Research On Dynamics Of Droplet Impacting On Microstructured Superhydrophobic Surfaces

The phenomenon of droplet impacting on solid wall exists widely in nature and engineering.With the development of science and technology,superhydrophobic surfaces has attracted more and more attention due to its superior performance in frost resistance,drag reduction and self-cleaning.Processing methods for superhydrophobic surfaces are becoming more and more mature.In this paper,the dynamic characteristics of droplet impacting on superhydrophobic surface with isotropic microstructures were studied with theory,experiment and numerical simulation.Firstly,the superhydrophobic surface with isotropic micro-pillars structure was fabricated based on 6061 aluminium plate.The apparent contact angle can reach 156.65 degree and rolling angle is less than 5 degree.Based on the energy conservation,a prediction model for the maximum spread factor of droplets impinging on an isotropic microstructured superhydrophobic surface was proposed.Two types of viscous dissipation were considered: one comes from the surface interaction between the liquid film and the top areas of the pillars and the other comes from the liquid flowing among the pillar arrays.The results show that the impact process of droplets is not only controlled by We,Re and Ca number,but also related to the apparent contact angle and superhydrophobic surface geometry.Through NAC Memrecam HX-3E high-speed camera,the process of droplet impacting on superhydrophobic wall with different diameters and velocities was recorded.It was found that the larger the droplet diameter and impact velocity,the larger the spread factor of droplet.The influence of droplet diameter on contact time is greater than that of droplet velocity.At the same time,the proposed prediction model of maximum spread factor is verified.In addition,the effects of inclination angle and impact velocity of droplets on the rolling-slip length of droplets impacting on inclined superhydrophobic surfaces are compared and analyzed.It is found that the longer the roll-slip length of the droplet on the inclined plane is,the greater the inclination angle of the inclined plane and the impact velocity of the droplet are.Flow-3D software was used for numerical simulation.Volume of Fluid(VOF)method was used to capture the free interface.The Morphology change of droplets impacting on the wall and the variation of internal pressure and velocity fields were analyzed.The regularity of morphological change was in good agreement with the theoretical model and experimental results.It is also found that the peak pressure occurred at the moment after droplet impact and when the liquid film edge contacted in the retraction stage,the internal velocity of the droplet was larger in the retraction stage and higher than the impact velocity.The variation of pressure and velocity near the wall with time is also analyzed.The pressure value near the wall increases sharply at the instant after impact,and when the liquid film retracted until the edge contacting,it increased slightly when the droplet was about to leave the wall.The velocity value near the wall was the largest at impacting moment,and then decreased gradually.Then it increased again when the bottom of the annular edge contacting at the retraction stage.But the direction was downward.The velocity was reversed as the droplet moving upward gradually.