Effect of Adjuvants on Physical–Chemical Properties, Droplet Size, and Drift Reduction Potential

Abstract

Adjuvants alter the physical–chemical properties of pesticide formulations, influencing either the droplet size or drift phenomenon. Selecting the appropriate adjuvant and understanding its characteristics can contribute to the efficiency of Plant Protection Product (PPP) application. This reduces drift losses and promotes better deposition on the crop. The objective of this study was to evaluate the effects of four commercial adjuvants based on mineral oil (Agefix and Assist), vegetable oil (Aureo), and polymer (BREAK-THRU) on the physical–chemical properties (surface tension, contact angle, volumetric mass, electrical conductivity, and pH), droplet size, and drift, using pure water as the control treatment (no adjuvant). Surface tension and contact angle were measured with a DSA30 droplet shape analyzer, while droplet size measurements were determined through a laser diffraction particle analyzer (Malvern Spraytec), using a single flat fan spray nozzle (AXI 110 03) operating at 0.3 MPa. Drift reduction potential was evaluated inside a wind tunnel with an air speed of 2 m s⁻¹. All adjuvants reduced surface tension and contact angle compared to water. volumetric median diameter (VMD) increased for Aureo, Assist, and Agefix, generating coarse, medium, and medium droplets, respectively, while BREAK-THRU formed fine droplets, similar to those generated by water. Aureo had the greatest reduction in Relative Span Factor ($RSF$), with a reduction of 30.3%. Overall, Aureo, Assist, and Agefix adjuvants significantly reduced the percentage of droplets <100 µm and increased those >500 µm. Drift reduction potential was achieved for all adjuvants, with Aureo showing the highest reduction of 59.35%. The study confirms that selecting the appropriate adjuvant can improve PPP application and promote environmental sustainability in agricultural practices.

1. Introduction

Plant Protection Product (PPP) application is a common practice in modern agriculture aimed to control pests, diseases, and weeds, thereby helping to safeguard crop yields and food security. However, improper use of these chemicals poses several concerns for the natural ecosystem and human health, including biodiversity loss, soil and water contamination, and operator exposure [1,2,3].

In this context, spray drift has always been an undesirable phenomenon, defined as the spray portion which unintentionally does not reach its target, thus leading to substantial off-target losses. These losses migrate into the surrounding environment, posing risks to human and animal health, contaminating water surfaces and adjacent crops, and increasing production costs due to the wastage of the spray mixture [4,5,6,7]. The main factors affecting drift relate to both environmental (temperature, relative humidity, wind speed, etc.) and operating (sprayer type, nozzle design and its orifice size, spray pressure, and application height) conditions. During the atomization process, agricultural nozzles can produce droplets with a wide range of sizes, emerging as one of the most critical factors to be considered during phytosanitary treatments. Besides being primarily determined by the features of the nozzle, droplet size also depends on the physical–chemical parameters of the spray mixture, such as surface tension and viscosity [6,8,9,10,11]. It is well documented in the literature that droplets with a diameter smaller than 150 μm are more susceptible to drifting away by the action of wind currents. The fraction of these droplets in the sprayed volume can be used as a preliminary estimation of drift sensitivity [12,13].

On the other hand, the role of agricultural adjuvants has been well documented by scholars over the years [14,15,16,17,18,19,20], demonstrating a remarkable influence on environmental dynamics, mostly in reducing significantly the drift and improving the biological efficacy of the treatment. The interaction of adjuvants with the environment (water, soil, non-target organisms) often determines the persistence and bioavailability of the active ingredient that each additive accompanies. Adjuvants that dissolve well in water ensure the active ingredient spreads effectively, reducing the need for excessive amounts or adjuvants that are more prone to keeping the active substance in place to prevent the phenomenon of leaching into groundwater, thus protecting the soil and water health. Plant-based adjuvants can degrade quickly within a short period of time after application, so helping the prevention of biodiversity. Considering these aspects, adjuvants would be essential for ensuring sustainable agricultural practices and minimizing ecological risks.

Moreover, the environmental fate of these compounds is strictly related to climatic factors, such as temperature and rainfall, that control degradation and run-off rates. For instance, polymers and emulsifiable oils serve as drift retardants by increasing droplet size while minimizing drift potential. Surfactant-based adjuvants added to the spray solution are designed to slightly produce fine droplets while reducing surface tension, which enhances stability and adhesion to plant surfaces. Importantly, adjuvants with the function of buffers and acidifiers can have direct control of hydrogen ion potential (pH), maintaining the spray solution at an optimal value [21,22,23,24,25,26].

Despite adjuvants offering immediate agricultural advantages in terms of efficacy of PPPs and drift minimization, it is necessary to mention their potential environmental and economic impacts, which need careful management and regulation. On the environmental side, adjuvants with pesticide formulations can significantly hinder the soil health and microbial communities, potentially destroying the natural ecosystem balance [27]. They may also contribute in a negative manner to non-target organisms, including beneficial insects and aquatic species, which can experience toxicity due to adjuvants’ residues, thus amplifying ecosystem disturbances.

Contrarily, from the economic point of view, using adjuvants can increase the cost-effectiveness of agricultural practices by reducing the overall amount of pesticide needed for effective pest control, with subsequent reduced application costs, and improving crop yields. These benefits translate into higher productivity and increased profits for farmers as they can achieve the same or better result with less product. Additionally, it is worth mentioning that the initial investment in adjuvants and the potential requirement for specialized equipment or training may constitute an economic limitation for farmers. However, the long-term financial implications of environmental degradation (such as soil infertility) and loss of biodiversity are often overlooked [28,29]. Some studies available in the literature have provided evidence that the toxicity of formulated products, including adjuvants, can increase their adverse effects. As an example, research carried out by Chen et al. (2018) [30] highlighted the toxicological risks of organosilicon surfactants on bees and humans.

With the increasing awareness about the potential risk of pesticides, several strategies for minimizing spray drift have been developed over the years, including the adoption of suitable low-drift nozzles (such as air induction nozzles); electrostatic spraying, allowing droplets to be directed to the target by charging them electrically; and using anti-drift spray additives. Adjuvants can significantly alter the physical–chemical properties (viscosity, surface tension, volumetric mass, etc.), affecting the stability and breakup process of the spray mixture components. They can change the dynamics of the droplet distribution, potentially influencing droplet size and drift [12,31].

