From singular jets to compound droplets: Viscosity and inertia control of immiscible drop-pool impact dynamics.

Abstract

Hypothesis: Secondary-droplet composition in immiscible drop-pool impact is hypothesized to be governed by the relative motion of the oil-air and oil-water interfaces formed at the tip and inside the jet, respectively. Increasing oil (droplet) viscosity should reduce this relative motion and favor compound oil-water droplets, while crown breakup may provide an additional high-inertia route to their formation.

Experiments: Silicone-oil droplet (2.7-2.8 mm in diameter) with dynamic viscosity in the range of 0.49-96.3 mPa · s, impacts a deep-water pool at 1.6-4.4 m/s, covering Froude and Ohnesorge numbers of 99-727 and 0.003-0.4, respectively. High-speed shadowgraphy and tracer-particle visualization, with tracers dispersed only in the water phase, are used to track both interfaces and identify secondary-droplet composition.

Findings: Based on jet morphology and secondary-droplet composition, four regimes were identified: (i) a short jet without pinch-off, (ii) a singular jet, (iii) an oil-dominated secondary-droplet regime, and (iv) a compound secondary-droplet regime. Following the inertia-driven crater, the subsequent collapse-to-jet transition was strongly affected by oil viscosity. Increasing viscosity limited the stretching of the oil-rich portion in the jet, resulting in the thick jet. This shifted the location of primary pinch-off from the internal interface (oil secondary droplet) to the water-containing lower portion of the jet (compound one). At sufficiently high impact inertia, crown breakup provided an additional route to compound-droplet formation. These new findings establish a direct link between the coupled kinematics of the oil-air and oil-water interfaces, pinch-off location, and secondary-droplet composition in immiscible impacts.