Study on Effects of Different Concentration Adjuvants on the Properties of Prochloraz Emulsion in Water Solution Droplets and Deposition

Abstract

Adjuvants are frequently incorporated into crop protection operations to modulate the droplet characteristics by diminishing the surface tension (ST) and contact angle (CA), thereby positively influencing the wetting and spreading behavior of the droplets. However, there are no quantitative conclusions on the extent to which the amount of adjuvant added affects droplet properties. Therefore, the decision to add spraying adjuvants in actual pesticide spraying operations relies on the operator’s experience. In this study, we investigated the effect of a surfactant additive (KAO A-200) on the droplet properties and deposition of prochloraz emulsion in water (PEW) solution for crop protection in unmanned aerial vehicle (UAV) aerial spraying. Three KAO A-200 additive concentrations of 0.42%, 0.83% and 1.67% and four solution concentrations of 2.5%, 3.33%, 4.17% and 5% of PEW were set to evaluate the droplet properties of PEW solution with ST and CA as assessment indicators. The results show that the average STs of adjuvant solution droplets tended to decrease as the concentration of KAO A-200 increased. According to the optimal concentration, the KAO A-200 addition concentration of 0.83% was therefore determined to be the most appropriate dosage. With the appropriate KAO A-200 dosage condition, the results show that the average STs increased as PEW solution concentration increased, while the average CAs of PEW solution droplets showed a first decreasing and then increasing trend. The “4.17% concentration PEW (22.5 g a.i./1.2 L) + 0.83% concentration KAO A-200” condition was selected as the optimized combination for crop protection UAV field aerial spraying tests. The test showed that the coverage rate of PEW solution droplets on the upper and lower layers of oilseed rape canopy increased by 71.47% and 41.55%, the deposition density increased by 71.91% and 98.45%, and the coefficient of variation in droplet deposition decreased by 44.41% and 48.13%, respectively. These results are significantly better than those obtained without the adjuvant addition.

1. Introduction

Oilseed rape (OSR, Brassica napus) is an important oil crop in China [1], but the occurrence of OSR sclerotinia disease has a detrimental impact on the yield of OSR seeds [2]. Fungicide spraying is a conventional and widely employed method for managing OSR sclerotinia disease, in which PEW is a commonly utilized fungicide [3,4,5]. The utilization of PEW can be achieved through the successful deposition and adhesion of PEW solution droplets on crop leaves. This is crucial for effectively controlling OSR sclerotinia disease. Failure to achieve proper deposition and adhesion may not only result in potential harm to the environmental pollution [6] but also to the health of animals and even humans. Studies have found that prochloraz can trigger multiple mechanisms that inhibit aromatase activity and lead to developmental toxicity in rats and dogs [7]. Prochloraz is also known as an endocrine disruptor, causing developmental toxicity with multiple mechanisms of action, including oxidative stress and DNA damage. It was concluded that it might be genotoxic in human cells [8]. Other studies have indicated that prochloraz impacts progesterone hormone levels and animal development [9,10]. It and its metabolites have caused adverse effects on the human nervous system and respiratory problems [11]. Thus, although prochloraz is widely used as a fungicide or pesticide in agricultural operations, it should be used as little as possible on agricultural produce.

