Droplet rebound and dripping during impact on small superhydrophobic spheres

While droplet impact processes on hydrophilic and hydrophobic spheres have been widely investigated experimentally and numerically, the impact behaviors of water droplets on small superhydrophobic spheres are studied numerically and theoretically in this research. The numerical model adopts the volume of fluid method (VOF) and is verified by comparing the simulation results with the experimental observations in the literature. The effects of Weber number and sphere-to-droplet diameter ratio on the droplet impact dynamics are discussed. The final outcomes of the impact droplets are classified into rebound and dripping types with the latter appearing at a larger Weber number or a smaller diameter ratio. As the Weber number and diameter ratio increase, droplet deformation during impact is reinforced with the maximum width factor of the rebound droplet becoming greater. The maximum width factor of the dripping droplet is nearly independent of the Weber number but is enlarged by the increasing diameter ratio. Moreover, a larger diameter ratio reduces the contact time of the rebound droplet but raises that of the dripping one. A theoretical model based on energy conservation is established to predict the boundary between the droplet rebound and dripping outcomes and is in good agreement with the simulation results. The diameter ratio limit for droplet dripping at a zero Weber number is also obtained. Our results and analyses provide insight into the interaction mechanism between the impact droplet and small spheres or particles.