Although there are adjuvants that can improve the characteristics of spray mixtures, information about their real potential is still being determined. However, adjuvants and their efficacy in enhancing droplet size (volumetric median diameter) and reducing the fine droplets, while minimizing drift, remain contentious among researchers and agricultural producers.

Over the years, numerous studies have explored the effects of spray adjuvants on pesticide performance. For instance, Katzman et al. (2021) [32], using a polyacrylamide-based adjuvant, examined its effect on pesticide drift using active air sampling under field conditions. The authors found an increase in the proportion of larger droplets, but also an increase in finer droplets, thereby increasing the airborne drift risk. Li et al. (2022) [33] compared the effects of different adjuvants, like mineral and vegetal oils, on droplet size and spray deposition distribution on the target. Findings demonstrated that adjuvants generally increased droplet size compared to water, with discrepancies depending on the type of adjuvant and nozzle used. Additionally, a study by Polli et al. (2021) [34], analyzing the physical–chemical properties of adjuvants on the droplet size distribution of a tank mix of glyphosate and dicamba solutions, observed an increase in volumetric mass and viscosity, and a reduction in contact angle and surface tension of the spray solution, caused by the adjuvants.

Adjuvants also affect droplet size distribution, notably including a decrease in volume median diameter (VMD). Further research conducted by Vieira et al. (2018) [35] highlighted that using different types of adjuvants (high surfactant oil concentrate, non-ionic surfactant, and acidifier and microemulsion drift-reducing agent) with glyphosate alone led to a pronounced decrease in the percentage of droplets <100 μm, compared with glyphosate solution. The study also indicated that glyphosate with polymer solution resulted in the highest percentage of droplets <100 μm, in relation to other solutions.

In later years, more recent studies have been conducted to deal with this issue. Tavares et al. (2023) [36] conducted a study aimed at investigating how different adjuvants (mineral oil, propionic acid, and orange oil) affected the physical–chemical properties, drop size spectra, and absorption of pesticide mixtures of the fungicide azoxystrobin with glyphosate applied to soybean crop. The results revealed that adjuvants could significantly influence the performance of the application, improving the absorption rates. In the study of Idziak et al. (2023) [37], multifunctional adjuvants were evaluated in order to study the effects on contact angle, surface tension, and electrolytic conductivity, as well as on the efficacy of sugar beet plants. The authors pointed out the consideration that the tested adjuvants had high sugar beet yields. These studies suggested that the ability of adding suitable additives to spray solution can affect the effectiveness of phytosanitary treatments. Therefore, understanding how adjuvants impact the physical–chemical properties and droplet size is essential for improving treatment efficiency and minimizing spray drift potential.

Advancements in spraying technologies for agricultural adjuvants have recently revolutionized the precision and efficiency of PPP applications. Among these, Unmanned Aerial Vehicles (UAVs), in conjunction with variable rate spraying (VRS), have gained great interest as potential solutions to ground-based sprayers, allowing targeted applications in inaccessible areas [38]. In recent years, several studies have been conducted on these innovative systems. Comprehensive research on comparing UAV technology with a conventional air blast sprayer was carried out by Li et al. (2021) [39]. Wang et al. (2024) [40] focused attention on the effects of adjuvants on pesticide performance, evaluating some physical–chemical properties of the spray solution, like surface tension, viscosity, and contact angle, with different adjuvant concentrations. Besides contributing to the advancement of UAV-based technology, the authors pointed out that adding adjuvants to the pesticide solution increased the droplet size, reduced drift, and enhanced deposition capability.

Similarly, the study of Hu et al. (2024) [41] explored the spray performance of three types of tank-mix adjuvants (plant oil, mineral oil, and a mixture of alcohol and ester) on physical–chemical properties. Experiments were performed in cotton crops via a UAV system and a framework for improving pesticide effectiveness was delineated. Lastly, in the investigation conducted by Zeeshan et al. (2024) [42], a series of laboratory and field experiments were performed to assess the effect of six formulations of acetamiprid and six adjuvants sprayed by UAV technology on physical–chemical properties (volumetric mass, viscosity, surface tension, and contact angle), droplet deposition characteristics, and drift. Overall, the combination of the specific formulation and adjuvant led to improved physical–chemical properties of the spray solution, while, in the field trials, this combinatorial effect showed a higher drop size and reduced spray drift.

The effectiveness of the PPP application process can be improved by adding suitable tank-mix adjuvants. This issue has emerged as a fundamental strategy to cope with poor foliar deposition or excessive spray drift. Indeed, selecting appropriate adjuvants can enhance the quality of application by changing the atomized droplet size, reducing the drift and increasing the amount of active ingredient on the crop surface. However, the effects of spray adjuvants and pesticide formulations have not been evaluated in depth in the drift mitigation scenarios. To examine how different adjuvant formulations affect the performance of spray drift management, this study aimed at thoroughly investigating and comprehending their impacts on physical–chemical properties, droplet size characteristics, and drift reduction potential. Unlike other studies focused on a single aspect, in this research the three issues were integrated into a single comprehensive study. This holistic approach provided deeper understanding among these factors, which were crucial for optimizing the PPP application. Additionally, a first drift estimation was given by assessing the percentage of the volume of droplets with diameter less than 100 μm, offering useful preliminary knowledge about the drift across the adjuvants. Gaining insights into these effects, this paper can assist producers and farmers in properly choosing adjuvants, thus contributing to a more sustainable agriculture [32,43].

2. Materials and Methods

2.1. Experimental Design

The experimental design followed a completely randomized approach, with five different treatments of spray solutions: four adjuvants (BREAK-THRU, Agefix, Assist, and Aureo) and a control treatment (water). The number of repetitions was chosen for each parameter measured, as detailed below. The randomization ensured that all treatments had the same probability of being allocated to each experimental unit. The control treatment was included throughout all experimental stages to serve as a comparative reference in the study of physical–chemical properties, droplet size distribution, and drift reduction potential.

