| The pests and diseases that appear in agricultural plant protection are not only unfavorable to the growth of plants,but also affect the harvest of crops.The rational application of pesticides is an important means to effectively slow down the infestation of pests and improve agricultural production efficiency.In the process of pesticide spraying in agricultural plant protection,spray droplets often break off the surface of the plant in the form of breaking,splashing,rebounding,etc.,and then enter the natural world,causing great pesticide waste and serious environmental pollution.Therefore,we are looking for safe,reliable and effective Measures to enhance the deposition of spray droplets on the surface of the target plant,improve the utilization rate of the liquid medicine,and reduce pesticide pollution are of great significance.Adding a very small amount of polyethylene oxide(PEO,Polyethylene oxide)as a water-soluble thickener in the liquid medicine can change its viscosity,surface tension and other physical parameters.For the convenience of presentation,a solution prepared from PEO with a relative molecular mass of 50,000 is abbreviated as PEO-5,and the rest is the same.To this end,two natural plants were selected to make the lotus leaf superhydrophobic plane and the clover superhydrophobic plane,and different types of PEO solutions were prepared.The impact experiments of deionized water droplets and PEO solution droplets were performed on the two types of surfaces.The experimental process Use a high-speed camera to record to observe the impact dynamics of droplets on the super-hydrophobic surface of plants,to obtain the laws of typical phenomena such as rebounding,breaking,splashing,adhesion and deposition of droplets on the super-hydrophobic surface of plants,and to explore the leaves that improve pesticide spraying The way of adhesion rate.At the same time,simulations of water droplets hitting a single smooth superhydrophobic surface and a microstructured superhydrophobic surface were carried out,and the experimental results were compared and discussed.The main research results are as follows:Experiments show that the addition of PEO material makes the viscoelasticity of the droplets significantly increase than that of water,and the surface tension is slightly reduced.The dynamic behaviors of water droplets and PEO-5 droplets hitting the two surfaces of lotus leaf and clover are similar overall.The lotus leaf has better superhydrophobicity.The contact angle of the water droplet on it is larger,and the droplet is more likely to rebound,but The clover surface has stronger adhesion and the pinning phenomenon is more obvious,which leads to a more asymmetrical dynamic behavior in the late stage of the droplet impact process.The behavior of water droplets and PEO-5 droplets impacting the surface of the lotus leaf is very similar.As the impact speed increases,there will be complete rebound,satellite droplet launching,partial rebound,droplet breaking,and liquid finger breaking to separate secondary droplets.occur.The impact of these two droplets on the clover surface is roughly similar to that of the lotus leaf.However,there is no satellite droplet launch when the droplets hit the clover surface,the pinning effect lasts longer,and the critical speed at which various phenomena occur is also different.The PEO-30 droplets hit the surface of the lotus leaf and the clover.With the increase of the impact speed,an oscillating bounce mode and an oscillating mode appeared respectively,and the critical velocities were 1.13m/s and 0.98m/s,respectively.PEO-100 droplets and PEO-400 droplets exhibit strong viscoelastic properties.When they hit the superhydrophobic surface of lotus leaf and clover,they adhere and deposit on the surface of lotus leaf no matter what the speed.The simulation analysis of the impact of water droplets and PEO droplets on the superhydrophobic surface was performed using the level set method.First,use the direct contact angle model for a single smooth surface to establish two-dimensional and two-dimensional axisymmetric simulation models.The simulation results can better restore the impact and spreading process of the water droplets in the experiment,and simulate the subsequent dynamic processes such as the retraction and rebound of the water droplets.The result has a certain deviation,mainly because the plane is absolutely smooth,which is not in line with reality;after the PEO droplet hits the smooth plane,it will be stretched and oscillated up and down.After a period of time,the energy dissipation is completely static on the surface,which is consistent with the experimental results.More consistent.A single smooth surface cannot show the characteristics of the surface microstructure of natural plants,so it has strong limitations.So on the basis of the direct contact angle model,the hypothesis of a single smooth surface was improved,and a micro-structured superhydrophobic surface was constructed.When the two droplets hit the surface of the microstructure,the solid-gas-liquid three-phase contact is in the Cassie-Baxter state.The simulation results reproduce the bounce of the droplet,the launch of the satellite droplet,the droplet breaking,and the up and down oscillations.phenomenon.Finally,the non-dimensional parameters such as spreading coefficient and contact time are analyzed and discussed with experimental results and simulation results.In general,the simulation and the experiment are more consistent,and the analysis results can provide a certain theoretical reference for the droplet impact dynamics;the greater the relative molecular mass and concentration of PEO when pesticides are formulated in agricultural plant protection,the greater the effect of enhancing the deposition.Obviously,the disadvantage is that the effect of pesticide atomization becomes poor;for plant leaves with super-hydrophobic characteristics,using PEO-30 with a concentration of more than 0.5%or PEO-100 and PEO-400 with a concentration of more than 0.2%can completely deposit the droplets. |
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