Surfactant-free droplet breakup dynamics under confinement

Understanding the droplet actuation in microfluidics, enabling biomedical diagnostics and single-cell analysis has been the subject of much interest in recent times. Among these operations, droplet splitting is critical for controlling reaction volumes, concentration gradients, and sequential fluidic processes. In prior studies, surfactants were used to lower surface tension and facilitate breakup. However, such additives can contaminate reactions in many applications. In the present work, we show clean, surfactant-free splitting of pure water droplets in oil, driven solely by geometric confinement and controlled flow conditions. The Y-junction design directs the continuous-phase flow to intensify curvature and Laplace pressure at the droplet neck, while the bifurcated outlets impose hydrodynamic resistance that stretches the droplet until the neck collapses, leading to precise and reproducible splitting. We show that self-similar viscous-capillary breakup dynamics and a finite-time singularity remain robust even under confinement, extending the universality of droplet breakup to microfluidic geometries. Our work combines multiphase computational fluid dynamics and experiments to map the effect of surface tension, viscosity, and junction angle on the splitting thresholds.