Impact Dynamics Of Droplets On Cone-shaped Superhydrophobic Surfaces

It is of great practical significance to understand the basic hydrodynamic characteristics of the bouncing droplet on the superhydrophobic surface according to its key characteristics and to customize the corresponding control strategy.Current research work focuses on controlling the contact time of bouncing droplets,which is beneficial on a range of applications,such as anti-icing,self-cleaning,etc.,by adjusting the contact time between the droplets and the surface.Another method to control droplet bouncing is from the perspective of energy optimization,which determines the long-term continuous dynamic process of droplet,is beneficial on charge and electricity generation using bouncing droplets.In this thesis,we experimentally,theoretically and numerically investigated the impact dynamics of pure water droplets on the superhydrophobic flat surface and the superhydrophobic cone-shaped surface with different cone angle(60°,90°,120°).The experimental results show that the contact time of the droplet on flat surface is approximately ～11ms,and restitution coefficients range from 70% to 85%.However,due to the continuous falling of the impinging droplet on a cone-shaped superhydrophobic surface which prolonged the spreading,the droplet undergoes almost inversion-symmetric spreading and retracting processes with contact time increased by30%-90%,and contact time increases as the cone angle decreases.Spreading of the impinging droplet is dominated by inertial force,and the maximum spreading factor is mainly determined by impact velocity.Besides,the impinging droplet rebounds with higher restitution coefficients on cone-shaped superhydrophobic surfaces.Such enhanced droplet rebound is beyond the prediction of existing theoretical models,in which the viscous boundary layer is recognized as the dominant channel of energy dissipation and thus an increase in the contact time would result in a lower restitution coefficient.In this thesis,the effect of the difference of cone angle ranging from 60° to120° on restitution coefficient is not significant.In this thesis,numerical simulation is performed to analyze the pressure field,internal flow field of the droplet,and energy transfer.It is revealed that the underestimate of the restitution coefficient on cone-shaped superhydrophobic surface is caused by weakened redirection of the impinging droplet which leads to a weaker effect of the tapered architecture on boundary layer formation,and weaker viscous flow near the moving edge.Further,by analyzing the distribution of the viscous dissipation,we verified the view that the boundary layer donates the major viscous dissipation during the impact process.