Study On The Motion Control Of Droplet Impingement On Annular Superhydrophobic Surface

The research on the movement control of droplets is of great significance in the fields of energy and environment,such as condensation heat transfer,fluid transportation,anti-icing,anti-fouling,self-cleaning and water collection.At present,the control of droplet movement needs external energy input such as light,heat,electricity and magnetism,which limits its application.Based on the structure,the circular groove surface was fabricated by micro milling machine.The movement characteristics of the droplet impacting the circular superhydrophobic surface were studied by experiments,and the directional bounce control of droplet was realized.The details are as follows:Firstly,in order to explore the influence of the circular groove structure on the droplet transverse transportation,droplets impacting on circular groove arrays superhydrophobic surfaces was studied.The circular groove structure was milled on the surface of aluminum alloy by micro milling machine,then etched by hydrochloric acid and modified by FAS-17 to get superhydrophobic properties.The whole process of droplets impacting on the surfaces were recorded by high-speed cameras.It was found that when droplets impact the circular groove arrays superhydrophobic surface at different velocity,there were 5 kinds of bouncing modes: pillar vertical rebound,pillar breakup vertical rebound,pillar breakup directional rebound,complete directional rebound and horizontal-breakup directional rebound due to the effect of circular groove.Compared with the impinging on a flat superhydrophobic surface,the directional bouncing of droplets can be realized due to the pea deformation in the process of droplet collision.Studies have shown that the droplet bounce distance and change rule are not related to the droplet diameter,but only depend on the structure parameters of the groove and the impact velocity.When the structural parameters of the groove are invariable,the greater the velocity of the droplet is,the greater the bounce distance is;The bounce distance increases with the curvature when the collision velocity is unchanged.With the increase of groove depth,the droplet directional bounce distance first increases and then decreases rapidly.The critical We of directional bounce is affected by the groove width,which increases with the decrease of the groove width.When the groove width is small enough,there is no pea deformation in the process of droplet collision,and no directional bounce occurs.Secondly,the experiments of droplets hitting the super-hydrophobic surface of a single convex ring was studied,and the phenomenon of center-breaking bowl-shaped ejecting mainly occurred.The ring structure restricts the spreading of the droplets,and the horizontal momentum is converted into vertical momentum,which the solid-liquid contact only occurs in the ring.Cylindrical protrusions in the ring can pierce the liquid membrane,promote center breakage and assist shrinkage,and reduce solid-liquid contact time.The analysis shows that the center-breaking ejecting of the single convex ring structure can reduce the heat exchange between the liquid and the solid.The cold water collision temperature measurement experiment proves that the structure is better than the single ring super-hydrophobic surface and the flat super-hydrophobic surface in reducing the heat exchange,which is expected to promote the development and application of the anti-icing surface.