Study on Spray Deposition Effect of a New High-Clearance Air-Assisted Electrostatic Sprayer

Abstract

This study evaluates the performance of a novel high-clearance air-assisted electrostatic sprayer designed for vineyards and investigates the impact of applied voltage on droplet deposition. This sprayer, which uses a new type of air-assisted electrostatic spray nozzle, could spray three rows of grapes at the same time, significantly improving work efficiency. Field test results show that the middle row of the high-clearance air-assisted electrostatic sprayer deposition effect was better than the left and right rows, and the minimum droplet deposition density inside the grape canopy was 26.4 deposits/cm². The droplet deposition effects of electrostatic spraying were effectively improved, and the average droplet deposition density of the canopy increased by 15.72%. Electrostatic spraying improves the deposition on the outer canopy but reduces deposition on the inner canopy, so electrostatic spraying reduces the penetration of droplets into the canopy. The sprayer’s design proves effective for large-scale operations, offering insights into electrostatic spray technology’s role in precision agriculture.

1. Introduction

Although various methods of pest and disease control have been extensively studied, chemical control remains the simplest and most effective method of pest and disease control in vineyards today [1,2,3]. However, outdated sprayers can lead to excessive pesticide application, resulting in pesticide residue levels exceeding safety standards in grapes and causing environmental pollution [4,5,6]. These outdated sprayers also cause low work efficiency and high labor intensity [7]. Therefore, pursuing low-volume, high-efficiency pesticide spraying technology is an important means of reducing pesticide pollution and saving labor [8].

Traditional sprayers, while effective for field crops, often struggle to achieve uniform coverage in vertically structured vineyards due to dense foliage, complex canopy architectures, and the need for high-clearance equipment [9,10]. However, the integration of electrostatic principles with air-assisted systems in high-clearance sprayers tailored for vineyards remains underexplored, particularly in multi-row applications where spatial variability in airflow dynamics and canopy interactions significantly influence outcomes [11,12].

Air-assisted sprayers, first conceptualized in the mid-20th century, revolutionized pesticide delivery by utilizing high-velocity airstreams to propel droplets into dense canopies [13]. Early studies demonstrated their superiority over hydraulic nozzles in orchard settings, with improved deposition on abaxial leaf surfaces and reduced drift [14,15]. However, challenges persist in vineyards characterized by tall, vertically trained vines (e.g., row heights > 2 m and row spacing ≥ 3.5 m), where conventional sprayers exhibit limited adaptability [16,17]. Field assessments revealed that only 30–45% of applied pesticides reached inner canopy layers in standard spray systems, with significant losses attributed to droplet evaporation and wind interference. Subsequent innovations, such as variable-rate nozzles and laser-guided targeting, partially addressed these issues but introduced complexities in calibration and energy consumption [18,19]. The introduction of electrostatic spraying represents an innovative shift. Research has shown that charged droplets exhibit more optimal trajectories toward plant surfaces due to Coulombic attraction [20]. Despite these advancements, practical applications in vineyards still face challenges, including leaf attenuation of the electrostatic field and humidity variations [12,21]. Additionally, the interaction between air-assisted and electrostatic effects has not been fully quantified, particularly in multi-row configurations, where the length of air ducts and the uneven distribution of airflow can have different impacts on droplet transport [22].

Electrostatic sprayers, initially developed for industrial coatings, have gained traction in agriculture for their ability to reduce chemical usage while maintaining efficacy [23]. Key studies established that droplet charging significantly improves deposition on hard-to-reach surfaces, such as the undersides of leaves, with coverage increasing by 40–60% compared to uncharged sprays [20]. In vineyards, electrostatic systems achieved 27% higher canopy coverage than conventional airblast sprayers, albeit with variability dependent on nozzle orientation and canopy porosity [12]. These findings align with computational fluid dynamics (CFD) models, which highlighted the role of droplet size and charge density in optimizing electrostatic deposition under turbulent airflow conditions [11].

However, practical limitations persist. Field trials revealed that electrostatic charging exacerbated drift in open-field vineyards under moderate wind speeds (>2 m/s), underscoring the need for integrated airflow control [24]. Recent innovations, such as induction-charged nozzles and adjustable electrode geometries, aim to mitigate these issues by fine-tuning electric field intensities [25]. Nonetheless, the scalability of such systems for high-clearance, multi-row vineyard operations—where sprayer height exceeds 2 m and row spacing exceeds 3.5 m—remains untested.

