Size-tunable microvortex capture of rare cellsElectronic supplementary information (ESI) available. See DOI: 10.1039/c7lc00355b

Inertial separation of particles and cells based on their size has advanced significantly over the last decade. However, size-based inertial separation methods require precise tuning of microfluidic device geometries to adjust the separation size of particles or cells. Here, we show a passive capture method that targets a wide size range of cells by controlling the flow conditions in a single device geometry. This multimodal capture device is designed to generate laminar vortices in lateral cavities that branch from long rectangular channels. Micro-vortices generated at lower Reynolds numbers capture and stabilize large particles in equilibrium orbits or limit cycles near the vortex core. Other smaller particles or cells orbit near the vortex boundaries and they are susceptible to exiting the cavity flow. In the same cavity, however, at higher Reynolds number, we observe small particles migrating inward. This evolution in limit cycle trajectories led to a corresponding evolution in the average size of captured particles, indicating that the outermost orbits are less stable. We identify three phases of capture as a function of Reynolds number that give rise to unique particle orbit trajectories. Flow-based switching overcomes a major engineering challenge to automate capture and release of polydisperse cell subpopulations. The approach can expand clinical applications of label free trapping in isolating and processing a larger subset of rare cells like circulating tumor cells (CTCs) from blood and other body fluids. A wide distribution of cell sizes can be selectively captured by tuning the flow rate in a micro-vortex flow.