2.2. Experimental Tests

The experimental trials were conducted in the Department of Agricultural Engineering of the Federal University of Viçosa, Viçosa, Minas Gerais State, Brazil.

Overall, the study was divided into three experimental steps, regarding (i) the determination of the physical–chemical properties of the adjuvants in solution with distilled water, (ii) the evaluation of the influence of adjuvants on droplet size distribution, and (iii) the assessment of the drift reduction potential of the tested adjuvants. In all three experiments, the adjuvant concentration was based on manufacturers’ label recommendations.

Four commercial products comprising emulsifiable concentrate (EC) and suspension concentrate (SC) formulations were used as adjuvants. The selection of these additives was based on their common use in Brazilian agriculture, being drift-reducing and highly recommended for mitigating spray drift risk. Despite this, different performances are expected due to the distinct active ingredients, which could lead to mitigation of the drift in different proportions. Moreover, all adjuvants are considered multifunctional and applicable in different environmental conditions to face common agricultural challenges.

The main characteristics of adjuvants, including trade names, active ingredients, manufacturers, formulations, and doses, are reported in

Table 1. Main characteristics of the adjuvants and doses used in the experiments.

Adjuvants	Active Ingredient	Manufacturer	Formulation	Dose % (V V−1)
BREAK-THRU®	Polymethyl-siloxane	Evonik (Essen, Germany)	SC	0.1
Agefix®	920 g L−1 mineral oil	Agecom (Mauá, Brazil)	EC	1.0
Assist®	782 g L−1 mineral oil	BASF (Calgary, AB, Canada)	EC	1.0
Aureo®	720 g L−1 Soybean methyl ester	Bayer (Leverkusen, Germany)	EC	0.5

EC: Emulsifiable Concentrate and SC: Suspension Concentrate.

.

To determine the physical–chemical properties, solutions were prepared using distilled water. Each adjuvant was mixed in a beaker with 0.6 L of distilled water. The solutions were homogenized with a magnetic stirrer (RC LABOR, São Paulo, Brazil) for 5 min at a rotation speed of 68.07 rad s⁻¹. For the experiments relating to the evaluation of the droplet size characteristics and drift reduction potential, pure water was used.

2.2.1. Physical–Chemical Properties

Surface tension, contact angle, volumetric mass, electrical conductivity, and hydrogen ion potential (pH) were determined as physical–chemical properties. The main instruments used to measure these properties are depicted in Figure 1.

Specifically, pH measurements were performed using a K39 portable pH meter (KASVI, Campina, Brazil), while electrical conductivity measurements were carried out through an mCa 150 benchtop conductivity meter (Tecnopon, São Paulo, Brazil). Both devices were calibrated using standard solutions with known conductivity and pH before evaluations. The beakers were filled with the treatment control (water) and the electrodes were immersed in the beaker until the pH and conductivity readings stabilized. After each reading, the electrode was cleaned with distilled water and dried with a paper towel [34].

The volumetric mass of the solutions was measured at (20 ± 0.5) °C, according to the ABNT NBR 13826 [44] standard. Measurements were made using a 25 mL pycnometer with a 0.1 mg precision balance. The volumetric mass of the solution was calculated by relating the mass of the solution (kg) to the volume of pycnometer (m³), according to Equation (1).

$$\rho ={\displaystyle \frac{m_1-m}{V}}$$

(1)

where $m_1$ represents the mass of the pycnometer after the solution was added, $m$ represents the mass of pycnometer without solution, and $V$ represents the volume of pycnometer.

Surface tension and contact angle were tested through the Drop Shape Analyzer device DSA30 (KRÜSS GmbH, Hamburg, Germany). Specifically, surface tension was measured using the pendant drop method, which exploits a drop suspended from a needle in a bulk liquid or gaseous phase. The shape of the droplet resulted from the relationship between the droplet volume and mass, adjusted according to the Young Laplace equation. The contact angle was determined using the sessile drop method [9,45]. The droplet-bearing surface was a standard surface (film paper).

For each solution, six repetitions were performed for the surface tension and contact angle measurements, whereas three repetitions were conducted for the other physical–chemical properties (volumetric mass, electrical conductivity, and pH).

2.2.2. Droplet Size Characteristics

For the droplet size measurements, the AXI 110 03 flat fan nozzle (Jacto, São Paulo, Brazil) was used at a pressure of 0.3 MPa. The droplet size distribution was evaluated using the Malvern laser particle analyzer (Malvern Instruments Ltd., Spraytec, Worcestershire, UK). A comprehensive description of the components of the laser instrument can be found in Privitera et al. (2024) [46], while in Figure 2 the experimental setup is shown.

During the measurements, the nozzle was installed on the spray boom, in the middle between the two units, at a distance of 0.3 m from the laser beam, with the jet adjusted to cross it perpendicularly [12]. The particle analyzer consists of two units (transmitter and receiver). Their reciprocal distance was set to 0.42 m. The collecting lens had a focal length of 750 mm and could read droplets ranging from 2 µm to 2000 µm. The laser equipment was configured to measure the droplet size of the nozzle for 5 s at a frequency of 2500 Hz, ensuring a minimum of 2000 valuable readings. From the Spraytec software (version 4.0) program, the volumetric diameters ($D_{v0.1}$, $D_{v0.5}$, $D_{v0.9}$), V < 100, V > 500 and Relative Span Factor ($RSF$) were extracted. Spraying tests were repeated five times for each solution.

$D_{v0.1}$, $D_{v0.5}$, and $D_{v0.9}$ represent the droplet diameters equal to or below the specified size, which accounts for 10%, 50%, and 90%, respectively, of the total droplet volume. V < 100 and V > 500 represent the percentages of the total volume of droplets with a diameter smaller than 100 µm and larger than 500 µm, respectively. The $RSF$ represents the homogeneity of the droplet size distribution within a spray. It was calculated according to Equation (2).

$$RSF={\displaystyle \frac{D_{v0.9}-D_{v0.1}}{D_{v0.5}}}$$

(2)

The lower the $RSF$, the more homogeneous the droplet size distribution for a spray. Conversely, the higher the value, the less uniform the distribution.