Modern vineyards increasingly adopt vertical trellising systems to maximize yield and mechanization compatibility, necessitating sprayers capable of operating at heights > 2 m without damaging canopy structures [16,26]. High-clearance sprayers, equipped with extendable booms and adjustable nozzles, have emerged to meet this demand. Despite these advancements, critical gaps remain. First, most high-clearance sprayers prioritize hydraulic or pneumatic atomization, neglecting the potential synergies between air assistance and electrostatic charging [12,27]. Second, operational parameters such as nozzle tilt angle, airflow velocity, and voltage settings are rarely optimized collectively, leading to suboptimal deposition uniformity [28,29]. For example, field tests showed that tilting nozzles downward by 15–20° improved mid-canopy coverage, but similar adjustments in electrostatic systems have not been systematically evaluated [30].

This paper presents a newly developed high-clearance air-assisted electrostatic sprayer characterized by several innovations: (i) a three-row simultaneous spraying configuration; (ii) a high-clearance chassis enabling undisturbed passage through vineyard rows; and (iii) an adjustable-angle electrostatic nozzle system. These features were designed to improve spray deposition, particularly on the outer canopy, while significantly enhancing operational efficiency in vertically structured vineyards. By bridging air-assisted electrostatic spraying with high-clearance engineering, this research advances sustainable viticulture. The sprayer’s design reduces pesticide dosage while maintaining efficacy—a critical need [31]. Furthermore, its multi-row operation aligns with labor-saving automation trends, offering scalability for large vineyards.

2. Materials and Methods

2.1. The Sprayer

The experiment used a high-clearance air-assisted electrostatic sprayer suitable for trellis-trained wine grape vineyards, equipped with two 800 L capacity tanks as shown in Figure 1. When the sprayer is in operation, the cab rises to the top. With a maximum ground clearance of 2.2 m, the sprayer can operate across a row of grapevines and simultaneously spray three rows of grapes, greatly improving operational efficiency.

The high-clearance air-assisted electrostatic sprayer is equipped with four-wheel steering control, which effectively reduces the turning radius. The maximum lifting height of the spray boom is 4 m, ensuring that the grape plants will not touch the spray boom when turning at the head of the field. The sprayer utilizes high-speed airflow atomization technology and induction charging technology.

The high-speed airflow required for atomization is provided by a Roots blower (model: Zhenglin ZLS-100L; power: 7.5 kW; airflow: 6.19 m³/min; pressure: 39.2 kPa; speed: 1540 rpm, Zhejiang, China). The Roots blower is driven by a diesel engine (model: Beilong 2V92; power: 13 kW; maximum speed: 3000 rpm, Chongqing, China). High-speed airflow is generated by a Roots blower, divided by a three-channel diverter, and delivered through flexible ducts to three spray suspension units, and each spray suspension unit works on a row of wine grapes. The sprayer uses two diaphragm pumps (model: Seaflo SFDP2-070-060-53; voltage: 24 V; maximum flow rate: 26.5 L/min, Fujian, China) powered by vehicle batteries to supply liquid. Each 800 L chemical tank is equipped with a diaphragm pump. The liquid is pumped out by the diaphragm pump, passes through a three-way solenoid valve (model: Yanake 3V210-08; voltage: 24 V, Nanjing, China), and is then distributed into three separate streams, each of which is delivered to one of the three air ducts on the same side. The sprayer uses two high-voltage electrostatic modules to charge the air-assisted electrostatic spray nozzles. The high-voltage electrostatic modules are manufactured by Dongwen (Tianjin, China), with an input voltage of 24 V, an output voltage of 0 to −5000 V, and a maximum output current of 1 mA. The switches for the diesel engine, the high-voltage electrostatic module, and the diaphragm pumps are all mounted in the cab.

Considering the losses of air pressure and liquid pressure in the system, before operation, the diesel engine and diaphragm pump of the sprayer were set to ensure that the output air pressure of the Roots fan is 0.7 bar (controlled by rotational speed), and the output liquid pressure of the diaphragm pump is 1.0 bar (controlled by controlling the reflux).

