Study On The Mechanism Of Droplet Self-Transportation On Wetting Gradient Surface

Directed droplet migration(that is,the use of surface tension to drive droplets to move in a specific direction)has a variety of applications in environmental protection and life sciences,such as fog collection,microfluidic devices,biochips,oil-water separation,etc.Precise control of droplets is of great significance to the precision control and microfluidic technology research in industrial production processes.Front Tracking Method(FTM)is a numerical simulation method that can accurately describe the geometry of complex interfaces.It tracks complex multiphase flow interfaces and deformations through a series of interconnected markers.In this paper,the front tracking method is used to study the numerical simulation of the self-transport behavior of droplets on the surface with a wetting gradient.A superhydrophobic confined channel model was established to study the migration characteristics of droplets on the confined surface and explore the migration and heat transfer process of droplets hitting a cold plate with a wetting gradient.In this paper,three-dimensional FTM method is used to simulate the droplet migration on the wet gradient surface.The influence of different parameters on the droplet migration velocity and shape change is revealed,and a surface roughness model is proposed to consider the effect of surface roughness on droplet migration.The results show that the wetting gradient and surface wettability can increase the driving force and wetted area of droplets,thereby accelerating their migration.Smaller channel width can increase the migration velocity of droplets.When the ratio of channel width to droplet diameter is 0.75,droplets will obtain twice the driving force during migration and thus obtain a greater maximum velocity.And because the slippage process of the droplet in the region II(the middle part of the surface)is avoided,the time for the droplet to complete the migration and completely enter the region III will be greatly shortened.Roughness reduces the droplet’s spread length and increases its height,thereby affecting the droplet’s migration time.When the roughness reaches 0.8,the droplet migration speed will drop to zero.And in the descending stage of the capillary number curve,the greater the roughness,the more violent the fluctuation on the capillary number curve will be.Excessive wettability limitation results in a small wetted area,which affects the driving force of the droplet.Through analysis,this paper identifies the optimal dimensionless wetting limit of 0.8,which provides guidance for achieving efficient droplet transport.In this paper,a numerical simulation of the migration and heat transfer process of droplets hitting a cold plate with a wetting gradient is carried out.The effects of dimensionless temperature difference between surface and droplet,surface wettability,initial impact velocity and wettability limitation on droplet heat transfer characteristics and temperature field distribution are revealed.The results show that the greater the dimensionless temperature difference between the droplet and the surface,the greater the heat flux that the droplet rises after falling on the surface,and the greater the decline in the descending stage of the heat flux curve.The contact angle of the surface has a huge impact on the heat transfer of the droplet migration process on the surface,and the droplet can reach thermal equilibrium faster on the hydrophilic surface with a small contact angle.A larger initial impact velocity will cause the droplet to rebound repeatedly on the surface at the initial stage,resulting in a larger change in the transient heat flux of the surface during the droplet impact stage.But as the droplet finishes rebounding and enters the migration stage,the impact of different impact velocities will be small.The influence of wettability limitation()on the droplet on the surface comes from the wetting area,and the dimensionless wetting area of =0.4 and=2.5 differs by as much as four times in the last stage.The larger the wettability limitation,the smaller the droplet’s maximum temperature drop,the slower the temperature drop,and the later the droplet’s maximum temperature reaches the inflection point near the surface temperature.