The objective of this study is to investigate the effects of surface tension, droplet size, and leaf angle on the dynamic wetting and spreading behavior of pesticide droplets on both adaxial and abaxial surfaces of rice leaf, and to provide a basis for realizing the “reduce application and increase efficiency” of rice spraying by regulating the dynamic wetting and spreading behavior of pesticide droplets on the rice leaf surface.

A full factorial experiment was designed to investigate the dynamic wetting and spreading behavior of single droplet on both adaxial and abaxial surfaces of rice leaf in this study. The Silwet-408 solutions with surface tension of 21.4, 33.2, and 43.7 mN·m^-1 were formulated by adjusting the concentration of Silwet-408 to replace the pesticide solutions. A droplet generator was used to produce the single droplets of 532, 627, 746, 830, and 957 μm. The leaf angles were set as 40°, 65°, and 85°.

There were significant effects of surface tension, droplet size, and leaf angle on the rate of change of droplet contact angle on the rice leaf surface (P<0.05), and the overall trend of the influences on the adaxial and abaxial surfaces was basically the same; increasing leaf angle or reducing droplet size or lowering surface tension could increase the rate of change of contact angle and promote the wetting and spreading of droplet. Among them, the effect of lowering surface tension was the most significant. When the surface tension was decreased from 33.2 to 21.4 mN·m^-1, which was close to the critical surface tension of rice leaf surface, the rate of change of the contact angle (advancing contact angle and receding contact angle) increased by 7.49 and 6.22 times for the adaxial surface, and 11.13 and 7.61 times for the abaxial surface, and the wettability of the droplets was changed from relatively poor or poor (80°≤contact angle<100° or contact angle≥100°) to medium or positive (60°≤contact angle<80° or contact angle<60°) within 75 s; when the surface tension was much larger than the critical surface tension of rice leaf surface, the rate of change of contact angle increased with the increase of leaf angle, and increased with the decrease of droplet size, but the effect of leaf angle was less than that of droplet size, and droplets of almost all particle sizes still maintained relatively poor or poor wettability (80°≤contact angle<100° or contact angle≥100°) after 75 s. In addition, the analysis of droplet wetting hysteresis phenomenon showed that the roughness of the rice leaf surface was relatively small, and the droplet wetting hysteresis phenomenon was not serious. The droplet with surface tension close to the critical surface tension of rice leaf was driven by the dynamic surface tension of the solid-liquid-gas three-phase system to wet and spread on the leaf surface, and the dynamic change of the contact angle with time could be fitted by the model θ=θ_e+Aexp(-Kt); although the droplet with a surface tension much higher than the critical surface tension of rice leaf could steadily adhere to the leaf surface without the phenomenon of roll-off, it was never able to break through the pinning effect and retention resistance of the leaf surface, and could not achieve wetting and spreading.

All three factors, surface tension, droplet size, and leaf angle, significantly affect the dynamic wetting and spreading behavior of pesticide droplets on the rice leaf surface. In the actual application scenarios, since the leaf angle cannot be adjusted artificially, the surface tension and droplet size of the pesticide solution can be adjusted according to the purpose of the application to regulate the dynamic wetting and spreading behavior of the droplet. The results of this study are helpful to understand the mechanism of dynamic wetting and spreading of pesticide droplet on the rice leaf surface, and can provide theoretical support and guidance for the rational selection of surface tension and droplet size in rice pesticide application scenarios.