Particle concentration evolution and sedimentation-induced instabilities in a stably stratified environment

We present the results of a combined experimental and theoretical investigation of sedimentation in a stratified fluid. The theory of sedimentation in a homogeneous ambient is extended to include the influence of a spatially varying particle settling speed. The results of an experimental investigation of latex particles settling in a stably stratified salt water solution are reported. Density variations in the suspending fluid reduce the particle settling speed which increases particle concentrations, thus enhancing the effects of hindered settling. A criterion is developed for the convective instability of an initially uniform suspension settling in a stably stratified ambient. If, as depth increases, the magnitude of the ambient density gradient decreases sufficiently rapidly, an initially uniform particle concentration will give rise to a statically unstable density profile. Experimental observations provide qualitative verification of this new stability criterion. (c) 2005 American Institute of Physics.