Electrostatic spraying has been widely used to enhance the droplet deposition under plant protection using UAV (unmanned aerial vehicles). The plant protection droplets can also move through the intense rotor under downwash and turbulence, when the droplets can carry sufficient electric charge. However, the CMR (charge-to-mass ratio) can attenuate markedly before reaching the crop canopy. Field-scale quantification of CMR has been constrained to deploy over a wide spray footprint, due to the conventional rigid Faraday cylinder collectors. In this study, a flexible AFC (aluminum foil container) was developed to collect the charged droplets under field conditions. The CMR was then quantified in contact charging UAV electrostatic spraying. A coupled attenuation model was further used to interpret the relative contributions of the charge loss and droplet mass loss during flight. A high-voltage generator module was integrated into a quadrotor spray platform and then configured for contact charging. The negative high voltage output was connected to a metal contact electrode inside the liquid tank in charge of the working solution. While the positive terminal was mounted on the landing gear and discharged to air, providing for the weak capacitive coupling that supported charge balance during hovering. The AFC collector followed the Faraday cage principle. But the rigid collectors were replaced with a flexible aluminum foil conductive layer supported by an insulating backing, allowing for the collector geometry to match the spray swath. Repeatability tests under identical spraying showed that the consistent CMR was comparable to a conventional Faraday cylinder in the field deployment. Field experiments were conducted at the charging voltages from 15 to 35 kV and the flight heights from 2.0 m to 5.0 m. Two spray adjuvants were evaluated at the same recommended concentration of 1‰, a surfactant-based product. Momentive Agrospred 910 and a modified plant oil product, Maifei, were used in the field test. Baseline CMR increased with the voltage and then reached 3.50 mC/kg at 35 kV. Field measurements indicated that there was an outstanding CMR loss in the rotor downwash, indicating the height effect. Once the flight height increased from 2.0 m to 5.0 m, the average CMR attenuation rates were 14.09 %, 24.23 %, 39.32 %, and 52.84% at 2.0, 3.0, 4.0, and 5.0 m, respectively. Rotor wind speed during measurements remained close to 14m/s. A shared background served as in the treatments. Model fitting showed that the exponential charge decay dominated the CMR attenuation from 2.0 m to 5.0 m, while there was a weak evaporation-driven mass loss. The charge attenuation coefficient, λ, was fitted approximately 0.16 at 35 kV, whereas the mass loss coefficient, K' prime, was approximately 4.0×10^-4, corresponding to less than 0.003 correction within 5.0 m. The λ increased from 0.08 to 0.16 over the voltages, as the voltage rose from 15 to 35 kV, with the intermediate values of 0.10, 0.12, and 0.14 at 20, 25, and 30 kV. Despite the high decay per unit distance at the higher initial charge, the higher voltage still produced the higher retained CMR in the typical operating heights. Both adjuvants increased baseline CMR for the retained CMR under UAV operation. Momentive increased the baseline CMR by about 17.71% at 35 kV, with the maximum of 4.12 mC/kg, whereas Maifei increased the baseline CMR by about 45.43 % with the maximum of 5.09 mC/kg. Attenuation rates were broadly comparable with/without adjuvants. The primary benefit of the initial CMR of the more charge remained after in-flight decay. Overall, the droplet charging model was established for contact charging, while the CMR attenuation model was for UAV electrostatic spraying. The AFC effectively captured the CMR variations dominated by flight height and rotor wind in the UAV crop protection spraying. The CMR can play an enhancing role in spray adjuvants. These findings can provide an experimental reference to optimize the UAV electrostatic spraying.

Electrostatic spraying is expected to integrate with unmanned aerial vehicle (UAV) platforms in modern agriculture. This approach represents an innovative plant protection technology, offering promising potential for improving pesticide utilization efficiency, deposition uniformity in precision agriculture. Electrostatic spraying imparts electrostatic charges to droplets through the application of high-voltage electric fields, thereby modifying their trajectories and enhancing adhesion to plant surfaces. This is particularly beneficial for achieving deposition of the abaxial (underside) surfaces of leaves, which are typically difficult to reach using conventional spraying techniques. This review aims to examine the development history, theoretical foundations, and current research on electrostatic spraying for crop protection UAVs. It systematically outlines the three mainstream charging mechanisms—corona, inductive, and contact charging—highlighting their physical principles, advantages, limitations, and the applicable voltage ranges. Inductive charging dominates current applications due to its relative safety and engineering simplicity. However, contact charging also presents distinct advantages over other mechanisms. The droplet charging process is relatively mild, avoiding intense discharges that may occur in corona charging systems. Since the sprayed liquid comes into direct contact with the electrode, a stable charge-transfer field is established, leading to more sufficient and uniform charging of droplets. In addition, the structural designs of inductive charging nozzles are discussed, including various electrode configurations (e.g., ring-type, cone-type, and embedded parallel plates), selection of electrode materials (such as copper, nickel, and stainless steel), and the integration of air-assisted mechanisms. Furthermore, key evaluation techniques are reviewed, including charge-to-mass ratio (CMR) measurements, droplet size characterization (e.g., volume median diameter, VMD), and deposition detection methods. However, standardized testing protocols remain lacking, and significant discrepancies persist between laboratory measurements and field performance—especially under complex field conditions involving wind, temperature, and humidity variations. The droplet behavior of electrostatic spraying is further analyzed through droplet trajectory modeling, with emphasis on three dominant electrostatic field interactions: 1) induced fields between droplets and plant targets, 2) repulsive fields among charged droplets, and 3) externally applied fields between the nozzle and the target. Each mechanism contributes differently to droplet motion, distribution, and deposition efficiency. Evidence from recent studies suggests that combining electrostatic spraying systems with UAV platforms can effectively improve spray characteristics, such as deposition density, spray width, total deposition, and pest control performance. However, their trajectory control capability remains limited and challenging:1) rotor-generated airflow interferes with droplet trajectories, weakens electrostatic adhesion, and accelerates CMR decay; 2) variations in UAV flight height affect the electric field distribution between the nozzle and the crop canopy, reducing deposition accuracy; and 3) environmental factors such as wind, temperature, and humidity introduce uncertainties that compromise field performance. To address these limitations, several recommendations for future research are proposed: 1) advance high-voltage contact or corona charging systems with enhanced safety features; 2) develop electrostatically optimized-size nozzles and adjuvants; 3) refine evaluation metrics by integrating CMR with droplet size distribution; and 4) conduct large-scale, crop-specific field validations. With the advancement of UAV technology, high-voltage electrostatics, and system miniaturization, UAV electrostatic spraying is poised to become a key tool for next-generation precision pesticide application, offering strong potential for reducing pesticide use and supporting sustainable agriculture.