Mechanism of particle dual-orbital motion in a laminar microvortex

Particle orbital motion in a hydrodynamic vortex confined in a microcavity is a relatively new issue of fluid mechanics. In this study, we use a high-speed microscopy system to visualize the phenomenon of particle two-orbital motion within a laminar microvortex. Specifically, a finite-size particle recirculates along a small inner orbit and a large outer orbit alternately and periodically. The influences of the inlet Reynolds number (Re = 110-270), particle diameter (d = 20 and 30 mu m), and microcavity size on the particle orbiting behaviors are investigated. The vortical flow field, orbital morphology, and particle velocity variations are characterized quantitatively to elucidate the mechanisms of particle recirculation along the dual orbits. The particle orbital motion results from the combined effects of hydrodynamic forces, particle slingshot effect, and particle-wall interactions in a complex way. The findings of this study could deepen the understanding of the particle orbital motion in a microvortex.