2.2. The Air-Assisted Electrostatic Spray Nozzle

The air-assisted electrostatic spray nozzle used in this paper utilizes high-speed airflow to atomize the liquid, and the droplets are charged by an induced electric field generated by a ring-shaped electrode [32]. The air-assisted electrostatic spray nozzle is fixed to the duct via a movable clip, and high-speed air flows into the nozzle from the side, as shown in Figure 2. The liquid enters the nozzle from the rear via the liquid pipe, and a high-voltage wire connects the ring electrode and high-voltage electrostatic module through reserved high-voltage line holes.

High-speed airflow is generated by the Roots blower and passes through an expansion-contraction passage inside the nozzle, further increasing its velocity. The flow of airflow in the nozzle can be regarded as an airflow process in a variable cross-section pipe, and the one-dimensional isentropic flow theory for variable cross-section pipes in gas dynamics can be applied [27]. During the design process of the nozzle, the internal airflow velocity of the nozzle is set to be less than the speed of sound. Therefore, when high-speed airflow enters the nozzle from the duct, it enters the expansion phase. During the expansion phase, the velocity of the high-speed airflow decreases, and the pressure increases. During the contraction phase, the velocity of the high-speed airflow increases, and the pressure decreases [27]. The following relationship exists between nozzle airflow inlet pressure (p₀) and outlet pressure (p_B):

$${\displaystyle \frac{p_B}{p_0}}={({\displaystyle \frac{2}{k+1}})}^{{\displaystyle \frac{k}{k-1}}},$$

(1)

where k = c_p/c_v is the ratio of the specific heat under constant pressure (c_p) to that under constant volume (c_v), k = 1.4.

Since the airflow in the air-assisted electrostatic nozzle is an adiabatic isentropic process, the formula for calculating the airflow speed in the atomization region (v_A) is

$$v_A={\displaystyle \frac{S_B}{S_A}}\sqrt{{\displaystyle \frac{2k}{k-1}}{\displaystyle \frac{p_B}{\rho }}\left[{({\displaystyle \frac{p_0}{p_B}})}^{{\displaystyle \frac{k-1}{k}}}-1\right]},$$

(2)

where S_B is the cross-sectional area of the nozzle outlet, S_A is the cross-sectional area of the atomization region, and ρ is the air density (ρ = 1.17 kg/m³).

When the diameter of the nozzle outlet is 4 mm, the cross-sectional area of the nozzle is 4π mm², and the cross-sectional area of the atomized area is about 3.75π mm². The air pressure at the inlet of the nozzle is 0.4 bar to 0.6 bar. Then, the air velocity in the atomized area is about 245~293 m/s.

The spray nozzle generates fine mist droplets under the action of high-velocity air flow [33]. These droplets are charged in the induced electric field formed by the ring electrode of the nozzle. The intensity of the electric field is influenced by the electrode’s geometrical parameters, material properties, and applied voltage. The dimensional parameters of the ring electrode of the air-assisted electrostatic nozzle are 30 mm outer diameter, 4 mm hole diameter, and 1 mm thickness, respectively. The ring electrode material is made of brass, with Fermi energy levels and work functions of 7.1 eV and 4.5 eV, respectively, enabling the generation of strong electric field intensities. When the applied voltage is 3 kV, the induced electric field strength in the atomization region reaches 165.27 kV/m, enabling the generation of fine droplets with a charge-to-mass ratio as high as 10.46 mC/kg. These data have been validated in previous studies [32].

2.3. Spray Suspension Unit Design

The high-clearance air-assisted electrostatic sprayer is equipped with three spray suspension units, each comprising two ducts. The distance between the two ducts of the two groups of spray suspension units on the outside of the spray boom of the sprayer is set at 2.2 m, and the distance between the middle group is set at 1.8 m. Each duct is equipped with a set of seven air-assisted electrostatic spray nozzles, with two sets of nozzles facing each other. Each spray nozzle is spaced 0.16 m apart to ensure uniform spatial distribution of the spray volume. The air-assisted electrostatic spray nozzles are mounted on the duct via movable clips, allowing the spray nozzle angle to be freely adjusted. A previous study has found that tilting the spray nozzle downward by 20° can effectively increase the droplet deposition in the grape canopy [30,34]. Therefore, the seven nozzles on each duct are installed at a downward angle of 20°.

