Bouncing drops over liquid surfaces

Droplet impact occurs frequently in nature and is used in many industrial and technological applications, such as the impact of raindrops on the surface of a pond, the spraying of liquid coatings on wet surfaces, and inkjet printing. Depending on the material properties of the impacting droplet and the underlying substrate, the impact can result in bouncing, floating, splashing or spraying. This thesis investigates a particularly interesting impact scenario, namely floating or bouncing droplets, where a liquid droplet impacts a liquid film at a sufficiently low velocity, resulting in the droplet never coming into direct contact with the underlying film throughout the impact process. The droplet-film impact dynamics is studied when the control parameters, namely the material properties of the droplet: radius, viscosity, impact velocity and of the film: thickness, viscosity are varied. The work is mainly experimental and uses digital holographic microscopy and shadowgraphy as visualization techniques, complemented by numerical simulations and some simple phenomenological models. The results in this thesis shed light on the spatio-temporal description of the thin film deformations after impact, the criteria for the bouncing or floating of droplets on thick films, and the influence of gravity on droplets of different sizes subjected to impact.