On the dynamics of head-on collision of sessile droplets on the traveling substrate

In this work, the dynamics of the head-on collision of two sessile water droplets, namely the moving droplet and pinning droplet, on a traveling substrate was experimentally investigated. Unlike the traditional scenario of sessile droplets collision, the traveling substrate propelled the moving droplet attached to it to move at a certain speed. The results revealed that there are only two distinct regimes of collision consequences: coalescence without separation and separation with separation. A series of time-resolved photographic images mapping all the collision regimes in the parameter space of Capillary number and size ratio were used to identify the controlling factors for different collision results. The breakup of the receding meniscus in separation regime was discussed in detail as it is caused by the change of the droplet morphology. The findings indicated that the length of the receding meniscus played a significant role in the droplet impact dynamics, and when it exceeded about 8.2 mm, the receding meniscus broke up. Furthermore, it was demonstrated that the sum of the volume of initial pinning droplet's receding meniscus and the volume of the moving droplet before collision is equal to the volume of the receding meniscus at its maximum length after collision. The critical Capillary number/size ratio for the transition between these two regimes was derived based on the volume principle before and after the collision. The model-predicated results show good agreement with the experiments. The findings could contribute to a better understanding of the droplet collision dynamics and the optimization of the anti-icing, self-cleaning and inkjet printing devices.