The determination of pesticide droplet deposition is influenced by two significant properties, namely surface tension (ST) and contact angle (CA) [12,13]. By incorporating spraying adjuvants into a fungicide solution, it is possible to decrease the ST and CA [14,15], thereby improving the wetting and adhesion properties of the droplets on the target leaves. Additionally, these adjuvants can facilitate the penetration of the waxy layer on the leaf surface, promoting the absorption of active components [16,17], particularly when the surface tension of the droplets is lower than the critical surface tension of the plant leaves. Song et al. [18] conducted a study on the impact of four adjuvants, namely methylated plant oil (Beidatong), alkoxy-modified polytrisiloxane (Silwet408), hyperbranched fatty alcohol ether-modified polymer (ND500) and polymer adjuvants (G2801), on the wetting and deposition of insecticide solutions on wheat leaves. The findings indicated that these adjuvants had a significant effect on reducing the ST of pesticides and enhancing the wetting property of wheat leaves. The addition of spraying adjuvants is limited, however. Improper utilization of organosilicon adjuvants can result in the degradation of the cuticle of leaf epidermal cells, leading to a decrease in its water retention ability and cellular damage and ultimately pesticide-induced harm [19,20]. Furthermore, incorrect or excessive application of adjuvants can also contribute to environmental contamination [21] and cause harm to sensitive organisms such as bees [22,23]. Gao et al. [24] conducted experiments to investigate the spreading ability of droplets of nonionic surfactant OP-10 solution with varying mass fractions (0, 0.005%, 0.01% and 0.1%). The results indicated that the spreading ability was highest at a mass fraction of 0.005%, suggesting that the addition of a small amount of OP-10 to the solution had a significant impact on the wetting performance of the solution. Xu et al. [25] conducted a study on the spreading and evaporation process of adjuvants on the surfaces of waxy and polychaete-inhabited leaves. They investigated four different concentrations of adjuvants and observed that the spreading area of droplets increased as the concentration of adjuvants increased. However, the spreading area reached a peak at a certain concentration, suggesting that the optimal amount of spraying adjuvant is not necessarily the highest. Furthermore, previous research has demonstrated that an excessive concentration of the adjuvant can lead to the occurrence of droplet “run-off” from the leaves. This phenomenon results in a significant loss of pesticides and a decrease in the deposition of pesticides on the intended target leaves [26,27]. For the “run-off” phenomenon, scholars have used the terms point of run-off (POR) and maximum retention (R_m) to describe the law of retention and loss of pesticide solution on leaves [28,29]. The POR is defined as the saturation point, i.e., the amount of pesticide solution carried by the crop leaves beyond which the pesticide solution will run off the leaf, and the R_m is the achieved after the pesticide solution runs off.

KAO A-200 is a variety of tank-mixed adjuvants launched by Shanghai Kao Company in China in recent years. It is called a special adjuvant for its ability to improve spraying droplet deposition in aerial spraying. Therefore, KAO A-200 was chosen to test its effect on improving pesticide properties and actual droplet deposition in aerial spraying. In this study, trials on KAO A-200 to investigate the STs and CAs were conducted in the laboratory using varying concentrations to determine the optimal dosage. Using the optimal KAO A-200 addition concentration, the STs and CAs of PEW solution droplets with different concentrations were further experimentally analyzed to determine the optimal concentration for PEW solution applications. Aerial spraying tests based on UAV were carried out to verify the significant enhancement of droplet deposition with the use of KAO A-200. This study provided references for accurately adding spraying adjuvants and reducing pesticide application.

2. Materials and Methods

2.1. Pesticide and Spraying Adjuvant and Additive Concentrations

The pesticide used in this study was prochloraz emulsion in water (PEW) fungicide with 45% active ingredient content from Sichuan Lier Crop Science Co., Ltd., Mianyang, China (pesticide registration number: pd20140295), as shown in Figure 1. KAO A-200 is a nonionic surfactant additive (Shanghai Kao Chemical Co., Ltd., Shanghai, China). The main components of KAO A-200 include alkyl polyoxyethylene ester and alkyl polyglycoside of hydroxyl ester.

The manufacturer recommended a dosage of 900 mL/ha (405 g a.i./ha) PEW. Considering that the recommended spraying volume for controlling sclerotinia disease in oilseed rape for PEW solution is 18 L/ha, the dosage of 60 mL (27 g a.i.) per 1.2 L solution, as recommended by the manufacturer, and the dosages of 30 mL (13.5 g a.i.), 40 mL (18 g a.i.) and 50 mL (22.5 g a.i.) per 1.2 L solution were used in the tests to study the effects of the added amount of PEW on the droplet properties. The adjuvant KAO A-200 added amount was set as three gradients, which were 5 mL, 10 mL and 20 mL per 1.2 L solution.

The dosages and volume/volume (v/v) concentrations of the PEW and KAO A-200 additives are presented in

Table 1. PEW and KAO A-200 additive concentrations.

