Development and Characterization of a Contact-Charging Electrostatic Spray UAV System

Abstract

Utilizing agricultural UAVs for pesticide and insecticide spraying is an effective measure for plant protection. However, achieving effective coverage on the back side of target is often challenging. To address this issue, this study combined a contact-charging spraying system with a UAV to develop an electrostatic plant protection UAV system. Upon activating the electrostatic component, strong electrostatic effects were observed at the nozzle, altering the distribution of the liquid flow; the distribution within the liquid flow became more homogeneous, while the edge regions experienced electrostatic repulsion, leading to changes in droplet size and an increase in droplet density. In the central area, droplet size reduced from 159 μm to 135 μm, while in the edge area, it changed from no value to 120 μm. During field tests using the UAV, the results showed an increase of 1.0 m in effective spray width (at a flight height of 4.0 m), indicating that the charges and propellor wind field contributed to the diffusion of droplets towards the edges. Additionally, the droplet density increased by an average of 19.7 droplets/cm², and the overall deposition increased by 0.12 μL/cm², resulting in an approximate three-fold increase compared to conventional spray, which aids in insect control and reduces pesticide usage.

1. Introduction

With the continuous evolution of drone technology, the application of unmanned aerial vehicles (UAVs) has experienced significant growth in fields such as agriculture, environmental conservation, and disaster management [1,2,3]. In the realm of agriculture, electrostatic spraying has garnered attention for its purported ability to enhance droplet deposition and improve spray quality [4,5,6].

In the USA, aerial electrostatic spray systems produced by Spectrum Electrostatic Sprayers Inc. (Houston, TX, USA) have gained extensive usage. These systems have demonstrated a four-fold increase in droplet deposition and superior insecticidal efficacy when deployed on fixed-wing aircraft, effectively controlling weeds and pests like cotton aphids, whiteflies, and red spiders [7,8,9,10,11]. In China, rotary-wing UAVs have emerged as versatile tools for plant protection and seeding, frequently employed for swift take-offs and efficient operations on small, scattered land parcels [12,13,14]. These UAVs harness the downwash airflow generated by their propellors to augment penetration while minimizing pesticides droplets drift due to environmental factors [15,16,17,18]. According to statistics, as of 2022, China’s inventory of agricultural UAVs exceeds 121,000 units, collectively covering an operational area exceeding 1 billion acres [19]. Consequently, the utilization of electrostatic spray systems on UAVs holds immense potential.

Driven by the widespread adoption of agricultural UAVs, researchers have undertaken extensive efforts to optimize the structure and parameters of these UAV spray systems to enhance their applicability [20,21,22]. UAV electrostatic spraying technology is a low-altitude, low-volume method for plant protection [5,23]. UAV Systems designed by Ru Yu et al. have demonstrated increased droplet density on the upper, middle, and lower layers of rice crops [24]. Additionally, research on helicopter electrostatic spray systems has demonstrated that electrostatics increase the deposition of liquid droplets on the back side of leaves, while charged droplets can produce a wrap-around effect [25]. However, the interactions between UAVs and electric fields, liquid dynamics, and propellor wake fields remain insufficiently elucidated. To uncover the underlying principles governing these interactions and to determine the optimal method of droplet charging, this study devised two electrostatic spray systems: one employing induction-based charging and the other employing contact-based charging. Subsequently, we conducted tests using one of these improved configurations on a UAV platform, evaluating the deposition performance of the contact-based electrostatic spray system.

In general, the expanding utilization of UAVs, especially in conjunction with electrostatic spray systems, represents a promising frontier across diverse sectors. Nevertheless, further research is imperative to comprehensively elucidate the intricate interactions among UAVs, electric fields, liquids, and propellor wake fields, thereby enabling the refinement of electrostatic spray technology for enhanced efficacy and applicability.

2. Materials and Methods

2.1. Induction and Contact-Charging Spray System

In order to compare the differences in spraying between ring-shape electrodes and parallel plate electrodes, as well as to investigate the adverse effects of over-high voltages on induction charging, three methods of electrostatic spraying were considered, induction, contact, and corona charging, with induction being the most commonly used. However, in pursuit of heightened charging efficacy, high voltages are typically applied. Nevertheless, electrodes often encounter several limitations. To examine the impact of over-high voltage on induction charging spraying, an experiment involving high voltage induction-charging spraying was designed, with a comparison between ring-shape electrodes and parallel plate electrodes. The principle of induction charging involves the following process: after atomization at the nozzle and passing through the electrode coverage area, electric charges in the liquid flow that share the same polarity as the electrode are repelled and subsequently charged with an opposite polarity upon leaving the nozzle. A high voltage electrostatic generator unit is capable of stably providing high voltage positive or negative charges in the range of 15–30 kV.

