Gravitational sedimentation of prolate particles with a non-uniform mass distribution

The dynamics of sedimenting particles under gravity is surprisingly complex due to the presence of effective long-ranged forces. When the particles have a well-defined symmetry axis and non-uniform density, recent theoretical predictions suggest that prolate objects will repel and oblate ones will weakly attract. We tested these predictions using mass polar prolate spheroids, which are composed of 2 mm spheres glued together. We probe different aspect ratios (κ) and center of mass offsets (χ) by combining spheres of different densities. Experiments were done in both quasi-2D and 3D chambers. Three situations were explored: single particle flipping, repulsion between pairs of particles, and collections of hundreds of particles in 3D. Particles sediment at low Reynolds numbers and are tracked optically. Single particle trajectories are fit using a reduced mobility matrix model that can be solved analytically for the trajectory in time. Pairs of particles exhibit effective repulsion, and their separation roughly scales as ~(κ-1)/χ^0.5, i.e. particles that are more prolate or have more mass asymmetry have stronger repulsion effects. In 3D, particles with χ>0 are distributed more uniformly than χ=0 particles, and the degree of uniformity increased with κ, indicating that the effective 2-body repulsion is manifested for a large number of particles.