Evolution of single-particle recirculating orbits within a hydrodynamic microvortex

Sized-based sorting and trapping of particles and cells from a mixture utilizing a hydrodynamic microvortex has been a flourishing area of inertial microfluidics in recent years. From the point of view of fluid mechanics, many fundamental issues remain unrevealed in this research area. Here, using a high-speed microscopic imaging system, we experimentally investigated the formation and evolution of isolated particle recirculating orbits induced by a hydrodynamic microvortex within a square microcavity (400 µm × 400 µm). The influence of the inlet Reynolds number (Re) over a wide range (88–244) on the evolution of recirculating orbits of particles with different diameters (d = 10 µm and 20 µm) at relatively very low concentration was systematically investigated to further previous studies. We also observed an intriguing phenomenon that a larger single-particle (d = 35 µm) always occupied the outer orbit, while a smaller single-particle (d = 20 µm) occupied the inner orbits at Re = 155. This result is contrary to previous reports and we explored the reason for it. Moreover, we quantitatively characterized the dimensionless particle orbit areas (A) and critical inlet Reynolds number (Re c ), which determines the formation of particle orbits. The results provide further insights into the fundamental understanding of particle behaviors of trapping and orbiting and a useful guideline for microvortex-based microfluidics applications.