Real-time droplet caliper for digital microfluidics

We developed an optical, microfabrication-free approach for performing real-time measurements of individual droplet characteristics (frequency of production, velocity, and length) flowing in a transparent microfluidic channel. Our approach consists in an interpretation of the differential signal produced by a pair of photodiodes connected head-to-tail due to the variations of illumination at the passage of a droplet. We checked the relevance of this zero-background method by comparing results to video measurements, and observed a very good agreement at rates up to the kHz range. Moreover, since the measured values are stored in a simple text file, flow characterization over very long times (several hours) becomes accessible. We applied this facility to perform three examples of long-term studies: stationary regimes, transient regimes, and the effect of an external forcing. Several unexpected features, like long-period fluctuations, can thus be evidenced.