Characterization of dynamic droplet impaction and deposit formation on leaf surfaces

Authors: DONG, XIANG - Chinese Academy Of Agriculture & Mechanical Sciences; Zhu, Heping; YANG, X - Chinese Academy Of Agriculture & Mechanical Sciences

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/21/2014
Publication Date: 1/5/2015
Publication URL: http://handle.nal.usda.gov/10113/61404
Citation: Dong, X., Zhu, H., Yang, X. 2015. Characterization of dynamic droplet impaction and deposit formation on leaf surfaces. Pest Management Science. 71:302-308.

Interpretive Summary: Reaction of foliar applied pesticide droplets on plant surfaces affects their physiological and biological effectiveness. The need to elucidate these droplet fates on leaf surfaces to increase the biological control efficiency was long recognized, but recent reports do not address this problem. This research investigated the dynamic processes of water-based droplet impact, rebound, retention and deposit formation on various types of leaf surfaces with different droplet sizes, impact velocities, impact angles, and non-ionic surfactant concentrations. Microscopic measurements of droplet impaction clarified the effectiveness of surfactants to enhance droplet retention on hydrophobic leaves and also revealed that water droplets did not rebound or splash on hydrophilic leaves. This research quantitatively described the fate of droplets on plant surfaces, and demonstrated that spray applications must be tailored for different plant surfaces and spray systems to achieve optimal biological effects. The accomplishment of this research provides scientific guidelines for spray applicators to stabilize the fate of spray droplets on targets to improve droplet retention and minimize droplet rebound and runoff, resulting in increased pesticide application efficiency.

Technical Abstract: Elucidation of droplet dynamic impaction and deposition formation on leaf surfaces would assist to optimize application strategies, improve biological control efficiency, and minimize pesticide waste. A custom-designed system consisting of two high-speed digital cameras and a uniform-size droplet generator was used to investigate 3-dimensional droplet impact processes. Leaves of Dracaena deremensis, Euphorbia pulcherrima and Zea mays and glass slides were targets. Distilled water amended with a non-ionic surfactant at concentrations of 0.25, 0.50 and 0.75% (v/v) were used as spray solutions. When water droplets impacted the glass slides and D. deremensis, rebound or splashes were not observed, but were observed on E. pulcherrima and Z. mays surfaces. Droplet spread area on D. deremensis surfaces increased 47% when droplet diameters and impact speeds increased from 175 to 490 µm and 3.5 to 7.0 m/s, respectively. However, the droplet spread area on D. deremensis decreased 15% when impact angles increased from 50 to 81°. Surfactant amendments improved droplet spread area on glass slides and D. deremensis and reduced droplet rebound or splashes on E. pulcherrima and Z. mays leaf surfaces. Percentage of droplets that either splashed on or bounced off Z. mays decreased from 56 to 37, 2 and 0% for surfactant concentrations of 0, 0.25, 0.50 and 0.75%, respectively. Water droplets did not rebound or splashes after they impacted glass and D. deremensis surfaces. The use of surfactants stabilized droplet deposition on both hydrophilic and hydrophobic leaf surfaces and reduced droplet rebound and runoff.