Dynamics of mass polar spheroids during sedimentation

The dynamics of sedimenting particles under gravity are surprisingly complex due to the presence of effective long-ranged forces. When the particles are polar with 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 probed different aspect ratios (kappa) and centre of mass variations (chi) by combining spheres of different densities. Experiments were done in both quasi-two-dimensional (2-D) and three-dimensional (3-D) chambers. By optically tracking the motion of single particles, we found that the dynamics were well described by a reduced mobility matrix model that could be solved analytically. Pairs of particles exhibited an effective repulsion, and their separation roughly scaled as (K - 1)/chi(0.39), i.e. particles that were more prolate or had smaller mass asymmetry had stronger repulsion effects. In three dimensions, particles with chi > 0 were distributed more uniformly than chi = 0 particles, and the degree of uniformity increased with kappa, indicating that the effective 2-body repulsion manifests for a large number of particles.

Automated summary

Experimental results confirm that polar prolate particles repel each other while oblate particles weakly attract. This effect is observed in both two-dimensional and three-dimensional experiments, and is dependent on the shape and mass asymmetry of the particles.