Influence of spray additives on droplet evaporation and residual patterns on wax and wax-free surfaces

Abstract

Evaporation time and wetted area of single droplets sizing from 246 to 886 µm at relative humidity (RH) ranging from 30 to 90% were measured with sequential images under controlled laboratory conditions. Droplets were placed inside an environmental-controlled chamber under a stereoscope and a high definition digital camera. The spray mixtures used to form droplets included different combinations of water, a polymer drift retardant, a surfactant, and two insecticides. The droplet evaporation was investigated on the surfaces of crabapple leaf surfaces, hydrophilic and hydrophobic glass slides, respectively. Adding surfactant into spray mixtures greatly increased droplet wetted area while droplet evaporation time was greatly reduced. For a 343 µm droplet on the crabapple leaf at 60% RH, the evaporation time decreased from 70 to 50 s and the wetted area increased from 0.366 to 0.890 mm2 after the surfactant was added into the spray mixture containing water and insecticide. Adding the drift retardant into the spray mixture slightly increased the droplet evaporation time and decreased the droplet coverage area. Also, changing the target surface from the hydrophilic slide to the hydrophobic slide greatly increased the droplet coverage area and reduced the droplet evaporation time. Increasing RH could increase the droplet evaporation time greatly but did not change the coverage area. The droplet evaporation time and coverage area increased exponentially as the droplet size increased.