| The directional drive of droplets and liquids can be applied to condensation heat transfer,mist water collection,heat pipes,microfluidic systems,fuel cells and other fields.Because of its wide range of applications,it has also become a research hotspot in recent years.Since the different surface wettability of the material will make the liquid have different dynamic behaviors on the solid surface,changing the surface wettability of the material is an effective way to achieve directional driving.Both the wetting gradient surface and the shape gradient surface have been verified by a large number of studies to have excellent performance in the field of directional driving.In order to further strengthen the directional driving of droplets and liquids,this paper proposes a combination of the wetting gradient surface and the shape gradient surface.A composite gradient wedge surface with a typical wedge pattern surface in,and the self-propelled properties of droplets on the composite gradient wedge surface were investigated by numerical simulation.After verifying that the composite gradient wedge surface can effectively drive the droplet to move,in order to further improve the capillary rise efficiency of the liquid in the copper groove,the composite gradient wedge surface was extended and applied to the capillary rise in the groove,and a wetting gradient rectangular copper groove was prepared.Copper grooves and composite gradient wedge-shaped copper grooves,and an experimental system were built for capillary rise experiments.In this paper,the fluid volume method was used to numerically study the self-driving of droplets on the composite gradient wedge surface,and the mesh independence and model verification were carried out for the computational model.By changing the wetting gradient on the wetting gradient surface(15 °/mm,10 °/mm and 15 °/mm),changing the wedge angle(20°,30° and 40°)on the wedge pattern surface and changing the composite gradient wetting gradients(15 °/mm,10 °/mm,and 15 °/mm)and wedge angles(20°,30°,and 40°)on wedgeshaped surfaces,comparing parameters such as droplet moving speed and displacement,and studying droplets.The self-propelled characteristics of the droplet were explored to explore the optimal working conditions of the droplet self-transport.The simulation results showed that the increase of the wetting gradient and the wedge angle can both accelerate the moving speed and increase the displacement of the droplet.The average velocity of the droplets on the 15 °/mm and 20° composite gradient wedge angle surfaces was the fastest,which was 37.7%(5 °/mm)higher than that of the single gradient surface and 175.5% higher than that of the single wedge pattern surface(20°).Although numerical simulation can easily and quickly simulate the self-actuation of droplets on wettability surfaces,it is difficult to control all experimental conditions are the same as the actual situation,and inevitably there are some errors.The self-driving of the droplet and the capillary rise of the liquid have the same dynamic theoretical basis,and the force is similar.To extend the application of composite gradient wedge surfaces,the technique was applied to capillary rise in grooves.And an experimental system of capillary rise was established in this paper.A single hydrophilic and wetting gradient rectangular trench was prepared by alkali oxidation assisted method,and capillary rise experiment was carried out at 30° inclination angle.The experimental results showed that when the groove size was the same,the smaller the wetting contact angle was,the larger the maximum height of water capillary rise was.Under the same surface contact angle,the smaller the width of copper groove,the higher the maximum height of capillary rose.When comparing the single hydrophilic and wetting gradient rectangular grooves,it was found that the capillary rise height of the rectangular groove with wetting gradient was greater than that of the single hydrophilic copper grooves under the same groove width due to the extra wetting gradient driving force.The height of capillary rise in copper groove with wetting gradient of 43.5°-4.9° was 35.9% higher than that of contact angle43.5°,and 14.82% higher than that of contact angle 4.9°.In order to further enhance the capillary rise of water in the groove,a wedge-shaped copper groove with extra shape gradient driving force was proposed.In the capillary rise experiment,it was found that small surface contact angle and small groove width can increase the water rise height.In addition,the composite gradient wedge copper groove is proposed by combining the wetting gradient and the wedge-shaped copper groove.Due to the increased shape gradient driving force and wetting gradient driving force,the rising velocity of water in the composite gradient wedge-shaped copper groove with a wetting gradient of 43.5°-4.5° and a groove width of 0.5 mm-1 mm in all copper grooves was the fastest,and the highest rising height can reach26.91 mm,an increase of 19.7% compared with 43.5°,an increase of 11.2% compared with4.9°.Compared with the wetting gradient rectangular copper groove,the average velocity of liquid capillary rise in the composite gradient wedge-shaped copper groove was increased by12.8%. |
