We conducted numerical experiments to investigate the mixing of stratified suspensions containing different types of particles. We used a point-force two-way coupling method. We studied the mixing behavior of stratified suspensions and we discovered two types of mixing: microscopic (individual-particle-level) and macroscopic (vessel-scale) collective mixing. In addition, we examined the vertical mixing speed of the stratified suspension. We used a simple theoretical model to analyze the fingering settling velocity. Then we introduced a nondimensional number representing the difference in collectivities of the upper and lower suspensions while accounting for particle terminal velocities. We discovered that the proposed nondimensional parameter has a negative sign that distinguishes the mixing form of only microscopic individual-particle-level mixing and a positive value that predicts the speed of macroscopic collective mixing of stratified suspensions.
