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Before a droplet can contact a surface during impact, it must first displacethe air beneath it. Over a wide range of impact velocities, the air iscompressed into a thin film, slowing the progress of the liquid toward thesurface. Below a critical impact velocity, the air film remains intact; thedroplet rebounds off of the air film without making contact. Above thiscritical impact velocity, the droplet always makes contact. The initiation ofcontact formation occurs via a topological transition, whereby the initiallyconnected gas domain is ruptured, and a liquid capillary bridge forms, bindingthe droplet to the surface. Here we probe this transition in detail around thecritical impact velocity using calibrated total internal reflection (TIR)microscopy to monitor the air film profile at high speed during the impactprocess. Two air film rupture modalities are observed: nucleated contacts,which are isolated and don't correspond to the global minimum air filmthickness, and spontaneous contacts, that occur always on a ring centered uponthe impact axis where the air film reaches its global minimum. Our measurementsshow that for impact velocities exceeding the critical velocity for contactinitiation, 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 timeduration of the air film prior to contact, analyzed for over 180 droplet impactexperiments, are consistent with a linear instability driven by van der Waalsforces. Impact events of water solution droplets show different statistics forcontact nucleation than the silicon oil; nevertheless, a critical impactvelocity above which contact always occurs is identified for both liquids.

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