Surfaces are rarely completely dry. If they are of intermediate wetting, such as most agricultural crop or plant leaves, they typically support sessile drops on them. Yet, very few studies focused on the outcome of impact of surfaces that are already pre-wetted with sessile drops.
In study on drop-on-drop impacts, we derive the kinematic and dynamics conditions and regime map selecting for the Crescent-moon fragmentation regime important for impacts on surfaces of intermediate wetting, and particularly contaminated leaves. The drop-on-drop impact dynamics is critical for understanding of retention and removal from spray impacts, and spreading and dispersal of chemicals and pathogens from plants to plants.
Using a combined geometric and momentum conservation argument, we also derive the expressions governing the base of the Crescent-moon and its bordering ligaments. For more, read: Wang, Y. and Bourouiba, L. (2018) Non-isolated drop impacts. Journal of Fluid Mechanics. 835:24-44.