Items	Dosage (mL/1.2 L)	v/v Concentration (%)
PEW	30 (13.5 g a.i.)	2.5
	40 (18 g a.i.)	3.33
	50 (22.5 g a.i.)	4.17
	60 (27 g a.i.)	5
KAO A-200	5	0.42
	10	0.83
	20	1.67

.

2.2. Determination of Surface Tension and Contact Angle

The optical contact-angle-measuring instrument, DSA100S (A. Kruess Optronic GmbH, Stuttgart, Germany), was utilized to determine the CAs and STs of the PEW solutions. The main parameters of the instrument are presented in

Table 2. The main parameters of DSA100S.

Items	Parameters
CA measurement range	0–180°
CA resolution	0.1°
ST measurement range	0.01–100 mN/m
ST resolution	0.01 mN/m
Optical system magnification	7 times
Sample observation field of view	3.2–22.5 mm (diagonal size)
Video system frame rate	1000 fp/s

, and a visual representation of the instrument can be seen in Figure 2. The measurements were taken using deionized water and adjuvants. During the experiment, a volume of 1 μL of solution was dispensed for the ST measurement with the hanging drop method, and 1 μL of solution was applied to a polyester card on the droplet sampling platform for the CA measurement with the sessile drop method via a Hamilton line syringe. The instrument’s video system uses cameras, facilitated by software ADVANCE 1.6. Each measurement was repeated three times, and the average value was taken as the test outcome. Test results were exported in EX-CEL format for further analysis.

The ST and CA data obtained from the DSA100S instrument were subjected to analysis using OriginPro 9.0 to determine the mean value and standard deviation for each parameter combination.

2.3. Field Crop Protection UAV Aerial Spraying Experiments

2.3.1. Experimental Site and Crop Protection UAV

The experimental site was situated within the Wujiang National Agricultural Modern Demonstration Zone (31.18022° N, 120.76267° E) in Suzhou City, Jiangsu Province, China. The trials were conducted on 8 April 2021, with a mean wind speed of 0.45 m/s and a mean temperature of 14.68 °C. The aerial spraying tests were conducted using the V25, a four-rotor electric crop protection UAV V25 (Wuxi Hanhe Aviation Technology Co., Ltd., Wuxi, China). The UAV is depicted in Figure 3. It is a fully autonomous crop protection UAV with a real-time kinematic Global Positioning System (RTK-GPS) and customization of flight routes, flight height, flight speed and aerial spraying rate. The primary technical specifications of V25 are shown in

Table 3. The main technical parameters of V25.

Items	Parameters
UAAS size	1235 mm × 1235 mm × 647 mm
Rotor diameter	838 mm
Battery capacity	20,000 mAh–51.8 V
Flight speed	1.0–7.0 m/s
Flight height	0.5–5.0 m
Tank volume	22 L
Aerial spraying rate	2.3–7.4 L/min

.

2.3.2. Experiment Design

The field experiments involved conducting aerial spraying tests with PEW solutions, both with and without the addition of KAO A-200. The analysis focused on the impact of the adjuvant on the deposition of droplets in PEW solutions. As depicted in Figure 4, the sample area was divided into three repetitions along the vertical direction of the flight route, with a 10 m interval. A total of 9 sampling points, labeled S1 to S9, were symmetrically distributed from left to right on both sides of the flight route, with a distance of 0.5 m between each point [30,31].

The water-sensitive paper (WSP) was positioned horizontally on both the upper and lower layers at each sampling point, without any overlapping, as shown in Figure 5. The WSP was placed at a vertical distance of 15 cm from the top canopy of the OSR plant and 30 cm from the ground [32,33].

Droplets were sampled using the WSPs. At the end of each spray test, the WSPs were gathered and placed into self-sealing bags, which were subsequently transported to the laboratory for further analysis. The analysis of droplet deposition density and coverage was conducted using Deposit Scan 1.2 (DS) software [34,35].

In the conducted experiments, the unmanned aerial vehicle (UAV) maintained a flight height of 1.5 m, a flight speed of 5.0 m/s, an effective spray amplitude of 4.0 m and a spraying volume of 18 L/ha.

