Oil droplet impacting on the bulk water: energy transport depending on the oil viscosity

We experimentally investigate the interfacial phenomena (crater and jet) driven by the oil droplet impacting on the bulk water, while varying its viscosity. The silicon oil droplet (diameter of 2.7 mm), whose viscosity is varied as 0.65-100 cP, was dropped at different heights from the water surface to achieve the impact velocity of 1.5 - 3.5 m/s. The interaction of droplet and water surface was captured with a high-speed camera. Upon impacting, a crater is formed below the surface and as it retracts due to surface tension, a vertical jet appears above the surface. Various jet phenomena appear depending on the viscosity and impact velocity, which are categorized into four regimes based on the pinch-off, jet thickness, and secondary droplet composition. To understand the transition between each regime, we analyzed the temporal energy transport between gravitational potential energy, surface energy, and kinetic energy. The results show that there is less energy conversion (i.e., higher loss to viscous dissipation) to kinetic energy at higher viscosities than at lower viscosities, which leads to differences in the geometry of the jet (the maximum height of jet, the presence of pinch-off, and so on).