  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 air is
compressed into a thin film, slowing the progress of the liquid toward the
surface. Below a critical impact velocity, the air film remains intact; the
droplet rebounds off of the air film without making contact. Above this
critical impact velocity, the droplet always makes contact. The initiation of
contact formation occurs via 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 (TIR)
microscopy to monitor the air film profile at high speed during the impact
process. Two air film rupture modalities are observed: nucleated contacts,
which are isolated and don't correspond to the global minimum air film
thickness, and spontaneous contacts, that 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 h min ~ 20 nm,
for two fluids - silicone oil and a water glycerol mixture. The height and time
duration of the air film prior to contact, analyzed for over 180 droplet impact
experiments, are consistent with a linear instability driven by van der Waals
forces. Impact events of water solution droplets show different statistics for
contact nucleation than the silicon oil; nevertheless, a critical impact
velocity above which contact always occurs is identified for both liquids.
