Inertial Focusing in Spiral Microchannels
The flow rate increases over time. At low flow rates, the red and green particles (which are different sizes) are mixed together. At higher flow rates, the green particles migrate toward the center of the channel while the red ones stay toward the inside. At even higher rates, the red ones migrate toward the center as well.
In this video, what appears to be a single stream of focused particles in the microchannel is revealed at the outlet to be three separate streams. The first two streams separate at the outlet near the top of the video. As the flow rate is changed, one stream is shown to split into two separate streams, indicating that the observed focusing is due to the overlap of three separate focal streams.
Martel, J. and Toner, M. "Inertial Focusing Dynamics in Spiral Microchannels." Physics of Fluids, 24, 032001 (2012).