Recent years see increasing studies of air entrapment during droplet impacting on a solid surface with many results. The dynamics of trapped air film during a droplet impact on a solid surface is investigated in this work by the phase field method in combination with a dynamic contact angle (DCA) model. The DCA model is established experimentally by capturing the droplet dynamics in analogy to the entrapped air evolution. By using the DCA model as the input, the simulation can accurately reproduce the experimental results. The effects of droplet viscosity and surface tension on the dynamics of the air film are then studied, and three possible regimes are identified, demarcated by an effective Ohnesorge number (Ohe). Regime 1 is the case where no daughter droplet is generated and the air bubble is always attached to the substrate, corresponding to the classical case at a high Ohe number (Ohe > 0.073). Regime 3 is a newly discovered regime in this work where a daughter droplet is generated and the air bubble is always detached from the substrate, corresponding to a low Ohe number (Ohe < 0.019) due to combined strong surface tension and vortex effects. Regime 2 is for moderate Ohe numbers where a daughter droplet is generated and the air bubble can either detach from or attach to the substrate. Different from conventional thought that the detachment in this regime is decided by a static contact angle, the DCA plays a leading role in determining the volume ratio of the daughter droplet to the gas bubble, and the combined effects determine the fate of the bubble. Such finding provides better insight on the entrapped air dynamics upon droplet impacting on a solid surface, an area of high engineering importance.

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