Crystallization process under strong spatial confinement and stepwise cooling in a nonvibrating granular system

We study the effects of strong confinement on crystallization in a 2D-dimensional magnetic granular system fluidized by an alternating magnetic field whose amplitude controls the effective temperature. The granular system comprises spherical magnetic particles confined within a prismatic box slightly tilted to create an energy gradient that produces particle sedimentation. Three different geometries were used: hexagonal, circular, and rectangular. The systems start from a fluid state and go to a solid state by following a linear or stepwise cooling down. It is found that confinement reduces the crystallization time compared to the case of no spatial confinement by walls, and an additional reduction is found when using a stepwise cooling profile. In the unconfined case, the system crystallizes in the hexagonal close-packed configuration. In contrast, strong spatial confinement imposes extreme restrictions on the final configuration of the particles. While the hexagonal container favors the hexagonal close-packed arrangement, the rectangular cell imposes the square order, and the circular cell favors the formation of necklace-like structures piled one over the other. The results of the orientational order parameter and the radial distribution function corroborate these observations. As the container size increases, the effects of the walls on bulk ordering decrease, now dominating the hexagonal close-packed order in all cases. However, defects in the structure are more numerous than those without walls.