Temperature and humidity were monitored during data collection. The ambient temperature was (23 ± 1) °C and the relative humidity was (70 ± 1)%. To monitor these variables, a portable digital thermo-hygrometer HT300 (Instrutherm, São Paulo, Brazil) was utilized.

2.2.3. Spray Drift Test

Spray drift tests were conducted in a wind tunnel under controlled conditions. The wind tunnel, located in the Agricultural Engineering department of the Federal University of Viçosa (Viçosa, Minas Gerais State, Brazil), had dimensions of 7.0 m in length, 1.4 m in width, and 1.5 m in height. The wind speed was manually adjusted to 2.0 m s⁻¹ and monitored with the TAFR-180 anemometer (Instrutherm, São Paulo, Brazil), following the recommendations of the ISO 22856 standard [47]. An axial fan, with a diameter of 0.9 m and 12 blades, generated artificial wind inside the tunnel and was connected to a 3.68 kW three-phase induction motor (Metalcore, São Paulo, Brazil) via a two-pulley system. A three-phase inverter (SIEMENS, Caxias do Sul, Brazil) was coupled to this motor, which was set to a frequency of 30 Hz to match the desired fan output speed. The distance between the axial fan and nozzle was 1.75 m. The air flow inside the wind tunnel was evaluated using the Reynolds number (Re), which, when considering an average speed of 2.0 m s⁻¹ and a hydraulic diameter of 1.448 m, was approximately Re ≈ 1.92 × 10⁵. This value indicated that the air flow was turbulent.

To determine the ground drift potential of each adjuvant, the AXI 11003 flat fan nozzle (Jacto, São Paulo, Brazil) was installed in a spray boom of length of 0.6 m and placed in the center of the spray boom at 0.5 m above the tunnel floor. The nozzle had an anti-drip valve with a solenoid valve flow control system. Furthermore, a pressure gauge was installed in the spraying system to control the working pressure during the spraying process.

Spray drift experiments were performed by assessing the spray solution volume (μL) on four polyethylene wires with a diameter of 2 mm and a length of 1 m. Four wires, spaced 1 m apart and starting 2 m from the spray nozzle, were positioned transversely to the airflow direction and fixed to metal supports at a height of 0.1 m, following the methodology proposed by ISO 22856 [47]. A schematic view of the wind tunnel is shown in Figure 3.

For quantitative characterization of the volume deposited on each wire, the artificial blue dye Brilliant FCF (Duas Rodas Industrial, Santa Catarina, Brazil) was used in each treatment as the dye at a concentration of 3 g L⁻¹ [7,34]. The optimal absorbance reading was achieved at this concentration, which was the minimum value capable of producing a detectable signal in relation to the equipment noise threshold.

Each adjuvant and water mixture were prepared according to the recommended concentration. After each spraying, all the wires were collected and stored in labeled plastic bags for subsequent analyses. Afterwards, about 0.05 L of distilled water was added to each bag to extract the dye deposited on the wires, and the bags were sufficiently shaken by hand. After this step, the washing water from each bag was pipetted and then poured into a cuvette for measuring the absorbance values through a SP-22 spectrophotometer (Biospectro, São Paulo, Brazil) at a wavelength of 630 nm.

For the conversion of absorbance values coming from each reading of concentrations (mg L⁻¹), a serial dilution was followed for each adjuvant solution to build standard calibration curves through sampling known concentrations of the tracer [48,49] (

Table 2. Models adjusted to estimate the dye concentration y (µg L⁻¹) as a function of the absorbance x of the adjuvant solutions, being used in y = ax + b.

Treatment	a	b	r2
BREAK-THRU	67.390	−0.9452	0.9989
Agefix	54.570	−0.2428	0.9991
Assist	59.305	−0.2911	0.9990
Aureo	68.457	−0.4477	0.9987
Water	99.193	−0.5199	0.9996

). Given the distinct chemical properties of the evaluated adjuvants, it was inferred that their molecular interactions with the dye and their capability to modify the electronic structure of the molecule can vary, thereby leading to different fluorescence intensity. Consequently, the construction of the calibration curves for each solution was preparatory for considering the molecular dynamics and accounting for these variations. This approach ensured precision in measuring the deposited volume on each wire.

The spraying process had a duration of 30 s, which was set via a chronometer and repeated three times for each spray solution. The temperature and relative humidity throughout the trials were kept constant at (20 ± 1) °C and (80 ± 5)%, respectively, according to the guidelines of the ISO 22856 standard [47].

Based on the concentration values obtained in each sampler and the volume of water used to wash the samplers, the amount of deposit existing on the collector wires (μL) was determined, being converted to the percentage of drift according to the methodology reported in ISO 22856 [47]. Furthermore, the values of the reference solution (water + dye) in µg L⁻¹ were considered, and the percentage of drift reduction was calculated for each adjuvant (Equation (3)) according to the methodology adapted from Itmeç et al. (2022) [50]:

$$PRDP={\displaystyle \frac{{PD}_r-{PD}_1}{{PD}_r}}$$

(3)

where ${PD}_r$ denotes the drift potential of the reference liquid (μg L⁻¹) and ${PD}_1$ represents the drift potential of the adjuvant (μg L⁻¹).

2.3. Data Analysis and Processing

Measurements of the various parameters were replicated from three to six times. Specifically, volumetric mass, electrical conductivity, pH, and spray drift were measured three times, droplet size was measured five times, and surface tension and contact angle were measured six times. The chosen number of replicates aligns with standard practices in agricultural experiments, ensuring sufficient statistical power to detect meaningful differences while accounting for practical limitations, such as time and resource constraints.

All experimental data were separately subjected to analysis of variance (ANOVA), which has adequate power with this sample size to identify significant differences.

When significant differences were observed, the means were compared using both Tukey’s Honestly Significant Difference (HSD) and Dunnet’s test at a 95% confidence level (p-value = 0.05). According to the statistical concepts, Tukey’s test was applied to make pairwise comparisons among the adjuvants, excluding the control group (water). In contrast, Dunnet’s test was used to compare multiple groups (adjuvants) against a single control treatment (water).

