Droplet impacting dynamics on a thin liquid film on the rice-leaf microstructured surface

Abstract

Although spraying pesticides onto stems and leaves is the most effective method to prevent plant pests and diseases, the splashing of the droplets upon impact with the leaf surface is one of the main ways that the liquid agent is lost during the application process. Moreover, the collision and impact of flowing droplets with the liquid film are complex owing to the presence of microstructures on the leaf surface. Therefore, understanding the droplet impact dynamics is crucial for improving the pesticide application process. In this paper, a simulated rice leaf with a microstructured surface was established, and the impacting dynamics of a liquid droplet on this surface with thin liquid film were investigated using numerical methods. Specifically, the influences of different droplet diameters, droplet velocities, liquid film thicknesses, and leaf surface microstructure dimensions on the impacting dynamics were analyzed. The results showed that the behavior of spreading, jetting, rebound, and splashing occurred upon when droplets impact the simulated surface of rice leaves with thin liquid film. Moreover, a larger droplet diameter and a higher velocity lead to an increase in both the maximum height and diameter of the formed crown. Additionally, the thicker liquid films result in crowns with a smaller base radius. When the jetting angle increases, the crown height increases while its base radius decreases. With the increase in the rib spacing of microstructure, the maximum crown height initially increases and then decreases, while the crown base radius decreases. However, the rib depth has no significant influence on the base radius of the crown. Equations for the quantitative expression of the transition relationships between the deposition, jetting, and splashing phenomena were established on the basis of the Reynolds number, Ohnesorge number, and Weber number as well as the dimensionless liquid film thickness.