Characteristic rupture height of the mediating air film beneath an impacting drop on atomically smooth mica

Before a droplet can contact a surface during impact, it must first displace the air beneath it. Over a wide range of impact velocities, the droplet first squeezes the air into a thin film, enhancing its resistance to drainage; this slows the progress of the liquid toward the surface. Indeed, below a critical impact velocity, the air film remains intact, and the droplet rebounds off of the air film without making contact. For impact velocities exceeding this critical impact velocity, the droplet always makes contact. The initiation of contact formation requires a topological transition, whereby the initially connected gas domain is ruptured and a liquid capillary bridge forms, binding the droplet to the surface. Here we probe this transition in detail around the critical impact velocity using calibrated total internal reflection microscopy to monitor the air film thickness and profile at high speed during the impact process. Two air film rupture modalities are observed: nucleated contacts, which are isolated and do not correspond to the global minimum air film thickness, and spontaneous contacts, which occur always on a ring centered upon the impact axis where the air film reaches its global minimum. Our measurements show that for impact velocities exceeding the critical velocity for contact initiation, the air film ruptures at a nearly identical height hmin approximate to 20 nm, for two fluids: silicone oil and a water-glycerol mixture. The height and time duration of the air film prior to contact are presented for over 180 droplet impact experiments. Impact events of water solution droplets show statistics for contact nucleation different from those for the silicone oil; this suggests that another mechanism may dominate contact nucleation during impact of the solution. Nevertheless, a critical impact velocity above which contact always occurs is identifiable for both liquids.