Hindered stokesian settling of discs and rods

When a homogeneous dispersion of solid particles at finite volume fraction (φ) in a fluid sediments under gravity, it settles with a velocity v slower than the terminal velocity v_s of a single particle sedimenting in an infinite fluid, as quantified by the hindered settling function H(φ) = v(φ)/v_s. This fundamental measure of the effect of collective hydrodynamic interactions on sedimentation has been extensively studied by experiments on suspensions of spheres, and by theory in the dilute and very dense limits. Here we experimentally study sedimentation of particles with an orientational degree of freedom at very low Re (~ 10^-4) and very high Pe (10⁹-10¹⁰). We prepare suspensions of disc-like and rod-like shapes with a few different aspect ratios and study the velocity of the sedimentation front and the volume fraction of the sediment. To better compare these shapes with spheres and among each other, we compare the hindered settling function H(s) at fixed interparticle separation, s, rather than as a function of φ. The 2D disc-like particles occupy a higher hydrodynamic volume than the 1D rod-like particles. Within these two types of shape, there is a further dependence on aspect ratio with H(s) showing a logarithmic dependence on the aspect ratio of the rods. We also report on orientational correlations of the particles in the sedimenting state.