How a droplet wets a surfrace: probing the stability limit of the intervening air

Before a droplet can contact a solid surface, it must displace the intervening air. Normally, this process is cut short by contact nucleation on surface defects - primarily because the lubrication stresses in the air film are feeble, and will only arrest the droplet within 1 micron of the surface; at this scale, defects are abundant on a typical solid, and provide ample opportunity for a capillary bridge to nucleate and bind the drop to the surface irreversibly. When such defects or nucleation sites are absent, however, the dynamics can be radically different - the air film will remain stable and can even cause the droplet to rebound below a critical impact velocity. Above the critical impact velocity, however, the droplet will rupture the air film, which is destabilized by the interfacial attraction of the liquid to the solid substrate. Here we probe the limit of the stability of the air film using a variety of different impact conditions. Surprisingly, impact velocity is not the only essential control parameter; the composition of the surrounding air, the pressure of the surrounding air, and the electro-static charge on the substrate can all profoundly alter the stability of the air film. Using rapid 3D microscopy, we characterize the stability landscape of the air film, and probe how various mechanisms affect the impact outcome.