Effects of leaf surface roughness and five adjuvants types on impacting droplet adhesion and spread

Authors: Abbott, Johnpaul; AMBROSE, A - College Of Wooster; Zhu, Heping

Submitted to: Journal of ASTM International
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/2020
Publication Date: 9/10/2020
Citation: Abbott, J.R., Ambrose, A.E., Zhu, H. 2020. Effects of leaf surface roughness and five adjuvants types on impacting droplet adhesion and spread. Journal of ASTM International, Pesticide Formulation and Delivery Systems. 40:128-139. https://doi.org/10.1520/stp162720190145.
DOI: https://doi.org/10.1520/stp162720190145

Interpretive Summary: Pesticide application is the most common method to protect crops against attacks from insects and diseases. However, effectiveness of spray systems is limited by the retention and spread of impacting spray droplets. This research considered how the retention and spreading of impacting water droplets on leaf surfaces was influenced by surface roughness, adjuvant class and concentration. Test results showed that some adjuvant classes were better than others, depending on the leaf surface roughness, instead of a one-size-fits all. It also revealed that the interaction between the adhesive and cohesive forces of the leaf surface and sprayed droplet was significant in retention and spreading of impacting droplets on targeted leaf surface. This scientific information could be used to improve the efficiency of pesticide spray applications, helping to reduce waste and environmental impact, while helping to improve yield and food security.

Technical Abstract: The adhesion and spreading of impacting spray droplets on leaf surfaces for pesticide applications is dependent on leaf surface roughness as well as the type and concentration of adjuvants contained in the spray solution. Higher adjuvant concentrations increase deposition while rougher surfaces decrease deposition and wettability. Quantifying and relating the effects of surface roughness and adjuvants can improve pesticide spray applications and modeling. This paper presents a preliminary study relating quantified leaf surface roughness to spray deposition and spreading for spray solutions containing different classes and concentrations of adjuvants. A 3D optical surface profiler and the areal roughness parameter for roughness height, Sa, were used to quantify surface roughness for different leaf types ranging in wettability from very easy to very difficult to wet (Contact Angles between 36° and 158°) and roughness from smooth to very rough. Spray solutions were composed of distilled water and adjuvant concentrations of 0.0, 0.10, 0.25, 0.50, 0.75, and 1.00% (v/v). The adjuvants tested were a crop oil concentrate, a modified seed oil, a nonionic surfactant, an oil-nonionic silicone surfactant blend, and organo-silicone. Spray was emitted from a streamed mono-sized droplet generator mounted on a horizontal motion track, traveling at a speed of 1.341 m/s. Droplet motion and impacts were recorded with three ultrahigh-speed video cameras and analyzed using 3D motion analysis software. Deposition was determined by comparing droplet volume before and after impact. For all the adjuvant classes there was complete deposition on smooth-easy to wet leaves at all concentrations. For all the adjuvant classes deposition on rough-hard to wet leaves increased linearly as concentrations increased, where approximately 70% deposition was achieved for the non-ionic and silicone adjuvants at 0.75% and 0.50, % concentration, respectively. Depositions of less than 70% were achieved for the crop oil concentrate, modified seed oil, and oil-silicone blend adjuvants.