Eur. Phys. J. E (2012) 35: 1
https://doi.org/10.1140/epje/i2012-12001-6

Regular Article

Particle-like and fluid-like settling of a stratified suspension

Division of Sustainable Resources Engineering, Faculty of Engineering, Hokkaido University, N13-W8, 060-8628, Sapporo, Hokkaido, Japan

^* e-mail: harada@eng.hokudai.ac.jp

Received: 13 July 2011
Revised: 30 September 2011
Accepted: 14 December 2011
Published online: 9 January 2012

Abstract

The gravitational settling of inhomogeneously suspended particles in a fluid has been investigated. Of particular interest is whether collective or individual motion of particles is dominant during their settlings, i.e., whether the particles settle as a continuous suspension or they settle individually relative to the surrounding fluid. We observed the settling of a stratified suspension which has the lower and upper concentration interfaces in a quasi-two-dimensional vessel. In some cases, the suspension behaves perfectly as a continuous fluid and the motion of the constituent particle is subject to bulk flow caused by the interfacial instability. In other cases, the particle behaves individually relative to the surrounding fluid. The existence of a concentration interface plays a significant role in these extreme behaviors of suspension. The transition from the collective to individual behaviors can be predicted quantitatively by a parameter which expresses the border resolution of the concentration interface.