Retention and spread capability of impacted droplets with surfactant and hydrocolloid based adjuvants

Authors: Abbott, Johnpaul; Zhu, Heping; Jeon, Hongyoung

Submitted to: Journal of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2021
Publication Date: 12/20/2021
Citation: Abbott, J.R., Zhu, H., Jeon, H. 2021. Retention and spread capability of impacted droplets with surfactant and hydrocolloid based adjuvants. Journal of the ASABE. 64(6):1883-1894. https://doi.org/10.13031/trans.14461.
DOI: https://doi.org/10.13031/trans.14461

Interpretive Summary: Effectiveness of pesticide application systems can be increased significantly by increasing the retention and spread of the impacting droplets on specifically targeted crops that are influenced by the interaction between the adhesive characteristics of the droplets and leaf surface fine structures. In this research, investigations were conducted on the adhesive retention and spreading capabilities of spray droplets containing different types of adjuvants on leaves. The surface roughness and wettability of leaves ranged from hydrophilic to superhydrophobic characteristics. The tested adjuvants represented the non-ionic, oil, organo-silicone, blended, or hydrocolloid polymer based additives commonly used in pesticide spray solutions. Droplets amended with adjuvants that could reduce the surface tension generally had greater retention rates than water-only droplets on hydrophilic and hydrophobic leaf surfaces. The organo-silicone adjuvant provided the best retention at the concentration of 0.75% or higher to achieve more than 90% retention on the extremely rough and superhydrophobic leaf surface. Addition of any adjuvants tested in the spray solutions improved the droplet spreading areas on the smooth and hydrophilic leaf surfaces. To achieve the droplet spreading area nearly 3 times larger than the initial droplets on these leaf surfaces, it required 0.10% adjuvant concentration in the spray solutions. The hydrocolloid polymer adjuvant improved droplet retention on the slightly hydrophobic leaves but was ineffective, providing no retention, on the superhydrophobic leaves; however, addition of this adjuvant to the spray solution retained the droplet settlement shape over time after initial spread on the hydrophilic leaves, thereby achieving similar benefits as other adjuvants to enhance adhesion and minimize run-off of spray deposits. The information from this research provided a scientific baseline for selecting or formulating optimal adjuvants to improve pesticide spray application efficiency for specific crops.

Technical Abstract: To optimize spray application efficiency, it is necessary to understand how the addition of adjuvants modifies deposition properties of spray droplets on leaf surfaces due to variations in adhesive characteristics, such as the roughness and wettability. Droplet retention and spread behaviors with seven commercially available adjuvants were tested and compared at different concentrations. Tests were conducted with three leaf surfaces ranging in roughness and wettability, from very smooth and hydrophilic to very rough and superhydrophobic. The adjuvants were formulated with non-ionic surfactant, crop oil, seed oil, organo-silicone, hydrocolloid polymer, or combinations of these agents as functional ingredients. Droplets of approximately 340 µm diameter were emitted from a streamed mono-sized generator. Droplet impact, and spread were recorded with a 3D imaging system consisting of three highspeed digital cameras and analyzed using 3D motion analysis software. Retention and spread were determined by comparing droplet liquid volumes and droplet cross sectional areas before and after impact, respectively. The surface tension of spray solutions and wettability of leaf surfaces strongly affected droplet retention. Droplets with lower surface tension were more likely to achieve high retention than those with higher surface tension. Also, droplet retention generally decreased with increasing leaf roughness-wettability. All droplets with and without adjuvants achieved 100% retention on the hydrophilic leaf surface. Addition of non-ionic, oil or organo-silicone based adjuvant in the spray solutions improved droplet retention on hydrophobic leaves while the organo-silicone based adjuvant achieved the highest retention on superhydrophobic leaf surfaces. Retention of droplets with the hydrocolloid polymer adjuvant were generally comparable to the other six adjuvants on the hydrophilic leaves and the hydrophobic leaves with intermediate roughness and wettability but failed to achieve any retention on the superhydrophobic leaves. To improve droplet retention and adhesion, selection of adjuvants representing the non-ionic, oil, organo-silicone, blended, or hydrocolloid based additives for enhancing spray solution performance must comply with leaf surface characteristics.