Sedimentation of particles with nonuniform density at low Reynolds number

Particle sedimentation plays a key role in the accumulation of geological deposits and is essential to many industrial processes. It may also provide a novel route towards self-assembly of particulate films if long-wavelength density fluctuations are suppressed. Recent theoretical predictions suggest that non-spherical objects with nonuniform density can accomplish this task [1]. Here we report experiments focusing on the settling dynamics of such particles at low-Reynolds numbers (∼10^-3). Our particles are composed of aluminum and steel spheres glued together in various configurations. A single doublet made of equal-sized aluminum and steel (EAS) balls aligns with gravity, contrary to a doublet made of two aluminum balls, which aligns with fluid flow. For two identical EAS particles, an effective repulsive force is observed during settling due to the competing tendencies to align to the fluid flow and gravity. For three or more pairs of EAS particles, more complex dynamics are observed that are likely due to the chaotic nature of many-body hydrodynamics, yet repulsion between the particles seems to play a key role. Preliminary experiments geared toward realizing hyperuniform structures from sedimentation will also be discussed.

[1] Goldfriend, et al., PRL, 118, 158005 (2017)