颗粒物质的剪切阻塞和反阻塞相变的实验研究

Granular materials are very special: they are highly dissipative, disordered, and discrete in nature, with multi-scale properties; due to the large particle sizes, thermal fluctuation plays a negligible role. As a classical far-from-equilibrium system, granular materials have become one of the frontiers of scientific research in recent years. Granular materials are ubiquitous in nature and are closely related to our daily lives. Understanding the physics of granular materials plays a key role in many industrial and geotechnical applications and it is fundamentally important for our scientific interest. One of the big puzzles about granular materials is the nature of jamming transition - - a system changes its characteristics from a liquid-like to a solid-like matter. The recent discovery of Shear-Jammed states has demonstrated the importance of the shear as a state variable in describing the jamming transition and it has greatly expanded the original scope of the Chicago-UPenn jamming diagram, which describes the jamming transition at zero temperature as a critical-like phenomenon controlled by a single parameter- - packing fraction. This diagram is conceptually simple but extremely powerful, allowing the study of jamming in different amorphous systems within the same framework. The nature of the Shear-Jamming is still not well understood and this forms one of our main focuses of this project. Our goal is to understand the mechanism of shear jamming and to discover the possible connections between shear jamming and isotropic jamming and between shear jamming at the quasi-static limit and rheology at the dynamical limit. The inverse process of jamming transition, the unjamming of granular materials is also full of mystery, which is very important in understanding the driving mechanism of many natural hazards and the physical principles of the plasticity of a material. Thus the study of unjamming forms another main focus of this project. We will apply photo-elastic techniques to analyze the unjamming transitions from the microscopic particle scales to the macroscopic system scales using a series of state-of-the-art experiments. We will analyze and compare several closely related classical models, such as the STZ, the soft mode analysis, and the dynamical system theory.

颗粒物质作为一个经典的远离平衡态的多体物理系统，在近年来成为科学研究的前沿之一。颗粒物理的一大难题是颗粒系统由类液态变成类固态的阻塞相变。近年来剪切阻塞态的发现拓展了原有的芝加哥-宾大相图，表明剪切作为描述阻塞相变的不可或缺的独立状态参量的重要性。原相图把不同的非晶态体系放在统一框架下研究，对零温度下的阻塞相变的描述归结为系统体积分数超过某临界密度时的临界行为。本项目的一大研究重点是探索剪切阻塞的机理和转变规律，寻求剪切和零剪切阻塞的内在联系，找寻准静态下的剪切阻塞和体系流变学的内在关联。作为反过程,反阻塞相变同样充满了奥秘。如何预测这一相变关系到深入理解相关地质灾害的形成机理和材料塑形的产生机制。对反阻塞相变的研究将作为本项目的另一重点。本项目将运用光弹性颗粒结合一系列独具匠心的实验系统去分析与之紧密相关的三个经典模型- - STZ，声子软模，和动力学理论的预测力，适用性，和它们的内在联系。

颗粒物质作为一个经典的远离平衡态的多体物理系统，在近年来成为科学研究的前沿之一。颗粒物理的一大难题是颗粒系统由类液态变成类固态的阻塞相变以及由类固态变成类液态的反过程。依托本项目的的支持，我们通过精密的实验研究探索颗粒物质阻塞态与剪切阻塞态的颗粒密堆积体系的静态力学特征，尤其是通过线性分析我们成功建立海森矩阵来理解无序颗粒体系的振动模式的能谱分布特点、模式的空间分布特点等。在此基础上，我们通过把密堆积的颗粒物质作为一个模型的玻璃体系，研究了凝聚态物理中的波色峰的物理特征与物理机制。这为理解颗粒物质阻塞态的结构与力学特征提供了坚实的基础。其次，我们通过研究恒压的密堆积颗粒的准静态剪切过程，研究了颗粒剪切阻塞态从类固态在向类液态在应变的作用下转变的动力学过程。通过研究颗粒团簇的微观动力学、能量、应变、应力的统计规律与宏观物理量之间的内在联系，我们探索了这一过程中类固态向类液态转变的微观机制与统计规律。再次，我们通过研究一个模型的滑坡体系，把颗粒体系从类固态到类液态的转变以及从类液态到类固态的转变的非线性动力学动力学进行了研究，并与颗粒的阻塞转变与反阻塞转变进行类比。同时，我们依托项目的支持对颗粒的流变学与颗粒漏斗的动力学堵塞开展了研究。通过项目的支持，我们对各个不同体系中的颗粒的类液态到类固态已经类固态到类液态的转变获得了比较全面深入的理解，为相关研究的进一步开展与新的研究生长点的形成提供了强大的基础。