Stokesian settling of anisotropic shapes

We study by experiments the sedimentation of suspensions of anisotropic shapes in the Stokes regime. In previous work that studied hindered settling, we measured the mean sedimentation velocities for suspensions of discs and of rods for a few different particle aspect ratios. Despite the orientational degrees of freedom available with nonspherical particles, the dominant factor in hindered settling appears to be proportional to the volume of the sedimenting particles. To understand the sedimentation process microscopically, we go beyond measurements of the mean settling velocity. Focusing on the case of rods, we measure the time-dependence and height-dependence of orientations and density fluctuations as rods sediment. We work in a semidilute regime, where the rod centres are separated by a distance of the order of the rod length, but much larger than the rod diameter. Starting from a well-mixed initial spatial distribution, the volume fraction quickly evolves into a robust steady state, with a stable gradient over height, and a well-defined and constant mean settling velocity. We also explore the relationship between the local density and fluctuations about the mean velocity.