2.3.3. Droplet Deposition Analysis

The WSPs were scanned as JPG images. The quantity of droplets per square centimeter is the droplet deposition density and the ratio of the area covered by deposited droplets to the area of the sampled WSP area is the droplet coverage rate. The droplet coverage density and droplet coverage rate on each WSP were calculated using DS. Additionally, the deposition uniformity and penetration rate were analyzed.

The evaluation of droplet deposition uniformity was conducted using the coefficient of variation (CV) in coverage rates [36]. The formula for calculating the CV is as follows.

$$CV=\frac{S}{\overline{X}}\times 100\%$$

(1)

$$S=\sqrt{{\sum }_{i=1}^n\frac{{\left(X_i-\overline{X}\right)}^2}{\left(n-1\right)}}$$

(2)

where $S$ is the standard deviation of the droplet coverage rates on the WSPs for each repetition, $X_i$ is the coverage rate of each WSP in every repetition, and $\overline{X}$ is the average value of $X_i$.

The droplet penetrability is expressed by the DPR, as calculated using the follow formula.

$$\eta =\frac{x_l}{x_u}\times 100\%$$

(3)

where $x_l$ and $x_u$ are the droplet coverage of the lower- and upper-layer WSPs for each sampling point.

3. Results and Discussion

3.1. STs and CAs of PEW Solution Droplets with Different KAO A-200 Concentrations

The 0.42%, 0.83% and 1.67% concentrations of KAO A-200 were added to the 5% concentration PEW solution at the recommended dosage (60 mL, 27 g a.i. per 1.2 L solution), respectively. Figure 6 shows the wetting changes in the 5% concentration PEW solution droplets with different concentrations of KAO A-200. As depicted in Figure 6, the droplets of the 5% PEW solution underwent a transition from their initial state to a state of steady spreading within approximately 60 s regardless of whether the KAO A-200 was added or not. However, upon comparing Figure 6a with Figure 6b–d, it becomes evident that the spreading speeds of the PEW solution droplets with adjuvants were faster, while the contact angles (CAs) were significantly reduced. Therefore, it is believed that the adjuvant KAO A-200 changed the droplet properties of the PEW solution. Furthermore, it was found that the 5% PEW solution droplets’ spreading time and CAs differed with adjuvant concentration, indicating that the concentrations of KAO A-200 had effects on the PEW solution. The analysis was conducted in conjunction with the results of specific ST and CA tests, as follows.

The ST is an important index in characterizing the physicochemical properties of pesticide solution droplets [37]. The ST of a droplet can be defined as the force exerted by the droplet or the shrinkage of the liquid surface. A smaller ST value indicates that the droplet has an easier time wetting the solid interface and spreading on the surface of crop leaves [38]. When the surfactant molecules in a solution surpass a certain threshold, they undergo a process of synthesis from individual ions or molecules into colloidal aggregates known as micelles. The critical micelle concentration (CMC) refers to the minimum concentration at which surfactant molecules aggregate in a solution to form micelles [39,40]. It is widely accepted in the scientific community that the ST remains constant even when the adjuvant in the solution surpasses the CMC. Figure 7 illustrates the ST changes observed in droplets of a 5% PEW solution with varying concentrations of KAO A-200. The surface tension (ST) values exhibited a decrease as the concentration of KAO A-200 additive increased. The ST values decreased as the KAO A-200 concentration increased, being 41.65 mN/m with 0% additive concentration, 39.41 mN/m with 0.42% additive concentration, 33.98 mN/m with 0.83% additive concentration and 33.89 mN/m with 1.67% additive concentration. These results are consistent with the conclusions of Song et al. [18] and Gao et al. [24], which are in line with expectations. The addition of adjuvant KAO A-200 could reduce the STs of 5% PEW solution droplets. The STs of 5% PEW solution droplets with 1.67% adjuvant concentration decreased by 18.2% compared with those without the adjuvant. When the additive concentration of KAO A-200 increased from 0.42% to 0.83%, the STs decreased from 39.41 mN/m to 33.98 mN/m, a decrease of 13.78%, while when the additive concentration of KAO A-200 increased from 0.83% to 1.67%, the STs decreased from 33.98 mN/m to 33.89 mN/m, a 0.26% decrease. The ST value changed very slowly and eventually stopped decreasing. As the STs of KAO A-200 were tested using a mixture of KAO A-200 and PEW, and only three concentrations of KAO were considered, the CMC of KAO A-200 could not be accurately determined. Considering the relatively high ST value achieved for a concentration of 0.42%, the additive concentration of 0.83% was considered the optimal concentration (OC) of KAO A-200 in the tests.

