Résumé
Pesticide spraying in agriculture involves atomizing a liquid stream through a hydraulic nozzle. The spray droplets results from the destabilization of a liquid sheet formed by the nozzle. Standard pesticide solution adjuvants as dilute solution of long polymer chains or dilute emulsions are known to influence the spray drop size distribution. Although being documented, these effects are not understood yet. In order to elucidate the physical mechanisms at the origin of the change on the drop size distribution, we investigate the influence of different complex fluids on the destabilization mechanisms of liquid sheets. We here form liquid sheets by the collision of a liquid drop on a small solid target. Upon impact, the drop flattens into a radial sheet expanding in the air bounded by a thicker rim. Different destabilization mechanisms of the sheet are observed depending on the fluid nature. A pure water sheet spreads out radially until it reaches a maximum diameter and then retracts due to the effect of surface tension. The destabilization mechanism is drastically modified when a dilute oil in water emulsion is used. The liquid sheet spreads out radially but holes perforate the sheet before the retraction, as already observed for some surfactant solutions [1]. The holes grow until they merge together and form a web of ligaments, which are then destabilized into droplets. To investigate the different sheet destabilization mechanisms, we use a fast camera imaging coupled to an original technique recently developed to access the time and space-resolved sheet thickness. Physico-chemical parameters of the dilute emulsion are modified to rationalize their influence on the perforation mechanism as for instance the influence of the emulsion concentration. We show in particular that the number of hole nucleation events per liquid sheet is directly governed by the emulsion concentration. In striking concordance, we find that the emulsion concentration directly controls the drop size distributions of a spray, as measured with a diffraction-based size analyzer, suggesting that experiments on liquid sheets are appropriate model experiments to gain an understanding on the physical mechanisms governing the spray drop size distribution. [1] A. Rozhkov, B. Prunet-Foch, M. Vignes-Adler, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 466, 2897-2916 (2010).