## 3 Results and Discussion
### Plate Shape
#### 3.1.3 Contact Angle of Plate Shape  
Figure 7 indicates the contact angles of AAO, AAO-O (oil impregnation), and AAO-O-SST (salt spray test). The contact angles of bare aluminum and bare aluminum-SST (salt spray test) were also investigated for comparison. The contact angle of a water droplet on the bare aluminum was \(74.4^{\circ}\pm 5.8^{\circ}\), and it was significantly decreased to \(24.8^{\circ}\pm 5.6^{\circ}\) after the salt spray test. The bare aluminum is susceptible to corrosion. After anodizing, the contact angle was dropped to \(21.0^{\circ}\pm 1.7^{\circ}\). It showed that the nanoporous structure enhanced the wettability of water by increasing surface roughness. After oil impregnation on the AAO surface, the contact angle was increased. The contact angle of a water droplet on AAO-O-SST was \(81.1^{\circ}\pm 1.9^{\circ}\). Although the salt spray test was conducted on the oil-impregnated AAO surface, the contact angle of the water droplet on AAO-O-SST was significantly increased. Figure 8 presents the schematic cross-section diagram of bare aluminum, bare aluminum-SST (salt spray test), AAO, AAO-O (oil impregnation), AAO-O-SST (salt spray test) surfaces. Bare aluminum  
Figure 6: Results of salt spray test for bare aluminum and AAO-O (oil impregnation on anodic aluminum oxide) surfaces. Figure 7: Contact angles of bare aluminum, bare aluminum-SST (salt spray test), AAO (anodic aluminum oxide), AAO-O (oil impregnation), and AAO-O-SST (salt spray test) surfaces of aluminum 7075.  
indicated a relatively flat surface, whereas the roughness was considerably increased after the salt spray test as shown in Figure 6. The results of the contact angle test showed that the contact angle of bare AI was decreased after the salt spray test. A surface of high roughness generally presents a low contact angle. Oxidized aluminum by the salt spray test increased the roughness of the surface, and the increased roughness reduced the contact angle. After anodizing, a nanoporous structure was generated. The produced nanoporous structure by anodizing decreased the contact angle. After oil impregnation on the AAO surface, the contact angle was increased. We assumed that the oil entirely covered the surface of the AAO. The contact angle on the surface of the AAO-O-SST was significantly increased after the salt spray test. It could be imagined that some oil remained on the nanoporous structure after the salt spray test as shown in Figure 8e. If there was no oil remaining on the surface of the AAO-O-SST, the contact angle would be similar to the contact angle of the AAO. When the oil layer was completely covered on the top of the surface of the AAO-O-SST, the contact angle would be similar to the contact angle of the AAO-O. The contact angle of the AAO-O-SST had a similar contact angle to the bare aluminum because the exposed nanoporous structure held the droplet. This study analyzed the interfacial tension between the proposed surfaces and water. The interfacial tension shows the adhesive force between the liquid phase of one substance and the liquid, solid or gas state of another element. When there is high interfacial tension, the water spreads on the surface. The high interfacial tension indicates a hydrophilic surface, whereas little interfacial tension shows a hydrophobic surface. Therefore, the high interfacial tension poses a low contact angle, and the low interfacial tension has a high contact angle. The interfacial tension was calculated by proposed equations from previous studies. We employed the formulas in the [PERSON] et al. study [30] to calculate the interfacial tension between the water and the proposed surfaces. The formula for the interfacial tension between the aluminum surfaces and the water is indicated in Equation (1). The formula for the interfacial tension between the AAO surfaces and the water is shown in Equation (2). Equation (3) shows the formula for the interfacial tension between the AAO-O-SST surfaces and the water.