Using water as a baseline for the statistical data analysis was particularly reasonable, as it allowed us to determine whether the adjuvants differed significantly from the control treatment. This approach provided a more accurate assessment of adjuvant performance, since Dunnet’s test more rigorously controls the type I error (false positive) caused by random data fluctuations when making specific comparisons.

All statistical procedures were performed using R software (version 4.2.3).

3. Results

3.1. Physical-Chemical Properties

None of the evaluated spray solutions exhibited significant variations in volumetric mass, with average values close to 1000 kg m⁻³ (Figure 4A).

The electrical conductivity values of the Assist and Aureo adjuvant solutions increased, with values of 406.39% and 229.38% over water, respectively. Conversely, the Agefix and BREAK-THRU solutions reduced the electrical conductivity values by 20.81% and 66.84%, respectively (Figure 4B). These variations resulted in statistically significant differences in the electrical conductivity values among the four adjuvant–water solutions.

The pH levels of the Aureo and Agefix adjuvants significantly differed from each other and the other adjuvants, showing a reduction of 40.49% and 11.43% with respect of water (Figure 4C).

All adjuvant solutions reduced both the surface tension and the contact angle compared to water. Significant differences in surface tension were observed among the adjuvants. The polymer-based BREAK-THRU adjuvant showed the greatest reduction in surface tension and contact angle, which decreased by 64.74% and 54.96%, respectively, when compared to water. This substantial reduction was due to the inherent characteristics of polymers, which suggested their ability to enhance wetting properties, thus promoting greater spreadability and coverage on surfaces. An unexpected outcome was found for the Agefix adjuvant, which showed a larger corresponding contact angle with respect to other additives with higher surface tension. This behavior may be due to the fact that the adjuvant had lower chemical affinity with the test surface (film paper), resulting in some variations in surface tension and in contact angle during measurements. Another possible explanation may be the uneven distribution of surfactants at the liquid–surface interface, thereby limiting the reduction in the contact angle (Figure 4D).

3.2. Droplet Size Measurements

Table 3. Results of ANOVA applied to the volumetric diameters, $RSF$, and percentages of the total volume of droplets with a diameter smaller than 100 μm and greater 500 μm.

Item	Source of Variation	df	SS	MS	F	Significant
$D_{v0.1}$	ST	4.00	11,095.00	2773.75	65.09	<0.0001
	Residual	20.00	852.30	42.61
	Total	24.00	11,947.30
$D_{v0.5}$	ST	4.00	36,010.00	9002.50	285.80	<0.0001
	Residual	20.00	630.00	31.50
	Total	24.00	36,640.00
$D_{v0.9}$	ST	4.00	26,340.70	6585.20	116.47	<0.0001
	Residual	20.00	1130.80	56.50
	Total	24.00	27,471.60
$RSF$	ST	4.00	1.30	0.32584	176.10	<0.0001
	residual	20.00	0.04	0.00185
	Total	24.00	1.34
V < 100	ST	4.00	1202.48	300.62	203.95	<0.0001
	Residual	20.00	29.48	1.47
	Total	24.00	1231.96
V > 500	ST	4.00	60.23	15.06	38.56	<0.0001
	Residual	20.00	7.81	0.39
	Total	24.00	68.04

ST: Spray treatment; df: degree of freedom; SS: Sum of squares; MS: Mean square.

reports the results of the ANOVA for volumetric diameters, $RSF$, V < 100, and V > 500. F values revealed significant differences among treatments for all quantities.

In more detail, Figure 5 illustrates the effects of the spray solutions on volumetric diameters ($D_{v0.1}$, $D_{v0.5}$, $D_{v0.9}$) and Relative Span Factor ($RSF$).

The BREAK-THRU adjuvant had no significant difference in $D_{v0.1}$ compared to water, recording the lowest value among the analyzed adjuvants. In contrast, Aureo had the highest $D_{v0.1}$, with a significant increase of 81.28% over water. The average increase for Agefix was 18.82%, whereas Assist showed a higher rise of 48.01% compared to water. However, for $D_{v0.9}$, BREAK-THRU presented values similar to those of water, but Assist and Aureo displayed significantly higher values, with increases of 13.31% and 18.39%, respectively. A similar increasing trend in $D_{v0.9}$ was observed for the Agefix adjuvant, which produced a less pronounced but higher value than water. $D_{v0.5}$ values were influenced by the type of adjuvant. The Aureo, Assist, and Agefix adjuvant solutions produced coarse, medium, and medium droplets, respectively, and were statistically different from water. Meanwhile, the BREAK-THRU solution presented droplets like those obtained with water, showing no statistical differences (Figure 5A).

The reproducibility of the results, expressed in terms of the average coefficient of variation (CV) values of the volumetric diameters, was highest with the mineral-oil based adjuvants (Assist and Agefix, with average CV values of 0.92% and 1.07%, respectively), followed by BREAK-THRU (CV = 3.18%). The Aureo adjuvant exhibited the lowest reproducibility with the highest CV (6.22%).

The uniformity of droplet size distribution within the spray was examined to explore the variation in the homogeneity of droplet size measured during the spraying process with the solutions. The average of $RSF$ values is shown in Figure 5B.

The Aureo additive presented the lowest $RSF$ value, less than 30.35% in comparison to water, thus revealing greater homogeneity in the droplet size distribution within the spray than all other adjuvant solutions. On the other hand, the BREAK-THRU solution had less homogeneity compared to other adjuvants and was not different from water.

When analyzing the reproducibility of the results, even for the $RSF$, the best performance was achieved with Assist and Agefix adjuvants, exhibiting average CV values of 1.37% and 0.57%, respectively. Conversely, the worst results were obtained with the Aureo adjuvant (CV = 4.55%), whereas intermediate results were obtained with the BREAK-THRU solution (CV = 3.70%).

