Singular jets from compound drop impact*

The impact of a water drop onto a solid surface can either lead to the deposition of the drop liquid or its rebound, depending on the wetting properties of the target surface. If the water drop is covered by an oil layer to form a compound drop, the water core can rebound from the target surface independently of the wetting properties. Under such conditions, the rebound dynamics can produce very thin and fast jets, similar to the singular jets observed for single phase drops impacting onto a superhydrophobic surface. We combine high-speed imaging experiments with high-resolution numerical simulations to study the formation of these jets. We demonstrate that the maximum jet velocity can reach two different peaks when varying the impact velocity. Furthermore, we show that the collapse dynamics of the cavity at the two peaks follows different scaling laws, due to two different mechanisms responsible for the singular flow at the two peaks. We explain how the complex dynamics of the two fluid interfaces are responsible for these differences, with the first jet due to the convergence of capillary waves, while the second jet is produced by the accumulation of oil above the recoiling water core.