Parallel Congruent Electrode Sensor to Detect and Characterize Droplets in a T-Junction Droplet Generator

In this article, we present a simple capacitive droplet sensing method that works with an on-chip droplet generator with real-time sensing capability of droplet composition and size. The sensing system works with the aid of a simple pair of electrodes along the channel wall forming a parallel congruent electrode (PCE)-based capacitor sensor. Using a PCE capacitor, we were able to characterize the droplet based on the material and its size. The droplet generator was classical T-junction-based and was regulated by our latest pneumatic-based control process. We present a numerical model that can simultaneously solve droplet generation and real-time capacitive sensing. The numerical model solves laminar two-phase flow, phase field, and electrostatics multiphysics simultaneously to predict droplet generation and capacitive sensing. We utilized the model to predict the best operating and geometric parameters. Our simulation showed that the electrode width to droplet size ratio of 1:0.95 was the best proportion for sensing droplet movement. This was verified experimentally. The two electrodes in the PCE position outperformed five electrodes in the coplanar interdigital electrode (IE) in the position for the same set of droplet generation. The change in capacitance value was observed using the PCE sensor for the variation in dispersed material and droplet size. The resolution of the sensing was 0.012 pF. The multilayer droplet generation, with simple and simultaneous sensing as well as regulation capability presented in this article, can be useful for the development of controls and sensing for precision droplet generators.