CAREER: Plasticity and Jamming

技术总结这个职业奖项支持旨在了解无定形固体的可塑性和堵塞转变之间的关系的理论和计算研究。人们普遍认为晶体的机械性能是由它们的缺陷控制的；然而，对于非晶态固体，还没有达成这样的共识。现在有越来越多的证据表明，在非热、排斥系统中存在两种不同类型的临界行为，例如软颗粒或颗粒材料的悬浮液。一方面，非晶态固体在零剪切速率驱动下，表现出与其他脱钉体系相似的动力学临界行为。另一方面，当围压为零时，排斥颗粒的受限随机堆积在其弹性响应中表现出临界行为。在低围压下慢剪切过程中这些行为之间相互作用的关键问题仍然是相对未被开发的。PI将对剪切作用下软、硬颗粒体系的悬浮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的应力聚集和松弛机制。将使用为过冷液体和玻璃开发的新兴工具来研究这些过程的时空结构。PI将解决在干扰过渡时在干扰以上系统中观察到的动态临界行为是否持续，以及更广泛地说，干扰过渡附近如何影响允许剪切的时空结构。教育活动沿着三个主要方向展开：1)本科生课程开发将涉及将数值模拟注入核心高级课程2)研究生课程开发将涉及为工程和理论力学专业学生设计和实施统计物理导论。3)中等教育外展将与匹兹堡超级计算机中心一起在匹兹堡公立学校的高中教师参与的计划范围内进行。非技术总结这个职业奖项支持理论和计算研究，旨在从根本上理解非晶态材料在外力作用下的行为。许多无定形物质在没有外力的情况下表现出与普通固体一样的特性。尽管它们缺乏任何潜在的晶体秩序，但它们保持着它们的形状。日常生活中的许多例子中有几个包括窗户(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，就像材料是液体一样，据说材料经历了“塑性屈服”。在晶体材料中，塑料的屈服几十年来一直被认为是由晶体结构中的扩展缺陷控制的，这种缺陷被称为“位错”。在非晶态固体中，对塑性屈服的微观理解才刚刚开始。在这些非晶态固体的某个子类中，例如沙粒，组成颗粒通过纯粹的排斥力相互作用。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，材料才能像固体一样行为。在这种受限状态下，粒子被称为“卡住”。在约束即将释放的过渡点，材料的弹性性质被认为是完全反常的，这与在普通固体中观察到的任何东西都不同。因此，重要的是要询问系统是否会在干扰附近通过相同的机制屈服。PI将进行大规模的计算机模拟和理论分析来解决这个问题。教育活动沿着三个主要方向进行：1)本科生课程开发将涉及将数值模拟融入核心的高级课程2)研究生课程开发将涉及为工程和理论力学学生设计和实施统计物理入门。3)中等教育外展将与匹兹堡超级计算机中心一起在匹兹堡公立学校的高中教师参与的计划范围内进行。

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.