Submitted to: Journal of ASTM International
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/20/2008
Publication Date: 10/5/2008
Publication URL: handle.nal.usda.gov/10113/56257
Citation: Yu, Y., Zhu, H., Ozkan, H.E. 2008. Evaporation of Pesticide Droplets under Various Relative Humidity Conditions. Journal of ASTM International. 6(1):1-8. Interpretive Summary: It is important to understand the evaporation process of pesticide droplets on targets to increase the control efficiency of foliar applied insecticide and fungicide spray treatments. In many cases, droplets are required to stay on the targets long enough for plants to absorb chemical ingredients while in some other cases longer evaporation may lead to use large droplets which can cause runoff and low efficiency. Spray additives are widely used in foliar applications to reduce the spray drift and increase the leaf wetness. It is unclear how these additives affect the droplet evaporation and how they react on different types of leaves under varied environmental conditions. Effects of spray mixture additive, droplet size, and relative humidity on the evaporation of droplets deposited on waxy and wax-free surfaces were investigated under controlled conditions in the laboratory. The droplet evaporation time was greatly reduced by adding a surfactant, or increased by adding a polymer drift retardant. Droplets on the wax-free surface had a shorter evaporation time than the droplets on the waxy surface. Droplet evaporation time was greatly increased as relative humidity was increased, and was exponentially increased as the droplet diameter was increased. The information from this research is very useful for choosing optimal spray techniques and formulating proper chemicals to obtain the maximal pesticide spray application efficacy for the target plants.
Technical Abstract: 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 environmental-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 leas 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 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 time of the same size droplets containing the same insecticide but mixed with the surfactant was 26, 47, 77, 156, and 251 s, respectively. The evaporation time of 575 µm droplets containing the drift retardant at 60% RH increased from 159 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.