Collective effects in the sedimentation of particles in a viscoelastic fluid

The sedimentation of a suspension of rigid spherical particles in a polymeric fluid is studied employing experiments and particle-resolved numerical simulations. It is shown that the settling process in a viscoelastic Boger fluid is time-dependent and inhomogeneous; both experiments and simulations exhibit the formation of particle-rich, fast-settling regions and particle-depleted regions with backflow. The settling process in a Newtonian fluid is investigated for comparison and is shown to be homogeneous, with velocity fluctuations that decay following random mixing. In the viscoelastic Boger fluid, the mean settling rate is higher than that measured in a similar Newtonian suspending fluid, and in our experiments, higher than that of a single particle settling in isolation. When a cross shear flow is imposed, the mean settling rate in the viscoelastic Boger fluid is drastically reduced; the magnitude of the reduction is greater than that measured for a single particle, suggesting that both fluid elasticity and the suspension's particle volume fraction have important effects.