The CA is the angle between the tangent line of the gas–liquid interface and the sol-id–liquid interface, as shown in Figure 6. The larger the contact angle is, the easier it is for droplets to roll off the crop leaves. So, CA is usually used to evaluate the wettability of pesticide solution on crop leaves’ surface. Figure 8 illustrates the variations in CAs of 5% PEW solution droplets with varying concentrations of KAO A-200. The test results show that the CAs did not have the same monotonic change trend as STs as the concentration of KAO A-200 increased. The CAs were 36.46° without the adjuvant, 22.09° with a 0.42% additive concentration, 26.24° with a 0.83% additive concentration and 26.86° with a 1.67% additive concentration, respectively. The adjuvant KAO A-200 generally reduced the CAs of the 5% PEW solution droplets. These results are inconsistent with the conclusions of Gao et al. [24] and Xu et al. [25]. According to the tests on the CAs of droplets on corn leaves conducted by Gao et al. [24], the maximum droplet CA was 129.2° without OP-10, and as the added concentrations of OP-10 increased, the CAs of the distilled water droplets decreased monotonously, being 120.01° at 0.001%, 79.6° at 0.005%, 65.8° at 0.01%, 54.8° at 0.05% and 47.4° at 0.1%, respectively. Xu et al. [25] took waxy and hairy leaves as research objects and tested the spreading and evaporation properties of crop oil concentrate (COC) containing modified seed oil (MSO), nonionic surfactant (NIS) and oil surfactant blend (OSB) solution droplets on them. The results indicated that higher concentrations of additives could achieve smaller droplet CAs, and the wetted area increased as the adjuvant concentration increased, correspondingly. As for the abnormal changes in CA value, we speculated that this might be related to the properties of the polyester card interface that came into contact with the droplets, which needs to be further tested and verified. Meanwhile, in their latest study, Marta et al. [34] found that the wettability and mobility of herbicidal ionic liquid drops varied depending on the plant species and demonstrated that alkyl chain elongation plays a significant role in the evolution of the surface properties of herbicidal ionic liquid.

3.2. STs and CAs of PEW Solution Droplets with Different Concentrations of KAO A-200 OC

Based on the OC of KAO A-200, as determined in Section 3.1, the optimal STs and CAs for 2.5%, 3.33%, 4.17% and 5% concentration PEW solutions were investigated at the KAO A-200 additive concentration of 0.83. The ST values increased but changed slowly as the PEW concentration increased under the KAO A-200 OC condition shown in Figure 9; even at the recommended 5% concentration for the PEW solution droplets, the ST was 33.98 mN/m. Some of the critical surface tensions of common crop leaves are shown in

Table 4. The critical surface tension of some common crops [41].

Crops	Critical Surface Tension (mN/m)
Cabbage-type rape	36.40 *
Rice	36.70 *
Wheat	36.90 *
Corn	47.40–58.00
Cotton	63.30–71.81
Pepper	43.38–45.27
Cowpea	39.00–43.38
Eggplant	43.38–45.27

Note: * indicates the critical surface tension was obtained using the Zisman method, while the others were estimated using the CAs.

. It can be seen that the STs of rice, wheat, cotton, pepper, etc., range from 36 mN/m to 72 mN/m. From the perspective of ST value, the STs of 2.5%, 3.33%, 4.17% and 5% KAO A-200 concentration PEW solutions were all smaller than those of the common crop leaves shown, and PEW solutions can easily adhere to the leaves of these crops in actual practice.

