Systems have been demonstrated that use scale-dependent electromagnetic forces ( 8– 11), microscale hydrodynamic effects ( 12– 14), or deterministic physical interactions and filters ( 15– 17). Recently, microfluidic systems have been shown to be very useful for particle handling with increased control and sensitivity. Traditional techniques of particle manipulation rely on laminar flow ( 4) or differences in either particle mobility or equilibrium position in a flow with a variety of externally applied forces ( 5– 7). 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.Ĭontinuous manipulation and separation of microparticles, both biological and synthetic, is important for a wide range of applications in industry, biology, and medicine ( 1– 3). Theoretical analysis indicates the physical principles are operational over a range of channel and particle length scales. Unexpectedly, ordering appears to be independent of particle buoyant direction, suggesting only minor centrifugal contributions. A fourth dimension of rotational alignment was observed for discoidal red blood cells. 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. 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. The migration is attributed to lift forces on particles that are observed when inertial aspects of the flow become significant. Contrary to this perspective, we observe that flowing particles migrate across streamlines in a continuous, predictable, and accurate manner in microchannels experiencing laminar flows. Under laminar flow conditions, when no external forces are applied, particles are generally thought to follow fluid streamlines.
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