ABSTRACT
Increasing societal and governmental concern about the worldwide use of chemical pesticides is now providing strong drivers towards maximising the efficiency of pesticide utilisation and the development of alternative control techniques. There is growing recognition that the ultimate goal of achieving efficient and sustainable pesticide usage will require greater understanding of the fluid mechanical mechanisms governing the delivery to, and spreading of, pesticide droplets on target surfaces such as leaves. This has led to increasing use of computational fluid dynamics (CFD) as an important component of efficient process design with regard to pesticide delivery to the leaf surface. This perspective highlights recent advances in CFD methods for droplet spreading and film flows, which have the potential to provide accurate, predictive models for pesticide flow on leaf surfaces, and which can take account of each of the key influences of surface topography and chemistry, initial spray deposition conditions, evaporation and multiple droplet spreading interactions. The mathematical framework of these CFD methods is described briefly, and a series of new flow simulation results relevant to pesticide flows over foliage is provided. The potential benefits of employing CFD for practical process design are also discussed briefly.
Subject(s)
Models, Chemical , Pesticides/chemistry , Plant LeavesABSTRACT
The coalescence of a pair of droplets on a surface is investigated experimentally with images from detailed flow visualisations revealing the morphology of the process. It is found that they merge and evolve to a final state with a footprint that is peanut like in shape, with bulges along the longer sides resulting from the effects of inertia during spreading. The associated dynamics involve a subtle interplay between (i) the motion of the wetting process due to relaxation of the contact angle and (ii) a rapid rise in free-surface height above the point where coalescence began due to negative pressure generated by curvature. During the early stages of the motion, a traveling wave propagates from the point of initial contact up the side of each droplet as liquid is drawn into the neck region, and only when it reaches the apex of each do their heights start to decrease. A further feature of the rapid rise in height of the neck region is that the free surface there overshoots significantly its final equilibrium position; it reaches a height greater than that of the starting droplets, producing a self-excited oscillation that persists long after the system reaches its final morphological state in relation to its footprint.
