Strategy for the Selection of Tank-Mix Adjuvants to Improve the Wettability of Unmanned Aerial Vehicle-Sprayed Liquids on Citrus Leaf Surfaces

The past decade has witnessed a growth spurt in aerial pesticide application by plant protection unmanned aerial vehicles (UAVs). However, compared to the traditional ground application, their operation characteristics of low capacity and high concentration put forward rather higher requirements for the wettability of pesticide solutions on the target crop surface, which was a critical link to the pesticide efficiency. Tank mix of adjuvants is a common measure to modify the spray liquid wettability during foliar pesticide application. Yet, given the wide variety of spray adjuvants available, how to select suitable ones based on crop leaf surface characteristics during pesticide application of plant-protection UAVs still lacks attention, especially for tree crops like citrus. Herein, the surface free energy of citrus leaf, the surface tension of tested liquids, and their contact angles on citrus leaf were determined, and the quantitative relationship between the contact angle and the surface tension and its components, as well as the citrus leaf surface free energy and its components, was analyzed. The surface free energy of the adaxial and abaxial surface of citrus leaf was as low as 30.31 mJ m^–2 and 27.50 mJ m^–2, respectively, showing that citrus leaves are relatively difficult to wet. However, addition with 1% (v/v) of the three tested spray adjuvants to the liquid significantly reduced the surface tension, especially the polar component, thereby substantially decreasing its contact angles on the citrus leaf surface. There was a significant positive correlation between the droplet contact angle on citrus leaf surfaces and liquid surface tension and its polar component, with total surface tension as the main variable and the polar component as the covariant. Our findings proposed a strategy for screening tank-mix adjuvants to improve spray liquid wettability on tree crop leaves during pesticide application by plant-protection UAVs.