Enhancing droplet rebound on superhydrophobic cones

Understanding the underlying hydrodynamics and developing strategies to control bouncing droplets on superhydrophobic surfaces are of fundamental and practical significance. While recent efforts have mainly focused on regulating the contact time of bouncing droplets, less attention was given to manipulating droplet rebound from the perspective of energy optimization, which determines the long-term successive dynamics. Here, we investigate the impact of water droplets on superhydrophobic cones at low Weber numbers, where ideally complete rebounds arise. In sharp contrast to flat superhydrophobic surfaces, an impinging droplet on a cone-shaped superhydrophobic surface undergoes almost inversion-symmetric spreading and retracting processes with prolonged contact time, and more strikingly, it rebounds with a higher restitution coefficient. Such enhanced droplet rebound is beyond the prediction of existing theoretical models, in which the viscous boundary layer was recognized as the dominant channel of energy dissipation and, thus, an increase in the contact time would result in a lower restitution coefficient; nevertheless, numerical simulations have confirmed the increase in the restitution coefficient. The quantitative energy and flow field analyses of our numerical results reveal that the suppression of the boundary layer in early impact and the weakening of the viscous flow near the moving edge in the subsequent impact phases, which were not accounted for yet in existing theoretical models, are the causes for the enhancement of droplet rebound on superhydrophobic cones.

Automated summary

Understanding the underlying hydrodynamics and developing strategies to control bouncing droplets on superhydrophobic surfaces are important. Previous studies focused on regulating contact time, but less attention was given to optimizing droplet rebound. Contrary to flat surfaces, droplets on cone-shaped superhydrophobic surfaces exhibit inversion-symmetric spreading and longer contact time, resulting in higher restitution coefficient. Numerical simulations reveal that the suppression of the boundary layer and weakening of viscous flow near the moving edge contribute to the enhanced droplet rebound.