Breakup kinetics of water-in-water droplets without obstruction in Y-shaped microchannel

The breakup of water-in-water droplets in a Y-junction microchannel was experimentally investigated, focusing on the unobstructed breakup mode due to its broad applicability. This process was divided into four stages: entering, squeezing, pinch-off, and filament rupture, based on the evolution of the neck width and its thinning rate. In the entering stage, neck thinning accelerates with increasing dispersed-phase flow rate Q_d, droplet length l₀, and two-phase viscosity ratio λ, and slows down with increasing continuous-phase flow rate Q_c. In the squeezing stage, the minimum neck width follows a power-law relation with time. The thinning rate increases with higher Q_d, l₀, and λ, whereas increasing Q_c inhibits thinning. In the pinch-off stage, neck width decreases linearly with time, and the thinning is enhanced by higher Q_c, Q_d, and l₀, but remains nearly independent of λ. The filament rupture stage follows a power-law thinning trend, and exhibits similar parameter dependencies, though satellite droplets may form at high Q_d and Q_c, and show increased instability at low viscosity. These findings offer insights into neck dynamics and interfacial rupture in low interfacial tension systems.