Relationship between condensed droplet coalescence and surface wettability

Droplet condensation, which has better heat transfer performance than film condensation, can improve the efficiency of various engineering applications such as power generation, water harvesting and air conditioning. With the growth of condensed droplets, coalescence among these droplets is certainly taking place. Since the coalescence behavior greatly influences the droplet growth, the relationship between the droplet coalescence and the surface wettability needs to be understood. In this study, condensation experiments on prepared surfaces with various wettability were performed to observe the droplet coalescence behavior. The relationship between the droplet coalescence and the surface wettability was discussed and a regional map was created to classify different droplet coalescence behavior with four regions divided. To make the divided regions understandable, a dimensionless number called the Hysteresis number was first defined to denote the relative importance of the contact angle hysteresis compared to the contact angle for a random surface. Besides the application for analyzing the droplet coalescence behavior, the Hysteresis number can be widely applied for analyzing surface-droplet interactions in various engineering fields with a high value indicating that the contact angle hysteresis plays an important role and cannot be ignored.