Collective and individual motions of particulate dispersion with concentration interface

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Collective motion of fine particles in liquid can be widely seen in engineering processes such as water treatment or sediment transport. We have studied whether collective or individual settling motion of particles reveals in liquid. The results showed that the concentration interface which is an ambiguous interface between suspended particles and pure fluid plays a significant role in these extreme behaviors. In cases of small particle size with high concentration, the interfacial instability occurs at the lower concentration interface and consequently the settling velocity is much faster than that of an isolated particle. On the other hand, in case of large particles with low concentration, the concentration interface is less distinct and the suspended particles settle individually. We showed that the transition from these collective to individual motions of suspended particles can be controlled by the border resolution of concentration interface.