Bouncing to merging transition in drop impact on liquid film: Role of viscosity

Fuels sprayed inside IC-engines can impact the internal surfaces and thus obviates the purpose of atomization. Furthermore, the subsequent film gasification also constitutes a completely different mode of fuel gasification that could fundamentally affect the fuel/air mixing and combustion processes. The accumulation and growth of the film depends on the outcome of subsequent drop impact on the initially formed film, and as such controlling the outcome of the impact is critical for engine operation. In our recent study (Tang, et al. Soft Matter 2016), we presented a regime diagram based on the Weber number We (ratio of impact inertia and surface tension) and the film thickness, delineating the bouncing and merging operating conditions and providing scaling for the transition boundaries of a single liquid (C14 alkane). Since liquid viscosity is a critical fluid property that fundamentally affects the impact outcome, through its influence on the fluid motion and viscous dissipation of the impact inertia, here we extend the study for liquids from C10 to C17 alkanes, covering a wide range of viscosity, to assess its effect on the regime diagram. Results show that while the regime diagram maintains its general structure, the bouncing regime becomes smaller for less viscous liquids. Consequently viscous effects are modeled and a modified scaling is proposed. This new scaling attempts to unify all liquids and provide a useful tool to manipulate the outcome of drop impact on liquid film.