Adjuvant effects on evaporation time and wetted area of droplets

Abstract

Appropriate adjuvant selection for pesticide applications is central to improve spray performances on waxy leaves and to reduce off-target losses. Evaporation and deposition patterns of 500 m sessile droplets with five classes of adjuvants on five different waxy plants were investigated. Droplets generated with a single droplet generator were deposited on target leaves placed in a controlled environmental chamber at 60% relative humidity and 25C temperature. Adjuvants tested were two types of oil-based Crop Oil Concentrate (COC) and Modified Vegetable Oil (MSO), two types of surfactant Nonionic Surfactant (NIS) and Silicone Surfactant, and a type of mixture Oil Surfactant Blend (OSB), plus the water-only droplets for comparison purposes. The Silicone Surfactant was removed from the test because its various properties were inconsistent with time. The five waxy plants were difficult-to-wet with the water contact angle greater than 90. The single water-only droplets did not spread with time and formed extremely tiny wetted areas on the leaf surface. The addition of the adjuvant to the spray solution significantly reduced the contact angle and increased the wetted area, but the change varied with the plant specie and the adjuvant class. In general, the MSO and NIS had obvious effects on enhancing droplet spread and maintaining the droplet in solution on the waxy leaf surface. The residual pattern of droplets after evaporation was formed with coffee ring. Droplets with oil-based adjuvants had more uniform residual pattern than the droplet with the surfactant adjuvant. Results of this study demonstrated that selection of the appropriate class of adjuvants can significantly influence deposit formation on the waxy plants leading to effectiveness of agrochemicals.