Wave Phenomena During Droplet Impact

Wave phenomena that govern different processes during droplet impact are introduced and discussed. These include surface waves, in particular capillary waves, that are important for the spreading motion of droplets impinging onto solid walls. The rim of the advancing thin liquid film is subject to an instability that causes the formation of well-known lobes (“ink blob”). Surface waves on a liquid surface are observed to enhance floating and bouncing of droplets impinging on such surfaces. Kinematic effects play an important role for splashing. In the case of high-speed impacts the compressibility of the liquid is no longer negligible. On impact shock waves are formed that increase the pressure. Expansion waves later reduce the pressure. Under certain circumstances tension waves occur that cause the formation of cavitation zones. Finally, the collision of meteoroids with planetary surfaces can be considered as a droplet impact. During planetary impacts conditions may be such that evaporation waves are formed. These issues are all discussed.