Influence of formulation properties on droplet size spectra and ground deposits of aerially-applied pesticides

The influence of the physical properties of the spray liquid on droplet size spectra and ground deposits of aerially‐applied pesticides was studied using heavy (high‐viscosity) oil‐ and water‐based suspensions of wettable powders (w.p.), a clear solution in a light (low‐viscosity) oil, and an emulsion‐suspension containing a bacterial control agent. The heavy oil provided a highly viscous spray medium, 52 to 64 mPa s (at a shear rate of 480 s−1), resulting in large droplet sizes and a high deposit on the ground sampling units. When the spray medium was thickened with petroleum jelly to keep the w.p. in suspension during overnight storage, the viscosity became excessively high (102 mPa s at 480 s−1), and the droplet spectrum was undesirably coarse and resulted in poor coverage of the spray plot and low deposits. The light‐oil‐based solution (ca 4 mPa s), provided a finer droplet spectrum and lower deposits than the heavy‐oil‐based formulations. The water‐based suspensions of the w.p. (ca 2 mPa s), showed Newtonian behaviour, whereas the emulsion‐suspension (ca 240 mPa s at 480 s−1) showed pseudoplastic behaviour. Consequently, the emulsion‐suspension provided a markedly different droplet spectrum from that of the suspensions of the w.p. Nevertheless, all three aqueous media provided deposits comparable to those of the light‐oil‐based solution, markedly lower than the heavy‐oil‐based suspensions. The study indicated that highly viscous Newtonian formulations should be avoided in pesticide applications using Micronair AU3000 atomisers at the volume rate (4–7 litre ha−1) used here if high atomisation efficiency is required.