Coverage Area and Fading Time of Surfactant-Amended Herbicidal Droplets on Cucurbitaceous Leaves

Abstract. Proper use of appropriate surfactants to control droplet behavior on leaf surfaces is critical to improving herbicide application efficacy for controlling paddy melons (). An esterified seed oil surfactant and a petroleum oil surfactant were investigated to modify spread areas and fading times of water and herbicidal droplets on watermelon leaves, which were used to simulate paddy melon leaves. A combination of two systemic herbicides (triclopyr and glyphosate) was also used to form a water-based spray solution for comparisons. The glyphosate herbicide was pre-formulated with a non-ionic surfactant. Tests were conducted with 250 and 450 μm sessile droplets at 30% and 60% relative humidity (RH) inside a controlled environmental chamber. The sessile droplets were produced with a single-droplet generator. Droplet fading times significantly increased with both droplet size and RH. Droplet maximal coverage areas increased only with droplet size but not RH. For the water-only solution, addition of seed oil surfactant increased the droplet coverage area by over 2.3 times, but addition of the petroleum oil surfactant did not increase the droplet coverage area. For the spray solution containing two herbicides, addition of the seed oil surfactant increased droplet coverage areas on leaves from 0.146 ±0.012 mm² to 0.201 ±0.014 mm² for 250 μm droplets and from 0.348 ±0.017 mm² to 0.537 ±0.024 mm² for 450 μm droplets. In addition, their fading times increased from 36 ±7 s to 43 ±6 s at 30% RH and from 53 ±10 s to 62 ±3 s at 60% RH for 250 μm droplets, and from 98 ±6 s to 107 ±5 s at 30% RH and from 145 ±5 s to 151 ±8 s at 60% RH for 450 μm droplets. In comparison, addition of the petroleum oil surfactant to the herbicide solution reduced the maximal coverage areas and shortened the droplet fading times but increased droplet penetration rates. Therefore, to improve spray coverage area and reduce droplet evaporation, herbicide spray solutions that already contain a surfactant could be amended with the seed oil surfactant. The petroleum oil surfactant could be used when enhancement of droplet penetration into leaf tissues is necessary.