Natural Convective Evaporation Study of Sessile Droplets

Energy and mass transfer to and from droplets in sprays occur in a variety of applications. These include industrial processes like cooling and coating, but also in pathogen transmission, where isolated or a spray of droplets made of complex fluids evaporate leaving pathogens and non-volatile solids suspended in air or deposited on surfaces. In this study, we investigate the evaporation process of sessile droplets under natural convection conditions. We conducted an extensive set of experiments in which individual sessile droplets (droplet positioned on top of a surface) were placed at ambient conditions, to study their evaporation. Using shadowgraphy, the droplet is photographed, and later dimensions like droplet height, contact diameter, contact angle and volume are extracted from the pictures. Using dimensionless numbers, we derived a basic correlation between the Sherwood number (Sh) and the Rayleigh number (Ra) for sessile droplets for natural convection conditions. Results show that the surface on which a droplet sits has a great influence on the evaporation process, because it can affect wettability parameters like the Contact Angle (CA), Contact Diameter (CD) or a combination of both, and these affect the mass transfer process. Toward the end of a droplet's evaporation, the droplet shape transitions from a nearly spherical cap to a thin film, making the development of a universal correlation for predicting Sh difficult. Nevertheless, this study contributes towards to solving some of the questions that remain about the physical processes that occur during the evaporation of small, individual droplets on a surface. Like, what is the role of natural convection, and surface wettability on the mass flow rate. The study also includes the development of a basic correlation between the Sherwood and Rayleigh numbers for the calculation of the mass transfer coefficient for sessile droplets.