A study on the impact velocity and drop size for the occurrence of entrapped air bubbles – Water on parafilm

The roles that the impact velocity (Vi) and the drop size (D0) play for water drops impinging on parafilm surfaces were systematically investigated for systems of entrapped air bubbles. Entrapped bubbles were successfully observed by using a high-speed video camera system from a 45-degree top view. A half-closed region on the Vi–D0 plot was identified for the formation of the entrapped bubble. The upper and lower boundaries of Vi tended to decrease as D0 increased. The entrapped bubble generally took the shape of a hemispherical bubble after settling down on the parafilm surface. Bubble sizes became smaller when Vi approached the upper and lower limits. A monotonically increasing correlation between Re and We at a fixed drop size has been reported in the literature. This monotonic relationship was shown to be dependent on the drop size. Moreover, two main different mechanisms for the formation of entrapped air bubbles were proposed for water on parafilm systems.

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► The formation of entrapped bubbles for impinging droplets is systematically studied.
► The roles of impact velocity and droplet size are investigated.
► The kinetic energy range for the formation of entrapped bubbles is identified.
► Two different mechanisms for the formation of entrapped bubble are reported.