The sedimentation of fine particles in a stratified suspension, which has both the upper and lower interfaces, is studied experimentally. We measure the settling velocity of particles and observe the diffusion behavior near the interfaces. Especially we examine whether the macroscopic or the microscopic natures of suspension is dominant during sedimentation, i.e., the particles settle as an particle assembly relative to surrounding fluid or as a continuous suspension. The experimental observation shows that the gravity-induced instability of suspension-fluid interface, which has been already reported, governs the particle motion for high concentration and small particle size. However, the microscopic nature of suspension comes out in case of larger particles. In our study, the dependency of the sedimentation behavior on the particle size and the concentration is discussed quantitatively.

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