Study On Spreading Characteristics Of Droplet Impinging Soybean Leaves Surface Based On High Speed Camera Technology

There are many hydrophobic plant leaves in nature.Due to the rebound and splash caused by droplets hitting the surfaces of these leaves,great losses are found in the agrochemical spraying,and thereby their utilization rate is very low.Therefore,it is very important to study the properties of droplets impinging on the surface of plant leaves to improve the utilization rate,reduce the dose and increase the efficiency of pesticides.By analyzing the biological characteristics of soybean plants in the Huanghuaihai area,the leaves are observed to have a non-smooth surface morphology with a large number of hairy and curved stem vein structures.After measuring the contact angle(128.76°),adhesion,shows the surface of the soybean leaves is hydrophobic,while the adhesion test show it is highly adhesive.In this paper,the high-speed camera is used to record the impact of droplets on the surface of soybean leaves at 5000 fps.Different impact speeds(0.89-1.8 m/s)and blade tilt angles(0-30°)are investigated on the impact.The droplets spread to the maximum stage during the 0～3 ms process,and then retrace in the following 3～8 ms.During retracement stage,the droplet diameter decreases gradually.Data analysis,the impact velocity ≤1.03 m/s and the blade inclination angle<20°,droplets deposit on the blade surface.However,when the blade inclination angle is>20°,the droplets roll off or bounce off the leaf surface.When the impact velocity is>1.03 m/s and the blade inclination angle is<15°,the droplets splash.When the inclination angle is>15°,the droplets roll off or rebound,Droplet impact deposition and fracture critical K value is 30.14.The results show that although the surface of soybean leaves are high adhesive surface,there is still serious droplet loss when the leaves are inclined and the impact velocities are larger than1.03 m/s.Secondly,in order to improve the deposition efficiency of droplets,this paper studies the surface spreading properties of droplets containing surfactants impinging on soybean leaves.According to the molecular structure and surface tension characteristics,three surfactants were chosen:sodium lauryl sulfate(SDS,characterized by a single hydrophobic chain,high equilibrium surface tension and high dynamic surface tension),trisiloxane(Silwet-77,characterized by multi hydrophobic chains,low equilibrium surface tension and High dynamic surface tension)and sodium bis(2-ethylhexyl)sulfosuccinate(AOT,characterized by double hydrophobic chains,low equilibrium surface tension and high dynamic surface tension).After initial concentration investigation,surfactants droplets at a concentration of 0.2%SDS,0.05%Silwet-77,0.1%AOT were selected to impact the soybean leaves.The addition of surfactants do not affect the time length of spreading stage,but profoundly increase the receding time span during the impact.Data analysis,When the impact velocity is>1.34 m/s,and the blade inclination angle is>20°.Splash and partial rebound occurs for the impact process of 0.2%SDS and 0.05%Sliwet-77 droplets.In contrast,0.1%AOT greatly suppress the rebound and the splash.Since 0.05%Silwet-77 droplets have a lower equilibrium surface tension,the droplet spreading diameter is the largest at 5s after the impact.The analyses show that AOT has a lower dynamic surface tension and can change the wettability of the substrate to a hydrophilic state in the re-spreading stage,so it has a significant effect on the retained droplets.While in the case of the impact of 0.05%Silwet-77 droplet,it has a lower equilibrium surface tension,and thus the droplet spreading has the largest diameter when the droplets have retained on the surface of the blade.0.05%Sliwet-77 droplet impact deposition and fracture critical K value is less than 49.13,while 0.1%AOT critical K value is greater than 49.13.The results show that the low dynamic surface tension is beneficial to reduce the impact loss of the droplets,and the low balance surface tension is helpful to obtain a larger spreading wetted area after the collision.Based on high-speed camera technology,this study reveals the surface spreading characteristics of droplets impinging on soybean leaves.The research results can provide some basic theoretical basis for mechanized precision application of soybean field in Huanghuaihai area.