Fragmentation of a drop impacting near the edge of a solid substrate: a key to rain-induced foliar disease transmission

Plant diseases are still responsible for significant losses in crops despite the use of chemicals and genetically modified organisms. Moreover, these mitigation techniques present drawbacks in terms of cost and sustainability and their intensive use tends to reduce their effectiveness. There is thus a global call for alternative methods of crop protection and for the development of risk predicting models and decision aid tools to minimize the use of these techniques. One of the most promising of these global models rely on the physics behind the spreading of diseases. For example, the Septoria Leaf Blotch that affects wheat, is mainly dispersed through rainsplash. Modelling the rain dispersal of such disease requires information concerning the pathosystem, the raindrop size and speed distributions but also about the plant characteristics to only cite a few. The disease is known to propagate from one plant to the next when a raindrop hits a leaf on which contaminated fluid sits. The splash of contaminated droplets carries the pathogens to the next plant. The physics of such impacts remains poorly understood, both in terms of drop-leaf interactions and drop fragmentation upon impact. Such understanding could however improve the robustness and accuracy of the risk predicting models. The aim of this experimental work was thus to investigate the fragmentation physics of drop impacts. The crescent moon is a particular impact configuration that was recently shown to be both frequent and efficient at ejecting droplets. It consists in the impact of a drop in the vicinity of a contaminated sessile drop. Upon impact the drop spreads on the substrate and pushes the sessile drop, which is stretched and develops into an asymmetric liquid sheet in the air that subsequently fragments into droplets. This phenomenon being fully three-dimensional, its investigation is challenging. We propose to study a simpler configuration that shares most of the features of the crescent moon: a single drop impacting close to the straight edge of a flat dry substrate. Upon impact, the drop spreads on the solid. The liquid, then, takes off at the edge and forms an asymmetric liquid sheet that presents similar characteristics to the three-dimensional crescent moon. However, the sheet now remains in the plane of the substrate, which facilitates its quantitative analysis. Three parameters are systematically varied: (i) the impact speed, which modifies the kinetic energy of the impacting drop and thus its tendency to fragment into droplets, (ii) the offset, i.e., the distance between the impact point and the edge of the substrate, which modifies the asymmetry of the liquid sheet and the subsequent ejection pattern, and (iii) the inclination of the substrate and the corresponding addition of a tangential speed upon impact. We quantified the liquid sheet kinematics by first looking at the spreading liquid on the solid. The maximum distance spread and the time to reach the edge are measured, as they represent the pre-history of the liquid sheet. The evolution of the sheet in the air is then quantified in directions normal and tangential to the edge. In particular, its maximum extension in both directions and corresponding time, are measured and scaling laws derived for