Proof-of-Concept Experiment of Microfluidic Flow Sensor Based on Microcavity- Secondary-Flow Observation

We propose a new type of microfluidic flow sensor, where the velocity measurements are not done on the fluid sample itself but on a second, immiscible fluid in contact with the sample. This allows us to use a low-cost, straightforward, and well-developed particle tracking velocimetry on samples otherwise unsuitable for tracer particles. Our proof-of-concept experimental setup consists of a microfluidic device with the main channel filled with the sample and rectangular side-cavities, adjacent and opened to the main channel, filled with immiscible fluid with tracer particles. The flow in the main channel induces a circular secondary flow of the cavity liquid. We observed and analyzed the side-cavity particle movement and found that its characteristic parameters show a good linear correlation with flow velocity in the main channel over the whole observed range, from approx. 600 μm/s (volumetric flow rate Φ_v ≅ 200 nl/min, Reynolds number Re ≅ 10^-3) to approx. 350 Mm/s (Φ_v ≅ 1200 nl/min, Reynolds number Re ≅ 10^-2.