Adjuvant Effects on Evaporation Time and Wetted Area of Droplets on Waxy Leaves

Abstract

The use of an appropriate adjuvant for pesticide applications is a critical process to improve spray deposit characteristics on waxy leaves and to reduce off-target losses. After deposition and evaporation, residue patterns of 500 ° m sessile droplets that incorporated four classes of adjuvants on five different waxy plants were investigated. Droplets were generated with a single-droplet generator and deposited on target leaves placed in an environmentally controlled chamber at 60% relative humidity and 25°C ambient temperature. Adjuvants tested were two oil-based types (crop oil concentrate, or COC; and modified seed oil, or MSO), a nonionic surfactant (NIS), and a mixture (oil surfactant blend, or OSB). Water-only droplets were also tested for comparative purposes. The five waxy plants were difficult to wet and had a water contact angle greater than 90°. The water-only droplets did not spread at all and formed extremely small wetted areas on the leaf surface. The addition of an adjuvant to the spray solution significantly reduced the contact angle and increased the wetted area, but the change or improvements varied with the plant species and adjuvant class. In general, MSO and NIS enhanced the droplet spread and maintained the droplet evaporation time on the waxy leaf surfaces. After evaporation, the residues formed patterns of coffee rings. Droplets with oil-based adjuvants had more uniform residual distribution in the deposition patterns than droplets with the surfactant adjuvant. Results of this study demonstrated that selection of the appropriate class of adjuvants significantly improved deposit formation on waxy leaves, leading to more effectiveness of pesticides.