  The gravity-driven motion of rigid particles in a viscous fluid is relevant
in many natural and industrial processes, yet this has mainly been investigated
for spherical particles. We therefore consider the sedimentation of
non-spherical (spheroidal) isolated and particle pairs in a viscous fluid via
numerical simulations using the Immersed Boundary Method. The simulations
performed here show that the critical Galileo number for the onset of secondary
motions decreases as the spheroid aspect ratio departs from 1. Above this
critical threshold, oblate particles perform a zigzagging motion whereas
prolate particles rotate around the vertical axis while having their broad side
facing the falling direction. Instabilities of the vortices in the wake follow
when farther increasing the Galileo number. We also study the
drafting-kissing-tumbling associated with the settling of particle pairs. We
find that the interaction time increases significantly for non-spherical
particles and, more interestingly, spheroidal particles are attracted from
larger lateral displacements. This has important implications for the
estimation of collision kernels and can result in increasing clustering in
suspensions of sedimenting spheroids.
