Simulation of band sedimentation of a log-normally distributed particulate suspension

We present Stokesian Dynamics simulation results for sedimentation of a suspension of spherical particles in the Stokes flow regime. Unlike in previous studies, the suspension is located in a band overlaying clear fluid and the suspension is polydispersed. The motivation for the work is to develop an efficient method to fractionate particles by size. The simulations show that for moderate particle concentrations the particle-rich region behaves macroscopically as an effective dense fluid with a clearly recognizable suspension/clear fluid interface; this interface develops vertical fingers that grow in time by a Rayleigh-Taylor instability. On the other hand, for very small particle volume fractions the particles sediment as individual objects with a velocity close to their Stokes velocity. The question is what sets the transition between these "effective fluid" and "particulate" regimes, as only in the latter regime fractionation is possible. In the talk, we will present visualizations of the particle dynamics as a function of the volume fraction, and statistics of the particle concentration calculated from the simulations, comparing against the case of monodispersed suspensions. We anticipate that an important feature is the convection of smaller particles by the larger ones.