CAREER: Nonlinear Waves and Fluctuations in Jammed Systems

非技术性：作为一个社会，我们非常善于把东西整理好：车库里指定的几排车停着，当地杂货店里整齐地堆放着橙子，或者办公桌上一捆捆完美地堆放着文件。我们这样做，也许是无意识的，因为有序的安排往往节省了最多的空间。同样，在物质的所有固体形式中，物理学家可能对晶体材料了解最多；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，但没有明确的顺序。这些材料经常以特殊的方式对外界刺激做出反应。想象一下沙漏底部的一堆沙子。看起来足够稳定的东西，在添加了一些额外的谷物后，可能会突然崩塌。甚至是混乱的交通堵塞：是什么决定了流动状态和僵硬状态之间的界限？我们的世界充满了类似的例子，系统存在于一个接近边际稳定的地区。这个项目旨在了解被卡住的无定形固体振动和波动的方式。通过使用悬浮在气态等离子体中的胶体粉尘等“模型”系统，首席研究人员和研究团队可以可视化粒子的单独运动，这些运动构成了非晶态系统中近乎毫不费力的结构重排。这些研究活动不仅揭示了玻璃等非晶态固体中常见的单个分子运动，而且提供了广泛物理系统中动力学和无序之间的联系。在这项研究工作的同时，该项目还包括在德卡尔布县的一所当地小学创建课后科学俱乐部，该县拥有第三大学校系统，也是格鲁吉亚最多元化的县。技术：该项目专注于无定形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的许多领域的学科。实验和模拟都被用来研究阻塞和玻璃系统中低频振动模式的动力学和几何约束的作用，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标度的极大增加。该项目涉及两个主要主题，第一个主题是无序固体中低频振动模式的起源。人们已经知道分子玻璃在低频率下有多余的振动模式，这已经有40年的历史了。研究这一现象的理想模型系统是悬浮在等离子体中的欠阻尼胶体粒子集合，通常被称为“尘埃等离子体”。这个系统被用来直接可视化和表征T=0振动模式，这在分子玻璃中是不可能的。第二个项目解决了干扰系统中的波动诱导力问题。干扰相变与二阶相变有许多相似之处，例如临界点附近有发散的长度尺度。通过实施几何约束来控制干扰附近可能存在的力链的频谱，类似于热力系统中的临界卡西米尔力，研究团队可以量化这一长度。该实验使用大小可随盐浓度变化的柔软、光滑的聚合物水凝胶颗粒作为“沙箱”，用于研究在没有颗粒摩擦的情况下的堵塞转变。在这两项研究工作的同时，该项目还包括在德卡尔布县的一所当地小学创建一个课后科学俱乐部，德卡尔布县拥有第三大学校系统，也是佐治亚州最多元化的县。

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.