Oil droplet impacting on the water surface: jet dynamics depending on the oil viscosity*

We investigate the evolution of Worthington jet and secondary droplets, driven by the oil droplet impacting on the water surface, while varying its viscosity. In detail, the silicon oil viscosity was varied in the range of 0.65-100 cP, and the droplet (the diameter of 1.5 - 2.3 mm) was dropped at different height from the water surface to achieve the impact velocity of 1.5 - 3.5 m/s. During the impact of oil droplet onto the surface, the resulting interface deformation, with a focus on the jet dynamics (the maximum height of the jet, the pinch-off time, and the height and number of secondary droplets, and so on), was measured with a high-speed camera at the speed of 4000-5000 fps. Interestingly, we found that the number of secondary droplets and their occurrence height was varied depending on the oil viscosity and the impact velocity of the droplet. With lower viscosity, the number of secondary droplets increased as the impact velocity decreases, and vice versa when the oil viscosity is higher. We will discuss further the mechanism of this phenomena.