Cumulative volumetric curves of the adjuvants under evaluation, including the control treatment, are shown in Figure 6. The graph was obtained considering the five repetitions of each treatment. The curves showed distinct behaviours among the various adjuvants, with the only exception for the BREAK-THRU treatment and water, whose curves did not show any relevant distinction and appearing almost superimposed. In addition, an evident similarity was observed in the curves of the mineral oil-based (Assist and Agefix) adjuvants. These trends indicated that the impact of BREAK-THRU adjuvant on the atomization process was less pronounced compared to other adjuvants. This suggested a limited influence on modifying the droplet size distribution and revealed a lower efficiency in increasing the droplet size. On the other hand, oil-based additives showed a clear tendency to generate coarser droplets, as evidenced by the shift of the curves to the right. Aureo emerged as the most efficient additive to reduce the drift risk. Overall, this disparity can be associated with the physical–chemical properties of oil-based adjuvants, which enhance their ability to form larger droplets during the atomization process.

Figure 7 reports the average percentages of the total volume of droplets with a diameter less than 100 μm (V < 100) and greater than 500 μm (V > 500) when different adjuvant additives were sprayed. Regarding the V < 100 percentage, Aureo, Assist, and Agefix adjuvants significantly reduced this percentage by 71.09%, 57.10%, and 20.31%, respectively, compared to water. In contrast, a slight reduction in this percentage (0.76%) was noted for the BREAK-THRU solution, which was not statistically significant in comparison to water. Furthermore, for the percentage of V > 500, the adjuvant solutions Aureo, Assist, and Agefix increased by 129.88%, 86.28%, and 42.07%, respectively, compared to water. On the other hand, the result for the BREAK-THRU solution was not statistically significant compared to water. As a general trend, solutions with higher $D_{v0.5}$ showed greater homogeneity in the droplet size, lower V < 100 percentage, and higher V > 500 percentage.

According to the trends of volumetric curves, it was observed that BREAK-THRU and water exhibited the highest proportions of fine droplets (<100 µm), evidenced by the steeper slope at the beginning of the curves, which was in line with the high V < 100 values (23.55% and 23.72%, respectively), indicating a higher risk of drift. Notably, Aureo and Assist adjuvants showed curves with a less steep initial slope, representing smaller proportions of fine droplets (V < 100 of 6.86% and 10.18%, respectively), which reduced the risk of drift; however, they exhibited a more prolonged slope after 500 µm, indicating greater formation of large droplets (V > 500 of 7.53% and 6.10%, respectively).

3.3. Drift Reduction Potential

The analysis of the drift effect while adding distinct adjuvants is depicted in Figure 8. As a general trend, all the tested adjuvants had lower drift than water at all evaluated downwind distances. The average percentages of total sprayed volume collected on polyethylene wires for the adjuvants BREAK-THRU, Assist, Agefix, and Aureo were 12.88%, 11.0%, 8.02%, and 5.16%, respectively.

In general, all adjuvant solutions demonstrated lower drift with respect to water. Specifically, water showed the highest initial drift of 12.56% at 2 m, reducing sharply to 4.7% at 5 m. In contrast, BREAK-THRU had an initial drift of 8.44% at 2 m, then reducing to 0.42% at 5 m. Assist began with an initial drift of 6.1% at 2 m, reaching a decrease of 0.84% at 5 m. Agefix, starting at 4.87% at 2 m, had the lowest drift at 5 m (0.35%), with a drift reduction potential of 50.51% (Figure 8B). Finally, Aureo exhibited the lowest initial drift at a distance of 2 m (2.25%), with the highest drift reduction potential of 59.35%. These results showed that, although all adjuvants contributed to the drift reduction, Aureo and Agefix demonstrated the best performance, making them ideal for applications requiring enhanced drift control and efficient deposition of agricultural pesticides.

4. Discussion

This paper investigates the effects of four commercially available surfactant and emulsion adjuvant solutions on physical–chemical properties, droplet size, and drift reduction potential.

Electrical conductivity plays a crucial role in the understanding of physical–chemical interactions and solution stability, significantly impacting the efficacy of PPP applications. The distinct values observed among the adjuvants (Assist, Aureo, Agefix, and BREAK-THRU) reflected significantly different behaviors in terms of ionic stability and molecular interactions. The presence, concentration, and valence of ions in the solution are crucial factors affecting electrical conductivity and surface tension [51].

Conductivity solutions, such as those prepared with Assist, may present greater electrostatic interaction, favoring the solubilization of salts present in systemic herbicides, such as glyphosate, and may also aid in the dispersion of copper-based fungicides. This performance is attributable to its chemical composition; Assist is rich in mineral oil and non-ionic surfactants, which reduce surface tension and improve the effectiveness of the treatment. However, its high ionic strength can increase the risk of flocculation in mixtures with cationic formulations, and it is essential to observe the mixing order to avoid precipitation. In addition, Assist favors the deposition of larger droplets in moderate wind conditions, reducing drift losses. According to the results obtained by Zheng et al. (2021) [52], a mineral oil-based solution had less expansion and smaller surface tension values compared to those of vegetable oil solution, leading to better spreading on the rice leaf surface. Their results were consistent with those of our research; in fact, Assist and Agefix (mineral oil-based adjuvants) reduced values of surface tension with respect to the Aureo additive (vegetable oil-based adjuvant), underlining more efficient pesticide use and efficacy.

Adjuvants with low electrical conductivity values, like Agefix and BREAK-THRU, enhance the chemical stability of sensitive formulations that are prone to degradation or inactivation under certain conditions. Agefix, based on mineral oil, reduces surface tension and minimizes losses due to runoff, which can optimize application. In turn, BREAK-THRU, a polymethylsiloxane-based adjuvant, has high spreading and adhesion capacity due to its contact angle, improving leaf coverage. Its low electrical conductivity can reduce ionic interactions, preserving the stability of sensitive mixtures, such as inoculants, and minimizing the inactivation of microorganisms. This property may be beneficial for applications involving natural extract-based pesticides or foliar fertilizers. When coping with pesticide solutions, ionic interference should be considered due to the number of ions in the solution [36].

Reducing surface tension and contact angle increases the spreadability of the liquid on plant surfaces, maximizing pest control effectiveness [53,54]. On the other hand, droplets with a higher contact angle are more susceptible to displacement, leading to irregular application and reduced treatment performance. In this sense, mineral oils are good allies in reducing surface tension and contact angle. Mineral oils have a low affinity for water, which causes them to approach and interact with water molecules on the surface, reducing surface tension, as observed in Agefix and Assist. In addition, they can also minimize product washing off leaves due to rain or irrigation after application, especially on waxy leaves, improving phytosanitary control. He et al. (2019) [55] highlighted that the reduction in surface tension and contact angle may vary depending on the specific properties of the solution.

