Single-particle trapping mechanisms into microcavities are still puzzling for size-based particle/cell sorting in microfluidics. Aiming to verify the prediction of particle-wall collision trapping mechanism, we explore the effects of the microcavity trailing wall on the single-particle trapping behaviors for various microcavity aspect ratios (λ = 0.5–5) and inlet Reynolds numbers (Re = 5–400), uncovering three new trapping phenomena, namely, contact trapping, collision trapping, and rapid trapping. We characterize the particle velocity variation during the trapping process. We also investigate the separatrix topology (streamlines between the microvortex and microchannel flow) and map the different particle trapping phenomena. The particle trapping results from the combined effects of the microcavity trailing wall, the separatrix topology, and the particle dynamics. The results provide new insight into the fundamental understanding of particle trapping mechanisms and could guide the applications of microcavity-based microfluidics.

The sorting of target particles from heterogeneous samples is challenging yet crucial for cell biology research and clinical diagnosis. Among various microfluidic methods, the use of cavity-based laminar vortex combined with inertial focusing is a powerful label-free passive technique for the selective sorting of large rare cells with high purity and concentration from billions of blood cells. However, this technology faces the challenge of improving the cavity holding capacity of trapped particles. This paper describes a round cavity-based vortex sorting method and presents a novel judgment criterion. The proposed round cavity achieves a holding capacity of entrapped target particles that is 2.2–7.8 times higher than that of rectangular cavities. By comparing the particle recirculating orbits and the simulated vortex morphology in round and rectangular cavities, a mechanism whereby particles/cells are held within the cavities is investigated. It is found that the area ratios (S = Ap/Ac) of the particle orbit area (Ap) to the cavity area (Ac) are 0.56 and 0.95 for the rectangular and round cavities, respectively. The results show that the round cavity provides more efficient space for recirculating particles and has better sorting performance. This round cavity-based vortex sorting method will be useful for clinical applications.