The CAs did not change monotonously with different concentrations of PEW solution and KAO A-200 OC shown in Figure 10. The maximum CA value was 27.79°, as observed for the PEW concentration of 2.5%, but the results show that when the PEW concentrations were 3.33% and 4.17%, the CAs decreased significantly to 15.13° and 16.48°, respectively. When the concentration of PEW was increased to the recommended level of 5%, the CA value also increased to 26.24°, which was found to be comparable to the CA value observed at a concentration of 2.5%.

The aforementioned test results provide information on the OC of the adjuvant KAO A-200, as well as the STs and CAs of different concentrations of PEW under the condition of KAO A-200 OC. The statistical data are presented in

Table 5. STs and CAs of PEW solution droplets with different KAO A-200 OC concentrations.

Concentration (%)	ST (mN/m)	CA (°)	Dosage (mL/1.2 L)
2.5	31.7	27.79	30 (13.5 g a.i.)
3.33	32.48	15.13	40 (18 g a.i.)
4.17	33.62	16.48	50 (22.5 g a.i.)
5	33.98	26.24	60 * (27 g a.i.)

Note: * the recommended dosage.

. The lowest ST was found at a concentration of 2.5% PEW, measuring 31.7 mN/m. In contrast, the contact angle (CA) reached its maximum value at 27.79°. The dosage of PEW used at this time was 30 mL (13.5 g a.i.)/1.2 L, which is only half of the recommended dosage. The minimum CA was 15.13° and achieved when the PEW addition concentration was 3.33%, and the ST at this time was 32.48 mN/m. Therefore, in terms of droplet deposition, the 2.5% PEW addition concentration is best from an ST perspective and the 3.33% PEW addition concentration is best from a CA perspective. However, it should be noted that the recommended dose of PEW is 60 mL (27 g a.i.)/1.2 L; an excessive reduction in the dose may compromise the efficacy of disease prevention and control. According to the statistical test results in Table 5, we found that the ST value was 33.62 mN/m when the PEW concentration was 4.17%, which is only 1.92 mN/m larger than the minimum level of 31.7 mN/m in the tests, and the CA value was 16.48°, which is 1.35° larger than the minimum level of 15.13° in the tests. The PEW addition dosage of 50 mL (22.5 g a.i.)/1.2 L was second only to the recommended dosage, 60 mL (27 g a.i.)/1.2 L. In order to ensure a good control effect, one should not reduce the active ingredient of pesticides too much [42,43,44,45]. Therefore, the PEW solution with a 4.17% concentration under KAO A-200 OC was selected for the field aerial spraying test after comprehensive consideration.

3.3. Effect of KAO-A-200 on Aerial Spraying Droplet Deposition

In the field aerial spraying experiments, 4.17% concentration PEW solutions both with and without a 0.83% KAO A-200 concentration were sprayed. According to the droplet statistical results on the upper and lower sampling WSPs, the droplet deposition density, the droplet coverage rate, the droplet deposition uniformity and the droplet penetrability were analyzed.

3.3.1. Droplet Deposition Density

Figure 11 shows the deposition densities of droplets on both the upper and lower layers, comparing treatments with and without the adjuvant KAO A-200. The droplet deposition density on the upper layer was higher than that on the lower layer under the influence of the OSR plant leaves, as expected. The droplet deposition densities increased both on the upper and lower layers when the adjuvant was used in the aerial spraying tests. The droplet deposition density increased by 98.45% on the lower layer, being higher than that of the upper layer by 71.91%. Therefore, the inclusion of an adjuvant can significantly enhance the deposition of pesticide droplets, particularly on the lower layers.

