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 ultrahigh-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 become smaller with reduced air pressure. Furthermore, we find 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.