Review and prospects of electrostatic spraying technology for plant protection UAVs

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.