Experimental Study of Evaporation-Induced Crystallization Process of Sessile Saline Droplets under Low Pressure

The evaporation-induced crystallization process of sessile saline(NaCl solution) droplets on different substrates, including copper, slide glass, and polytetrafluoroethylene(PTFE),was experimentally studied under low ambient pressure.The effects of surface properties and ambient pressure were analyzed.The results showed that white salt crystals were easy to precipitate at the contact line under lower ambient pressure.Due to the large surface energy of copper, the contact surface was covered with salt crystal, the droplet contact diameter remained almost unchanged during evaporation, and the contact angle decreased gradually.On the slide glass, when the ambient pressure increased, the droplet evaporation caused a contraction, accompanied by the growth of salt crystal and the fluctuation of the droplet contact angle.On the surface of PTFE,bubbles were easy to form at the contact surface, and the growth and bursting of bubbles led to an obvious fluctuation of contact angle.The Pe number was introduced to analyze the influences of external mass transfer and internal ion diffusion.The research results are helpful to the industrial application of desalination.