A spray volume distribution scanner was used to measure the spatial spray volume of seven nozzles on a single duct. There are many horizontally arranged droplet collection slots vertically arranged on the scanner, which can collect all droplets in a horizontal area with a height of 0.12 m uniformly, as shown in Figure 3. The vertical arrangement of these collecting tanks can effectively measure the vertical uniformity of spray volume. The spray volume distribution scanner collected the spray volume from seven nozzles in sequence and transferred it to 14 measuring cylinders, with each cylinder collecting the spray volume at a height of 0.12 m. Figure 3c shows the distribution of spray volume after ten minutes of spraying by seven nozzles with a spray width of 1.56 m. The coefficient of variation is 0.11, ensuring that the spray volume is evenly distributed.

2.4. The Vineyard

The field trial was conducted at vineyards of Yuchuan Farm, located at the eastern foot of the Helan Mountains in Yinchuan City, Ningxia Hui Autonomous Region, China (38.13° N, 105.96° E). The vineyard planting pattern belongs to the modern fruit orchard dwarf intensive planting pattern, using the hedge planting pattern. The row spacing of grapes is approximately 3.5 m, with a maximum height of 2 m, a crown height of approximately 1.6 m, and a crown thickness of approximately 0.6 m.

During the test period, the weather was favorable with light winds and no rain. The daytime temperature in the field ranged from 20 to 24 °C, with a relative humidity of 46 ± 3%. Wind speed measurements were taken using a wind speed meter (testo 416, Germany) every five minutes, with a total of ten measurements. The maximum wind speed recorded was 2.2 m/s, and the average wind speed was 1.3 m/s. During the test period, the grapes were in the full leaf stage, and the grape leaf area index was approximately 3.63.

2.5. The Test Design

The main purpose of these tests was to evaluate the performance of the high-clearance air-assisted electrostatic sprayer and to investigate the effect of applied voltage on droplet deposition. In the vineyard, three rows of grapes with a row spacing of 3.5 m were selected to ensure that the grape plants were located at the center of the spray suspension unit during operation, as shown in Figure 4a. The ground was leveled to ensure that the boom would not tilt when the sprayer passed over it. In the three rows of grapes, the grape canopy at the same position was selected as the test points. Ignoring the obvious protruding branches that have not been trimmed, a tape measure was used to measure the height of the grapevine and the thickness of the canopy. The average thickness and height of the grape canopy at the test sites were 0.56 m, 0.65 m, 0.67 m, and 1.67 m, 1.54 m, and 1.62 m, respectively.

During the tests, water was sprayed from the high-clearance air-assisted electrostatic sprayer. The sprayer passed through the test sites at a constant speed of 3 km/h. The experimental variable was the applied voltage, with two levels: 0 kV and 3 kV. For each voltage level, three replicate trials were conducted to ensure the reliability of the results.

Water-sensitive paper (26 × 76 mm, Teejet Company, Glendale Heights, IL, USA) was used to collect droplets sprayed by the high-clearance air-assisted electrostatic sprayer [35]. Water-sensitive papers were arranged in three layers—upper, middle, and lower—in the grape canopy. The water-sensitive papers in the middle layer were located at the average height across the whole grape row. The lower layer of water-sensitive paper was located 0.5 m below the middle layer, and the upper layer of water-sensitive paper was located 0.5 m above the middle layer. Four water-sensitive paper placement points were selected on each layer, with two sheets of water-sensitive paper placed on both sides of each placement point. The arrangement of water-sensitive papers was symmetric relative to the axis of the row. Eight sheets of water-sensitive paper were placed on each layer. Therefore, when viewed from the rear of the sprayer, from left to right, the water-sensitive papers on each layer in sampling points are numbered S1, S2, …, S8, as shown in Figure 4b,c. To ensure the orientation and dimensional accuracy of the water-sensitive papers, clamps with extension rods were used to arrange the water-sensitive papers, as shown in Figure 4d. Each clamp held two overlapped water-sensitive papers with their non-absorbent surfaces facing outward. The sheet facing the nozzle was defined as the ‘positive’ paper, whereas the sheet facing away from the nozzle was defined as the ‘negative’ paper. After each test, when the water-sensitive papers were completely dry, the water-sensitive papers would be sequentially numbered in a collection bag for subsequent processing.

