Thin film instability driven dimple mode of air film failure during drop impact on smooth surfaces

Abstract

Air film stability underneath a drop is crucial for drop contact dynamics upon impact. An unstable film leads to the drop contacting the surface and subsequent spreading or splashing. Apart from previously reported film and kink contact modes, here we present the experimental evidence for a dimple failure mode of an air film, driven by a thin film instability when a drop impacts onto an atomically smooth surface. The dimple failure occurs beyond the inertial-capillary timescale and is initiated when the dimple inverts at the drop's central axis. For the same impact Weber number, the dimple failure observed in low-viscosity drops is absent at a higher viscosity, due to damping of capillary waves.