Continuous inertial focusing, ordering, and separation of particles in microchannels

TLDR

In laminar microchannel flow, particles are generally expected to follow fluid streamlines. Lift forces arising from inertial flow, amplified by symmetric or asymmetric channel geometries, bias particles to specific equilibrium positions, producing continuous, three‑dimensional streams of ordered particles. The authors demonstrate that particles migrate across streamlines in a continuous, predictable, and accurate manner, achieving >80‑nm lateral precision, longitudinal chaining, rotational alignment of discoidal red blood cells, and size‑dependent ordering independent of buoyancy, enabling high‑rate, force‑free bioparticle separation, cytometry, and filtration.

Abstract

Under laminar flow conditions, when no external forces are applied, particles are generally thought to follow fluid streamlines. Contrary to this perspective, we observe that flowing particles migrate across streamlines in a continuous, predictable, and accurate manner in microchannels experiencing laminar flows. The migration is attributed to lift forces on particles that are observed when inertial aspects of the flow become significant. We identified symmetric and asymmetric channel geometries that provide additional inertial forces that bias particular equilibrium positions to create continuous streams of ordered particles precisely positioned in three spatial dimensions. We were able to order particles laterally, within the transverse plane of the channel, with >80-nm accuracy, and longitudinally, in regular chains along the direction of flow. A fourth dimension of rotational alignment was observed for discoidal red blood cells. Unexpectedly, ordering appears to be independent of particle buoyant direction, suggesting only minor centrifugal contributions. Theoretical analysis indicates the physical principles are operational over a range of channel and particle length scales. The ability to differentially order particles of different sizes, continuously, at high rates, and without external forces in microchannels is expected to have a broad range of applications in continuous bioparticle separation, high-throughput cytometry, and large-scale filtration systems.