Influence of droplet size and velocity on droplet impact process on waxy leaf surfaces

Abstract

Abstract. A custom-designed system consisting of two high-speed digital cameras, a uniform-size droplet generator, a constant-speed linear track unit, light sources and a 3-dimensional (3-D) image analytical program was used to investigate the impact processes. Leaves of Dracaena deremensis, Euphorbia pulcherrima and Zea mays and glass slides were targets for the droplets. 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. The control treatment was distilled water. When water droplets impacted the glass slides and D. deremensis leaf surfaces, rebound or splashes were not observed, but were observed on E. pulcherrima and Z. mays surfaces. Droplet spread area on D. deremensis leaf 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 leaf surfaces decreased 15% when impact angles increased from 50 to 81°. Non-ionic 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 leaf surfaces decreased from 56 to 37, 2 and 0% for surfactant concentrations of 0, 0.25, 0.50 and 0.75%, respectively. Therefore, the use of surfactant stabilized droplet deposition on both hydrophilic and hydrophobic leaf surfaces and reduced droplet rebound and runoff.