The interaction of droplets with plane liquid surfaces

Kurzfassung

The impact of droplets on surfaces comprises a rich variety of fluid mechanical phenomena. Many processes of droplet impact are dominated by the kinetic and surface energy of the approaching droplets. However, the compressibility of the liquid, multiphase flows and capillarity can also be of great importance. The significance of these aspects to the various results of droplet impacts will be discussed, concentrating on the interaction of droplets with plane liquid surfaces. The topics addressed are related both to problems of practical importance such as erosion and planetary impacts, and to problems of fundamental character in free surface flows and capillarity. The initial phase of high-speed drop impacts is governed by the compressibility of the liquid. On impact shock waves are formed in the liquid increasing the pressure inside the drop. Here, the collision of a drop with a liquid surface is considered. Using a classical approach of gas dynamics the geometry of the colliding surfaces is taken into proper account. It is shown that a strong oblique shock will be formed whose occurrence is forced by the flow conditions ahead of the shocks.Finally, the high pressures within the drop are relieved by expansion waves. Here, the particular geometry of the drop can lead to the formation of tensile stresses causing the inception of cavitation. Another aspect of droplet impact is free surface flows. At small impact velocities droplets impinging on the surface of a pool provide an example of a free surface flow in which inertial, gravitational, viscous and surface tension forces are all important. Depending on the particular circumstances different features such as splashing, entrainment of air and coalescence of drops with the pool can be observed. We will report on the results of an experimental study of the transitional regime between coalescence and splashing which separates two main impact scenarios. Furthermore, the importance of capillary effects to the coalescence of drops will be addressed. The results are finally discussed in the light of vortex ring formation which is characteristic of drops coalescing with a free surface.