Experimental investigation of evaporation dynamic of sessile droplets in pure vapor environment with low pressures

In order to understand the evaporation dynamic of sessile droplet in pure vapor environment, this paper presented a series of experimental observations on the evolution of thermal patterns and the variation of evaporation rate during water and ethanol droplet evaporation at low pressures. The contact radius of droplet is fixed at 2.5 mm and the height varies from 0.4 to 1.6 mm. Results show that the surface temperature distributions for both water and ethanol are uniform at a high pressure ratio. When the pressure is reduced, a ring-like thermal pattern appears on the water droplet surface, while the gear-like pattern and the cellular flow happen on the ethanol droplet surface. Interestingly, the gear-like pattern is static when the substrate temperature is low, while it will oscillate at a high substrate temperature. Meanwhile, the cellular flow is affected by the droplet height. The evaporation is promoted by reducing the pressure. Moreover, due to the decrease of the evaporation surface area and the increase of surface temperature, the evaporation rate decreases first and then increases with the decrease of the droplet height.