model" systems such as colloidal dust suspended in a gaseous plasma, the principal investigator and research team can visualize the individual motion of particles which constitute the near-effortless structural rearrangements in amorphous systems. Not only do the research activities shed light on the individual molecular motions common in amorphous solids such as glasses, but they provide a connection between dynamics and disorder in a broad range of physical systems. In parallel to this research effort, the project includes the creation of an after-school science club at a local elementary school in Dekalb county, which hosts the 3rd largest school system and is the most diverse county in Georgia.Technical:This project focuses on the nature and origin of mechanical rigidity in amorphous solids, a subject which permeates many areas of condensed matter physics and materials science. Both experiments and simulations are used to investigate the dynamics of low-frequency vibrational modes and the role of geometric confinement in jammed and glassy systems, where small changes in temperature or density result in an enormous increase in the kinetic timescales of motion. The project addresses two major topics, the first of which is the origin of low-frequency vibrational modes in disordered solids. It has been known for 40 years that molecular glasses have excess vibrational modes at low frequencies. An ideal model system to study this phenomena is an under-damped ensemble of colloidal particles suspended in a plasma, commonly known as a "dusty plasma". This system is used to directly visualize and characterize the T = 0 vibrational modes in a way not possible in molecular glasses. The second project addresses fluctuation-induced forces in jammed systems. The jamming transition has many similarities to 2nd-order phase transitions, such as a diverging length scale near the critical point. By implementing geometric confinement to control the spectrum of force chains that can exist near jamming, in analogy to critical Casimir forces in thermal systems, the research team can quantify this length. The experiments use soft, slippery, polymer hydrogel particles whose size can be varied with salt concentration as a "sandbox" for studying the jamming transition in the absence of particle friction. In parallel to both research efforts, the project includes the creation of an after-school science club at a local elementary school in Dekalb county, which hosts the 3rd largest school system and is the most diverse county in Georgia.