Vapor absorption phenomenon into sessile liquid desiccant droplets

This paper reveals the vapor absorption phenomena of liquid desiccant solution droplets on solid surfaces based on heat and mass transfer analyses. Vapor absorption is function of the pressure difference between the ambient air and the partial pressure of the vapor in the vicinity of the droplet liquid-gas interface. Due to the presence of salt ions (Li⁺, Br^-) in the droplet, the vapor pressure at the droplet surface is significantly reduced, and hence vapor absorbs accordingly. Depending on the different ambient conditions, the droplet behavior differs. On hydrophilic glass substrate, the droplets spread slowly with monotonously decreasing contact angle and increasing contact radius. While on hydrophobic PTFE substrate, the droplets spread much less, and an “advancing stick-slip” phenomenon is observed at high ambient humidity. Different from water droplet, the solute concentration of the LiBr-H2O droplet will change along with time due to water absorption, which then causes a decrease in the vapor pressure difference. Therefore, the vapor absorption process will slow down due to the decreasing driving force, which corresponds with the observed droplet performance. Moreover, the volume expansion ratio, i.e., final volume to initial volume ratio, of the liquid desiccant droplets only depends on the ambient relative humidity, which is supported by the equilibrium relationship between the ambient air and the desiccant solution. Finally, droplets on hydrophilic glass substrates can reach equilibrium with the ambience faster than those on hydrophobic PTFE substrates, which is explained by the apparently shorter characteristic length for solute diffusion within droplets on hydrophilic substrates.