Effect of Fine Structures Formed by Nanoimprinting Using Anodic Porous Alumina Mold on Surface Hydrophobicity

Abstract

A surface with a simple nanopillar array and hierarchical micro-/nano structures consisting of microconvexes and a surface with a nanopillar array were fabricated by nanoimprinting using two types of anodic porous alumina mold. On both surfaces, the height of polymer nanopillars could be controlled by adjusting the anodization time for the preparation of the anodic porous alumina mold. Regarding water-repellent properties, a higher contact angle was observed on the surface with a hierarchical structure than on the surface with a simple nanopillar array. In the measurement of the water droplet sliding angle, the surface with a hierarchical structure showed higher performance characteristics than the surface with a simple nanopillar array. This is because the surface with a hierarchical structure can more easily trap air between the substrate surface and the water droplet than the surface with a simple nanopillar array. The hierarchical micro-/nano structures could be prepared repeatedly by nanoimprinting using a single mold. The method developed here for preparing a superhydrophobic surface is used for various applications, such as those requiring superhydrophobic surfaces with sliding angles.

Preparation of Polymer Nanopillar Arrays with Controlled Tip Shapes and Their Application to Hydrophobic and Oleophobic Surfaces

“…Therefore, the results shown in Figure 2b indicate that contact angle is unaffected by the height of nanopillars. 32 Figure 3 shows the effect of nanopillar diameter on wettability in a 500-nm-period nanopillar array, which had the highest contact angle of n-hexadecane. Figure 3a shows SEM images of polymer nanopillar arrays fabricated using three types of anodic porous alumina template with different pore sizes.…”

“…The results showed that the droplet contact angle did not change significantly when the pillar height was more than 1 μm. Therefore, the results shown in Figure b indicate that contact angle is unaffected by the height of nanopillars …”

“…Surfaces with fine patterns formed with hydrophobic or oleophobic materials exhibit higher water- and oil-repellent properties than smooth surfaces of the same materials. − Liquid-repellent surfaces based on fine patterns have attracted considerable attention because of their potential for various applications such as self-cleaning, anti-biofouling, and antifogging surfaces. − It has been reported that nanostructured surfaces fabricated by various methods, such as sol–gel methods, hydrothermal syntheses, and chemical vapor deposition (CVD) methods, can function as hydrophobic surfaces. − In the case of oil with lower surface tension than water, it is not easy to form oil-repellent surfaces on the basis of fine patterns, but it has been reported that inverse tapered or overhanging pillar array structures can be used to form oil-repellent surfaces. − Since the water- and oil-repellent properties based on the surface fine structures strongly depend on the surface geometry, precise control of surface structures is important to optimize surface wettability. We have reported the formation of polymer nanopillar arrays by nanoimprinting using anodic porous alumina as a mold and their application to superhydrophobic surfaces. − Anodic porous alumina is a nanohole array material obtained by the anodization of Al in an acidic electrolyte. When anodized under appropriate conditions, anodic porous alumina with an ordered arrangement of pores of uniform size and shape can be fabricated .…”

Continuous spinning of polymer nanofibers with uniform diameters using an anodic porous alumina spinneret with holes of different diameters

“…20 Our previous study has shown that anodic porous alumina has holes of more uniform diameters and shapes as the regularity of the hole arrangement increases. 21 First, Al plates (99.99% in purity) that were electropolished using perchloric acid and ethanol (1 : 4 in volume ratios) at 0.1 A cm −2 for 4 min were anodized for 17 h in 0.3 M oxalic acid at 17 °C under a constant voltage of 40 V. As a result of this treatment, the arrangement of holes at the bottom of the anodic porous alumina becomes self-organized and regular. The oxide film formed by the long-term anodization was selectively dissolved by immersion of the sample in a mixed solution containing 6 wt% phosphoric acid and 1.8 wt% chromic acid at 70 °C for 4 h to obtain an Al plate with dimples.…”

“…20 Our previous study has shown that anodic porous alumina has holes of more uniform diameters and shapes as the regularity of the hole arrangement increases. 21 pattern was anodized in 0.3 M oxalic acid for 3 h at 17°C under a constant voltage of 40 V. This first anodization step generated anodic porous alumina with an interhole distance of 100 nm and a thickness of 20 µm, which acts as a support layer. After this first anodization, the sample was immersed in 10 wt% phosphoric acid at 30°C for 25 min to increase the hole diameter.…”