The critical approach height for liquid-solid contact initiation beneath an impacting droplet

Before an impacting droplet can contact a solid surface, it must displace the air beneath it. As the droplet approaches within microns of the surface, the air fails to drain, and instead compresses. Due to the low viscosity of the air, this process occurs in the final microns before a mm-scale droplet should impact the surface; for a typical impact speed of 1 m/sec, this implies a micro-second timescale for air pressurization. To optically observe such diminutive length and fleeting time scales requires precise microscopy methods with sensitivity below the wavelength of light; frustrated Total Internal Reflection (TIR) microscopy and interferometry are two candidate methods that have been employed to this end. In this work, we present calibrated measurements of the air-liquid profile beneath an impacting droplet impinging upon an atomically smooth mica surface using TIR. We find that the droplet will not wet the surface spontaneously unless it reaches a critical distance from the surface of approximately 20-30 nm, +/- 7 nm. We analyze the air film with linear stability analysis for viscous dewetting of the air layer, and find that the Hamaker constant falls within a range of values for strongly interacting surfaces, consistent with the strongly wetting nature of the liquid on the solid surface.