Spray droplet sizes from aqueous liquid sheets containing soluble surfactants and emulsified oils

Agricultural sprays produced from the atomization of a nozzle-generated liquid sheet produce a wide range of droplet sizes, which impacts crop coverage and spray drift. While the operating conditions and nozzle type are main factors to achieve optimal droplet sizes, the chemical composition of the sprayed mixture also has substantial effect on the droplet size distribution. Particularly, the presence of surfactants and emulsified oil droplets found in agricultural adjuvants can influence droplet sizes, where surfactants tend to decrease droplet sizes and emulsion droplets tend to increase droplet sizes. However, the coupled, mechanistic level understanding of surfactants and emulsified oil droplets together remains mainly unknown. In this study, model spray systems of water, emulsified mineral oil, and surfactants TritonX-100 (water-soluble) and Span 80 (oil-soluble) at varied concentrations are sprayed through a flat-fan nozzle in a low-speed wind tunnel. A laser diffraction setup is used to measure the size distribution of spray droplets as a function of surfactant and oil compositions. The results show a non-monotonic size dependence on surfactant concentration, and importantly that the sprayed droplet sizes are linked with both the oil emulsion size and the aqueous phase dynamic surface tension and surfactant’s critical micelle concentration. The results also show that the oil phase surfactant has no significant impact on the sprayed droplet sizes. While motivated by the agricultural industry, the new insight into surfactant and oil emulsion synergism on sprayed droplet sizes has potential broader applications in multiphase printing, coating, and painting.