Analytical and experimental study on binary droplet evaporation: Inhibitory effect of adding mineral oil adjuvant to water

This work fundamentally explains the inhibitory effect of adjuvant on water droplet evaporation in pesticide application. An analytical droplet evaporation model coupled with the instantaneous distribution of temperature and species concentrations in the vapor film boundary layer accompanying with the droplet diameter decrease is developed to describe not only the droplet evaporation but also the surrounding boundary layer evolution process. Thermal and concentration boundary layer thicknesses as well as the property variations at the liquid-vapor interface are updated at each time step during the calculation. An adjuvant factor is defined to assess the inhibitory effect of the adjuvant on water droplet evaporation. The results show that adding a tiny amount of mineral oil adjuvant can inhibit water droplet evaporation. The analysis of droplet evaporation evolution demonstrates that the inhibitory effect is mainly controlled by mass diffusion where the concentration gradient surrounding the liquid-vapor interface dominates the overall phase change. A regression formula is introduced to express the dependence of droplet lifetime on temperature and humidity. The proposed model is proved to be suitable for the mixing condition with a tiny amount of adjuvant, which can be used to describe the mechanism of evaporation inhibition of pesticide adjuvant in plant protection.