(24b) A New Particle Morphology Characterization to Account for Surface Angularity and Experimental Validation through 3D Printing

Granular materials is pervasive in nature, and is known as the second-most manipulated material in industry. It has significant importance in the fields such as construction, agriculture, aerospace and pharmaceutical applications, just to list a few. However, complex behavior of granular material is still poorly understood, e.g., phase transition from solid to liquid-like behavior and vice versa, due to its particulate nature at the grain scale, where the particle morphology plays a critical role in the inter-particular interaction.

Majority of particulate research community have accepted the concept of defining particle morphology at three different scales: global form (at large scale), local angularity (at intermediate scale), and surface texture (at small scale). The global form is the first-order morphological property that characterizes the overall particle shape compared to sphere (or how much it is elongated). A number of morphology descriptors have been developed with some variations including Sphericity, Shape Factor, Flat & Elongated Ratio, etc. Local angularity is the second-order morphological property that characterizes the sharpness of particle corners, for which a set of morphology descriptors also has been developed such as Roundness, Angularity Factor, and Angularity Index. On the other hand, surface texture has been often mechanically characterized in terms of surface roughness (i.e., effect of surface texture) such as inter-particle friction angle.

This approach that characterizes the particle morphology using the three factors at three different scales has been widely perceived as an effective method. However, there still exists a gap in the body of knowledge, in particular describing the local angularity (i.e., morphology at the intermediate scale). The existing local angularity descriptors mainly focus on the “corner” angularity (i.e., sharpness of particle corners), therefore do not robustly capture the “surface” angularity that increases with the presence of a series of local surface concavities. For example, exiting descriptors characterize tetrahedron as highly angular due to its sharp corners, while quantifying the angularity of gear-like particle shapes relatively low. Although it may be reasonable we call the shape with sharp corners “geometrically” angular, it may not be the case when it comes to “mechanical” properties, because the gear-shaped particles would provide a higher shear strength and modulus than those of the tetrahedrons due to a stronger interlocking mechanism if the bulk materials are mechanically tested.

The objective of this study is to develop a new particle morphology descriptor that can robustly characterize the “surface” angularity in addition to the “corner” angularity. A set of 3D numerical particle models are developed and characterized using the existing descriptors and the proposed descriptor for comparison of the quantified angularities. The bulk specimens are then developed for each particle models and mechanically tested for validation of the proposed descriptor. A key feature of the validation approach adopted in this study is employment of 3D printing technique to produce 3D synthetic particles from the 3D numerical particle models. The printed synthetic particles are used for the experimental validation, while bypassing the high computational costs that incur in discrete element simulation of numerical particles with complex geometries. The initial results in the study will be highlighted and discussed.