Crop leaves naturally exhibit a curved morphology and primarily display bending deformation and vibrational responses under wind load. The curved surface structure of leaves plays a critical role in the deposition and retention of pesticide droplets. In this study, wind tunnel experiments combined with high-speed photography and digital image analysis were conducted to systematically investigate the curvature and flexibility distributions of three typical crop leaves: walnut, peach, and pepper, across a range of wind speeds. The results indicate that with increasing wind speed, all three types of leaves gradually transition from smooth, uniform bending to a multi-peak pattern of pronounced local curvature, with increasingly prominent nonlinear deformation characteristics. Moreover, once the wind speed exceeds the critical threshold of 6 m/s, the primary deformation region generally shifts from the leaf base to the tip. For example, the maximum curvature of walnut leaves increased from 0.018 mm⁻¹ to 0.047 mm⁻¹, and that of pepper leaves from 0.031 mm⁻¹ to 0.101 mm⁻¹, both more than double their original values. In addition, all three types of leaves demonstrated a distinct structural gradient characterized by strong basal rigidity and high apical flexibility. The tip flexibility values exceeded 1.5 × 10⁻⁵, 4 × 10⁻⁴, and 5.6 × 10⁻⁴ mm⁻²·mN⁻¹ for walnut, peach, and pepper leaves, respectively. These findings elucidate the mechanical response mechanisms of non-uniform flexible crop leaves under wind-induced bending and provide a theoretical basis and data support for the optimization of air-assisted spraying parameters.

Conventional spraying often results in poor deposition on the abaxial (lower) leaf surface and within the middle-to-lower canopy, where pest and disease pressures are typically highest. In this study, we evaluated the performance of electrostatic spraying using basil (Ocimum basilicum), cucumber (Cucumis sativus), and chili pepper (Capsicum annuum) leaves as target surfaces. A high-speed imaging system was employed to map droplet distributions on the abaxial surface, while a neighborhood-matching algorithm combined with droplet tracking was used to quantify the motion of individual droplets near the leaf. At the steady-state stage (frame 4500, 2.25 s), the number of charged droplets detected beneath the abaxial surface increased by 112% (basil), 132% (cucumber), and 213% (chili pepper) compared with non-electrostatic spraying. Smaller charged droplets exhibited higher horizontal velocities and smaller deflection angles in their trajectories near the leaf, indicating a stronger tendency to migrate toward the target surface and into the canopy interior. These findings demonstrate that electrostatic forces substantially enhance abaxial deposition and provide practical guidance for optimizing parameters for electrostatic spraying, such as droplet size, to improve spray efficiency in agricultural applications.

Electrostatic droplets can enhance deposition on target leaves. To comprehensively investigate the main factors affecting droplet adhesion and splashing after electrostatic droplet impact on pepper leaves, a specialized experimental platform was developed to capture the impact behavior of charged droplets on pepper leaves. The following four variables were examined: the applied voltage of the electrode ring, droplet size, impact velocity, and the relative distance from the impact point to the leaf tip (expressed as a percentage of the total leaf length). The results showed that all four factors had significant effects on droplet adhesion. The effect sizes (partial η²) of the four factors, in descending order, were impact velocity (0.935), applied voltage (0.907), impact position (0.895), and droplet size (0.505). Compared with non-electrostatic droplets, the increase in the maximum spreading factor of charged droplets was positively correlated with droplet size, impact velocity, and applied voltage but showed no significant correlation with the relative distance to the leaf tip. The critical velocity for splashing of electrostatic droplets decreased to a certain extent but increased with higher applied voltages. The influence of the four factors on droplet splashing decreases in the following order: impact velocity (odds ratio ≫ 1), charging voltage (odds ratio = 1.246), droplet size (odds ratio = 1.023), and impact position (odds ratio < 1). Among these, impact velocity has the most significant effect on splashing behavior. This study reveals the adhesion and splashing mechanisms of electrostatic droplets on flexible pepper leaves, providing theoretical support for the design of electrostatic spraying systems and the selection of operational parameters. The findings offer a scientific basis for optimizing droplet deposition, minimizing splash losses, and improving application efficiency in precision spraying.

Investigating the dynamic response of pepper leaves at different growth stages under droplet impact is of great significance for optimizing spray deposition processes. In this study, high-speed imaging combined with mechanical modeling was employed to systematically analyze the inclination angle variation and bending deformation behavior of pepper leaves at the seedling, flowering, and fruiting stages under droplet impact. The temporal evolution of petiole inclination angle and leaf blade inclination angle under different impact velocities and impact positions was investigated, and the energy transfer mechanism of bending energy during droplet impact was quantitatively analyzed. The results showed that pepper leaves exhibited a typical damped oscillation response after droplet impact. As the droplet impact velocity increased, the degree of leaf deformation increased significantly and the vibration duration was prolonged markedly. When the impact velocity increased from 0.89 to 1.53 m s⁻¹, the maximum vertical displacement (Δh_max) near the leaf tip increased from 0.8 to 1.9 mm at the seedling stage, from 0.6 to 3.1 mm at the flowering stage, and from 1.2 to 4.5 mm at the fruiting stage. Along the leaf length direction, both the maximum inclination angle variation and vertical displacement gradually increased from the near-petiole region toward the near-tip region because of reduced local stiffness and weaker structural constraints. In contrast, the maximum inclination angle variation generally decreased with increasing growth stage, mainly due to the increase in leaf mass and enhanced structural support. Based on the bending energy model, the conversion of droplet kinetic energy into leaf structural deformation energy was further analyzed. The results demonstrated that the bending energy of leaves increased significantly with increasing droplet impact velocity. When the droplet kinetic energy increased from 3.4 μJ to 8.0 μJ, the maximum bending energy increased from 0.063 μJ to 0.21 μJ. Among different regions, the near-tip area exhibited the highest bending energy response because of its higher flexibility and lower bending stiffness. These findings provide new insights into the interaction mechanism between droplets and flexible leaves and offer a theoretical basis for improving spray deposition efficiency and optimizing spraying strategies for pepper plants at different growth stages.