Rebound dynamics of a low-velocity droplet impacing a thin liquid film on a curved surface

This study presents an experimental and theoretical investigation on the dynamics of droplets impacting on thin liquid films over cylindrical surfaces at low velocities. For different impact velocities and cylinder-to-droplet diameter ratios, various impact regimes were observed, including complete rebound, partial rebound, rebound breakup, and coalescence. The Weber number ranges for droplet rebound under different conditions were identified. The results show that the rebound process of droplets on cylindrical surfaces exhibits asymmetric characteristics, with asymmetry increasing as the diameter ratio decreases. The restitution coefficient during rebound was investigated when the diameter ratio is much greater than 1, and it was found that droplets exhibit better elasticity when rebounding on thin liquid films. Additionally, a new model was proposed to describe the droplet spreading time. Based on this, energy balance models were established to predict the maximum spreading scale of droplets for the conditions where the diameter ratio is around 1 and much greater than 1, respectively. The model results show good agreement with the experimental data. Finally, the energy conversion during the spreading process was analyzed based on the current model.