Double Contact During Drop Impact on a Solid Under Reduced Air Pressure

Drops impacting on solid surfaces entrap small bubbles under their centers, owing to the lubrication pressure which builds up in the thin intervening air-layer. We use ultra-high-speed interference imaging, at 5 Mfps, to investigate how this air-layer changes when the ambient air-pressure is reduced below atmospheric. Both the radius and the thickness of the air-disc becomes smaller with reduced air pressure. Furthermore, wefind the radial extent of the air-disc bifurcates, when the compressibility parameter exceeds 25. This bifurcation is also imprinted onto some of the impacts, as a double contact. In addition to the central air-disc inside the first ring contact, this is immediately followed by a second ring contact, which entraps an outer toroidal strip of air, which contracts into a ring of bubbles. We find this occurs in a regime where Navier slip, due to rarefied gas effects, enhances the rate gas can escape from the path of the droplet.