Review of mass and momentum interactions during drop impact on a liquid film

Research on liquid drop impact, especially during the past two decades, has been motivated by a need for better predictive capability in many industries. This paper will review published works concerning mass and momentum interactions during drop impact on a liquid film. First, both experimental and numerical methods for capturing the evolution of the impact will be highlighted. This will be followed by a detailed description of the impact, including formations of the ejecta sheet, crown sheet, and splashing of secondary droplets during high-velocity impact. Other topics reviewed are impact on curved wetted surfaces, multi-drop impact, and the phenomena of spreading, coalescence and rebound in low-velocity impact. Each of these phenomena is discussed in terms of underlying physical mechanisms and predictive correlations and/or models. Despite significant past efforts to understand and characterize these phenomena, it is shown that much uncertainty remains, especially in regards to the interfacial features around the drop-film neck region during the earliest stages of the impact. Recent state-of-art advances in both experimental and numerical methods are shown to play a crucial enabling role in future research. The review is concluded with recommendations concerning future work that is needed to address poorly understood and/or contradictory issues.