Motion of non-spherical microparticles with center of gravity offset in a circulating flow

The motion and structure of non-spherical particles with center of gravity offset in a circulating flow are analyzed numerically using a dumbbell model. The calculation is performed by the Stokesian dynamics which accounts for multiple-body hydrodynamic interparticle interactions based on the Stokes equation. A dumbbell is found to rotate or orient, depending on the center of gravity offset and the strength of the circulating vortex. The cluster deforms and rotates, and its formation shows a sequence of modes such as rupture, isolation, and aggregation. Three types of particle systems are used: (a) rotating particle groups, (b) orienting particle groups, (c) bimodal groups of rotating and orienting particles. In particle system (b), each particle forms face-to-face aggregations and the size of clusters becomes large compared with those of particle system (a). In particle system (c), the particles of each species, i.e., rotating particles and orienting particles, gather together and form clusters which consist of the same kind of particles. It appears that the local separation of two particle species results from the motion of particle pairs of two species, where the rotating particle rolls over the orienting particle.