| Insights on microscopic processes of droplet impact on plant leaf surfaces are important to gain knowledge on advances in nozzle design and pesticide spray applications. Understanding mechanism of dr oplet r etention a nd s pread on pl ant s urfaces c an a ssist d evelopment o f e fficient p esticide application technologies to increase biological control efficacy and reduce pesticide off-target losses. The objectives of this research were to develop a sophisticated fast response system to study the3-dimensional dynamic processes of spray droplet impact, rebound and retention on1eaf surfaces, systematically in vestigate the dy namic i mpact of dr oplets on v arious t ypes of l eaf s urfaces under controlled e xperimental c onditions, a nd r esolve t he underlying mechanisms of dr oplet i mpact processes on leaves with the assistance of CFD simulations.To achieve the objectives, a system was developed to assess the dynamic processes of droplet impact, r ebound and r etention on1eaf s urfaces w ith t hree-dimensional (3-D) i mages. T he sy stem consisted of a uniform-size droplet generator, two high speed digital video cameras, a constant speed track, a leaf holder, and light sources. The droplet generator produced uniform single droplets of100to800μm and was mounted on the track from0.1to1.0m above the target holder. A step motor on the track drove the droplet generator at constant speeds of1.6t o10km/h to simulate nozzle travel speeds. The video cameras captured droplet impact images from two different view angles for3-D droplet im pact a nalyses at the s peed up t o50,000frames pe r s econd and i mage r esolution up t o1,280x800pi xels. A3-D i mage s oftware w as us ed t o m easure t he dr oplet s ize, motion a nd a ngle before and after impact. Experiments were conducted with four different types of leaves (Dracaena deremensis, Euphorbia pulcherrima, Pelargonium hortorum, Zea mays) a nd four different surfactant-amended water solutions. The surface tension of the four solutions ranged from0.0043to0.0073N/m. Droplet diameters ranged from100to500μm, and droplet impact velocities ranged from2to8m/s. Leaf surface inclinations against the horizontal plane were-30°,0°and30°.Five m otion p rocesses o f droplets af ter i mpact o n1eaf s urfaces w ere observed:co mplete retention, split retention, slide retention, splash and rebound. Test results showed that leaf surfaces with c ontact angles larger than120°(E.pulherrima and Z.mays) tended to create droplet rebound, splash an d s lide r etention w hile1eaf s urfaces w ith co ntact an gles s mailer t han60°(D. deremensis) tended t o ha ve dr oplet c omplete r etention.It w as f ound t hat s plashes h appened w hen dr oplets impacted on t he surface already wetted by previous droplet deposits. Only complete retention was observed w hen dr oplet i mpacted on t he hairy pl ant1eaf s urface. A lso, b ecause of v ariations i n morphologies on leaf surfaces, the droplet impact process performed differently at different locations even on the same leaf surface. During the complete retention process on the D. deremensis leaf surface, a330μm droplet at the impact speed of4.6m/s first spread out rapidly to form a thin film, and then shrank after film r eached t he m aximum co ntact ar ea. For a230μm d roplet i mpacted o n t he E. pulherima, the impact process was slide retention and the final contact length200μm which was smaller than the droplet diameter because of the high hydrophobic level on the leaf surface and absorption by leaf tissues. When a420μm droplet impacted on the E. pulherima at the impact speed of3.3m/s, the droplet completely bounced out and the rebound speed was1.2m/s.Test results from the droplets impacted on D.deremensis at the impact speed between4.2and5.0m/a nd i mpact a ngle be tween63and68°de monstrated t hat the m aximum s pread ar ea o f droplets increased as droplet diameter and impact velocity increased, but it decreased as the impact angle increased. The maximum spread area was also influenced by the impact angle between the droplet velocity and target surface. Increasing target surface inclination angle could increase the maximum spread area. Droplets with lower liquid surface tension had smaller contact angle on leaf surfaces and produced greater maximum spread area. Reducing liquid surface tension of droplets could reduce total number of dr oplet s lide r etention, s plash a nd r ebound on E.pulherrima and Z.mays leaf su rfaces. When the solution surface tension was lower than0.0043N/s, the droplets would become complete retention after they impacted on E.pulherrima and Z.mays leaves.The m aximum spread area on leaf surfaces i ncreased as normal W eber number and tangent Weber number. However, the tangent Weber number had more influence on droplet impact process than the normal Weber number. Based on different droplet impact processes, the Weber number could be divided into t hree regions:r etention (Wen>5Wep), t ransition (2.5Wep<Wen<5Wep) a nd r ebound (Wen<2.5Wep). T he o rder o f m ost i mportant factors t hat affected d roplet i mpact processes w ere droplet diameter, droplet speed, impact angle, and liquid surface tension. With the help of multiple linear regressions, a linear regression model of maximum spread diameter was established for droplet diameter, speed, impact angle and liquid surface tension at5%level of significance.To compare with the experimental results, the retention and rebound process of single droplets after impacted on surfaces were simulated with a CFD software Ansys Fluent by using the Volume of Fluid (VOF), RNG k-s turbulence model and Naive-Stokes model. The simulated process of droplet retention accommodated with impact, spread, shrink and secondary spread phases, and the simulated process of droplet rebound accommodated with impact, spread, shrink and rebound phases. Whether droplets would have retention or rebound relied on the droplet takeoff energy after the shrink phase.In conclusion, this research provided a new system that was able to systematically investigate the3-D dynamic processes of spray droplet complete retention, split retention, slide, splash and rebound on leaves in its entirety under controlled experimental conditions. The information from this research offered a m eans to manipulate controllable spray parameters to increase pesticide spray application efficiency and minimize pesticide waste by improving droplet impact processes and avoiding droplet rebound and runoff losses. |
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