Evaporation of Pesticide Droplets on Surfaces under Various Relative Humidity Conditions

It is important to understand the evaporation process of pesticide droplets on targets for increasing the efficiency and efficacy of foliar applied insecticide and fungicide spray applications. Evaporation characteristics of five droplet sizes (246, 343, 575, 762, and 886 μm) under three relative humidity (RH) conditions (30, 60, and 90 %) were studied in a laboratory. Sequential images of evaporating droplets placed inside a small environmentally controlled chamber were obtained using a stereoscope for determination of droplet evaporation time and residual patterns. The spray mixtures included different combinations of water, a nonionic colloidal polymer drift retardant, an alkyl polyoxyethylene surfactant, and an insecticide. The droplet evaporation was investigated on the surfaces of hydrophilic and hydrophobic glass slides which represented ideal non-waxy and waxy leaf surfaces, respectively. Among the spray mixtures investigated, the droplets containing the drift retardant had the longest evaporation time, and the droplets containing the surfactant had the shortest evaporation time after these additives were added into insecticide mixtures. The mean evaporation time of 246, 343, 575, 762, and 886 μm droplets containing water and the insecticide without additives at 60 % RH on the hydrophilic surface were 43, 77, 133, 226, and 384 s, respectively. The mean evaporation times of the same size droplets containing the same insecticide but mixed with the surfactant were 26, 47, 77, 156, and 251 s, respectively. The evaporation time of 575 urn droplets containing the drift retardant at 60 % RH increased from 159 s to 224 s when the deposition surface changed from the hydrophilic slide to the hydrophobic slide. The evaporation time of droplets greatly increased as RH increased, and also increased exponentially as the droplet size increased. Therefore, spray additives, target surface fine structure and RH greatly influenced the evaporation time of spray droplets.