本文介绍了颗粒物质在自由下落时的聚集成团现象，并通过分析一系列的实验现象来探究颗粒流聚集成团的原因。通过分析得出，颗粒物质聚集形成团簇的原因是颗粒间有内聚力的作用，并证实了内聚力是由于范德瓦尔斯相互作用和毛细力所导致的，其大小在nN量级。 同时，本文还利用分子动力学方法对该现象进行了计算机模拟。通过分析颗粒间的受力模型，确定颗粒间接触时的法向力和切向力。此外，引入实验中探测到的内聚力来完善颗粒间的作用力。计算中利用了Gear五阶算法来预测颗粒下一时刻的位置、速度等时间衍生量，并对预测结果进行修正使其更符合实际情况。数值模拟所用的是Fortran程序，通过设定合理的参数，确实出现了颗粒物质的聚集成团现象，这也说明颗粒间存在内聚力。 颗粒物质的集体行为跨越了固体、液体和气体，目前对颗粒物质的研究尚且没有一个完整的框架。通过研究颗粒流的断裂成团现象，可以对颗粒理论在一定程度上进行完善。

Granular media often appear to behave like ordinary fluids. The granular flow out of a circular nozzle starts out spatially uniform and then, further downstream, breaks up into well-defined clusters. This paper investigates why granular streams rupture and cluste. By analysis on series of experiments, we confirm that tiny, short-ranged cohesive forces are responsible for the clustering. And the nanoNewton cohesive forces arise from a combination of van der Waals interactions and capillary bridges between nanometer-scale surface asperities. Molecular Dynamics plays the most important role among the simulation methods for granular systems. Based on the knowledge of Molecular Dynamics and the model of forces between particles, we also simulate the clustering process though computer. For two-dimensional systems, the normal and tangential forces can be described if two particles are in mechanical contact. Then, we introduce the cohesive force to accomplish the interactions. By applying the Gear algorithm of five order, we predict the particle positions, velocities, and higher-order time derivatives, and correct the prediction in a proper way. The simulation is carried out in Fortran programs. After setting proper parameters, the rupture and cluster are observed in the computer simulation. By comparing the phenomena in the computer simulations with which in the experiments, we have identified that the cohesive forces do exist. For granular matter is the most familiar form of matter in existence on the Earth，researches on freely falling granular streams will enrich our knowledge about the force theory of granular media. These would benefit our society to some extent.