Dynamics of optoelectrowetting-induced unequal-sized droplet coalescence in a parallel-plate confined space

Parallel-plate optoelectrowetting (OEW) is a cutting-edge platform for high-throughput biochemical analysis; however, its currently extremely narrow gap height leaves the droplet coalescence dynamics within the confined space unclear. Combining a high-speed imaging system with three-dimensional finite element simulation, this study systematically analyzes the OEW-induced asymmetric droplet coalescence dynamics within an extremely narrow space of 130 μm. Based on quantifying the contact angle variations in both light and dark areas within the 0–600 V voltage range, this work investigates the coalescence details in the confined space under a typical wetting state (where the light area is hydrophilic and the dark area is hydrophobic). The study not only accurately captures the sub-millisecond-scale transient response of the contact angle after electrification and the dynamic evolution of the liquid bridge completed in only 0.5 ms, but also finds that the size difference between droplets leads to a significant offset of the liquid bridge toward the smaller droplet. Research on key parameters indicates that decreasing the initial droplet spacing can significantly accelerate the coalescence process, whereas increasing the voltage causes the dark-area contact angle to approach saturation, leading to strong lateral spreading that reduces the rising velocity of the liquid bridge. This study reveals the complex dynamics mechanism of OEW droplet coalescence under the interaction of electric fields and fluids within a confined space.