Résumé
Agricultural spraying involves atomizing a liquid stream through a hydraulic nozzle forming a liquid sheet, which is subsequently destabilized into drops. Standard solution adjuvants as dilute oil-in-water emulsions are known to influence the spray drop size distribution. We will present model laboratory experiments that aim to elucidate the physical mechanisms causing the changes of drop size distribution. Model laboratory experiments based on the collision of a liquid drop 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 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. Thanks to an original technique that we recently developed to access the time and space-resolved thickness of the sheet, we measure that the formation of a hole within the sheet is preceded by a localized thinning of the liquid film. We show that this thinning results from the entry and spreading of emulsion oil droplets at the air/water interface. The oil droplet spreading, due to Marangoni driven surface tension gradient, drags subsurface fluid with it. This subsurface flow causes a local film thinning which can ultimately rupture the film. Quantitative analysis of the spreading dynamics unambiguous confirms the physical mechanism at the origin of our observations.