These characteristics highlight the importance of selecting the ideal adjuvant based on the specific needs of the crop, environmental conditions, type of pesticide, and active ingredient to be mixed. In this sense, the findings obtained cannot be observed as a rule, since the interactions of tank mixtures are distinct [56].

Furthermore, the reduction in contact angle promoted by adjuvants can improve spray coverage on target surfaces and decrease the application volume rate, as highlighted by recent studies [57]. This reduction in application rate not only reduces production costs, but also minimizes losses to the environment, reducing the risks of contamination caused by agricultural pesticides. However, it should be considered that droplets with smaller contact angles and greater dispersion can reduce drying time and, consequently, the absorption time of active ingredients [58], which can compromise the effectiveness of the treatment. However, when selecting adjuvants for tank mixing of agricultural pesticides, it is important to consider parameters such as electrical conductivity, pH, type of formulation (suspension concentrate, emulsion, or wettable powder), and the chemical nature of the adjuvants (non-ionic, anionic, or cationic) when choosing tank mixes. Detailed analysis of ionic stability and molecular interactions is essential to minimize incompatibilities, optimizing the efficacy of pesticides and ensuring environmental sustainability.

According to İtmeç et al. (2020) [50], the reduction in liquid surface tension led to an increase in droplet size. However, this cannot be considered an absolute and simple truth, as several factors, such as viscoelasticity, can affect droplet size, as reported by Zhang and Xiong (2021) [11]. Likewise, Polli et al. (2021) [34] indicated that adjuvants increase volumetric mass and viscosity, which can affect the viscoelastic properties of spray solutions and, consequently, the increase in $D_{v0.5}$. However, Kooij et al. (2018) [59] stated that the change in surface tension altered the location of the fluid breakup, so fluid with a lower surface tension appears to be more unstable, favoring the formation of smaller droplets. In this research, the effect of volumetric mass caused by the adjuvants was not pronounced, with adjuvants with lower surface tension being associated with smaller droplet size.

In the study conducted by Milanowski et al. (2022) [9], oil-based adjuvants altered $D_{v0.1}$ diameter by emulsifying the liquid with oil, which increased the diameter and helped stabilize the $D_{v0.5}$ value. This effect may have contributed to the increased droplet size of the oil-based adjuvants (Aureo, Agefix, and Assist). Furthermore, this process can vary depending on the properties of the oil used, as observed with the oil-based adjuvants studied in our research. The effect of increased $D_{v0.5}$ of oil-based adjuvant droplets has also been reported by Makhnenko et al. (2021) [60] and Milanowski et al. (2022) [9]. The emulsification of the droplets reduced the percentage of V < 100 of the Aureo, Assist, and Agefix adjuvant solutions compared to water.

A smaller $D_{v0.5}$ diameter is associated with greater dispersion in $D_{v0.1}$ and $D_{v0.9}$ diameters, emphasizing the relationship between these volumetric diameters and, simultaneously, a more excellent dispersion contributes to an increase in $RSF$ values. Such values close to one are preferable, as they indicate greater homogeneity and possible uniformity in droplet size distribution when applied to plants [9]. The reduction in $D_{v0.1}$ and $D_{v0.9}$ with the use of the adjuvants Assist and Aureo is advantageous since they contribute to less variation in the droplet spectrum. On the other hand, BREAK-THRU adjuvant may be preferable when droplet homogeneity is not crucial for adequate control. Some studies using polymeric adjuvants in solutions have indicated high droplet variability, as observed with the BREAK-THRU adjuvant [13,35,45,61].

In our research, larger droplets were produced by Aureo and Assist adjuvants, indicating the highest values for volumetric diameters ($D_{v0.1}$, $D_{v0.5}$, $D_{v0.9}$). This result suggests that these adjuvants minimize the risk of pesticide drifting to non-target areas, such as neighboring crops or water bodies, which is important in sensitive areas where contamination could have severe environmental impacts. On the other hand, BREAK-THRU and Agefix additives generated lower values for volumetric diameters, resulting in a prevalence of finer droplets that can be useful for crops with higher leaf density. Besides this, these droplets allow better penetration into the inner part of the crop canopy, facilitating efficient pest and disease control. In light of these findings, the adjuvants tested can be proposed to be used with pesticides with different mechanisms of action. For instance, Aureo can be used in combination with a systemic herbicide, as the translocation in plant tissues may be ensured by larger droplets through roots or stems. In contrast, BREAK-THRU can be sprayed with a contact pesticide that controls a pest due to the direct contact with the leaf surface area, which is ensured by smaller droplets on the target.

Concerning the homogeneity in droplet size distribution, the best results were obtained when using Aureo and Assist adjuvants, which showed the lowest $RSF$ values. This ensures a consistent PPP application across the entire crop field, minimizing the possibility of under- or over-application.

The percentage of V < 100 varied depending on the composition of the adjuvant. An increase in the percentage of V < 100 is a reference indicator for drift and evaporation, which is unfavorable for the sustainability of pesticide application technology. de Moraes et al. (2019) [62] indicated that, within the field of agricultural pesticide application technology, percentages of V < 100 below 15% can be considered low. In this study, all the considered adjuvants presented low percentages of V < 100. However, despite this being considered, the BREAK-THRU adjuvant demonstrated a low drift reduction potential, suggesting that the percentage of V < 100 affected the drift behavior of adjuvants. Generally, larger droplets are associated with reduced drift to non-target areas, leading to a decrease in drift percentage as V < 100 decreases. Additionally, it notably reduces the percentage of V < 500, as these droplets may run-off and fall on the ground, compromising the efficiency of treatment.

