Gravity enables self-assembly

Crystallization of granular assemblies has broad implications for rapid and scalable creation of architected materials with applications ranging from structural materials to microarchitected battery electrodes. While significant advances have been made in understanding colloidal self-assembly at nano to micro scale, the governing mechanisms for organization of dry assemblies of hard spheres remain unclear. Here, we investigate crystallization of mono-size hard spheres with and without imposed vibration. Using X-ray computed tomographic analysis coupled with discrete-element simulations, we unravel the roles of gravity and imposed vibration on the three-dimensional self-assembly of the dry spheres. We use these insights to introduce gravity-mediated epitaxial crystal growth with slow pouring of balls on seeding templates. Contrary to vibration-induced crystallization, this method can form large single crystals with both close-packed and rather surprisingly, nonclose-packed metastable particle arrangements. Our results provide insight for the scalable manufacture of defect-free granular assemblies that can be used as space-holding templates to manufacture cellular materials, such as inverse opals and other related topologies. Key points: Self-assembly of hard spheres is a critical step for the scalable manufacture of micro-architected solids. Via a combination of vibration experiments, 3D X-ray tomographic observations, and simulations, we elucidate the critical role of gravity in the self-assembly of hard spheres. We design seeding templates that can not only induce the self-assembly into stable close-packed crystal structures but also rather counterintuitively into metastable single crystal structures.