Embedded Droplet Velocity Sensor for Lab-on-PCB Systems: A Case Study on Velocity and Monodispersity in Flow-Focusing Systems

Droplet velocity measurement in miniaturized Lab-on-PCB (LoPCB) platforms is essential for monitoring flow regime stability, monodispersity, and downstream processing in applications such as nanoparticle synthesis and clinical diagnostics, yet conventional optical techniques require bulky external instrumentation unsuitable for autonomous operation. This work presents a fully embedded opto-electrical velocity sensing system that performs sensing, signal conditioning, and data processing on-board, without requiring external instrumentation. The sensor architecture was designed following a Model-Based Systems Engineering (MBSE) workflow and validated through simulation prior to microcontroller deployment. The maximum detectable velocity, set by photoresistor response dynamics, increases linearly with droplet length. Experimental validation was performed in a 1.4 mm flow-focusing geometry using water-in-oil emulsions across flow-rate ratios (ϕ) from 0.5 to 2.0 under a fixed continuous-phase flow rate (Qc). The system achieved an average deviation of 5.32% from optical reference values across 195 droplets, with per-condition deviations ranging from 0.74% to 12.07% and below 6.4%, excluding the highest-dispersion condition. Embedded analysis of 797 droplets revealed a linear trend between velocity and droplet length (R2=0.993) and a power-law relationship between velocity and monodispersity (R2=0.912) over the 3.7–7.4 mm/s range. These results demonstrate suitability for autonomous, real-time droplet characterization in Lab-on-PCB platforms.