Dynamics of the cavity and the surface film for impingements of single drops on liquid films of various thicknesses.

This paper presents experimental and numerical investigations of single drop impacts onto liquid films of finite thickness. The dynamics of the drop impingement on liquid surface films, the shape of the cavity, the surface film dynamics and the residual film thickness are investigated and analysed. The shape of the penetrating cavity within the surface film is observed experimentally using a high-speed video system. Additionally, the thickness of the liquid film between the expanding, receding and retracting cavity and the solid wall is monitored in time using an optical sensor based on chromatic confocal imaging. The effects of various influencing parameters, such as the drop impingement velocity, liquid properties (surface tension and viscosity) and the initial liquid film thickness, on the time evolution of the cavity and film dynamics are investigated. Complementary to the experiments direct numerical simulations of the drop impacts and cavity expansion are performed using a volume-of-fluid free-surface capturing model in the framework of the finite volume numerical method. The numerical predictions of the film thickness dynamics agree well with the experiments for most phases of the impingement process. Finally, a scaling analysis of the residual film thickness between the cavity and the solid wall is performed for various impingement parameters.