Hydrodynamics during an immiscible compound droplet impact on a liquid pool

A numerical model based on the volume of fluid method is adopted to numerically study the hydrodynamics of an immiscible compound droplet impacting on a liquid pool. This numerical simulation achieves good agreement with the experimental results for both the evolutions of interface and cavity depth after droplet impact. By conducting the numerical simulation, three impact regimes are identified, namely, engulfment, bursting, and splashing, and a regime map with splashing threshold is plotted to quantitatively represent them. Under both bursting and splashing regimes, the inner and outer droplets have similar deformation behaviors during impact. The changes in impact velocity and inner droplet size have a greater effect on the hydrodynamic behaviors of the compound droplet under the bursting regime than that under the splashing regime. Larger inner droplet sizes can significantly reduce the deformation of the droplet and cavity. Moreover, to provide valuable guidance for controlling the compound droplet impacting on the liquid pool in the related real applications, a scaling correlation with a modified Weber number is proposed to predict the maximal spreading of the droplet.