Abstract
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, straight-forward, 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. $600~\mu \text{m}$ /s (volumetric flow rate $\Phi _{v} \cong 200$ nl/min, Reynolds number $\textit {Re} \cong 10^{-3}$ ) to approx. $3500 ~\mu \text{m/s}\left ({\Phi _{v} \cong 1200~ \text{nl/min}, ~\text{Re} \cong 10^{-2}}\right)$ .
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