Hypothesis

The addition of a thin, hydrophilic, porous, coating to an impermeable solid will lead to more rapid evaporation of liquid droplets that impinge on the solid. The droplet will imbibe quickly, but the progress normal to the interface will be limited to the thickness of the coating, and therefore the liquid will spread laterally into a broad disk to expose a large liquid–vapor interface for evaporation.

Experiments

Liquid droplets of volume 2.5–25 µL were placed on solids and then both the mass and area of each droplet were monitored over time. We compared data for smooth, impermeable hydrophilic glass to the same glass that was coated in thin (35–109 µm) porous, hydrophilic-glass layer fabricated from glass beads.

Findings

The droplet was imbibed (wicked) into the coating within seconds, and the liquid spread laterally to form a thin, broad, disk. Critically, evaporation of a droplet was enhanced by a factor of 7–8 on the thin coating. The evaporation rate was not proportional to the reciprocal thickness of the coating. The ability to enhance evaporation of small droplets on a solid may have practical applications, for example, in speeding the death of microbes.