Phenomenological study and comparison of droplet impact dynamics on a dry surface, thin liquid film, liquid film and shallow pool

In the process of droplet impact on a dry surface or a liquid layer, several intriguing phenomena may occur. Due to the lack of comparative studies, this paper aims at phenomenological observation and analysis of droplet impact on a dry surface, thin liquid films, liquid films and shallow pools in the range of h* from 0 to 1.768, with Weber (We) number from 125 to 437, h* being the ratio of the liquid thickness to droplet diameter. For this purpose, a dyed water droplet was released on the substrate and a color high speed camera captured the details of the process including the interaction of the two like liquids, but with different colors. The times of reaching the maximum spreading diameters were compared for different choices of h* and We number. In addition, wave formation on the liquid film and the mixing of the like and miscible liquids were studied, briefly. A range of phenomena were observed at varying We number and h*. On a dry surface, at low We numbers, spreading occurred in the deposition mode, and the lamella at maximum spreading was pinned to the surface, whereas depinning and receding was observed at high We numbers. When the dry surface was replaced with a liquid layer, a range of impact phenomena emerged, for the same We numbers. On a liquid layer, depending on the We number and h*, crown formation and splash, surface waves, and the formation and breakup of the central Worthington jet were observed and discussed. Our experimental observations resulted in identifying three distinct regimes of thin liquid film (TLF), liquid film (LF) and shallow pool (SP), where in each regime similar impact behavior is observed, different from those observed in other regimes. Among the results, we observed that, in the range of parameters studied here, spreading is the fastest in the TLF regime, faster than spreading on a dry surface, and it generally slows down with an increase in the liquid film thickness.