The water-sensitive papers at sampling points S1, S3, S6, and S8 faced the nozzle relative to the nearest nozzle and were defined as upper. Correspondingly, the water-sensitive papers at sampling points S2, S4, S5, and S7 faced the nozzle with their backs to the nozzle and were defined as lower. Distinguished by proximity to the nozzles, the water-sensitive papers at sampling points S1, S2, S7, and S8 were defined as outer, and the corresponding S3, S4, S5, and S6 were defined as inner. In the following expressions, individual sampling points were used to represent the water-sensitive paper at the sampling point. For example, S1 represents the water-sensitive paper at the sampling point S1.

2.6. Field Test Data Analyses

In field tests, the deposition coverage and deposition density of the droplets on the canopy were determined using the water-sensitive papers, which were then used to evaluate the effectiveness of the sprayer operation [31]. A scanner (M7628DNA, LENOVO, Beijing, China) was used to scan the water-sensitive papers into images using 600 dpi and 8-bit grayscale. The processed images were analyzed using “DepositScan 1.0” software to obtain droplet deposition density and droplet coverage [36].

The water-sensitive papers were arranged in three layers, with eight sheets of water-sensitive paper in each layer. Papers positioned at the same sampling point across the upper, middle, and lower layers were considered as replicate measurements. For example, water-sensitive papers at point S1 on the upper, middle, and lower layers in the same test were treated as repeat tests. Since each test was repeated three times, nine replicate tests were conducted at the eight sampling points from S1 to S8. When processing test data, to reduce errors, remove the two largest and two smallest data points from the nine data points obtained at each sampling point, and use the remaining five data points for analysis.

This article investigates the droplet deposition density (D) and the droplet coverage rate (C). The deposition density represents the number of droplets per unit area on water-sensitive papers, while the droplet coverage represents the ratio of the area of water-sensitive papers occupied by droplets after deposition. The high-clearance air-assisted electrostatic sprayer was tested on three rows of grapes simultaneously. Observing from the rear of the sprayer, the three rows of grapes were marked from left to right as the left row, middle row, and right row, and designated as L, M, and R, respectively. In this paper, droplet coverage and droplet deposition density are denoted by “C_ij” and “D_ij”, with i standing for L, M, and R, and j for S1, S2, …, S8. For example, the droplet coverage at point S1 in the left row can be denoted by “C_LS1”, and the droplet deposition density at point S8 in the right row can be denoted by “D_RS8”. The formula for calculating the average droplet coverage of the grape canopy is shown in Equation (3), and the formula for calculating the average droplet deposition density is consistent with it.

$$\overline{C_i}={\displaystyle \frac{{\displaystyle \sum _{j=1}^8{C}_{ij}}}{8}},$$

(3)

To assess the effectiveness of droplet deposition within the canopy, we used the ratio of the average droplet deposition coverage in the middle canopy to the average droplet coverage in the canopy and expressed this ratio as the spray penetration coefficient (SP) [21]. The sampling points located in the canopy interior are S3, S4, S5, and S6, so the spray penetration coefficient (SP) is calculated as shown in Equation (4).

$$S{P}_i={\displaystyle \frac{{\displaystyle \sum _{j=3}^6{C}_{ij}}}{{\displaystyle \sum _{j=1}^8{C}_{ij}}}}$$

(4)

3. Results

3.1. The Performance of the High-Clearance Air-Assisted Electrostatic Sprayer

Following the spraying operation, water-sensitive papers were collected and analyzed. The data for each sampling point are presented in Figure 5. Droplet coverage and droplet deposition density were highest at S1 and S8, followed by S3 and S6, while S4 and S5 had the fewest. The main reason for this is that S1, S3, S6, and S8 were upper, and S1 and S8 were closer to the nozzles with respect to S3 and S6, so the droplets are more likely to be deposited on S1 and S8 [30]. Comparatively, S2, S4, S5, and S7 were lower, with relatively few droplets deposited in the lower despite the presence of auxiliary airflow disturbing the grape leaves and the backside adsorption effect of the electrostatic spray [36]. Nevertheless, the minimum droplet deposition density at each sampling point was 26.4 deposits/cm².

