Experimental study on sessile droplet evaporation using laser interferometry

Abstract

Evaporation of a sessile droplet is a significant phenomenon prevalent in various industrial applications, such as inkjet printing and spray cooling. For a deeper insight into the evaporation mechanism of a sessile droplet, it is often required to measure the local evaporative flux at the droplet surface. However, it is quite challenging due to the invisible nature of the phase change process. As a result, many researchers have explored droplet evaporation problems using indirect methods. Meanwhile, a novel method utilizing laser interferometry to directly measure vapor distribution and local evaporation flow rate outside the droplet has been proposed. However, studies employing this method are scarce. Thus, in this study, we developed laser interferometry, ranging from hardware configurations to post-processing methods of raw data, to investigate sessile droplet evaporation in various circumstances. Utilizing laser interferometry, we first investigated the effect of gravity on the evaporation of a sessile droplet by measuring the evaporation process varying the molecular weight and droplet orientations. Results indicated that influenced by gravity-induced vapor distribution, the local evaporative flux and evaporation rate were suppressed for droplets on the substrate. In contrast, the opposite was observed for droplets under the substrate. Lastly, to explore the selective evaporation caused by the difference in volatility between different components in a multicomponent droplet, the external vapor distribution was measured using laser interferometry. The results showed that the signal intensity of the vapor distribution formed due to selective evaporation is proportional to the initial concentration of the droplet. However, it was found that the duration of the selective evaporation was not proportional to the initial concentration. This is because the effect of droplet shape on evaporation is negligible, while the effect of vapor pressure depression has a dominant effect on the evaporation phenomenon.