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 $\sim 25$. 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.

中文翻译：

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在降低的气压下对固体的液滴撞击过程中的双重接触

由于在薄薄的中间空气层中形成的润滑压力，撞击在固体表面上的水滴将小气泡夹在其中心下方。我们使用5 Mfps的超高速干涉成像来研究当环境气压降低到大气压以下时，空气层如何变化。随着气压的降低，空气盘的半径和厚度都变小。此外，当压缩参数超过$〜25$。这种分叉也以双重接触的形式被印在某些撞击物上。除了在第一环形触头内的中央空气盘外，第二环形触头紧随其后，第二环形触头捕获外部环形空气带，该环形空气带收缩成气泡环。我们发现，这种情况发生在由于稀薄气体效应导致的Navier滑移会提高气体从液滴路径逸出的速率的情况下。