Evolution of single-particle recirculating orbits within a hydrodynamic microvortex

Abstract

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 (Rec), 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.