Methods for evaluating leaf surface free energy and polarity having accounted for surface roughness

The effect of Aceria litchii (Keifer) infestation on the surface properties of litchi leaf hosts

BACKGROUND

The wettability of target crop surfaces affects pesticide wetting and deposition. The structure and properties of the leaf surface of litchi leaves undergo severe changes after infestation by Aceria litchii (Keifer). The objective of this study was to systematically investigate the surface texture and wettability of litchi leaves infested.

RESULTS

Firstly, the investigation focused on the surface structure and physicochemical properties of litchi leaves infested with Aceria litchii. Subsequently, different levels of Contact Angle (CA) were measured individually on the infested litchi leaves. Lastly, Surface Free Energy (SFE) and its polar and dispersive components were calculated using the Owens-Wendt-Rabel-Kaelble (OWRK) method. The outcomes revealed distinctive 3D surface structures of the erineum at various stages of mycorrhizal growth. At stage NO. 1, the height of the fungus displayed a peaked appearance, with the skewness value indicating a surface characterized by more crests. In contrast, at stages NO. 2 and NO. 3, the surface appeared relatively flat. Furthermore, post-infestation of litchi leaves, the CA of droplets on the abaxial surface of diseased leaves exhibited an increase, while the SFE value on the abaxial surface of leaves decreased significantly, in contrast to the abaxial surface of healthy leaves.

CONCLUSION

The infestation behavior of Aceria litchii changed the surface structure and chemistry of litchi leaves, which directly affected the CA value of foliar liquids and the SFE value of leaves, changing the surface wettability of litchi leaves from hydrophobic to superhydrophobic. This study provides useful information for improving the wetting and deposition behavior of liquid droplets on the surface of infested leaves. © 2024 Society of Chemical Industry.

Enhanced dosage delivery of pesticide under unmanned aerial vehicle condition for peanut plant protection: tank-mix adjuvants and formulation improvement

BACKGROUND

Suspension concentrate (SC) is one of the most widely used formulations for agricultural plant protection. With the rapid development of unmanned aerial vehicle (UAV) plant protection, the problems of spray drift, droplet rebound and poor wettability in the application of SC from UAVs have attracted wide attention. Although some tank-mix adjuvants have been used to enhance dosage delivery for UAV, their effects and mechanisms are not fully clear, and few formulations are specifically designed for UAV.

RESULTS

The type and concentration of tank-mix adjuvant affect the dosage delivery of SC. MO501 can significantly reduce DV<100μm , and inhibit droplet rebound on peanut leaves at concentrations ≥0.5%. Silwet 408 can achieve complete wetting and superspreading after adding ≥0.2% concentrations, but only ≥0.5% can inhibit rebound. XL-70 shows excellent regulation ability even at low concentration, and 0.2% concentration can simultaneously suppress impact and promote spreading. Besides, the formulation oil dispersion (OD) can significantly reduce the driftable fine fraction and inhibit rebound at dilution ratios of ≤250-fold, thus enhancing dosage delivery.

CONCLUSION

SC is prone to rebound on hydrophobic leaf surfaces and shows poor wetting and spreading properties. Appropriate types and concentrations of tank-mix adjuvants and formulation improvement are two effective strategies for improving the dosage delivery of pesticides, whereas the addition of inappropriate adjuvants may cause potential risks instead. These findings provide guidance for the rational selection of tank-mix adjuvants and potential applications of OD for UAV plant protection. © 2023 Society of Chemical Industry.

Importance of adjuvant formulation properties in predicting wetting on leaf surfaces

BACKGROUND

Leaf wettability can be a barrier to retention of agrichemical sprays. Adjuvants are used to modify leaf wetting by sprays to enhance retention. A previous study developed a model that accurately predicted nonadjuvant formulation wetting (contact angle) on both synthetic and leaf surfaces. Model inputs were the surface properties, roughness and polarity, as measured by the wetting tension dielectric method, coupled with the formulation properties, surface tension and dielectric constant. Preliminary work has indicated that the wetting ability of adjuvant formulations on different surfaces could be modelled in a similar way if the effect of adjuvants on solution polarity could be accurately quantified.

RESULTS

The wetting of nine agrichemical adjuvants, at a range of concentrations, were measured on seven synthetic and 14 leaf surfaces. A novel method was developed to quantify the interfacial dielectric polarity (IDP) of adjuvant formulations. Adjuvant concentration did not change the IDP indicating the surface-active surfactant molecules migrate to the interface, loading until saturation. Formulation properties of surface tension and IDP were found to be strong predictors of wetting in conjunction with surface properties of the substrate. The previously developed unaltered comprehensive wetting model could predict the wetting of adjuvant formulations on synthetic and leaf surfaces (R2  = 0.9) using these inputs.

