Spreading, fingering, and deposition in volatile liquid mixtures

When a volatile liquid drop is placed on a wetting surface, it rapidly spreads and forms a circular, thin film before evaporating. Surprisingly, when even trace amounts (~0.05%) of a less volatile liquid is present in the volatile liquid, the contact line destabilizes, leading to the formation of structures resembling “fingers” or “pearls”. We have characterized this phenomena using isopropanol with various contaminants (ethylene glycol, glycerol, dodecane, water, and acetone) on surfaces of varying wettability. Although pearls always form due to enhanced evaporation at the contact line, we find that fingers only form when the contaminant has a higher surface tension than the isopropanol, and partially wets the surface. The characteristic size of the structures increases with contaminant concentration. In addition, we find that the local vapor pressure of the isopropanol strongly affects the wetting characteristics of the contaminant. Once the isopropanol evaporates, striking droplet patterns of the contaminant are left behind. Some resemble crystalline lattices of various drop sizes, or the contaminant liquid may form a sub-micron thin film. These highlight the role of trace impurities in volatile liquids, and the deposition patterns they leave behind.