CAREER: Plasticity and Jamming

该奖项支持理论和计算研究，旨在理解非晶固体的塑性和干扰过渡之间的关系。人们普遍认为晶体的力学性能是由其缺陷控制的；然而，对于非晶固体，还没有达成这样的共识。现在有越来越多的证据表明，在软颗粒或颗粒状材料的悬浮液等非热排斥系统中存在两种不同的临界行为。一方面，非晶固体表现出一种类似于其他脱屑系统的动态临界行为，因为它们是在消失剪切速率下驱动的。另一方面，当围压消失时，排斥性颗粒的密闭随机填料的弹性响应表现出临界行为。在低约束压力下的慢剪切过程中，这些行为之间的相互作用的关键问题仍然相对未被探索。PI将对剪切作用下软硬颗粒体系的悬浮液进行大规模并行计算机模拟，以确定在不同颗粒刚度、堆积密度和剪切速率下应力积累和松弛的机制。这些过程的时空结构将使用为过冷液体和玻璃开发的新兴工具进行研究。PI将解决在干扰以上的系统中观察到的动态临界行为是否在干扰过渡中持续存在，以及更一般地说，与干扰过渡的接近如何影响允许剪切的时空结构。教育活动主要有三个方面：1)本科课程的发展将涉及数值建模融入高年级的核心课程。2)研究生课程的发展将包括为工程和理论力学学生设计和实施统计物理导论。3)中学教育外展将与匹兹堡超级计算中心联合开展，涉及匹兹堡公立学校的高中教师。这个职业奖支持理论和计算研究，旨在对非晶材料在外力作用下的行为有一个基本的了解。许多非晶材料在没有外力的情况下表现得像普通固体。尽管它们缺乏任何潜在的晶体秩序，但它们保持着自己的形状。日常生活中的许多例子包括窗户（由玻璃或有机玻璃制成）、蛋黄酱和海滩上的沙子。当外力变得足够大时，构成这些固体的颗粒开始流动，就好像材料是液体一样，这种材料被称为“塑性屈服”。在晶体材料中，几十年来人们一直认为塑性屈服是由晶体结构中被称为“位错”的扩展缺陷所控制的。在非晶固体中，对塑性屈服的微观理解才刚刚开始出现。在这些无定形固体的某一类中，例如沙粒，其组成粒子通过纯排斥力相互作用。在这种排斥的情况下，只有当外力限制粒子并迫使它们相互作用时，材料才能表现得像固体一样。在这种受限状态下，粒子被称为“堵塞”。在约束即将被释放的过渡点上，材料的弹性特性被认为是完全反常的，不像在普通固体中观察到的任何东西。因此，重要的是询问系统是否会通过相同的机制在干扰附近屈服。PI将执行非常大规模的计算机模拟和理论分析来解决这个问题。教育活动主要有三个方面：1)本科课程的发展将涉及数值建模融入高年级的核心课程。2)研究生课程的发展将包括为工程和理论力学学生设计和实施统计物理导论。3)中学教育外展将与匹兹堡超级计算中心联合开展，涉及匹兹堡公立学校的高中教师。

TECHNICAL SUMMARYThis CAREER award supports theoretical and computational research aimed at understanding the relationships between plasticity of amorphous solids and the jamming transition. It is generally accepted that the mechanical properties of crystals are controlled by their defects; however for amorphous solids, no such consensus has been reached. There is now mounting evidence for two different kinds of critical behavior in athermal, repulsive systems such as suspensions of soft particles or granular materials. On one hand, amorphous solids exhibit a kind of dynamical critical behavior similar to other depinning systems as they are driven at vanishing shear rate. On the other hand, confined random packings of repulsive particles exhibit critical behavior in their elastic response as the confining pressure vanishes. The key issue of the interplay between these behaviors during slow shear at low confinement pressure remains relatively unexplored.The PI will perform massively parallel computer simulations of suspensions of soft and hard particle systems under shear to identify the mechanisms for stress buildup and relaxation in various regimes of particle stiffness, packing density, and shearing rate. The spatio-temporal structure of these processes will be studied using emerging tools developed for supercooled liquids and glasses. The PI will address whether the dynamically critical behavior observed in systems above jamming persists at the jamming transition and, more generally, how proximity to the jamming transition affects spatio-temporal structures that allow shear.The educational activities lie along three main thrusts:1) The undergraduate curriculum development will involve the infusion of numerical modeling into core upper division coursework.2) The graduate curriculum development will involve the design and implementation of an introduction to statistical physics for engineering and theoretical mechanics students. 3) The secondary education outreach will be done in conjunction with the Pittsburgh Supercomputing Center in the context of a program which involves high school teachers from the Pittsburgh public schools. NON-TECHNICAL SUMMARYThis CAREER award supports theoretical and computational research aimed at a fundamental understanding of how amorphous materials behave when subjected to external forces. Many amorphous materials behave like ordinary solids without external forces. Even though they lack any underlying crystalline order, they hold their shape. A few of the many examples from everyday experience include windows (made of glass or plexiglass), mayonnaise, and sand on the beach. When the external forces become sufficiently large, the particles which make up these solids start to flow as if the material were a liquid and the material is said to undergo "plastic yield". In crystalline materials, plastic yielding has been understood for many decades to be governed by extended defects in the crystal structure known as "dislocations". In the amorphous solids, a microscopic understanding of plastic yielding is only beginning to emerge.In a certain subclass of these amorphous solids, e.g. the grains of sand, the constituent particles interact through purely repulsive forces. In this repulsive case, the materials can only behave like a solid when external forces confine the particles and force them to interact with each other. In such a confined state, the particles are said to be "jammed". At the transition point when the confinement is just about to be released, the elastic properties of the material are known to be completely anomalous, unlike anything observed in ordinary solids. Therefore, it is important to ask whether the system will yield via the same mechanism near jamming. The PI will perform very large-scale computer simulations and theoretical analysis to address this question.The educational activities lie along three main thrusts:1) The undergraduate curriculum development will involve the infusion of numerical modeling into core upper division coursework.2) The graduate curriculum development will involve the design and implementation of an introduction to statistical physics for engineering and theoretical mechanics students. 3) The secondary education outreach will be done in conjunction with the Pittsburgh Supercomputing Center in the context of a program which involves high school teachers from the Pittsburgh public schools.