Single and double toroid formation during oil droplet impact on an air-water interface at low Reynolds number

The impact of an oil droplet on an air-water interface is explored for low to moderate impact velocities. A computational fluid dynamic framework has been employed with appropriate boundary conditions to uncover the finer features of post-impact dynamics of such ternary systems. Simulations reveal that the impacting oil droplet opens up a crater on the water surface, which initially expands and then collapses during the evolution. Simultaneously, the oil droplet flattens, spreads, stretches, immerses, or dewets on the crater surface to manifest interesting metastable or unstable flow morphologies. At lower impact velocities, we observe the formation of oil droplets or air bubbles entrapped in water, oil lens, oil toroids, and compound droplets to name a few. The interfacial tension, density and viscosity contrasts across the interface play key roles in the formation of such flow morphologies. Energy analysis of the droplet impact reveals that a part of the kinetic energy of the droplet gets converted into surface energy, which, in turn, facilitates the interfacial deformation, formation of new interfaces, and metastable flow morphologies, such as single or twin toroids. All the different flow morphologies are categorized into eight fundamental regimes, which are mapped with the variation of Reynolds number and capillary number. The results provide insight into the complex physics associated with ternary phase drop impacts, and the different flow morphologies shown in the present study can be of significance in the production of double or Janus emulsions, as well as the development of next-generation microfluidic devices for bio-analysis, drug delivery, and multiphase reactions.

Automated summary

This study investigates the impact of an oil droplet on an air-water interface at low to moderate velocities using computational fluid dynamics simulations. The results reveal different flow morphologies, such as entrapped oil droplets or air bubbles in water, oil lens, oil toroids, and compound droplets, depending on the impact velocity. The interfacial tension, density, and viscosity differences across the interface play key roles in determining the flow morphologies. The findings provide insights into the physics of ternary phase drop impacts and have implications for the development of microfluidic devices and emulsion production.