Sommaire:

One of the major environmental issues related to pesticides spraying on cultivated crops is the drift process. Because of the wind, small droplets may drift away from the targeted crop and cause contamination. One way to reduce the drift is to control the spray drop size distribution by reducing the proportion of small drops. In this context, anti-drift additives have been developed, including dilute oil-in-water emulsions. Although being documented, the effects of oil-in-water emulsions on spray drop size distribution are not yet understood. The objective of this thesis is to determine the physical mechanisms at the origin of the changes of the spray drop size distribution for emulsion-based sprays. Spray drops result from the destabilization of a liquid sheet formed at the nozzle exit. With a model experiment, based on the impact of a single tear of liquid on a small target, we investigate the destabilization mechanisms of liquid sheets and compare the results obtained to the drop size distribution of sprays produced with agricultural nozzles. With dilute oil-in-water emulsions, the liquid sheet is destabilized by the nucleation and growth of holes within the sheet that perforate it during its expansion. The physical-chemical parameters of the emulsion, such as the emulsion concentration, the chemical nature of the components and the emulsion drop size distribution, are varied to rationalize their influence on the perforation mechanisms.

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