Humidity accelerates contact of solvent drops on mica substrates

Before a droplet can contact a solid surface, it must displace the surrounding air. For solvent droplets we find that liquid-solid contact is accelerated in humid air. The more rapid contact time is a direct consequence of an interfacial instability beneath the droplet; the humid air causes an elevated water condensation rate on the solvent surface, ultimately resulting in a solutal Marangoni instability. We characterize the growth rate and timescale of this instability, and explore its consequences when air pressure is reduced.