Analyzing the dynamics and structure of non-uniform spheroids in graviational sedimentation

Gravitational sedimentation may be the simplest way to steer a particle. In a suspension of many particles, a linear stability analysis shows particles with a non-uniform mass density will self-organize into hyperuniform structures via an effective repulsive force (Goldfriend et al., Phys. Rev. Lett., 2017). Here we experimentally test these predictions using non-uniform composite particles made from 2 mm individual spheres. We vary both the aspect ratio (κ) and center of mass offset (χ) by combining spheres of different materials. Particles sediment at low Reynolds number and are optically tracked in both 2D and 3D chambers. We quantified the mobility matrix for individual particles by fitting the data with simulated trajectories based on the Stokes equation. We find that two prolate (κ>1) particles with uniform density (χ=0) tend to constantly flip during sedimentation. However, when χ>0, we find that pairs of particles experience an effective repulsive force ~ (κ-1)/χ, so that particles with a small offset in their center of mass experience the largest repulsion (the small gravitational torque suppresses flipping). We also show that these repulsive effects can be readily seen in 3D suspensions of hundreds of particles with χ>0.