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This review concentrates on the fluctuations of the velocities of sedimenting spheres, and on the structural instability of a suspension of settling fibers. For many years, theoretical estimates and numerical simulations predicted the fluctuations of the velocities of spheres to increase with the size of the container, whereas experiments found no such variation. Two ideas have increased our understanding. First, the correlation length of the velocity fluctuations was found experimentally to be 20 interparticle separations. Second, in dilute suspensions, a vertical variation in the concentration due to the spreading of the front with the clear fluid can inhibit the velocity fluctuations. In a very dilute regime, a homogeneous suspension of fibers suffers a spontaneous instability in which fast descending fiber-rich columns are separated by rising fiber-sparse columns. In a semidilute regime, the settling is hindered, more so than for spheres.
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Supplemental Movie 1: Sedimentation of a suspension of spheres having Φ ≈ 0.3 in a test tube at low Reynolds number, showing a sharp front between the clear fluid and the suspension (Credits: B. Metzger & É Guazzelli). Download movie file (MP4)
Supplemental Movie 3: Sedimentation of a dilute suspension of fluorescing fibers within a laser sheet, showing the evolution of the streamer structure (Credits: B. Metzger, J. E. Butler & É Guazzelli) (accelerated movie). Download movie file (MP4)
Supplemental Movie 4: (Left) Point-fiber simulations with bottom wall (Credits: D. Saintillan, E. Darve & E.S.G. Shaqfeh) and (right) experiments (Credits: B. Metzger, J. E. Butler & É Guazzelli) under similar conditions (accelerated movie). Download movie file (MP4)
Supplemental Movie 5: Bottom view of a cloud of colored spheres settling in silicon oil (Metzger et al. 2007c). Download movie file (MP4)
Supplemental Movie 6: Side and bottom view of a simulation of a sedimenting cloud of point particles in an infinite fluid. The initial number of particles is 3000, but only half of them are plotted (Metzger et al. 2007c). The timescale is the Stokes time of the cloud. Download movie file (MP4)