Surfactant-free splitting of water droplets in a microchannel.

Precise manipulation and splitting of liquid droplets in confined geometries is essential for lab-on-a-chip platforms used in chemical synthesis, point-of-care diagnostics, and biological assays. Here, we demonstrate and analyze passive droplet splitting in a Y-junction microchannel by combining junction-angle optimization, droplet neck-thinning scaling analysis, and surfactant-free breakup of pure water droplets in oil, an integrated framework that has remained largely unexplored in prior passive splitting studies. In prior studies, surfactants were used to lower interfacial 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. Time-resolved tracking of the neck width L(t*) demonstrates a power-law collapse L ∼ (t*)α with α∈[0.93, 0.985], consistent with breakup proceeding through a self-similar, finite-time pinch-off dynamics in the viscous-capillary regime. Our work combines three-dimensional multiphase computational fluid dynamics (CFD) and experiments to map the effects of oil-water interfacial tension (0.01-0.05 N m-1), viscosity (0.02-0.20 kg m-1 s-1), and junction angle (30°, 45°, 60°, and 90°) on the splitting thresholds. The 45° Y-junction yields the lowest critical splitting velocity across all tested conditions, reflecting optimal concentration of viscous-capillary stress at the neck.