Controlling Liquid Atomization using Dilute Emulsions: Mitigation of Pesticide Spray Drift

Spray transport is mainly influenced by the velocity of the surrounding gas flow and by the distribution of drop sizes. In many applications, like spray painting, coating and crop protection, the exact location of spray deposition plays a very important role. In modern agriculture, misdirected pesticide spray is a major problem, which causes severe damages to surrounding ecosystems. This phenomenon is called spray drift. The susceptibility of a spray to drift is mainly influenced by its drop size distribution, since the trajectories of smaller drops can heavily be affected by the airflow. On the other hand, large drops have a negative effect on the coverage with pesticide. The present work aims towards controlling drop sizes during atomization with dilute emulsions. Aerodynamic fragmentation of a single drop at relatively low Weber numbers, corresponding to the drop bag breakup, is employed as a model system to evaluate the ability of various adjuvants to control the atomization process. For this purpose, a compact, open-circuit wind-tunnel is designed. The process of aerodynamic fragmentation is then recorded by a high-speed video system. It is observed that drop breakup is heavily promoted even at very small quantities of emulsion. A theoretical model is presented which describes the film thickness evolution during the aerodynamic fragmentation process. It is shown that a polyethersiloxane emulsion can considerably increase the film thickness at the instant of rupture as well as the mean child drop sizes. Next, the breakup morphology of liquid sheets originating from agricultural flat fan nozzles is analyzed and discussed qualitatively and quantitatively using a high-speed video system and appropriate image processing algorithms. It is demonstrated that the polyethersiloxane drops cause the nucleation of a large number of holes in the liquid films. This effect leads to a much more regular breakup and reduces the length of the intact sheet. It is shown that the additives influence the breakup of a single drop and the atomization in agricultural sprays in the same manner. Then, the effect of emulsion particles on final spray characteristics is quantified by image based drop size measurements. Compared to pure water, the volume fraction contained in drift-prone drops can be reduced substantially by the addition of small amounts of polyethersiloxane. Additionally, the more regular breakup induced by the emulsion decreases the polydispersity of drop sizes and thereby improves the coverage characteristics of the spray. Finally, it is shown that the addition of certain additives is very beneficial to the overall pesticide spray quality. The reduction of liquid contained in small and in large drops decreases the hazards caused by off-target pesticide deposition and improves the coverage characteristics. The crop protection process can thus be made more efficient and less precarious