(91c) Packing Structure Analysis of Flexible Rod Particles about Aspect Ratio, Bending Stiffness and Surface Energy

Packing structure analysis for granular materials is essential for understanding the particulate system. Powder packing in vessel is one of the fundamental process in many industrial processes, from raw materials to making new products. Particularly in certain industries, such as pharmaceuticals and battery, packing structure could be important enough to determine the performance of the drug or battery. Therefore, understanding the packing structure has great advantages in the view of industry and academy.

Many studies have been carried out to analyze the packing structure in a vessel. In the early days, the particle packing structure with monosized distribution was mainly investigated. To analyze the packing structure, characteristics such as porosity, radial distribution function (RDF) and coordination number (CN) were typically adopted. However, due to CN difficult to measure in the experiment, many studies have used computational approaches to deal with particle packing matter. Among the computational approaches for packing structure analysis, Discrete Element Method(DEM) is most widely adopted. Studies on the effect of particle size, particle shape, cohesion, and friction on particle structure were conducted using the discrete element method. The particle cohesion can be represented by the forces such as van der Waals, electromagnetic force, and liquid bridge. The relation with structure and cohesion has been studied. Among the various cohesion forces, Van der Waals force and JKR model have been widely used in previous studies.

Since the particle shape has quite great relationship with the cohesiveness, many researches are still focusing on the influence of the particle shape. The traditional discrete element method assumes spherical particles, but most of the real particles are not spherical. In order to overcome these restriction, there have been many studies analyzing the packing structure of various particle shapes by improved computational algorithm. Particularly, elliptical particles, which is the most easily feasible shape in DEM, have been adopted. The most commonly used particle for the analysis along with the ellipse is rod-shaped particles. Various methods connecting two or more particles are suggested to form a rod-shaped particle. In early, rigid rod and completely flexible rod models were developed. Rod models with finite resistance are recently developed. Bonded particle model, Euler-Bernoulli beam model, bead-spring model, Worm like chain (WLC) model are typically used.

In this study, particle packing structure in a cylindrical vessel is achieved by flexible rod particles based on DEM. Three features are selected as the factors affecting the packing structure of the particles: flexibility, surface energy, and aspect ratio. For the rod flexibility, the WLC model connecting two or more spherical particles to form a single rod is implemented. Surface energy is applied using the Hertz-Mindlin with JKR model. The aspect ratio can be controlled by the number of particles constituting the rod particles in the WLC model.

In order to investigate the packing structure macroscopically and microscopically, the porosity, RDF and CN is analyzed. The porosity distribution is also investigated to confirm the spatial distribution of porosity. The results of this study are as follows.

The results show that all three factors (bending stiffness, surface energy, and aspect ratio) gives RDF the effects. Among the factors, aspect ratio indicates most noticeable effect. The peak points of RDF were different according to aspect ratio. The peak points for flexibility and surface energy in the same aspect ratio are similar, but a certain peak point could disappear as the flexibility or the surface energy increases.

As for the porosity and CN, the porosity increases and the CN decreases as the bending stiffness increases. However, when the bending stiffness is above a certain value, it no longer affects porosity and CN. As the surface energy increases, porosity increases and CN also tends to increase.

Except for spherical particles with aspect ratio of 1, the void distribution appears to be concentrated at the center of the cylinder regardless of flexibility, surface energy, and aspect ratio. Even spherical particles are concentrated on the center of vessel, but the extent is relatively small. As a result of the surface energy, high surface energy makes particles entangled in the center, and the porosity of the wall surface and the top surface becomes much larger. As for the flexibility, as the bending stiffness increases, the voids at the center of vessel and the voids around the vessel wall increase due to the effect of the shape, and the height of the bed increases.

In summary, all of three factors such as the flexibility, surface energy, and aspect ratio affect the packing structure. The effect of particle shape is the largest. However, when the aspect ratio is high, the effect of the particle flexibility is considerably large and the effect of surface energy was small. When the aspect ratio is small, the effect on the surface energy is greater than the effect on the flexibility.