Comparing the operational effectiveness of each row of the high-clearance air-assisted electrostatic sprayer, the data showed that the droplet coverage and droplet deposition density in the middle row were higher than those in the left and right rows. In particular, the droplet coverage was more obvious. The average droplet coverage and average droplet deposition density data of each row were presented as shown in

Table 1. The droplet coverage and droplet deposition density data of each row.

	Row	Applied Voltage 0 kV	Applied Voltage 3 kV
Average Droplet Coverage (%)	Left row	7.50	7.85
	Middle row	9.39	9.89
	Right row	7.54	7.64
Average Droplet Deposition Density (deposits/cm2)	Left row	89.7	101.7
	Middle row	101.6	124.2
	Right row	85.75	94.7
Spray Penetration Coefficient (SP)	Left row	0.33	0.30
	Middle row	0.42	0.38
	Right row	0.38	0.32

.

As shown in Table 1, droplet coverage in the middle row was at least 24.5% higher than in the left and right rows. The maximum improvement in droplet coverage reached 29.5% under the 3 kV voltage condition. When the applied voltage was 0 kV, the increase in droplet deposition density was relatively modest, averaging 15.9%, whereas at 3 kV, the average increase reached 26.7%. These results indicate that the droplet deposition performance in the middle row was superior to that in the outer rows. This enhancement can be attributed to two primary factors. First, the distance between the spray nozzles and the grape canopy in the middle row was smaller, which reduced droplet drift and enhanced deposition [37]. Second, the flexible ducts supplying the middle row were significantly shorter—approximately 2 m—compared to those for the left and right rows, which were at least twice as long. The increased duct length in the outer rows likely led to reduced air pressure and decreased nozzle exit velocity, ultimately lowering the efficiency of droplet deposition [38].

Moreover, these structural factors also contributed to reduced droplet deposition at the inner sampling points adjacent to the central row. Notably, the middle row exhibited the highest spray penetration coefficients, reaching 0.42 and 0.38 under the 0 kV and 3 kV voltage treatments, respectively.

3.2. The Effect of Applied Voltage on Droplet Deposition

As shown in Table 1, droplet deposition was more effective at an applied voltage of 3 kV than at 0 kV, with noticeable improvements in both average droplet coverage and deposition density [39]. Throughout the canopy of wine grapes, the average droplet coverage and average droplet deposition density increased by 3.89% and 15.72%, respectively. Interestingly, despite the higher deposition efficiency, the spray penetration coefficient was lower under the 3 kV condition compared to 0 kV. To further investigate the influence of applied voltage on droplet distribution, droplet coverage, and deposition density, data were statistically analyzed for both the inner and outer sides of the grape canopy. The results are presented in

Table 2. The droplet coverage and droplet deposition density data on both inner and outer sides in different rows.

	Row	Outer		Inner
		0 kV	3 kV	0 kV	3 kV
Average Droplet Coverage (%)	Left row	10.01	10.92 ↑	5.00	4.78 ↓
	Middle row	10.9	12.26 ↑	7.95	7.51 ↓
	Right row	9.3	10.33 ↑	5.75	4.95 ↓
Average Droplet Deposition Density (deposits/cm2)	Left row	110.1	132.6 ↑	69.4	70.9 ↑
	Middle row	119.1	169.2 ↑	84.1	79.1 ↓
	Right row	104.2	130.9 ↑	67.3	58.4 ↓

“↑” represents an increase in data, and “↓” represents a decrease in data.

.

As shown in Table 2, increasing the applied voltage from 0 kV to 3 kV enhanced both the average droplet coverage and droplet deposition density on the outer canopy. In contrast, both parameters tended to decrease in the inner canopy. An exception was observed in the left row, where a slight increase in droplet deposition density on the inner canopy may be attributed to statistical variability. These results indicate that an applied voltage of 3 kV effectively improved deposition performance on the outer canopy while reducing droplet coverage and density on the inner canopy. To further investigate deposition patterns at the sampling-point level, data from the same sampling points across all rows were aggregated. Changes in droplet coverage and deposition density between the 0 kV and 3 kV treatments are illustrated in Figure 6 and Figure 7 for the outer and inner canopy sides, respectively.

