Experimental and numerical studies of droplet impact on flowing liquid films

Droplet impact on flowing liquid films constitutes an important research area due to its manifold applications both in industry and day-to-day living. Previous studies have, however, ignored the contributions of stochastic waves to the drop impact dynamics. In this project, an experimental study of droplet impact on controlled flowing liquid films is carried out. The aim of the study is to provide an understanding of the contributions of the spatial structures of waves to drop impact dynamics on flowing films. The experimental facility consists of a falling film rig which comprises film flow, film control, and droplet-generation units, as well as a high-speed imaging system. In a preliminary study, the effect of film control on the dominant wave propagation modes is investigated. Two classes of waves are identified, namely the gamma I and II wave families, which are characterised both qualitatively and quantitatively and confirmed to be in good agreement with previous studies in the literature. Studies on the interaction patterns between doubly-excited planar pulse waves on an uncontrolled flowing film surface are then carried out to provide insight into the interaction patterns of waveforms on flowing liquid films. Distinct interaction modes are found to be of central importance to understanding the complex wave interactions which could lead to interfacial ‘turbulence’. The effect of film control on the impact dynamics of both low and high-inertia drops is then studied. In both studies, the impact surface is divided into the “wave hump”, “flat film”, and “capillary waves” regions. For low-inertia drop impacts, film control is observed to have a qualitative and quantitative effect on the length of liquid columns formed in a partial coalescence outcome, the pinch-off time, as well as the size of the ejected daughter droplets. Qualitative differences included a complete change of the outcome, with other outcomes such as ‘bouncing’, ‘sliding’, and ‘total coalescence’ observed in the low-inertia drop impact scenario. For high-inertia drop impacts, the effect of film control is studied in the morphology of the crown produced in a splash outcome as well as the distinctive attributes of the ejected droplets. Significant quantitative differences are observed in the features of the crown such as its structure, diameter, height, wall thickness, facing-direction, coalescing time, and baseline propagation modes, as well as the number and size distribution of the ejected secondary droplets. Finally, numerical studies of the flow situations investigated experimentally are also carried out using two novel codes, one using interface-capturing and adaptive, unstructured meshes, the other employing a hybrid interface-tracking/level-set technique on structured meshes. The numerical results obtained show very good agreement with the experimental studies both qualitatively and quantitatively.Open Acces