Droplets: drags, coalescence and impact

Droplets spawn and perish continuously. The splashing of rain or the generation of aerosols during the breaking of waves are examples of drop formation in nature. In industry, droplets serve the need for controlled delivery of material. In agriculture, e.g. in pesticides crop spraying, drop impact and the subsequent splashing and droplet rebound is important. Drop impact occurs in e.g. metal spray deposition and direct fuel injection internal combustion engines. In many of these applications, splashing is an unwanted side effect after impact; it decreases deposition efficiency and may lead to widespread contamination. \ud We show that the superposition of two Rayleigh-Plateau-unstable modes is an efficient and robust method to generate a periodic stream of droplets, that allows for control of the droplet sizes and distances. The method is efficient because it uses the available surface energy of the continuous jet to induce the coalescence. The method is robust because the wave numbers of the employed Rayleigh-Plateau-unstable modes are close to the fastest growing mode. By tuning the phase difference between the two modes, the coalescence pattern is controlled. \ud We show experimental, numerical and modeling results of perpendicular droplet impact on a deep pool. We have extended the work of Oguz and Prosperetti to the regime beyond the terminal velocity under ambient conditions for droplets smaller than a millimeter in size. The dynamics of the cavity are well described by a 1D Rayleigh-Plesset model including surface tension of the cavity. The cavity is closed by a capillary wave which has a velocity set only by the droplet size. Next to the observed regular bubble entrainment, there also exists a region in which the second to last wave entrains a bubble, this is also confirmed experimentally. \ud We present an experimental study of oblique drop impact onto a quiescent deep liquid pool. We performed quantitative experiments where oblique drops impact onto a deep liquid pool for a wide range of We and impact angles α and analyzed the splashing behavior, the cavity formation and the cavity collapse