Impact of a core–shell compound droplet on a solid surface

Droplet impact on solid surfaces is fundamental to many natural and industrial processes, from water distribution in agriculture to precision technologies like inkjet printing and fuel injection. Recent studies have increasingly focused on the complex dynamics of multi-component, core–shell droplets, driven by their widespread presence in fields such as targeted drug delivery, biofuels, and 3D printing. Understanding the outcome of the impact of compound droplets and their maximum spreading on a solid surface is needed. This research investigates the influence of the controlling parameters, namely a broad range of core size, the core and shell viscosities, and Weber number on the impact outcome and the maximum spreading. Experiments of water-in-oil compound drops impacting on glass surface were conducted up to the range of impact parameters below splashing threshold. An equivalent Weber number (\(\overline{\text{We} }\)) was introduced to account for the core–shell interfacial energy. Results reveal that the size of the core and the viscosity of the shell play critical roles in determining impact behavior. Larger cores tend to enhance prompt splashing and rebound, while thicker shells dampen the rebound of the vertical jet formed by the core. Viscous cores significantly damped the rebound while had no influence on splashing. The maximum spreading factor is vastly affected by the shell layer viscosity rather than the core’s. The size of the core influences the maximum spreading in two different ways, varying the compound drop viscosity and increasing the core–shell interface. A quantitative framework for compound droplets impact on solid surfaces is established, focusing on impact outcome and maximum spreading. Distinct outcome regime boundaries and transitions are mapped within the parameter space of controlling parameters, while their influence and controllability on maximum spreading are systematically evaluated.