3.3.2. Droplet Coverage Rate

Figure 12 shows the droplet coverage rates of the upper and lower layers. In the aerial spraying treatment conducted without the adjuvant, the droplet coverage rates were 3.33% on the upper layer and 2.07% on the lower layer. However, when an adjuvant was introduced, the droplet coverage rates significantly increased to 5.7% and 2.93% on the upper and lower layers, respectively. The PEW solution droplets, when combined with the adjuvant, exhibited significantly higher coverage rates on both the upper and lower layers. Specifically, there was a 71.47% increase in coverage on the upper layer and a 41.55% increase in coverage on the lower layer. When analyzed in conjunction with the results of droplet coverage density, it is observed that the increase in coverage rate on the upper layer was similar to the increase in coverage density, with values of 71.47% and 73.91%, respectively. However, the droplet coverage rate on the lower layer showed only a slight increase, which was not as significant as the increase in droplet coverage density. The reason for this is that a higher quantity of droplets deposited per unit area corresponds to a higher droplet coverage rate. The droplet coverage rate is determined by the ratio of the droplet coverage area to the sampling WSP area. In addition to the coverage density, the coverage rate is also influenced by the size of the droplets and their spreading efficiency on the WSP.

3.3.3. Droplet Deposition Uniformity

Figure 13 shows the droplet deposition uniformity expressed by CV. Higher CV values indicate a lower level of uniformity in the deposition of droplets. It is evident that the uniformity of droplet deposition on the upper layers was superior to that on the lower layers in both aerial application treatments, with and without the inclusion of additives. The coefficient of variation in droplet deposition exhibited a decrease of 44.41% and 48.13% on the upper and lower layers, respectively. The addition of the additive significantly improved the droplet deposition uniformity. Therefore, the inclusion of an adjuvant can significantly enhance the uniformity of pesticide droplet deposition.

3.3.4. Droplet Penetrability

Figure 14 shows the droplet penetrability rates with and without KAO A-200 treatments. The droplet penetrability rate without the adjuvant was 62% higher than the rate with the adjuvant of 51.3% with the adjuvant. The reason for this was that the PEW solution droplets’ coverage of upper and lower layers increased during the aerial spraying after adding the adjuvant, but the droplet coverage rate on the upper layer increased more relatively.

4. Conclusions

Effective control of pests and diseases relies on achieving optimal pesticide droplet deposition. Therefore, it is crucial to maximize droplet deposition density and rate during spraying applications as well as enhance droplet penetration within the crop canopy to target pests and diseases occurring in the middle and lower sections. Improving the properties of pesticide droplets is a widely recognized and cost-effective approach to achieving the aforementioned objectives. The addition of adjuvants is a commonly employed method of enhancing the physical and chemical properties of pesticide solutions.

The ST and CA indices play a crucial role in characterizing the physicochemical properties of pesticide spraying solutions. From the perspective of the rational use of adjuvants, the relationship between the adjuvant concentration and the ST and CA of the PEW solution were studied first to determine the OC of KAO A-200, and the STs and CAs of PEW solution droplets of varying concentrations were studied under the KAO A-200 OC condition. Finally, considering the ST, CA and PEW dosage, the 4.17% PEW solution under the KAO A-200 OC condition was selected for field aerial spraying tests to evaluate the effect of the adjuvant on the PEW solution droplet depositions.

In summary, the OC of the adjuvant KAO A-200 is approximately 0.83% (v/v). The use of excessive adjuvant can result in wastage and increased costs associated with pesticide application. With the incorporation of the adjuvant, the coverage rate of PEW solution droplets exhibited a significant increase of 71.47% and 41.55% on the upper and lower layers, respectively, during field aerial spraying. Additionally, the droplet deposition density experienced a notable increase of 71.91% and 98.45% on the upper and lower layers, respectively. Furthermore, the coefficient of variation in droplet deposition demonstrated a substantial decrease of 44.41% and 48.13% on the upper and lower layers, respectively. The ST and CA of the pesticide solution typically decreased simultaneously below the OC of the adjuvant. However, experimental tests reveal that the ST decreased while the CA increased in the presence of a 5% concentration of PEW and 0.83% concentration of KAO A-200. The underlying cause of this phenomenon was thought to be associated with the surface characteristics of the polyester card. Therefore, it is imperative to take into account both the appropriate concentration and the characteristics of the target crop leaves when utilizing adjuvants, as this would significantly improve the effectiveness of disease control.