A schematic representation of the experiment in Figure 1 shows that when Switch 2 is open and Switch 1 is closed, the contact-charging method is employed. In this configuration, the high voltage end is directly connected to the bottom of the water tank, causing the solution within the tank to become charged. Consequently, the entire liquid in the water tank, water pipes, and the droplets emitted from the nozzle acquire a charge polarity matching that of the high voltage electrode. When Switch 2 was closed and Switch 1 was open, the induction charging method was employed. The atomization process of droplets was recorded and analyzed using digital cameras and high-speed cameras. The testing conditions included the application of a 30 kV charging voltage, with the process of charging and atomization being captured and analyzed through the cameras.

The experiment utilized two types of induction-charging electrostatic nozzles with ring-shape electrodes (manufactured by Kofon Jiahua Company, Beijing, China) and parallel plate electrodes (from Harper Adams University, Shropshire, UK). The composition of the electrostatic spraying system is illustrated in Figure 2 and includes a high voltage electrostatic generator unit, a water tank, a pump unit, water pipes, a spray bar rod, and a contact-charging nozzle. In the case of the induction-charging method, the high voltage electrode of the electrostatic generator unit was connected to the electrode, forming a charged electric field between the parallel plates or ring-shape electrodes. The pump unit supplied the nozzle with liquid from the water tank. As droplets pass through the space enclosed by the parallel plate electrodes or ring-shape electrodes, they acquire an opposite charge polarity to that of the induction electrode in accordance with the principle of induction charging. The other pole of the high voltage electrostatic generator unit was grounded or connected to the target, establishing an electric field between the nozzle and the ground or target.

2.2. Laser Droplet Size Measurement Instrument

The DP-02 droplet size laser analyzer, manufactured by Omec Company (Zhuhai, China), is composed of a collimated laser generation device, a signal acquisition device, and a data processing system. The collimated laser generation device comprises a laser tube, a beam expansion system, a collimating lens, and a Fourier lens. The signal acquisition device primarily consists of a main detector, auxiliary detectors, a voltage stabilizer, and data acquisition circuits. The data processing system is comprised of a computer and associated software programs. When light encounters tiny particles in its path, scattering phenomena occur. Larger particles result in smaller scattering angles, whereas smaller particles yield larger scattering angles. By measuring the distribution of scattered light, droplet sizes can be detected. The DP-02 droplet size laser analyzer can detect volume mean droplet diameter (D(4, 3)), surface mean droplet diameter (D(3, 2)), specific surface area (S.S.A), volume median droplet diameter (D50), and boundary droplet sizes (D10, D90). D50 is a crucial indicator for assessing spray quality.

Table 1. Parameters of DP-02 droplet size laser analyzer.

System Details	Parameter
Power Supply	~220 V/50 Hz
Laser	Type: He-Ne
Output Power	(2~3.5) mw
Wavelength	0.66328 μm
Number of Independent Detectors	48
Measurement Range	1–1500 μm
Median Droplet Size Repeatability Precision	±3%
Data Sampling and Analysis Time	≤2 min

shows its operational parameters, and Figure 3 shows the indoor test site.

2.3. Contact-Charging UAV Spray System

The second-generation optimization of a UAV electrostatic spray system [21] was developed in consideration of the challenges encountered in the first-generation version, where bipolar charging often resulted in the rapid neutralization of positive and negative liquid flows in the air. In the design of this prototype, a unipolar negative contact-charging method was employed, with the positive pole naturally discharging into the air to dissipate residual charges within the system. The high voltage electrostatic generator unit provides two types of output voltages—positive and negative. The negative output terminal is linked with contacting electrodes in the bottom of two water tanks, charging the water inside with a “-” charge, while the positive output terminal is exposed to the air for discharge, Figure 4a. The voltage-adjustable high voltage electrostatic generator unit is controlled for activation and deactivation using a remote-control device. Prior to the UAV’s takeoff, the voltage is calibrated through a voltage-adjusting knob. The high voltage generator unit switch is then activated after the UAV’s spraying operation to ensure system safety. Insulation plays a critical role in ensuring the effectiveness of the contact-charging electrostatic spraying method. During the assembly of the UAV electrostatic spray system, the water tank and water pipes are made of insulated PP material. The link wires between the high voltage generator and water tanks are coated with two layers of insulation material. At each connection point, including between the water tank and water pipes, and between the pump unit and water pipes, high voltage insulating tape and adhesive are used for bonding to prevent leakage and discharge during the transportation of fluids. The UAV parameters are shown in

Table 2. Parameters of contact-charging UAV spray system.

