Repulsive Interactions between non-Brownian particles with non-uniform density during sedimentation

We report experimental results on the sedimentation of non-Brownian particles with variable density distributions at low Reynolds number (10^-3). Particle doublets with uniform density were composed of equal-sized aluminum-aluminum spheres or steel-steel spheres, whereas doublets with non-uniform density were composed of equal-sized aluminum-steel spheres. We observed an effective in-plane repulsive force during the settling of two aluminum-steel doublets due to their tendency to align with gravity, in agreement with a recent prediction [1], in contrast to settling dynamics of two doublets with uniform density that align with the fluid flow. For 10 or more interacting doublets in 2D channel flow, non-uniform particles tended to organize into equally-spaced paths determined by the particle size, whereas particles with uniform density had much stronger local interactions and often crossed paths and flipped orientation. For 100 or more particles settling in a cylindrical tube, the resulting distribution of particle positions at the bottom after sedimentation was more uniform for aluminum-steel doublets, suggesting that the 2-body repulsive force plays a role in the many-body interactions during sedimentation.

[1] T. Goldfriend, et al., PRL. 118, (2017)