Sedimentation of small particles:: how can such a simple problem be so difficult?

Although sedimentation can be considered as one of the simplest examples of suspension flow, much remains unknown about the fundamental properties of sedimenting suspensions. The problem that one encounters lies in the long range nature of the multibody hydrodynamic interactions between particles. This will be illustrated for sedimenting suspensions of spheres, of non-spherical particles such as fibers, and for sedimenting clouds of particles.