Successive droplets impacting on non-wetting surfaces: Impact force and contact time

Successive droplet impact on non-wetting surfaces (NWSs) remains poorly understood compared to single droplet impact. In this work, the impact outcomes, contact time, and impact forces of successive droplets impacting NWSs are investigated via direct numerical simulations. Four typical outcomes, including single bounce, double bounce, spindle-shaped, and arrow-shaped jets, were observed by varying the droplet impact spacing, Weber number (We), and Ohnesorge number. A positive correlation between the contact time and both the impact spacing and We was revealed. It was found that, unlike the single droplet impact process, the force evolution over time in the successive droplet impact process showed a trimodal peak characteristic. The generation mechanism of the two peak forces (F1* and F2*) among these is similar to that of single droplet impact, while the another peak force originates from the appearance of the trailing droplet. Interestingly, the variation of impact spacing does not change the F1* but intensifies or suppresses the F2*. Additionally, the energy analysis shows that the collision process is highly dissipative, and the viscous dissipation of the whole process can account for up to 80% of the total initial energy. Meanwhile, it is observed that the post-collision viscous dissipation increases with increasing impact spacing. These findings reveal the dynamic and mechanical characteristics of successive droplets impacting on NWSs and provide an important theoretical basis for a deeper understanding of multi-droplet interactions.