System Details	Parameters
Wheelbase	2028 mm
Maximum Takeoff Weight	80 kg
Water Tank Capacity	30 L
Maximum Effective Payload	45 kg
Total Battery Capacity	28,000 mAh
Flight Speed	2–8 m/s
Flight Time	≥15 min
Effective Flight Height	2–8 m
Normal Spray Width	3–9 m
Remote Control Radius	2 km
Number of Working Nozzles	2
Voltage of High Voltage Generator	30 kV

.

2.4. Experimental Design

2.4.1. Droplets Size Test

To evaluate the changes in droplet size under the influence of electrostatics in different spray areas, a laser droplet size analyzer was employed to measure the droplet sizes in various spray detection points. To facilitate this evaluation, seven detection points were selected based on changes in the atomization angle of the spray before and after electrostatic activation. These detection points, shown in Figure 5, were defined relative to the nozzle, with the center point, A0 (0, 0), located directly beneath the nozzle at a distance of 50 cm. The coordinates of the remaining detection points were as follows: A1 (15, 0), A2 (25, 0), B1 (10, 20), B2 (20, 20), C1 (5, 40), and C2 (15, 40).

2.4.2. Electrostatic UAV Deposition Test

Experimental Materials: The materials used in the experiment included an electrostatic spray UAV, a tracer dye (red), test paper cards (with position numbers and an indication of the front/back, such as A1+ for the front side of the card at position A1 and A1-for the back side), tripods and clamps (17 groups), and sealed bags (labeled with test numbers, with one set of test paper cards placed in each sealed bag). The experimental site and layout are shown in Figure 6.

Experimental Conditions: The temperature was 9–13 °C, the wind speed was 0.3–1.5 m/s, and the air humidity was 58%.

Arrangement of Test Paper Cards: Test paper cards were positioned at intervals of 0.5 m, with each interval representing a sampling point. The sampling points were named as 0, −0.5, −1.0, −1.5, −2.0, −2.5, −3.0, −3.5, −4.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 m from the midpoint. Each sampling point had two layers of test paper cards, which were secured in place using tripods and swivel clamps.

The UAV was operated in GPS flight mode, executing a single flight along the central axis. The flight was conducted at altitudes of 3.0 and 4.0 m above the ground, with a flight speed of 2 m per second. Electrostatic charging voltages were applied at two settings: 0 kV and 30 kV, respectively. After the experiments were concluded, the test papers were collected and sealed in bags. They were then taken back to the laboratory and scanned using a scanner. The data were analyzed for droplet deposition rate, droplet density, and other parameters using the Image J software from the United States [27].

3. Results and Discussion

3.1. Adverse Effects of Over-High Voltage on Parallel Plate Electrodes Inductive Charging Nozzle

Figure 7 illustrates the electrostatic phenomenon of inductive charging electrostatics dispersed in a system based on parallel plate electrodes. Under a charging voltage of 30 kV, when the high voltage charge was activated, compared to conventional spraying, the spray plume was repelled and expanded, generating smaller size droplets that were expelled outward from the spray plume. This demonstrates the potential for optimizing droplet dispersal characteristics through electrostatic spraying. However, due to the over-high voltage, the droplets were reversely attracted to the induction electrode, resulting in a change in the charging method. This generated a strong electrostatic effect, causing droplets to accumulate on the nozzle, indicating that the current electrode structure design is not suitable for charging voltages exceeding the typical 4 kV.

When a high voltage charge of 30 kV was employed, the coverage area of the spray plume was expanded. However, due to limitations in the nozzle’s structural design, the over-high charging voltage led to strong adhesion of the spray plume, causing it to be attracted to the parallel plate electrode, thereby transitioning from an induction charging method to a contact-charging method. On one hand, this led to the dispersion of the spray plume, resulting in intense atomization effects. On the other hand, due to the accumulation of liquid on the nozzle surface, the electrode fixture and induction electrode formed a unified structure, causing droplets to rapidly move under the influence of the high voltage electric field, resembling intermittent emissions with rapid descent. The repeated occurrence of fine and large droplets hindered uniform deposition, preventing the achievement of the desired optimization of droplet distribution uniformity.

