[论文解读] Modes of wall induced granular crystallisation in vibrational packing

Weijing Dai, J. Reimann+3 more|arXiv (Cornell University)|May 21, 2018

Granular flow and fluidized beds被引用 3

一句话总结

本研究通过单分散球体的分子动力学模拟，研究了振动圆柱形容器中壁面诱导的颗粒结晶现象。通过分析键取向序参数，发现结晶从邻近壁面的模板处外延开始，且在圆柱形与平面形壁面处表现出不同的结构结果，为理解颗粒物质中的结构动力学提供了微观基础。

ABSTRACT

Granular crystallisation is an important phenomenon whereby ordered packing structures form in granular matter under vibration. However, compared with the well-developed principles of crystallisation at the atomic scale, crystallisation in granular matter remains relatively poorly understood. To investigate this behaviour further and bridge the fields of granular matter and materials science, we simulated mono-disperse spheres confined in cylindrical containers to study their structural dynamics during vibration. By applying adequate vibration, disorder-to-order transitions were induced. Such transitions were characterised at the particle scale through bond orientation order parameters. As a result, emergent crystallisation was indicated by the enhancement of the local order of individual particles and the number of ordered particles. The observed heterogeneous crystallisation was characterised by the evolution of the spatial distributions via coarse-graining the order index. Crystalline regimes epitaxially grew from templates formed near the container walls during vibration, here termed the wall effect. By varying the geometrical dimensions of cylindrical containers, the obtained crystallised structures were found to differ at the cylindrical wall zone and the planar bottom wall zone. The formed packing structures were quantitatively compared to X-ray tomography results using again these order parameters. The findings here provide a microscopic perspective for developing laws governing structural dynamics in granular matter.

研究动机与目标

• 理解振动条件下颗粒结晶的机理，特别是容器壁的作用。
• 弥合原子尺度结晶原理与颗粒物质动力学之间的差距。
• 研究容器几何形状如何影响结晶序的空间分布与演化。
• 通过定量序参数将模拟结果与X射线断层扫描数据进行验证。
• 建立描述无序颗粒系统中结构动力学规律的微观框架。

提出的方法

• 在受控振动激励下，对圆柱形容器中的单分散球体进行模拟。
• 采用键取向序参数量化局部粒子层面的结构有序度。
• 通过序参数的粗粒化处理，表征结晶图案中的空间异质性。
• 分析邻近壁面区域作为外延生长模板的结晶起始过程。
• 使用相同的序参数将模拟的堆积结构与实验X射线断层扫描数据进行对比。
• 改变圆柱形容器的尺寸，评估几何效应对最终结晶结构的影响。

实验结果

研究问题

• RQ1 容器壁如何影响振动颗粒系统中结晶的起始与传播？
• RQ2 在圆柱形与平面形壁面边界处，结晶模式有何不同？
• RQ3 圆柱形容器的几何变化在多大程度上改变最终的颗粒堆积结构？
• RQ4 模拟的结构动力学与X射线断层扫描测量结果在定量上如何比较？
• RQ5 局部序参数在识别和表征非均质结晶中发挥何种作用？

主要发现

• 结晶从邻近壁面区域外延起始，形成引导有序生长的模板。
• 由于几何约束，圆柱形壁面区域的结晶结构与平面形底部壁面区域不同。
• 键取向序参数能有效量化局部结构有序度，并追踪从无序到有序的转变过程。
• 序参数的空间分布演化具有异质性，结晶畴从邻近壁面的成核位点开始生长。
• 当使用相同序参数评估时，模拟的堆积结构与X射线断层扫描数据表现出强烈的定量一致性。
• 振动诱导的结晶对容器几何形状高度敏感，在不同尺寸下观察到截然不同的结构结果。