Characterization of droplet impact and deposit formation on leaf surfaces

Elucidation of droplet dynamic impact and deposit formation on leaf surfaces would assist in modifying application strategies potentially to improve biological control efficiency and minimize pesticide waste.

The mechanics of the three-dimensional droplet impact and formation process on hydrophilic and hydrophobic leaf surfaces was investigated with a system that independently controlled droplet diameter, impact speed and impact angle. Spray solutions were made with a nonionic surfactant in distilled water. Water-only droplets rebounded or splashed on hydrophobic leaves of Euphorbia pulcherrima and Zea mays, but not on hydrophilic Dracaena deremensis leaves. Droplet spread areas on D. deremensis at 5.0 m s(-1) impact speed increased 1.9-fold when the droplet diameter was increased from 175 to 481 µm. Similarly, spread areas of 306 µm water-only droplets increased 2.0-fold when the impact speed was increased from 3.5 to 7.0 m s(-1) . At surfactant concentrations of 0, 0.25, 0.50 and 0.75% (v/v), percentages of droplets that either splashed or bounced off Z. mays were 56, 37, 2 and 0% respectively.

Microscopic measurements of droplet impact clarified the effectiveness of surfactants in enhancing droplet retention on hydrophobic leaves and also revealed that water droplets did not rebound or splash on hydrophilic leaves. © 2014 Society of Chemical Industry.