The phenomenon of charge accumulation and the emission of large droplets due to over-high charging voltages in the contact-charging process suggests that the high voltage electric field provides significant potential energy for droplet movement. This assists in increasing droplet velocity and creates a strong spatial electric field that drives droplets to achieve a wrap-around effect on the spraying target. It can be concluded that the efficiency of contact-charging surpasses that of induction charging due to the voltage intensity being the determining factor.

3.2. Adverse Effects of Over-High Voltage on Ring-Shape Electrodes Inductive Charging Nozzle

Figure 8 illustrates the phenomenon of liquid electrostatic atomization in a system based on ring-shape electrodes in over-high voltage. Figure 8(1) depicts the nozzle not spraying and with electrostatics turned off, while Figure 8(2) shows the image of conventional spraying, where the inner wall of the ring-shape electrode and the plastic cylinder wall are very dry. Figure 8(3) captures the moment immediately after electrostatic activation, where droplets are strongly attracted by the electric field formed by the ring-shape electrode and adhere to both the electrode and plastic cylinder. This indicates that the adhesive force of the electric field is too strong, surpassing the effects of induction charging. Figure 8(4) represents an image taken shortly after electrostatic activation, where the entire nozzle unit was wetted by the spray, displaying similar phenomena to the induction nozzle with parallel plate electrodes.

Through testing with the ring-shape electrode induction nozzle, a similar conclusion is drawn, namely that induction charging nozzles should not utilize excessively high 30 kV voltages. This results in lower charging efficiency for induction charging compared to contact-charging.

3.3. Effect of Contact-Charging Electric Field on Liquid Flow Distribution

Figure 9a displays the liquid flow in conventional spray under a high-speed camera. In conventional spray, the liquid flow is mainly influenced by the pump, resulting in intermittent wave-like flow patterns. A significant portion of this liquid flow is concentrated in the central area directly beneath the nozzle, indicating that the hydraulic force generated by the water pump plays a major role in the initial stages of spraying.

Upon electrostatic activation (Figure 9b), the liquid flow in the central area(as can be seen in the blue box area) tended to become more uniform, indicating that electrostatics caused electrostatic repulsion within the liquid flow. This disperses the main liquid flow from the central area towards the periphery, demonstrating that electrostatics can lead to a more even spatial distribution of the spray.

From the images captured by the camera in Figure 9c–f, depicting conventional spraying and contact-charging spraying, it is evident that upon electrostatic activation, a strong electrostatic effect occurs at the edges of the liquid flow, causing it to disperse towards the surroundings. This portion of the droplets exhibited strong adhesion properties. When a light source was brought close to the spray liquid flow at this point, the dispersed droplets took the light source as a target. Under the influence of the electric field, they moved toward the light source in a highly directional manner, displaying an almost perfect distribution of electric field lines. This phenomenon demonstrates that the charged droplets experience a strong wrap-around effect, propelling them to move along the electric field lines. It highlights the significant potential of electrostatic spraying in enhancing deposition on the back side of the target.

In order to achieve better electrostatic charge efficiency, a more suitable charging method is often required. Through a comparison between inductive and contact-charging experiments, this study explains the reason for choosing the contact-charging method in electrostatic spray systems. Over-high voltage is often unsuitable for inductive charging methods. Due to inherent limitations of the inductive electrode (whether in parallel plate or ring-shape), the electrode attracts opposite charges in the liquid flow, and the excessive attraction force provided by high voltage causes the droplets to be adsorbed onto the electrode, wetting it, thus contradicts the theory of inductive charging. Furthermore, when the droplets are attracted and gathered on the electrode, the electrode and the liquid flow almost form a unity (same polarity), transitioning to the contact-charging method. Driven by the high voltage electrostatic generator unit, the potential energy of the large droplets accumulated on the electrode is converted into kinetic energy, accelerating their descent, and leading to instability in the charging process. Additionally, these large droplets adversely affect the uniformity of the spray. As shown in Figure 9d,f, in contrast, the contact-charging method produced more uniform and more adhesive droplets, contributing to achieving a more uniform deposition effect. Under high voltage conditions, the contact-charging method exhibited greater stability compared to inductive charging. The generation of small droplets, optimization of spray flow internal distribution, and the generation of wrap-around effects enable the advantages of electrostatic effects to be ideally manifested.

3.4. Effect of Charging Voltage on Droplet Size

Electrostatics also have an impact on the droplet size as shown in

Table 3. Droplet size changes in different spray area.

