Impact and adhesion of surfactant-amended water droplets on leaf surfaces related to roughness

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

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/31/2020
Publication Date: 12/1/2020
Citation: Abbott, J.R., Zhu, H., Ambrose, A.E. 2020. Impact and adhesion of surfactant-amended water droplets on leaf surfaces related to roughness. Transactions of the ASABE. 63(6):1855-1868. https://doi.org/10.13031/trans.14027.
DOI: https://doi.org/10.13031/trans.14027

Interpretive Summary: The pesticide spray applications are significantly affected by leaf wettability which depends, in part, on the fine scale surface roughness of the leaf. It has been shown that a 3D optical surface profiler is a reliable tool to measure leaf surface roughness and that the 3D topography parameter, arithmetic mean length, is a reliable metric relating roughness and leaf wettability. Taking these findings a step further this research examined the relationship between impacting droplet deposition and leaf surface roughness in terms of the arithmetic mean length. This research shows that deposition decreases as surface roughness increases, but only for intermediate and rougher surfaces, with roughness lengths greater than 1.6 micrometers. Complete deposition occurs, otherwise, for smooth leaves, i.e., leaves with roughness lengths shorter than 1.5 micrometers. The research will benefit scientists and engineers looking to improve ways to better understand and increase water droplet adhesion on plants for pesticide spray applications.

Technical Abstract: Spray efficiency depends on droplet retention and spread, which is partially affected by the fine-scale surface roughness of the targeted leaves. A 3D optical surface profiler was used to measure the arithmetic mean roughness length (Sa), providing a reliable metric for leaf surface roughness. The relationship between leaf surface roughness and droplet deposition was examined by correlating droplet adhesion with the Sa of six leaf types for spray solutions at five surfactant concentrations (0.0 – 0.75%) and eight initial droplet horizontal travel speeds (0.5-4.5m/s). Leaf roughness and wettability in terms of contact angle (CA) ranged from smooth and easy-to-wet to rough and difficult-to-wet (37° < CA < 159° and 1.11µm < Sa < 2.47µm). Deposition was determined by comparing initial droplet volume to the residual liquid volume after impact. Droplet size and initial travel speed were controlled by a streamed mono-sized droplet generator mounted on a horizontal track. Droplet motion and impacts were recorded with a 3D stereoscopic system consisting of three ultrahigh-speed video cameras and analyzed using 3D motion analysis software. Complete deposition occurred when Sa < 1.51µm regardless of surfactant concentration and initial travel speed. When Sa > 1.6µm deposition decreased for increasing Sa and for initial travel velocity and improved as surfactant concentrations increased. High surfactant concentrations were required to achieve deposition for the roughest leaves. Good deposition can reduce costs and environmental impact. For rough leaves, deposition is best achieved at low initial travel speeds and high surfactant concentrations. These constraints become less necessary for smoother leaves.