CONCLUSIONS

Wetting of adjuvant formulations can be modelled for a wide range of surfaces and this model is expected to advance the selection, and development, of adjuvants to target specific surfaces generating the desired wetting outcome. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Suspension Concentrate crop protection formulation design and performance for low spray volume and UAS spray application

BACKGROUND

Unmanned aerial systems (UAS) are providing interesting disruptive solutions for spray application of crop protection products with very-low spray volumes (VLV) down to 8 L/ha that offer improved sustainability through reduced water volumes and reduced soil compaction. However, the efficacy of products can be reduced by the significantly lower crop/plant spray coverage and formulation designs that can compensate for this are highly important here.

RESULTS

Suspension Concentrate (SC) formulations designed for VLV use containing and delivering low dose rates (g/ha) of organosilicone alkoxylate high-spreading surfactants were found to result in leaf coverage of VLVs comparable to those observed at higher spray volumes. High spreading was observed on textured leaf surfaces containing sub-micron sized epicuticular wax crystals. Greenhouse fungal disease studies showed enhanced efficacy with these SC formulations compared to standard SC formulations without these additives and maintained the observed increase in efficacy when applied at VLV. Alternatively, SC formulations without high spreading formulants but containing uptake promoting nonionic surfactants showed enhanced cuticle penetration through isolated cuticles at VLV in comparison to higher spray volumes, with coffee-ring spray deposit microstructures present at VLVs. Similarly, greenhouse studies showed enhanced efficacy that was maintained at VLV relative to SCs without these additives.

CONCLUSION

At VLVs, SC formulations applied at relatively low dose rates (g/ha) of formulants (adjuvants) enhancing spreading on the leaf surface and/or uptake of the active ingredient(s) maintained good spreading, uptake and biological efficacy in greenhouse studies overcoming the coverage limitations of SC formulations without these additives. This result is unexpected considering the low dose rate of adjuvants used. © 2023 Bayer AG. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Importance of leaf surface and formulation properties in predicting wetting outcomes

BACKGROUND

Leaf wettability is a major hurdle for the retention of agrichemical sprays that is combated, in part, by using adjuvant modified formulations. Scientists must understand the properties of the leaf surface and the formulation that govern wetting to intelligently select or formulate products to target specific pests.

RESULTS

A comprehensive database comprising 11 synthetic surfaces and 54 leaf surfaces (species, adaxial and abaxial sides, cultivars, and plant age) using 35 formulations (neat solutions and adjuvants solutions at different concentrations) was compiled. Surface properties of the physical roughness and chemical polarity, as quantified by the wetting tension dielectric method, and formulation properties of surface tension and polarity, as quantified by dielectric constant, were found to govern wetting. A comprehensive wetting model was developed that employed these variables and was capable of accurately predicting the wetting outcome (R2  = 0.86) on all the leaf and synthetic surfaces investigated. This model adequately predicts adjuvant formulation wetting despite exact formulation polarity being unknown.

CONCLUSIONS

Wetting can be modelled for a wide range of surfaces and solutions. The comprehensive wetting model developed shows potential to better predict the wetting outcome of adjuvant formulations should a method to quantify the formulation dielectric constant be developed. This research provides a significant advancement in the understanding of the properties governing wetting, which may aid the selection and development of adjuvants to target specific surfaces. © 2022 Society of Chemical Industry.

Effect of Adjuvants on the Wetting Behaviors of Bifenthrin Droplets on Tea Leaves

The hydrophobic epicuticle wax on fresh leaves of tea tree (Camellia sinensis (L.) 0. Kuntze) leads to the loss of pesticide droplets, reducing efficacy. In this study, four adjuvants were selected to improve the diffusion and adhesion of bifenthrin droplets on the surface of tea leaves at different growth stages. The coupling effect of bifenthrin and adjuvants on the time-dependent and concentration-dependent wettability of droplets was investigated, and the difference in the wettability between bud and leaf was explained by observing the surface morphology. It was found that adjuvants effectively reduced the contact angle of droplets and accelerated the diffusion speed, and the above phenomenon became obvious with the increase in the adjuvant concentration. The wetting promotion of young leaves was more significant due to the reduced epicuticle wax and the greater roughness compared with fresh buds. The surface tension of the pesticide mixture was negatively correlated with the cosine of contact angle after adding the four adjuvants regardless of the growth stage of tea leaves. The contact angle of 0.2% Silwet L-77 decreased to 0° within 10 s, but the extreme wettability led to the decrease in adhesion with the increase in concentration. However, the wettability and adhesion on the surface of tea leaves were simultaneously suitable with more than 0.1% nonionic surfactant. The minimum concentration of the optimal adjuvant proposed in this study can provide an experimental basis and guide more efficient plant protection in tea gardens.

