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Résumé:

Agricultural spraying involves atomizing a liquid stream through a hydraulic nozzle, thus forming a liquid sheet that is subsequently destabilized into drops. Standard adjuvants as dilute oil-in-water emulsions are known to influence the spray drop size distribution: the fraction of small drops responsible of spray drift is reduced. Although being documented, the physical mechanisms at the origin of the size increase of the drops remain unclear. To elucidate the mechanisms causing the changes on the drop size distribution, we investigate the influence of dilute emulsions on the destabilization mechanisms of liquid sheets. Model laboratory experiments based on the collision of a liquid tear on a small solid target are used to produce and visualize liquid sheets. With dilute oil-in-water emulsions, the liquid sheet is destabilized by the nucleation of holes in the sheet that perforate it during its expansion. The emulsion concentration and the size of the oil droplet of the emulsion are varied to rationalize their influence on the sheet destabilization mechanisms. The results obtained with the model laboratory experiments are compared to the measurement of the drop size distribution resulting from the destabilization of a conventional agricultural spray. The very good correlation between the number of perforation events and the volume fraction of small drops in the spray suggests (i) that the model experiment on liquid sheet is appropriate to investigate and gain an understanding of the physical mechanisms governing the spray drop size distribution and (ii) that the perforation destabilization mechanism of liquid sheets, which dominate for dilute emulsions, is at the origin of the increase of size of the spray. Finally the analysis of the of the kinetics of formation and growth of the holes allows to identify the physical origin of the perforation.