Sedimentation of suspensions in shear-thinning fluids

We present an experimental study of the sedimentation of non-Brownian monodisperse suspensions in polymeric shear-thinning fluids. In such media and contrary to what is observed in Newtonian fluids, no sedimentation front appears; particle aggregation moreover leads to the formation of vertical particle-rich structures and of an inhomogeneous particle volume fraction along the cell height. Once these structure formed, a very efficient settling occurs whereas the process further slows down at long times. We show that the temporal variations of the mean volume fraction at a given height can be described using two characteristic times. We have systematically studied the evolution of these times with the polymer concentration and the particle volume fraction of the suspension. Contrary to the well known results in Newtonian fluids, the more concentrated in particles is the suspension, the more efficient is the settling within intermediate times. The time needed for a complete separation of fluids and particles can however be longer than in a Newtonian fluid owing to the slow variations of the volume fraction at long times. Finally, the formation of the vertical structures is also studied according to the different parameters.