# Experimental Study on the Condensation Heat Transfer on a Wettability-Interval Grooved Surface
## 3 Results and Discussion
### Droplet Dynamic Behavior
#### 3.1.2 Spontaneous Suction and Directional Drainage via Grooves  
The spontaneous suction effect was a phenomenon in that when the droplets on the hydrophobic ridge were sucked by the liquid film in the hydrophilic groove, they could be sucked into the hydrophilic groove and merge with the liquid film spontaneously. This process, driven by the surface tension difference as shown in Figure 6a, increased the removal efficiency of droplets on the hydrophobic ridge. In addition, the spontaneous suction effect and the coalescence of droplets on the hydrophobic ridge would interact with each other, resulting in a chain effect. As shown in Figure 6b, when the droplets on the hydrophobic ridge were sucked into the groove, the volume of the liquid film in the groove would swell instantly, and then came into contact with the small droplets in the adjacent hydrophobic ridge, causing the coalescence of the small droplets and the liquid film. Finally, they all shed from the surface through the hydrophilic microgroove. The formation of the liquid film in the groove was a prerequisite for the occurrence of the spontaneous suction phenomenon. If there was no liquid film forming in the groove, the spontaneous suction phenomenon hardly happened. If the droplets on the adjacent hydrophobic ridges could not be sucked into the groove and shed in time after coalescence, the formed large droplet would further grow into a liquid block across multiple hydrophobic regions. The formation of the liquid block would increase the thermal resistance of the surface, which would deteriorate condensation heat transfer performance. It should be pointed out that the effect of directional drainage through grooves could inhibit the  
Figure 5: Droplet growth on the wettability-interval grooved surface: (**a**) dropwise condensation on the hydrophobic ridge; (**b**) droplets on the edge of the ridge and the maximum droplets on the ridge; (**c**) schematic diagram of suction effect and the maximum droplet radius. Figure 6: Spontaneous suction effect on the wettability-interval grooved surface. The blue circle indicates the liquid film movement, while the red circle shows the droplet and the red arrow indicates the direction of droplet movement: (**a**) spontaneous suction of signal droplet; (**b**) spontaneous suction effect induced droplets coalescence. further growth of the liquid block into a flooded liquid film similar to that on a plain hydrophilic surface. Directional drainage via grooves was a phenomenon that which after a liquid block covering multiple ridges formed on the surface, the condensate confined in the groove would flow downward along the groove due to gravity when it accumulated to a certain volume. Meanwhile, the liquid block outside the grooves continued to flow into the groove under the action of surface tension and the thickness of the liquid block gradually decreased until it completely shed through the groove. The schematic diagram is shown in Figure 7a. As a result, with the help of directional drainage, the critical volume required for the departure of the large liquid block was reduced. In other words, the wettability-interval grooved surface can effectively remove the condensate on the surface and inhibit the growth of the liquid block into the flooded liquid film. The removal of the liquid block from the surface by directional drainage is shown in Figure 7b, the whole process lasted about 2 s.  
The directional drainage behaviors usually act on the condensate together with gravity. When the liquid block pinned to the surface grew to a certain volume through the coalescence of droplets and liquid film, the whole liquid block would slip downward due to gravity. In the process of shedding, the liquid block merged the downstream droplets, and the volume continued to increase. At the same time, the directional drainage behaviors could help reduce the volume of the large liquid block during the shedding process until the large liquid block vanishes and is transferred into the liquid film in the groove.