Although Machado et al. (2019) [43] indicated that spray mixtures with higher surface tension had the potential to produce larger droplets and consequently reduced drift, this premise could not be generalized. Therefore, it is essential to consider environmental factors and the interaction between the solution and the spray tip [5,6]. Makhnenko et al. (2021) [60] highlighted that the effects of surface tension could vary depending on the type and structure of the adjuvant. This variation could affect the levels of liquid turbulence at the spray nozzle and consequently influence V < 100.

When pesticides are applied near sensitive crops, water bodies (i.e., lakes, rivers), or under adverse weather conditions (such as high winds), adjuvants such as Aureo and Assist are more appropriate, since they had lower V < 100 values, so lowering the proportion of fine droplets (<100 µm) within the spray that are prone to drift away. This result contributes to a safer application with less environmental impact, especially in places that require greater precision. On the other hand, in situations where uniform coverage is a priority, such as in early-stage disease control or on targets with dense foliage, adjuvants like BREAK-THRU may be advantageous. Its use during spraying yielded higher percentage of V < 100, producing more fine dropletsthat better enhance penetration into the canopy and efficiency in hard-to-reach areas. However, careful management of drift risk is essential, particularly in unfavorable weather conditions, such as low relative humidity and high temperatures, which can increase the evaporation of smaller droplets [7].

In addition, the V > 500 values highlighted that some oil-based adjuvants like Aureo and Assist produced a higher proportion of large droplets (>500 µm), reducing the concern about drift but potentially causing uneven coverage and pesticide waste, especially for small or complex targets. In contrast, in our research, BREAK-THRU generated lower V > 500 values, suggesting its effectiveness for applications requiring uniform coverage, while minimizing drift risk. It is noteworthy to address the complex effect of chemical adjuvants and the need to further understand their behavior under field conditions. As reported by Katzman et al. (2021) [32], adding the polymer-based additive (polyacrylamide-based adjuvant) to the spray solution led to a shift in the volumetric diameters towards larger droplets and to a significant increase in the proportion of fine droplets (<100 µm). Such trends could explain how the polymeric adjuvant could have a dual function of reducing the sediment drift outside of the targeted area while increasing the airborne pesticide drift. In our research, the polymer-based adjuvant (polymethyl-siloxane active ingredient) produced mainly higher V < 100 values, by maintaining a finer volumetric distribution with respect to the other tested adjuvants. However, there was a slight difference in volumetric diameters between our results and those of Katzman et al. (2021) [32]. These discrepancies may be associated firstly with the different polymeric active substances used and varying environmental conditions during the experiments. Importantly, Katzman et al. (2021) [32] carried out their experiments in a field using an UAV system equipped with a straight boom holding hollow cone nozzles (TeeJet D12-46) with specific operating parameters (e.g., flow rate, working pressure, spray height), which may have affected the observed outcomes. In both studies, the significant impact of polymer-based adjuvants on droplet size distribution was underlined, but variations in experimental conditions and adjuvant composition are crucial factors for comprehending their field behavior during pesticide application. By integrating strategies, such as drift reduction technologies or flow modulation, it is possible to achieve a balance between efficiency and safety during phytosanitary treatments.

Under this scenario, understanding the drift behavior of adjuvants helps reduce environmental impacts by preventing contamination of sensitive areas, such as riparian forests and water sources, while improving the spray deposition on the reference target. Our study identified Agefix and Aureo adjuvants as having low drift and being suitable for high-precision applications or use near sensitive crops. Using these products contributes to greater safety in adverse weather conditions, reducing drift losses, optimizing the use of pesticides, and reducing operating costs. Moreover, they help meet environmental regulatory standards, promoting sustainability and good agricultural practices.

5. Conclusions

Adjuvants play a crucial role in improving PPP application. With various adjuvants available, each owning different properties, it is noteworthy to evaluate their specific effects on physical–chemical properties, droplet size distribution, and drift potential. The study allowed the following final conclusions to be drawn:

• The electrical conductivity of Assist and Aureo adjuvants increased by 406.39% and 229.38% with respect to pure water, respectively. Aureo and Agefix solutions exhibited reductions in pH by 40.49% and 11.43%, leading to significant changes in the characteristics of the solutions. Additionally, surface tension and contact angle were significantly reduced, with the BREAK-THRU adjuvant showing the most substantial reductions of 64.74% and 54.96%, respectively.
• Focusing on the main spray characteristics included in this work ($D_{v0.1}, D_{v0.5}, D_{v0.9}$, $RSF$, percentages of V < 100 and V > 500), it emerged that the Aureo adjuvant demonstrated an increase of 81.28% in $D_{v0.1}$ compared to water, while Assist showed an increase of 48.01% in $D_{v0.5}$. Conversely, the BREAK-THRU adjuvant exhibited no significant differences in $D_{v0.1}$ in relation to water, highlighting the heterogeneity in the effect of adjuvants on the droplet size spectra. The analysis of the percentage of droplets smaller than 100 µm (V < 100) indicated that Aureo, Assist, and Agefix adjuvants reduced this percentage by 71.09%, 57.10%, and 20.31%, respectively. In turn, the BREAK-THRU solution had a reduction of only 0.76%, which was not statistically significant in relation to water. Accordingly, all adjuvants had a lower percentage of V < 100, suggesting effectiveness in reducing spray drift.
• Regarding the spray drift measurements, adding adjuvants to the spray solution affected the droplet formation process, and ultimately played a fundamental role in minimizing drift risk. The Aureo adjuvant demonstrated the greatest drift reduction potential, of 59.35%, followed by Agefix (50.51%) and Assist (40.35%). The total spray volume collected on the polyethylene wires revealed that the solutions with BREAK-THRU, Assist, Agefix, and Aureo collected 12.88%, 11.0%, 8.02%, and 5.16%, respectively, reinforcing the efficiency of the adjuvants in minimizing drift.

Since all of the experiments were conducted in controlled environments, in future developments further studies could be carried out in the field on cotton or coffee crops, which are widely diffused in the Brazilian landscape, as well as on Mediterranean crops. Potential progress in drift control could be explored using different nozzle-pressure combinations or adjuvants. Additionally, research could be focused on the long-term effect of these formulations and adjuvants on crop health. This would be necessary to validate the performance of adjuvants under real field conditions and provide improvements for agricultural applications, better showcasing the validity of the study in a field setting.