Effect Of Wall Wettability On Droplet Impacting Limited Liquid Surface

The phenomenon of liquid droplet impacting on liquid surface is common in nature,life and incdustry,and its research history can be traced back to 1875.Previous studies have focused on the impact of droplet on a wide liquid surface.The effects of droplet height,droplet size,liquid properties and other factors on the impact phenomena and characteristics were mainly considerated.Since 2012,a few literature begun to report the droplet impacting on liquid surface with limited areas surrounded by wall.Here,the effect and mechanism of wall wettability on droplet impacting on limited liquid surface are further researched.In this paper,three types of test tubes with inner wall behaving super-hydrophilic,hydrophobic and super-hydrophobic are prepared by nano-spraying technology.The contact angles of wall inside test tubes are 5.4°,98.8°and 146.5°,respectively.Correspondingly,the water-air interfaces inside the test tubes behave concave,horizontal and convex.The experimental phenomena of droplet impacting on these water-air interfaces are recorded by high-speed camera.In the experiment,the principle of specular imaging is used to make sure that droplet can impact the center of the liquid surface in test tube.The inner diameter D of test tubes is selected as 8,10 and 13 mm.The droplet diameter d is controlled as 2,3 and 4 mm.The range of droplet impacting height is from 50 to 900 mm.The experimental results show as follows.1.When the ratio of tube diameter to droplet diameter D/d is small,canopy bubble phenomenon is easy to occur instead of droplet jetting after the impacting of dropleat on liquid surface.2.The formation mechanisms of canopy bubbles inside super-hydrophilic and super-hydrophobic test tubes are different.For super-hydrophilic test tube,cavity forms after the droplet impacting the liquid surface to ycild thick liquid film adhered on superhydrophilic wall.Canopy bubble is generated after the contact of liquid film adhered on the circular wall.For super-hydrophobic test tube,during the form process of cavity after droplet impacting on liquid surface,the gas-liquid contact line at the top of the cavity is contracted inward actively to seal air inside cavity and gerate canopy bubbles.3.When D/d is large,the droplet impacting phenomenon changes from coalescence to jetting and returns to coalescence again by increasesing the the impacting height.What is more,the impacting height range to form jetting enlarges with the changes of wettability inside test tube from super-hydrophility to hydrophobicity and super-hydrophobicity.In order to reveal the influence mechanism of wall wettability on the liquid jetting conditions after droplet impact,this paper utilizes energy analysis based on the simplified model that the cavity formed by droplet impacting is treated as as a cone.The results show that cavity size and energy dissipation decreases with the changes of wettability inside test tube from super-hydrophility to hydrophobicity and super-hydrophobicity,under the condition of same droplet diameter,test tube diameter and impacting height.This analysis verifies that the more hydrophobic the wall is,the easier is to generate droplet jetting.