Site	A0	A1	A2	B1	B2	C1	C2
Conventional	137	144	NV	144	141	124	137
Electrostatics	123	128	120	126	110	109	103
Change	14	16	120	18	31	15	34
Trend	↓	↓	-	↓	↓	↓	↓

Note: NV stands for no value, indicating that no numerical values were detected in this area.

. Testing droplet sizes at different positions revealed that while the liquid flow became more homogeneous, the droplets were further dispersed and atomized into smaller droplets. However, the extent to which electrostatics affected droplet sizes varied at different positions.

The area with the greatest variation in droplet size was A2, which initially had no droplets (no value) during conventional spray. Upon electrostatic activation, the spray angle increased and also resulted in changes in droplet size. The changes in A0, A1, B1, and C1 were 14/16/18/15 μm, showing minimal differences. This analysis was primarily attributed to these four areas being situated in the central area of the spray, where hydraulic forces play a major role.

On the other hand, the changes in the outer edge areas of the spray were 120, 31, and 34 μm, and the variations in droplet size were significantly larger than in the central area. This indicates that the droplets in the edge areas are more affected by electrostatics. Additionally, the droplet size changes followed the pattern A2 > B2 > C2, indicating that the closer the area is to the nozzle, the greater the influence of the electric field force. This suggests that the electric field intensity decreases as the distance from the nozzle increases. In contrast, the droplet sizes in the central area were noticeably larger than those at the edge, indicating that smaller droplets are more influenced by electrostatics.

3.5. Effect of Electrostatic on UAV Spray Width

Effective spray coverage is a crucial parameter for assessing agricultural UAVs, and the droplet density determination method is a primary means of evaluating low volume sprayers. According to current research, the droplet density exceeding 15 droplets/cm² is referred to as the effective spray width for crop protection UAV [26].

In Figure 10, through the measurement of deposit densities distributed laterally across sampling points, a significant enhancement in deposition at the central sampling point (point 0) due to electrostatic spraying technology was observed. The average deposit density at this central point was 150 droplets/cm², which was significantly higher than the 65.5 droplets/cm² achieved with traditional spraying techniques. Furthermore, electrostatic spraying exhibited greater variability in deposit density at all sampling points compared to traditional methods. After reaching its peak in the central area, the deposition density with electrostatic spraying significantly decreased as the sampling points moved away from the center, indicating that the effectiveness of electrostatic spraying was most pronounced within a certain range from the nozzle.

UAV electrostatic spraying resulted in higher droplet densities at all coordinate points compared to conventional spraying, with an average increase of 18.7 droplets/cm². This demonstrates that UAV electrostatic spraying can effectively enhance droplet deposition density. The general trend in droplet distribution is characterized by the highest density in the central region, gradually decreasing towards both sides of the UAV. The effective spray swath for conventional spraying was 3.5 m, while electrostatic spraying extended it to 4.5 m, indicating an increase of 1.0 m. This suggests that UAV electrostatic spraying can enhance lateral movement of droplet deposition.

In Figure 11, the analysis of droplet deposition on the front and back sides of a uniform target under electrostatic spraying mode is depicted. Overall, in the central region (−2 to +1.5), the droplet density on the front side exceeded that on the back side. This is primarily due to the downward pressure caused by the airflow. However, an interesting observation is that in the peripheral areas beyond this central region, the deposition on the back sides of the target surpassed that on the front sides. This phenomenon can be attributed to the changing flow dynamics influenced by the propellor’s reduced effect in the peripheral region. In these areas, the electric field force becomes dominant, and with the lateral movement of the airflow, droplets first move to the back side and then to the front side of the target. On average, the droplet density on the front side was very close to that on the back side, with of 36.9 droplets/cm² on the front and 32.5 droplets/cm² on the back, respectively.

Figure 12 illustrates the deposition characteristics of electrostatic spraying on the front side of the target at different heights. From the graph, it can be observed that at a flight height of 4.0 m, the droplet deposition density on the front side of the target in the central region (−1.5 to 0.5) was slightly higher than that at 3.0 m. The reason for this change is that with increasing height, under the influence of airflow, droplets tend to disperse. As the height increases, the distribution area of droplets becomes larger, leading to a decrease in density. However, the increase in flight height also results in an expansion of the effective spraying width. For instance, at a 3.0 m flight height, the effective spraying width is 3 m, whereas at a 4.0 m flight height, it increases to 4.5 m. Therefore, raising the flight height appropriately can enhance the effective spraying width of electrostatic spraying.

