# Experimental Study on the Condensation Heat Transfer on a Wettability-Interval Grooved Surface
## 3 Results and Discussion  
### Droplet Dynamic Behavior  
#### 3.1.1 Comparison of Droplet Dynamic Behaviors on Functional Surfaces  
As indicated by the above contact angle measurement, the functional surfaces presented diverse wettability. To thoroughly understand the effect of surface wettability on the condensation characteristics in humid air, the condensate dynamic behaviors on different surfaces (plain hydrophilic surface, plain hydrophobic surface, hydrophilic grooved surface, hydrophobic grooved surface, and wettability-interval grooved surface) in humid air were compared and analyzed. The efforts are especially devoted to revealing the droplet growth and shedding characteristics. Generally speaking, the filmwise condensation on the plain hydrophilic surface goes through four stages: nucleation, liquid film growth, merging, and shedding. On the plain hydrophilic surface, due to the large nucleation density and rate, the morphology of condensate was a thin liquid film. After a period of time, the small liquid film gradually merged into the large ones. The coalescence process is shown in Figure 4a. Because of the existence of high surface adhesion, the liquid film did not easily fall off and gradually merged into a larger liquid film covering the surface, and finally fell off due to gravity. Droplet condensation occurs on the plain hydrophobic surface and usually goes through four stages: nucleation, droplet growth, coalescence, and shedding. The nucleation sites of the droplets were randomly distributed on the surface, and the small droplets grew into large droplets by either direct condensation or coalescence. The coalescence process is shown in Figure 4b. The droplet started to fall off when it reached a critical droplet radius where gravity could overcome the surface tension and adhesion force. The shedding droplets swept the downstream droplets and refreshed the condensation surface. Then, it was followed by new nucleation sites and repeated dropwise condensation. The droplets on the plain hydrophobic surface could slip off rapidly, and the liquid film covering the majority of the plain hydrophilic surface did not appear on plain hydrophobic surface. On the hydrophilic grooved surface, liquid membranes appeared on the ridges and grooves during condensation. The liquid membranes gradually merged into a large liquid film covering the surface, which was similar to the plain hydrophilic surface, and the coalescence process is shown in Figure 4c. Finally, the flooded liquid film slipped off the surface due to gravity. The microgrooves were beneficial for the liquid film slip compared with the plain hydrophilic surface. On the hydrophobic grooved surface, the nucleation process was different from that on a plain hydrophobic surface. The droplets nucleated preferentially at the edge of the groove due to the edge effect [32]. After growing to a certain volume, the droplets started to merge as shown in Figure 4d. The merged droplets on the ridge could stand on the top of the groove and did not collapse into the groove. The small droplets inside the grooves would also be absorbed by the large droplets on the ridge, so it was difficult to form the flowing liquid film in the grooves. Similar to the plain hydrophobic surface, when the droplets on the hydrophobic grooved surface reached the critical volume, they would fall off due to gravity and sweep the downstream droplets on the condensation surface; then, the new condensation process started again on the clean surface. On the surface with uniform wettability, the sweeping behavior driven by gravity was the main way to remove the condensate. On the wettability-interval grooved surface, the process of droplet nucleation was similar to that of a hydrophobic grooved surface, where nucleation sites preferentially occurred at the edge of the groove and were randomly distributed on the hydrophobic ridge, as shown in Figure 5a.