As shown in Figure 6, all outer sampling points exhibited increases in both droplet coverage and deposition density under the 3 kV condition compared to 0 kV. At sampling points S1 and S8, representing the upper outer canopy, droplet coverage increased by an average of 6.39%, while deposition density increased by 23.22%. The disparity in the magnitude of increase between these two parameters suggests limited improvement in coverage despite enhanced droplet deposition. In contrast, sampling points S2 and S7, representing the lower outer canopy, showed more consistent increases: droplet coverage increased by 47.34% and deposition density by 57.90%. This indicates that electrostatic atomization at 3 kV facilitated the formation and deposition of fine droplets, significantly improving both deposition metrics on the lower outer canopy. When no voltage was applied (0 kV), both S2 and S4 showed low initial values for coverage and density. Upon application of 3 kV, the electrostatic force promoted the deposition of a large number of fine droplets at these points, resulting in a significant and proportionate increase in both coverage and deposition density, without substantial disparity between the two.

As shown in Figure 7, when the applied voltage increased from 0 kV to 3 kV, both droplet coverage and deposition density at sampling points S3 and S6—located on the upper inner canopy—decreased significantly. This contrasts with the trend observed at the outer canopy sampling points, where both parameters generally increased. In comparison, sampling points S4 and S5—representing the lower inner canopy—exhibited only minor reductions in coverage and deposition, with changes that were largely negligible. As discussed earlier, the application of voltage enhances atomization and promotes greater droplet deposition on the outer canopy. As a result, substantial deposition occurred externally, reducing the number of droplets capable of penetrating into the inner canopy. This redistribution of droplets likely contributed to the observed decrease in internal droplet coverage and deposition density under the 3 kV treatment. The spray penetration coefficient declined under the applied voltage of 3 kV, reflecting reduced droplet transport into the inner canopy region, which is also observed by others [27].

Furthermore, it is important to consider the role of canopy structure and climatic conditions in influencing electrostatic deposition. The decrease in droplet deposition within inner canopy layers under 3 kV treatment may be attributed not only to droplet shielding by the outer foliage but also to the attenuation of electrostatic field intensity caused by increased leaf density. Additionally, environmental factors such as relative humidity can influence charge retention on droplets and the effectiveness of electrostatic attraction. These findings suggest that the in-field performance of electrostatic spraying systems may vary with canopy density and weather conditions, which should be considered in future optimization efforts.

4. Conclusions

A high-clearance air-assisted electrostatic sprayer, equipped with a newly designed electrostatic spray nozzle and capable of simultaneously operating on three vineyard rows, was tested in vineyards located at the eastern foot of the Helan Mountains in Ningxia. The key findings are as follows:

(1)

The middle row of the high-clearance air-assisted electrostatic sprayer deposition effect was better than the left and right rows, with the maximum droplet coverage and droplet deposition density increased by 29.5% and 26.7%, respectively. And the minimum droplet deposition density inside the grape canopy was 26.4 deposits/cm².

(2)

When the applied voltage was 3.0 kV, the average droplet coverage and the average droplet deposition density of the grape canopy increased relative to the applied voltage of 0 kV. Throughout the canopy of wine grapes, the average droplet coverage and average droplet deposition density increased by 3.89% and 15.72%, respectively.

(3)

When the applied voltage was 3.0 kV, the droplet coverage and droplet deposition density of the outer grape canopy increased significantly relative to the applied voltage of 0 kV, but decreased the droplet coverage and droplet deposition density of the inner canopy. And it also resulted in a lower spray penetration coefficient. This suggested that while electrostatic spraying improved surface deposition, it limited spray penetration efficiency.

Future studies should explore ways to further improve electrostatic spraying performance and adaptability in field conditions. First, the observed trade-off between enhanced outer canopy deposition and reduced internal penetration highlights the need for the optimization of voltage levels and nozzle arrangements. Additionally, operational factors such as forward speed and system pressure should be investigated to achieve better droplet deposition efficiency and spray uniformity under varying vineyard conditions.