Rebound dynamics of double-droplet impact on a superhydrophobic spherical surface

The impact of double droplets on a fixed canonical curved surface is a common yet complex phenomenon, in which the intrinsic relationship among inter-droplet interactions, rebound dynamics, and geometric constraints of the surface has not been fully elucidated. To address this, the three-dimensional multi-relaxation-time Shan–Chen lattice Boltzmann method is employed to simulate simultaneous and non-simultaneous double-droplet impacts on a superhydrophobic spherical surface. This study elucidates the formation mechanisms of the complete-coalescence rebound (CCR), partial-coalescence rebound (PCR), and no-coalescence rebound regimes, evaluating the effects of the Weber number and droplet spacing on contact time, restitution coefficient, and centroid offset. For simultaneous impacts, the results show that the contact time exhibits a U-like dependence on the horizontal spacing, reaching a minimum in the PCR regime. This is attributed to a fundamental transition in the retraction mechanism: from the inefficient “sides-first, center-last” liftoff in the CCR regime to the highly efficient “center-led, sides-follow” detachment in the PCR regime. The restitution coefficient decreases with increasing horizontal spacing. For non-simultaneous impacts, both horizontal and vertical spacing enlarge the contact time and centroid offset. Moreover, a threshold Weber number (We ≈ 41) is identified, above which lamella rupture driven by strong inertia markedly prolongs the contact time.