## 3 Results and Discussion
Figure 5b is a lateral-view image of the anodic film, showing a straight channel with hemispherical closed barrier layer formation on the channel bottom. In Figure 5c, the aluminum substrate has been removed from the independent AAO film, and the anodic film presents a hemispherical closed barrier layer on the bottom. Because the closed barrier layer may affect the gas or liquid circulation in advanced applications, the barrier layer can be removed. In Figure 5d, the barrier layer is removed in a both-side open anodic channels. The anodic film structure is supported by a continuous wall, and both ends of the anodic tube are open to allow for air or liquid flow in the anodic tubes. However, in our experiment, 6061 Al tubes were utilized as the raw material for anodization. AAO obtained from 6061 Al has a worse microstructure than AAO prepared from pure Al. Figure 6 shows photos of the tubular AAO film formed with 6061 Al tubes. Figure 6a shows the raw material, the 6061 Al tube, which has a rough surface before mechanical and electrical polishing. Figure 6b shows that the 6061 Al tube presents a flat surface that is suitable for the formation of high-quality AAO after electro-polishing (EP) process. In Figure 6c, the AAO film on the EP Al further presents golden color by oxidized in oxalic acid solution with a DC voltage of at 40 V. Figure 6d shows  
Figure 4: Step-by-step images of tubular AAO fabrication process: (**a**) initial 6061 Al tube and their outward appearances after each procedures, including (**b**) mechanical grinding; (**c**) electrolytic polishing; (**d**) the first anodization; (**e**) ordering pattern; and, (**f**) the second anodization; and, (**g**) the finished tubular AAO film with Al rings on both ends. tubular AAO film partially with the inner EP Al surface; therefore, the Al substrate could be entirely removed by dipping in CuCl\({}_{2}\) aqueous solution. The image shows that part of the Al substrate was removed, and the partially exposed AAO film is visible. In Figure 6e, the exposed AAO film with the Al rings on both sides further obtained after the Al substrate was totally removed. In future applications of tubular AAO, the Al rings on both sides can function as connectors. Figure 6f demonstrates that the red ink solution exuded through the AAO nano-pores. Figure 5: Schematic diagram of tubular AAO micro-morphology; (**a**) AAO film on the aluminum substrate and its (**b**) enlarged image; AAO film (**c**) without aluminum substrate; and, (**d**) barrier layer. Figure 6: Photos of tubular AAO film formed from 6061 Al tubes: (**a**) raw 6061 Al tube material; (**b**) electro-polished (EP) 6061 Al; (**c**) AAO formation on the 6061 Al tube; (**d**) partially removed 6061 Al substrate from AAO film; (**e**) tubular AAO film without 6061 Al substrate; and, (**f**) red ink exudation through AAO film. Figure 7 shows tubular AAO with Al connectors for the further characterizations of gas transmembrane pressure or liquid exudation tests. In Figure 7a, the schematic drawing of tubular AAO film with nano-channels structure is presented and both ends are connected with metal aluminum tubes. Figure 7b demonstrates the exploded view of a gas connector with tubular AAO, and the gas connectors on the tubular AAO in Figure 7c. The practical testing setup of gas connectors on the tubular AAO is shown in Figure 7d. For the pressure bearing testing of tubular AAO films, the barrier layer at the AAO bottom was kept, and then Ar gas was input into one of gas connector from the storage bottle, while the other end of gas connector was sealed. The pressure bearing capacity of tubular AAOs depended on their thickness. AAO film with a thickness of 100 \(\upmu\)m can resist an argon pressure up to 8 atm; however, 30 \(\upmu\)m AAO film can only withstand 3 atm of Ar gas. For the aeration test, AAO's barrier layer was removed, and Ar gas was injected into one gas connector (the other one was sealed). As aerating tubular AAO was put in the water, a lot of bubbles leaked out on the AAO surface.