Droplet impact hydrodynamics on curved surfaces

Abstract

Droplet impact, dynamics, wetting, and spreading behavior on solid surfaces impose rich and interesting physics, in addition to extensive understanding of processed employing droplets and sprays. The physics and mechanisms become more interesting and insightful when the geometry and wettability of the surface provide additional constraints to the fluid dynamics. Post-impingement morphology and dynamics of water droplets on various curved surfaces, having dimensions comparable to that of the droplet, have been explored in the thesis. Top and side views of the impaction phenomenon have been captured using the high-speed imaging technique. The surface concavity or convexity, target-to-droplet size ratio, surface wettability and impact Weber number are systematically varied in order to note interesting outcomes. The focus of the thesis is the quantitative determination of the influence of these parameters on the post-impact spreading of the liquid on the target surface and the phenomenological description of their outcomes. The post-collision hydrodynamics have been quantified along the azimuthal and axial direction, employing various variables, namely, the spreading factor, the wetting fraction, non-dimensional film thickness at the pole and axial jetting velocity. The observations indicate that the spreading factor and the wetting fraction increase but film thickness at the pole decrease with increasing impact Weber number and increasing target convexity. Whereas opposite variations are found true for the increasing target concavity. The observations also revel the occurrence of axial jetting hydrodynamic phenomenon in concave surfaces unlike on convex surfaces. This is because gravity force assists the extension of impacted droplet in the azimuthal direction for increasing convexity of the target whereas opposes for increasing concavity of the target. Further, analytical expressions for maximum wetting fraction, maximum spread angle, temporal evolution of liquid film thickness and jet velocity have been produced on different target geometries. The findings of the thesis may be applied in cooling, coating, spray painting and wetting of intricate structures and complexly designed engineering components.