Pesticide droplet interfacial behavior and drying deposition characteristics on apple leaf surfaces at different growth stages: role of nonionic surfactant self-assembly.

The physicochemical properties of pesticide solutions can be effectively altered via surfactant addition, which significantly influences the wetting, bouncing behavior and deposition efficiency of pesticide droplets. We combined high-speed imaging, contact angle measurements, and infrared thermography to track droplet impact, spreading, and drying on apple leaves at three growth stages-flower bud, young fruit, and fruit enlargement. Deep-focus microscopy observations revealed that leaf surfaces at different growth and developmental stages possess trichomes of varying densities, which exert a non-negligible influence on droplet bouncing, wetting, and drying deposition behaviors. Cryo-TEM showed that AEO-4, AEO-5, and AEO-7 assembled into spherical micelles, while AEO-6 formed vesicles in pesticide solutions. The small micellar structures facilitated droplet penetration into trichome structures on leaf surfaces, whereas the vesicle elasticity reduced droplet bouncing. AEO-6 enhances Marangoni convection within the droplet, accelerates the drying rate, and promotes uniform deposition, whereas the micelles formed by AEO-4 and AEO-5 are susceptible to capillary flow within the droplet and the micelles formed by AEO-7 are more susceptible to gravitational effects, leading to non-uniform deposition upon drying and a "coffee ring" pattern. Through a comprehensive analysis of factors such as the self-assembly morphology of surfactants, the heterogeneity characteristics of leaf surfaces at different growth stages, and the Marangoni flow patterns within droplets, the wetting behavior on adaxial versus abaxial leaf surfaces varied markedly with trichome density. Notably, AEO-6 vesicles demonstrate superior performance, via an energy storage-release mechanism, reducing the coffee-ring effect and bouncing behavior, and enabling more uniform pesticide deposition. This offers a theoretical foundation for customizing surfactant selection based on leaf growth stage and surface location.