Surface properties of Tetranychus urticae Koch (Acari: Tetranychidae) and the effect of their infestation on the surface properties of kidney bean (Phaseolus vulgaris L.) hosts

BACKGROUND

The wettability of the target surfaces affects the wetting and deposition of pesticides on them. The properties of leaf surfaces change after infestation by Tetranychus urticae Koch. Studying the surface wettability of T. urticae and the changes in leaf wettability after infestation is important to guide the use of acaricides.

RESULTS

The body surface of T. urticae is an ellipsoidal crown covered with dense cuticle striations and hairs arranged in different directions, which makes the surface of T. urticae rough and hydrophobic. The abaxial surfaces of the leaves are rougher and more hydrophobic than the adaxial surfaces. After infestation by T. urticae, the faded spots were sunken on the adaxial surface and raised on the abaxial surface, where they had formed new wide peaks and valleys. The adaxial surface became obviously rougher and more hydrophobic, while the roughness of the abaxial surface became slightly larger, and the change in hydrophobicity was not obvious. The contact angles of the studied commercial acaricide on these surfaces were greater than 65° and were affected by the infestation. Reducing the surface tension can allow for better wetting of these surfaces and eliminate changes in leaf wettability.

CONCLUSION

The surfaces of kidney bean leaves became more hydrophobic after infestation by T. urticae with hydrophobic surface. The wettability of the acaricide solution should be adjusted according to the changes in leaf wettability. This study has important theoretical guiding significance for improving effective deposition of acaricide.

Roughness measurement of leaf surface based on shape from focus

BACKGROUND

Surface roughness has a significant effect on leaf wettability. Consequently, it influences the efficiency and effectiveness of pesticide application. Therefore, roughness measurement of leaf surface offers support to the relevant research efforts. To characterize surface roughness, the prevailing methods have drawn support from large equipment that often come with high costs and poor portability, which is not suitable for field measurement. Additionally, such equipment may even suffer from inherent drawbacks like the absence of relationship between pixel intensity and corresponding height for scanning electron microscope (SEM).

RESULTS

An imaging system with variable object distance was created to capture images of plant leaves, and a method based on shape from focus (SFF) was proposed. The given space-variantly blurred images were processed with the proposed algorithm to obtain the surface roughness of plant leaves. The algorithm improves the current SFF method through image alignment, focus distortion correction, and the introduction of NaN values that allows it to be applied for precise 3d-reconstruction and small-scale surface roughness measurement.

CONCLUSION

Compared with methods that rely on optical three-dimensional interference microscope, the method proposed in this paper preserves the overall topography of leaf surface, and achieves superior cost performance at the same time. It is clear from experiments on standard gauge blocks that the RMSE of step was approximately 4.44 µm. Furthermore, according to the Friedman/Nemenyi test, the focus measure operator SML was expected to demonstrate the best performance.

Simulating spray droplet impaction outcomes: comparison with experimental data

BACKGROUND

A suite of plant retention spray models has been developed to simulate spray retention using virtual surfaces (either single leaves or whole plants) and their outputs compared with experimental data for the equivalent spray scenarios.

RESULTS

The results for a single formulation (0.1% v/v lecithin mixture in water) and difficult to wet plant species Chenopodium album L (common lambsquarters) are presented. They include experimental observations with single leaves, as well as simulations of virtual impaction events, conducted to provide for the first time estimates of f (the proportion of theoretical impact drop diameter at shatter). With this factor prescribed, multi-plant simulations using a range of nozzle types and droplet sizes (volume mean diameter (VMD) range 241 to 530 μm) are compared with equivalent experimentally determined spray retention by real plants. The simulations demonstrated that impaction resulted predominantly in shatter with the production of daughter droplets, and that retention is mainly due to re-capture of these droplets. Overall the simulations show the same trends as experimental retention results from different nozzle applications, but at best predicted retention results were 68% to 79% of experimental percentage retention, depending on plant spacing.

CONCLUSIONS

Retention is the result of some primary drop capture but predominantly by recapture of shatter droplets as the modelling illustrates. The value of f affects the droplet shatter outcome and can result in fewer, more energetic daughter droplets, or more droplets but with lower energies. However, this effect alone cannot explain the discrepancy between actual and simulated results. Possible operational influences are discussed. © 2020 Society of Chemical Industry.

