用密堆积光弹性颗粒研究非晶固体塑性的微观物理机理

Amorphous solids are ubiquitous in nature, including metallic glasses, polymer glasses, concentrated emulsions, colloidal glasses, foams, and granular materials, spanning a broad range of scales. In crystalline materials, the plasticity has been well understood in terms of dynamics of dislocation, i.e. flow defects in the crystals where the flow defects can be directly visualized under a microscope. In a contrast, the plasticity in amorphous materials, i.e. glass, is still poorly understood due to the disordered nature of the materials. In this project, we will perform experimental studies of the plasticity of a 2D glass in the athermal quasi static limit where the 2D glass is made of densely packed bi-disperse granular disks with very low friction. Starting from a densely packed homogeneous and isotropic initial state, we apply shear deformation to the system. For a sufficiently small strain, the response of the system is linear and elastic like; when the strain is large enough, the plasticity of the system gradually develops and eventually the shear bands are fully developed. Our experiment allows the access of all the important geometrical and mechanical quantities such as movements of particles, inter-particle contacts, and contact forces. The analysis of particle movements allows us to track the individual T1 events – the localized, smallest, and simplest structural rearrangements (flow defects) in 2D systems – that form the cores of shear transformation zones in 2D. Meanwhile the analysis of contacts and contact forces allows us to study how the stress is distributed surrounding each individual T1 events and to uncover the means of mutual interactions between different T1 events in space. In this study, we are particularly interested in how to relate the local plastic deformation to the macroscopic response of the system and also in the development of the shear bands.

非晶固体在自然界中广泛存在，如金属玻璃、高分子玻璃、乳浊液、胶体玻璃、泡沫、和颗粒物质等，涵盖广泛的尺度。人们对于晶体塑性通过位错动力学的研究而了解的很清楚，因为位错可以通过显微镜直接观测。反之人们对非晶固体（玻璃）的塑性由于材料的无序而了解的有限。我们将通过实验研究二维玻璃在零温准静态下的塑性。二维玻璃由紧密堆积的双分散低摩擦的颗粒圆盘组成。从紧密堆积的均匀各向同性的初态出发，对体系施加剪切应变。对足够小的应变系统宏观响应是弹性的；当应变到一定程度，系统塑性逐渐发展并伴随宏观剪切带的逐渐形成。我们可以从微观上观测颗粒运动、颗粒间相互接触的位置、相互接触力等重要的几何和力学物理量。从而我们能跟踪微观的每个作为二维剪切应变核心的塑性单元—T1事件；通过接触点和接触力的测量能使我们掌握它们间的相互作用。该研究有助于理解系统的微观塑性行为和宏观行为之间的深刻联系，从而理解无序固体塑性的物理机理。

颗粒物质体系具有离散、无序、零温的特点，是一种典型的非平衡态物质体系，和玻璃态物质有着极大的相似性。对颗粒物质体系的塑性形变的微观物理机理的研究有助于促进非平衡态统计物理学的发展，对于揭示颗粒物质和玻璃态物质的相互关系和异同点，促进颗粒物质的非平衡态统计物理的发展，具有重要的科学理论意义和工程应用价值。..在项目的支持下，我们首先从分子玻璃塑形理论的角度出发，着眼于塑性单元的实验观测。我们主要对颗粒塑形形变基本单元T1事件展开统计分析。我们发现T1事件与剪切带的形成以及与系统宏观的应力应变曲线的演化有着紧密的关联。通过对颗粒尺度的应力、能量、应变的系统分析与统计，我们发现颗粒物质的能量涨落与软玻璃流变学理论所提出的有效温度之间的对应关系并建立了颗粒物质的塑形形变与玻璃老化过程的内在联系。..其次，我们从颗粒物质的接触网络的视角出发，通过分析接触网络的最小组成单元元胞的结构与动力学在循环剪切的驱动下的统计规律，我们发现元胞的统计分布是稳定性与熵相互竞争的结果。我们同时在非平衡态系统中发现元胞的演化在稳态下的细致平衡的规律。然后，我们也对颗粒静态密堆积的结构与力学的正则模式进行了系统的分析，我们掌握了无序密堆积颗粒体系的波色峰的形成的物理机制。对波色峰与范霍夫奇点，以及波色峰与安德森局域化的区别于联系进行了探索。..最后，我们对各向同性的颗粒密堆积的力学的长程关联进行了定量的测量，从实验上首次观测到了应力长程关联的-2次方的幂指数衰减规律。同时，我们对应力的长程关联的空间对称性与颗粒剪切带形成的初始的局域化直接的深刻联系进行了实验的论证与观测，我们发现剪切带的形成与应力的空间对称性的破缺有着深刻的联系。.同过项目的资助，我们初步揭示并理解了颗粒物质塑形形变的物理机理，并开展对博士研究生科研能力的重点培养和优秀本科生的科研能力的培养,为团队发展打好了基础。