## 4 Results and Discussion
### The Effect of Air Speed on Water Splash  
The results of splashed water measured are shown in Figure 9, Figure 9a for the overall view whereas Figure 9b-d are locally expanded scales to show the results of uncoated and hydrophilic coating in more detail. Three trends can be observed. Firstly, water splash will occur if the air speed is large enough, irrespective of surface coating conditions. Specifically, the threshold air speeds for water splashing are smaller than 2.5 m/s, about 2.5 m/s, and about 3.0 m/s for the hydrophobic coated, uncoated, and hydrophilic coated fin surfaces. Secondly, water splash of the hydrophobic coated surfaces is the largest, uncoated second, and hydrophilic coated the smallest, for all the fin cases, irrespective of fin pitches and coating conditions. That is, the hydrophilic surface fin coating reduces the amount of splashed water significantly by about one to two orders and about two times with respect  
Figure 8: Distributions of Nu and Eu versus Re. (**a**) Uncoated heat exchanger; (**b**) Hydrophilic coated heat exchanger. (**c**) Hydrophobic heat exchanger.  
to the hydrophobic coated and uncoated surfaces, respectively; thus, it is an effective method of mitigating the water splash situation. Thirdly, critical air speeds for which the amount of water splash increases dramatically exist for both hydrophobic coated and uncoated surfaces, the former being about 3.5 m/s and the latter about 4 m/s. In contrast, for the hydrophilic coated surfaces, no such speed is noticed as the increase is gradual. Hence, by hydrophilic surface coating, the splash amount can be greatly reduced and even completely prevented for smaller air speeds; the result is consistent with those in the literature [8; 10; 18].  
Figure 9: _Cont._  
Figure 9: Effects of fin surface coating on water splash. (**a**) Splash water of hydrophilic, hydrophobic, and uncoated heat exchangers. (**b**) Enlarged splashing water situations. (**c**) Enlarged water splash of uncoated heat exchangers. (**d**) Enlarged water splash of hydrophilic heat exchangers.  
The results of the roughness of about 0.7 \(\upmu\)m (Figure 1) and the smaller contact angle of 35\({}^{\circ}\) (versus 85\({}^{\circ}\)) of the hydrophilic coated fins suggest that that water droplet can sink into the more-or-less uniformly protruded roughness structure of the coated surface. This could lead to two effects. One is the reduced heat transfer capability of the coated surface and the other is the reduced water splash amount. For the pinned water in the submicron structure, heat is transferred mainly by conduction. Thus, the coated fins will transfer heat less effectively because the thermal conductivity of water (about 0.6 W/m-K) is considerably smaller than that of the Al alloy. But this reduced heat transfer can be partially compensated by the enlarged surface area due to roughness. Hence, the overall effect would not be significant as illustrated in this study. In contrast, the pinned water droplet into the coated surface does prevent water splash. Namely, by a thin and rough coating, the reduced heat transfer can be minimized while having enough surface roughness to hold the water droplet for the coating to be effective for practical applications.