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The settling of heavy spherical particles in a column of quiescent fluid isinvestigated. The performed experiments cover a range of Galileo numbers ($110\leq \text{Ga} \leq 310$) for a fixed density ratio of $\Gamma = \rho_p/\rho_f= 2.5$. In this regime the particles are known (M. Jenny, J. Du\v{s}ek and G.Bouchet, Journal of Fluid Mechanics 508, 201 (2004).) to show a variety ofmotions. It is known that the wake undergoes several transitions for increasing$\text{Ga}$ resulting in particle motions that are successively: vertical,oblique, oblique oscillating, and finally chaotic. Not only does this changethe trajectory of single, isolated, settling particles, but it also changes thedynamics of a swarm of particles as collective effects become important evenfor dilute suspensions, with volume fraction up to $\Phi_V =\mathcal{O}\left(10^{-3}\right)$, which are investigated in this work.Multi-camera recordings of settling particles are recorded and tracked overtime in 3 dimensions. A variety of analysis are performed and show a strongclustering behavior. The distribution of the cell areas of the Vorono\"itessellation in the horizontal plane are compared to that of a randomdistribution of particles and shows clear clustering. Moreover, a negativecorrelation was found between the Vorono\"i area and the particle velocity;clustered particles fall faster. In addition, the angle between two adjacentparticles and the vertical is calculated and compared to a homogeneousdistribution of particles, clear evidence of vertical alignment of particles isfound. The experimental findings are compared to simulations.

Columnar structure formation of a dilute suspension of settling spherical particles in a quiescent fluid