Oil Adjuvants Enhance the Efficacy of Pyraclostrobin in Managing Cucumber Powdery Mildew (Podosphaera xanthii) by Modifying the Affinity of Fungicide Droplets on Diseased Leaves

Adding adjuvants improved the affinity of fungicide droplets to cucumber leaves infected with powdery mildew (Podosphaera xanthii) and subsequent efficacy of fungicide treatments in reducing the disease. The affinity of oil adjuvants was quantified by adhesional tension and "work of adhesion". Oil adjuvant-fungicide mixtures were applied to plants in field experiments to evaluate their effectiveness in disease prevention. Both the adhesional tension and work of adhesion of the adjuvants at selected concentrations increased on powdery-mildew-infected cucumber leaves more than on healthy cucumber leaves. The adjuvant GY-Tmax (GYT) displayed the best surface activity or "surfactivity" in enhancing the affinity and adherence of droplets to powdery-mildew-infected cucumber leaves, while epoxidized soybean oil (ESO), methyl oleate, and biodiesel exhibited much lower effects in terms of the surface tension, contact angle, adhesional tension, and work of adhesion. Field experiments determined that the combination of GYT at 1,000 mg liter^-1 and pyraclostrobin (150 g a.i. ha^-1) was most effective (91.52%) in controlling cucumber powdery mildew. Pyraclostrobin with ESO was also highly effective (ranging from 77.54 to 89.65%). The addition of oil adjuvants, especially GYT and ESO, to fungicide applications can be an effective strategy to enhance the efficacy of pesticides in controlling plant diseases by modifying the affinity of fungicide droplets to symptomatic leaves.

Impact factor assessment of the uptake and accumulation of polycyclic aromatic hydrocarbons by plant leaves: Morphological characteristics have the greatest impact

Polycyclic aromatic hydrocarbons (PAHs) have toxic, teratogenic, mutagenic and carcinogenic effects on living organisms. Plants can function as pollutant bioindicators and bioaccumulators due to their wide surface distribution and specific responses to atmospheric pollutants. However, various plants exhibit significant differences in their capacities to accumulate PAHs. At present, research has mainly focused on the effects of leaf morphology and physiological characteristics, and few studies have evaluated the effects of the leaf surface on PAH accumulation. We aimed to assess the factors impacting the uptake and accumulation of PAHs by leaves. We selected 8 common tree species in Shanghai, China, and used supercritical fluid extraction technology to determine the content of PAHs in their leaves. Specific measurements of leaf area, width/length, wax content, and stomatal density were applied to index the morphological and physiological characteristics; surface roughness, surface free energy, polar components, and dispersion components were compiled into an adsorption performance index. Principal component analysis (PCA) and canonical correlation analysis (CCA) were used to assess the effects of different leaf characteristics on PAH accumulation. We found that the mean concentrations of ΣPAHs ranged from 300 to 2000 ng·g^-1 and that the proportions of different benzene rings were significantly different among the different tree species. Leaf morphology and physiological characteristics had more significant effects compared to surface adsorption. CCA showed a significant negative correlation between leaf morphological characteristics and wax content, but had no significant correlation with surface adsorption. Low-molecular-weight PAHs were found to be mainly affected by the morphological characteristics, while medium- and high-molecular-weight PAHs were influenced by wax content and adsorption. Our conclusions provide a theoretical basis for the establishment of a reliable plant atmosphere-monitoring system and a method for screening tree species with strong PAH adsorption capacity.

Influence of Multiple Factors on the Wettability and Surface Free Energy of Leaf Surface

The wettability of plant leaves directly reflects leaf hydrophilicity, which is the key factor that influences the adhesion of liquid pesticide as well as affects plant protection products (PPP) efficacy. Generally, the wettability of leaf surface is quantified by the contact angle and surface free energy (SFE), which are mainly dependent on leaf surface properties, liquid properties and other spraying parameters. Therefore, the aim of this paper was to investigate the SFE of rice and rape leaves with the variation of leaf status, leaf surface, and probe liquid as well as the influence of droplet falling height, solid surface, and PPP concentration on the wettability. The results showed that: (1) the dispersive components of SFE of rice and rape account for a large proportion which are closely related to their hydrophobicity—the abaxial of rape new leaf and the adaxial of rape old leaf are easier to wet comparing with rice and rape leaves in other statuses; (2) the increase of droplet falling height had a significant effect on improving the wettability between wax surface and adjuvant solution, while it had little improving effect on the wettability between wax surface and water; (3) the wettability of different solid surface varied greatly, and the order of wettability from good to bad is water-sensitive paper (WSP), wax, rape leaf, and rice leaf; (4) the effect of PPP concentration on the leaf surface wettability is significant, the contact angle decreased with the increase of PPP concentration, and the wettability of microemulsion is better than that of suspending agent and wettable powder. In conclusion, the SFE and wettability of crop leaf surface determine the suitable type of PPP, studying the influence of multiple factors on leaf surface wettability can provide a reliable reference for providing scientific guidance as well as improving the effective utilization of PPP.