Circle packing on spherical caps

Abstract

We have studied the packing of congruent disks on a spherical cap, for caps of different size and number of disks, N. This problem has been considered before only in the limit cases of circle packing inside a circle and on a sphere (Tammes problem), whereas all intermediate cases are unexplored. Finding the preferred packing configurations for a domain with both curvature and border could be useful in the description of physical and biological systems (for example, colloidal suspensions or the compound eye of an insect), with potential applications in engineering and architecture (e.g., geodesic domes). We have carried out an extensive search for the densest packing configurations of congruent disks on spherical caps of selected angular widths (θmax=π/6, π/4, π/2, 3π/4, and 5π/6) and for several values of N. The numerical results obtained in the present work have been used to establish (at least qualitatively) some general features for these configurations, in particular the behavior of the packing fraction as function of the number of disks and of the angular width of the cap, or the nature of the topological defects in these configurations (it was found that as the curvature increases, the overall topological charge on the border tends to become more negative). Finally, we have studied the packing configurations for N=19, 37, 61, and 91 (hexagonal numbers) for caps ranging from the flat disk to the whole sphere, to observe the evolution (and eventual disappearance) of the curved hexagonal packing configurations while increasing the curvature.