A continuous flow, passive separation device was designed using an equivalent circuit to create variable flow rates for hydrodynamic focusing to drain channels and collection outlets. By varying the diameter of the sample inlet connection into the reservoir, the particle position was influenced significantly, which enabled desired separations. Additionally it was noted that the relative, horizontal position of the inlet also had a significant influence on particle position within the device. A dimensionless number, the Characteristic Sample Inlet, was developed to relate geometric properties of the inlet reservoir to downstream particle distribution. It was found that a 2:1 ratio between inlet reservoir and sample inlet diameter, and placed at the top of the reservoir yielded the best separation results. Fluid velocity profiles in the reservoir were explored using Comsol Multyphysics. The experimentally observed particle trajectories and COMSOL predictions were in good agreement. Based on Comsol models a dimensionless parameter to relate the unique velocity profiles within the inlet reservoir to downstream separation of particles was also developed. A mixture of 10, 5.5, and 3.0 µm particles were separated to three distinct collection outlets at 73.4%, 64.7%, and 52.8% respectively. Therefore this project shows that passive separations of particles can be achieved simply by alerting the ratio of inlet hole relative to inlet reservoir diameter, and by placing the inlet hole at the top of the reservoir.

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