| Hydrophobic surfaces have broad application prospects in industrial production and people’s lives.The impact of droplets on superhydrophobic surfaces has also received a lot of research and attention.In recent years,it has been found that when the droplets impact the super-hydrophobic surface with square column array,it would produce pancake bounce.Compared with ordinary super-hydrophobic surfaces,it can further reduce the contact time of liquid and solid,improve the performance of superhydrophobic surfaces.And the pancake bounce of droplets expands the application of superhydrophobic materials in self-cleaning,anti-fog,corrosion resistance,frost resistance,non-destructive liquid transportation,etc.,and has a considerable impact on anti-freezing rain and anti-icing.This paper systematically studies the bounce process of droplets impacting the super-hydrophobic surfaces from experiment and numerical calculation.The superhydrophobic surface with square column array is prepared,and the pancake bounce is generated by the droplet impact experiment.The influence of the square column size and the physical parameters of the droplets on the pancake bounce is explored,and the causes and control methods of the pancake bounce are comprehensively studied by numerical calculation and theoretical analysis.The aluminum alloy is processed by mechanical wire cutting,laser etching,and fluoride modification methods to obtain super-hydrophobic surfaces with microstructures and super-hydrophobic surfaces with millimeter square column arrays.The contact angle of the superhydrophobic surface is greater than 150°,and it has very good hydrophobic properties.An experimental platform is built and a high-speed camera is used to record the bounce process of droplets with different speeds impacting the super-hydrophobic surfaces at a frame rate of 6000 per second.The results show that the larger droplet spread when the impact speed is increased,but the contact time remains basically unchanged.A numerical calculation model with microstructure is established,calculate by the VOF method.The calculated results are in good agreement with the experimental results of the four droplet bouncing processes of falling,spreading,retracting and bouncing,as well as the variation of spreading factor and spreading height with dimensionless time.And the complex pressure field and velocity field in the process of droplet impact are analyzed,the movement mechanism of droplet bounce is further explored.It is found that the pressure and velocity inside the surface is symmetrically distributed along the central axis during the entire process of droplet impact.During the spreading process,two symmetrical velocity vortices are formed on the edge of the ring.After spreading to the maximum,the droplet shrinks toward the center under the action of surface tension.The droplet impact experiments under various working conditions produce pancake bounce,which is formed because the liquid infiltrated between the square columns provides sufficient capillary force to lift the entire droplet.Because there is no retraction stage,the contact time of the pancake bounce is much shorter than that of the ordinary bounce.On the same superhydrophobic surface,the larger diameter and velocity of the droplets,the easier it is to produce a pie-like bounce.And it is found that the size of the square column array of the super-hydrophobic surface also has an effect on the pancake bounce.Numerical calculation of the pancake bounce process shows that the ring edge is lifted and thickened as a whole during the retraction process.Through analysis,it is found that the formation of the pancake bounce need meet the two criteria of time scale and energy criterion: the fluid must return to the surface at an appropriate time,and it must have enough kinetic energy.The correlation between time scale and energy criterion and superhydrophobic surface characteristics is established,and the theoretical model of pancake bounce is put forward and verified. |
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