Furthermore, Figure 13 analyzes the deposition of droplets on the back side of the target at different spraying heights. Overall, at a flight height of 4.0 m, the droplet deposition density on the back side of the target was higher than that at 3.0 m, with an increase of approximately 14.0 droplets per square centimeter.

3.6. Effect of Electrostatic Spray on Deposition

In Figure 14 and Figure 15, regarding the indoor deposition of droplets, as can be observed, in terms of droplet density, the front side exceeds the back side. However, it can be noted that the back side of the target mainly consists of small droplets distributed compactly, while the front side has larger droplets. It is evident that smaller droplets on the front side are more susceptible to the influence of the electric field force and are easier to be controlled by it, thus facilitating deposition on the back side of the target. However, during the UAV tests, the droplet density decreased, primarily due to the effect of the wind field causing dispersion.

Table 4. Deposition difference between outdoor UAV conventional spray and electrostatics spray.

		Conventional	Electrostatics
Droplets Density (Droplets/cm2)	Front sides	18.2	36.9
	Back sides	11.9	32.5
Deposition (μL/cm2)	Front sides	0.05	0.17
	Back sides	0.03	0.06

compares the deposition of droplets under electrostatics and conventional conditions. In general, when compared to conventional UAV spray, UAV electrostatic spray increases deposition by threefold, with a corresponding increase in droplet density of 19.7 droplets/cm² overall. This indicates that UAV electrostatic spray can effectively increase target deposition, thereby improving the efficiency of pesticide application.

3.7. Effect of UAV Propellor Downwash Airflow on Droplet Movement

Although the droplet density on both the front and back sides of the target increased to varying degrees in the field-simulated target tests, it is evident that their adhesion was inhibited. This is primarily due to the downwash airflow generated by the UAV’s propellors being the main influencing factor, followed by liquid forces and electric field forces. The electric field force mainly increased the droplet density and was primarily influenced by electric field forces, hydraulic force, and wind conditions. Hydraulic force had an initial atomizing effect, while the electric field force increased the droplet density and reduced droplet size. The combined action of the electric field force and the wind field affected the movement of the droplets.

In the edge areas, the droplet density on the back side of the target exceeded that on the front side. In the central area, the droplets were primarily influenced by the wind field. According to the characteristics of the downwash airflow, some of the airflow generated from the propellors would wrap-around and carry droplets to various parts of the target, while another part would expand to both sides. The droplets transported by this airflow would first reach the back side of the target. As the airflow’s effect diminished, the electric field began to play the main role, causing the droplets to be adsorbed onto the target’s surface.

The increase in effective spraying width is primarily due to the action of electrostatic atomization, which increases the coverage area of the liquid flow. Subsequently, the liquid is transported under the influence of the wind field. Ultimately, this leads to an increase in the effective spraying width of the droplets to varying degrees.

4. Conclusions

The inductive charging method is currently applied in most agricultural electrostatic spraying scenarios. However, the structural principle of the induction charging electrode cannot guarantee effective insulation and charging efficiency at over-high voltages, limiting its development with higher voltages. This study provides further evidence for this theory, as there were many maladaptations observed in the induction nozzle under over-high voltage conditions. This poses a challenge for researchers who wish to further improve charging efficiency.

In this study, the strong atomization and wrap-around effect of the contact-charging electrostatic spray were vividly evident. Under the influence of the high voltage electric field, several notable changes occurred. Firstly, the distribution of droplets within the liquid flow became more homogeneous, reducing the variability in droplet size. This, in turn, resulted in the production of smaller droplets, which translated into a uniform and orderly distribution on the deposition surface, particularly at the target. In terms of adhesion, any target that came into close proximity to the cloud of droplets formed an electric field between itself and the nozzle, creating a wrap-around effect. This phenomenon is highly significant in laboratory conditions, providing strong evidence of the electrostatic potential of the contact-charging electrostatic spray.

When the contact-charging electrostatic spray system is integrated into a UAV and operates in the presence of downwash airflow, the droplets undergo accelerated motion. They initially reach the target and the ground, and then disperse outward in both directions. The wind carries these droplets and, through the combined effects of the wind and electrostatic forces, the deposition quantity and density of droplets on both the front and back sides of the target are increased significantly. The integration of the UAV with the electrostatic spray system results in higher droplet density on the target, thereby enhancing pesticide utilization while reducing environmental and soil contamination caused by off-target pesticide droplets. This highlights the substantial potential of this UAV electrostatic spray technology in plant protection.