## 2 Heat Exchanger Fabrication  
In this study, three types of fin surfaces of the fin-and-tube heat exchanger were examined. The first was the usual uncoated fin made of aluminum alloy. The second was the surface-coated hydrophilic blue fin using the same aluminum alloy as the substrate which is typically used in industry for corrosion prevention. The third was the surface-coated hydrophobic fin for preventing excessive corrosion using the same Al substrate. The compositions of the aluminum alloy included Al (about 99 wt%) as the main material and the rest was a total of about 1 wt% of Si, Fe, Cu, Mn, and Zn. The tensile strength and thermal conductivity of the alloy were about 118 N/mm\({}^{2}\) and about 220 W/m-K, respectively. The symbol wt% represents the weight percentage. The hydrophilic surface coating (with a thickness of 5 \(\upmu\)m) consisted of two layers. One was the corrosion-resist base layer which was an inversion-coated aluminum chromate film (with a thickness of about 0.13 \(\upmu\)m). On top of the base layer was the spray-coated hydrophilic film which consisted of a mixture of organic and inorganic materials. The coated film passed 160 h in the flow and salt spraying tests for 500 h according to JIS Z 2371 [15] to ensure its reliability. Practical field applications show that the coating still performs well for more than three years. The hydrophobic surface coating was essentially the same as that of the hydrophilic coating, except that the hydrophilic layer was replaced by a corrosion resistance layer to further prevent corrosion. That is, in terms of the capability of corrosion prevention, hydrophobic coating provides the best results, followed by hydrophilic coating, and then uncoated surfaces. In terms of cost, the hydrophilic coating is more expensive, whereas the uncoated ones are cheaper. In other words, if corrosion were the key issue, a hydrophobic coating is necessary, as is currently practiced in industry. However, hydrophobic coating could lead to the undesirable effect of water splash. Thus, a hydrophilic coating which costs slightly more can prevent corrosion and might also mitigate water splash and deserves further investigation. The surface morphologies of the uncoated and coated aluminum alloys are depicted in Figure 1 by AFM (Atomic force microscope, Nanosurf easyScan2, Liestal, Switzerland). The surface of the uncoated Al alloy, as shown in Figure 1a, exhibits the ridge-valley line type roughness resulting from the fabrication process and has a shiny appearance due to the metal reflectivity of the Al. In contrast, the coated hydrophilic surface, as shown in Figure 1b, shows a more uniformly distributed protrusive type roughness (roughness of about 0.7 \(\upmu\)m) and has a dull light blue appearance. Moreover, the observable color difference between the uncoated and hydrophilic coated fin surfaces prevents the misuse of either one in practical applications. On the other hand, the surface hydrophobic coated surface, as shown in Figure 1c, is geometrically similar to that of the uncoated one and could be misused. The contact angle (\(\theta_{CA}\)) is the water droplet angle at the interface of the water droplet, air, and solid surface as illustrated in Figure 2a. It is always used to quantify the wettability of a solid surface. The contact angle of the fin surface was measured by an image method utilizing the Young-Laplace equation ([PERSON], et al. [16]) using a water droplet size of 5 \(\upmu\)L. Typical results for the three types of surfaces are illustrated in Figure 2b-d, respectively. The contact angles of the uncoated, hydrophilic coated, and hydrophobic-coated surfaces are 85 \(\pm\) 5\({}^{\circ}\), 35 \(\pm\) 5\({}^{\circ}\), and 95 \(\pm\) 5\({}^{\circ}\), respectively. A difference of about 50\({}^{\circ}\) in the contact angles between the uncoated and hydrophilic-coated aluminum fin surfaces is significant for the present study. The contact angle of 35 \(\pm\) 5\({}^{\circ}\) of the coated surface also meets the suggestion of [PERSON] et al. [9].