Droplet Shape and Wetting Behavior under the Influence of Cyclically Changing Humidity

Relative humidity (RH) plays a crucial role in wetting and spreading phenomena by affecting the evaporation rate, evaporation modes, and spreading dynamics via precursor film formation, surface modification, and surface tension alteration. We examined the effect of the periodically varied relative humidity (RH) between low (20%) and high (85%) levels on the wetting of the droplet of nonhygroscopic (pure surfactants) and hygroscopic (ethylene glycol, glycerol) liquids on a hydrophobic surface. It was revealed that the changing RH induces two modes of transition between the wetting states of the droplet: with hysteresis and without hysteresis. Droplets of both nonhygroscopic and hygroscopic liquids exhibit shape hysteresis during the first cycle: (i) droplets of surfactants irreversibly spread saving an initial volume; and (ii) ethylene glycol and glycerol droplets irreversibly absorb the moisture, increasing the volume and the base diameter. Further, cyclically changing the RH results in the droplet breathing effect, i.e., the nonhysteresis transition of the droplet shape between two wetting states corresponding to the minimum and maximum RH levels. In the case of the glycerol droplet for three cycles of the RH variation, the volume hysteresis (the droplet volume increases in each cycle) was observed. This is determined by the moisture absorption due to high hygroscopicity of glycerol. We also revealed that for all liquids studied, the droplet spreading at each increase in RH started at reaching the RH threshold level.