Coalescence characteristics of vibrated drops on a wettability gradient surface

The coalescence characteristics of two vibrated droplets at a certain distance on a microstructured surface with gradient wettability are investigated by a high-speed camera in this work. The results show that the volume ratio of the two droplets has a significant effect on the vibration modes. With the change of the volume ratio, the droplet exhibits different vibration modes, such as the pumping mode (PM), the rocking mode (RM), or the pumping-rocking mixed mode (PRM). In addition, the coalescence time of the two droplets varies with the volume ratio. When the volume ratio is close to 1, the two vibrated droplets are in synchronous pumping mode (SPM) and the coalescence time is the shortest. When the volume ratio is far away from 1, the two droplets may show the rocking mode (RM), the asynchronous pumping mode (APM), or the pumping-rocking mixed mode (PRM). At this point, the coalescence time gets increased, especially for the small volume ratios. Finally, the movement characteristics of the three-phase contact lines are discussed, and a theoretical model is proposed to analyze the coalescence process. This work provides a new method to remove droplets rapidly, which is essential to enhance the heat transfer performance of dropwise condensation.