Numerical simulation of concentration interface in stratified suspension : Continuum–particle transition

Behavior of concentration interfaces, which are apparent interfaces between a suspension and pure liquid, was studied using numerical techniques. Two types of numerical simulations were used to classify whether collective or individual behavior occurred in the ratio of the average particle separation to the wavelength of the fastest growing perturbation. The first is Lagrangian tracking of individual particles in fluid, and the second type is interface tracking of two immiscible continuum phases. These two extremely different approaches represent the dual nature of the concentration interface: immiscible with no interfacial tension and miscible with no diffusion. These results reflect the experimentally-observed behavior of particles, which is both collective and individual. Sealing of the concentration interface by particle-induced flow is crucial to collective motion of suspended particles. A proposed dimensionless parameter describes quantitatively the transition from collective to individual settling of suspended particles