Resolving Early-Stage Bridge Dynamics in Oil Droplet Coalescence Using High-Speed Microscopy*

Droplet coalescence impacts the dynamics of emulsification, aerosol formation, and oil-spill dispersion. The earlier regime of coalescence determines how surface tension, viscosity, and inertia redistribute fluid mass.  Yet it remains not well characterized for viscous oils where bridge evolution is dominated by the surface-tension–viscosity balance. Recent theoretical models predict a universal crossover from viscous to inertial scaling as the bridge radius grows, but optical data below ~50 µm remain limited.

Using a high-speed camera-microscopy system, the coalescence of oil droplets (R ≈ 0.7 mm) in both air and saltwater is visualized at sub-micron spatial and microsecond temporal resolutions. The setup combines a high-speed camera with a 50× long-working-distance objective and synchronized pulsed illumination, enabling direct observation of bridge radii down to ≈ 30 µm, extending optical access into a range previously accessible only through electrical measurements. Complementary wide-field imaging with another high-speed camera captures overall droplet geometry and contact-line motion, linking microscopic bridge dynamics to macroscopic deformation.

Experiments quantify how viscosity, interfacial tension, and ambient medium influence bridge growth across oils of differing properties. The results provide quantitative benchmarks for modeling oil coalescence in environmental and industrial multiphase flows.