Abstract
Optimizing phytosanitary spraying on field crops is a major environmental and health issue. It is in this context that, in partnership with Compagnie Fruitière, a leading European banana producer, we are studying the physico-chemical interaction mechanisms of model phytosanitary formulations with banana leaves. These formulations have been developed to combat Black Cercosporiasis of banana, one of the most serious diseases affecting banana production, causing major losses in yield and fruit quality. After contact with a banana leaf, we compare the impact, spreading and impregnating properties of a mineral oil alone, of direct emulsions of this oil currently used by our partner, and of original inverse emulsions of this same oil.First, we characterize the stability and droplet size distribution of the dispersed phase, as well as the rheological properties of the different emulsions.The impact of individual drops of the different formulations on a banana leaf is visualized using a high-speed camera. Analysis of the impact dynamics reveals that, at short times (<10ms), drops of direct emulsion exhibit a splashing phenomenon that generates the emission of secondary droplets from the initial drop after impact on the leaf, then retract; in contrast, reverse emulsions spread out without destabilizing or retracting, in a manner quite similar to pure oil drops. The absence of splashing could be an advantage of reverse emulsions over direct emulsions, both in terms of reducing disease dispersal and the phytosanitary risk of soil pollution.The evolution of the spread of a controlled volume of sprays of these different formulations on the banana leaf is quantified. In all cases, an anisotropic spread was observed, favored in the direction of the leaf's secondary grooves. At long times (t<35min)) all formulations exhibit a slow evolution of the spray coverage rate following a power law ta, α=0.27±0.05 compatible with Tanner's law for the spreading of an individual drop. However, the spreading of direct emulsion droplet sprays is preceded by a latent period (t<2min) when the coverage rate does not evolve.Oil and emulsion impregnation is studied by infrared micro-spectroscopy. Following the deposition of a thick film of oil on the surface, infrared mapping shows that the oil accumulates mainly in the leaf parenchyma. Impregnation dynamics studies suggest Fick diffusion of the oil preceded by an effective diffusion coefficient D = (1.2±0.8) ×10-12 m2/s, preceded by a lag time of around 3.6 hours. Under the same experimental conditions, for inverse emulsions, oil impregnation is observed, but absent in the case of direct emulsion. This result suggests that the oil/sheet interface plays a major role in the impregnation process. Spray impregnation of emulsions is also quantified, confirming a preferred impregnation mechanism for reverse emulsion.