Study On Asymmetric Dynamics Of Droplets Impacting On Superhydrophobic Surfaces

It is of great significance to study the dynamic characteristics of droplets impacting on superhydrophobic surfaces in self-cleaning,corrosion protection and anti-icing.At present,the researches mainly focus on droplets impacting on the stationary or isotropic superhydrophobic surface,while the researches on droplets impacting on the moving or anisotropic superhydrophobic surface are less.Therefore,in this study,the dynamic characteristics of asymmetric changes of droplets were systematically explored through droplets impacting on moving and anisotropic superhydrophobic surface.Firstly,droplets impacting on moving superhydrophobic surfaces was studied.It was found that when droplets hitting the moving superhydrophobic surface at different velocity,there were 5 kinds of bouncing modes: pillar rebound,pillar breakup rebound,semi-stretched rebound,stretched rebound and breakup rebound due to the effect of surface velocity.Droplets slip after impinging on a moving superhydrophobic surface compared with impinging on a stationary superhydrophobic surface.When the impact velocity is small,the slip distance increases first and then decreases with the increase of surface velocity.The reason is that the driving force of slippage will change from shear force to lift force with the increase of surface velocity.When the impact velocity of droplets is large,the slip distance increases rapidly first and then slowly with the increase of surface velocity.This is because the driving force of slippage changes from shear force to the resultant force of shear force and lift force with the increase of surface velocity.The results show that both the impact velocity and the surface velocity have effects on the spreading characteristic of droplets.The maximum normal spreading diameter of droplets is independent of the surface velocity,which is consistent with the calculation formula of droplets impacting on stationary superhydrophobic surfaces.The maximum tangential spreading diameter is related to both velocities,and increases with the increase of the other velocity when one velocity remains unchanged.It is found that the contact time of droplets impinging on the moving superhydrophobic surfaces is independent of the impact velocity,but decreases first and then stabilizes with the increase of the surface velocity.The decrease of contact time is due to the asymmetric deformation of the droplet caused by the shear force after the droplets impacting on the moving surfaces,which leads to the acceleration of droplets contraction and the decrease of contact time.Secondly,the experiments of droplets impinging on superhydrophobic rectangular grooves were carried out.The variety of droplet morphology was studied.The effects of droplet impact velocity,groove width and groove depth on the spreading characteristic of droplets were investigated.When the width and depth of the groove remain unchanged,the larger the impact velocity,the larger the maximum spreading diameter of the droplets in the direction perpendicular to the groove and parallel to the groove.When the impact velocity and the groove depth remain unchanged,the larger the groove width,the smaller the maximum spreading diameter of droplets in the direction perpendicular to the groove and parallel to the groove.When the impact velocity and groove width remain unchanged,the greater the depth of the rectangular groove,the smaller the maximum spreading diameter of droplets in the direction perpendicular to the groove,and the larger the spreading diameter in the direction parallel to the groove.Finally,the effects of droplet impact velocity,groove width and groove depth on contact time were studied.