Interaction between a pair of particles settling in a stratified fluid.

The anisotropic structure of fluidized suspensions is governed by their microstructures which are in turn determined by the dynamics of particle pair interactions. Here, we present a numerical simulation of particle interaction in linearly stratified fluids. It is shown that unlike homogeneous fluids, stratification results in the attraction of particles settling abreast. The attraction between the particles is characterized by the combined effects of buoyancy, inertia, and diffusion. The interaction of the particles settling in tandem can be fundamentally altered due to the presence of the background density gradients and the drafting-kissing-tumbling behavior in a homogeneous fluid can be replaced by drafting-kissing-separation or drafting-separation phenomenon depending on the strength of the stratification. In the case of weak stratification, drafting-kissing-tumbling occurs, however, a prolonged kissing time is observed and the rate of change of the orientation of particles is reduced. It is shown that the formation of the buoyancy-induced vortical structures and the generation of stratified jets behind the particles are the fundamental mechanisms in governing the dynamics of the particle pair interaction in stratified fluids.