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Submillimeter-Sized Bubble Entrapment and a High-Speed Jet Emission during Droplet Impact on Solid Surfaces.

When a droplet impacts a solid surface, the entrapment of a submillimeter-sized bubble and the emission of a high speed jet can be observed at low impact velocities. In this work, we show that bubble entrapment occurs only on sufficiently hydrophobic surfaces within a narrow range of impact velocities. The bubble is entrapped on hydrophobic surfaces, whereas it is trapped into the top of the droplet on superhydrophobic surfaces. The collapse of the air cavity formed during droplet impact, which is dominated by inertia and influenced by surface wettability, is the cause of the bubble entrapment. The velocity of liquid jets emitted after cavity collapse for drop impact with and without bubble entrapment scales with their sizes according to different power laws, which is explained by simple scaling analyses.