Granular materials are ubiquitous in nature and industrial production. Investigating the structure of packing is crucial for understanding the physical properties of granular materials. Owing to their symmetry and simple geometry, spherical particles have long served as an ideal model for studying granular packing, yielding numerous research outcomes.

In recent years, the influence of particle shape on packing structures has drawn considerable attention. Non-spherical particles, characterized by complex shapes, tend to interlock and form stable structures. Their significant geometric cohesion notably affects the stability and porosity of granular packing.

To investigate the structural evolution and compaction mechanisms of three-dimensional concave particles (hexapod-shaped) under external tapping, focusing on the role of geometric cohesion in enhancing mechanical stability, we employ hexapod-shaped particles that are composed of three mutually orthogonal spherocylinders in this study. The granular system subjected to consecutive tapping can reach a stationary state. In the densifying process of the system, packing structures with different volume fractions will be formed. Meanwhile, by combining with X-ray tomography, we can obtain the microstructure.

The findings reveal that the volume fraction of “hexapod” particle packing is significantly lower than that of hard-sphere systems. The compaction curves of “hexapod” particles across varying tapping intensities are accurately described by the Kohlrausch-Williams-Watt (KWW) law, which is consistent with hard-sphere system, suggesting a relaxation process governed by heterogeneous modes. Furthermore, both the volume fraction of the steady-state granular packing and the average contact number exhibit an inverse relationship with tapping intensity, increasing as the intensity decreases. A detailed statistical analysis of contact points indicates that the compaction process of “hexapod” particles is predominantly influenced by two factors: the augmentation in the number of neighboring contacting particles and the modification of contact forms. These factors collectively enhance the degree of interlocking among hexapods within the system. Specifically, the compaction process is primarily propelled by the escalation in neighboring contacts and the refinement of contact types, particularly the increase in cylinder-cylinder (cc) contact. This rise in cc contact significantly enhances mechanical stability through strengthening geometric interlocking.

This study reveals the structural evolution characteristics of non-spherical particles in the compaction process and provide important experimental support for understanding the unique mechanical and dynamic properties of concave particle packing. This research not only enriches the experimental data of granular packing structures but also offers a new perspective for exploring the universal laws of packing for particles of different shapes. This study is to lay a more solid foundation for the theoretical research and industrial applications of granular materials, thereby promoting technological progress and innovation in related fields.