Micro-nano hierarchical pillar array structures prepared on curved surfaces by nanoimprinting using flexible molds from anodic porous alumina and their application to superhydrophobic surfaces

Flexible molds with micro-nano hierarchical structures on the surface were fabricated by a two-step template process using anodic porous alumina as a starting material. The obtained flexible molds could be used to form micro-nano hierarchical pillar arrays on the surface of glass tubes and convex lenses by photo-nanoimprinting. The contact angle characteristics of the surfaces with hierarchical pillar arrays were measured, and it was confirmed that they exhibit superhydrophobic properties with a water-droplet contact angle exceeding 150°. The flexible molds obtained in this study can be used repeatedly and efficiently to form micro-nano hierarchical surfaces with superhydrophobic properties on the surfaces of substrates with various curvatures.


Introduction
Nanostructured surfaces exhibit various functions depending on their surface morphology. [1][2][3][4] Nanoimprinting is an effective method for preparing various functional nanostructured surfaces, such as antireective, water-repellent, and antibacterial surfaces, because it enables the efficient patterning of the controlled ne structures on the substrate surface. [5][6][7][8][9][10][11][12][13][14] Nanoimprinting molds are usually fabricated by a combination of electron beam lithography and dry etching. 15,16 However, these methods have some problems, such as the difficulty in fabricating molds with large-area patterns and high-aspect-ratio structures. We have been investigating the formation of ordered ne patterns by nanoimprinting using an anodic porous alumina mold obtained by the anodization of Al in an appropriate acidic electrolyte. [17][18][19] Anodic porous alumina is characterized by its ability to effectively form ordered nanohole array structures with high aspect ratios, in addition to large areas. 20 Nanoimprinting using anodic porous alumina as a mold is suitable for preparing structures with high aspect ratios and large sample sizes.
In the present work, we have expanded the nanoimprinting process using anodic porous alumina molds to the preparation of micro-nano hierarchical structures on curved surfaces. The preparation of ne patterns on curved surfaces is important for the functionalization of the surfaces. As one possible process for the preparation of the ne patterns on the curved surfaces by nanoimprinting, we have demonstrated the nanoimprinting process by using a mold prepared by the anodization of an Al substrate with a given curved surface. 21,22 The problem with this process is that applicable samples are limited to substrates with curvatures identical to the mold curved surface. To overcome this problem, we have introduced exible molds originating from the anodic porous alumina used as a starting structure. The use of a exible mold composed of polydimethylsiloxane (PDMS) has been reported in the preparation of ne patterns of polymers on various curved surfaces. [23][24][25] Since the exible mold can deform itself to follow the surface shape of the substrate, it is possible to form ne patterns on various curved surfaces using a single mold. In contrast, the sample areas and aspect ratios of the obtained structures were limited in the previously reported process based on mold preparation processes. In addition, to the best of our knowledge, there are no reports of the formation of micro-nano hierarchical structures, which are effective for superhydrophobic surfaces, 26 by nanoimprinting using exible molds. Here, we describe the preparation of highaspect-ratio hierarchical pillar arrays on curved surfaces by nanoimprinting using a exible mold fabricated from anodic porous alumina and their application to superhydrophobic surfaces for the rst time. In the present work, the exible molds with hierarchical structures were prepared by a two-step template process using anodic porous alumina as a starting material. First, anodic porous alumina with hierarchical structures was prepared by an integrated process composed of anodization and selective patterning. 27,28 Next, polymer negatives of anodic porous alumina with the hierarchical structures were formed, and nally, the exible mold was obtained by molding using the polymer negative as a mold. This process can effectively form high-aspect-ratio micropillars with nanopillar arrays at their tips on curved surfaces. Although the formation of hierarchical structures by nanoimprinting has been reported, a method that can form hierarchical pillar arrays with aspect ratios exceeding 5 on curved surfaces has not been reported. Surfaces with hierarchical pillar arrays formed with high aspect ratios are considered to trap a layer of air between water droplets and the substrate surface more easily than surfaces with hierarchical structures formed with low aspect ratios. According to the Cassie-Baxter model, a surface with air trapped between water droplets and the substrate surface functions as an excellent water-repellent surface. 29 Therefore, this process is expected to be an efficient method for making curved substrate surfaces superhydrophobic. In addition, this process can form a hierarchical pillar array structure with a high aspect ratio, similar to the hierarchical structure found on the sole of a gecko foot, and is expected to be used for preparing structural adhesive surfaces on curved substrates. 30 Experimental Fig. 1 shows a schematic diagram of the process of preparing a exible mold with a micro-nano hierarchical structure on the surface and nanoimprinting using the obtained exible mold. In this study, the ordered anodic porous alumina used as a starting material was prepared by two-step anodization, as reported previously. 31 An Al sheet (99.99% purity) was polished electrochemically in a mixture of perchloric acid and ethanol (1 : 4 in volume ratio) at 0.1 mA cm À2 for 4 min to achieve a mirror surface. The electropolished Al sheet was anodized in 0.1 M phosphoric acid under a constant voltage of 195 V at 0 C for 16 h to obtain an ordered hole arrangement. The oxide layer formed by the rst anodization was dissolved selectively in a mixture of 1.8 wt% chromic acid and 6 wt% phosphoric acid, and then anodized again for 2 h under the same conditions to prepare the ordered anodic porous alumina with an interhole distance of 500 nm and a lm thickness of 8 mm. Aer anodization, a resist mask with micropatterns was formed on the surface of the anodic porous alumina by a stamping process. For the formation of the resist mask, a polydimethylsiloxane (PDMS) stamp with a square pattern of micrometer-size protrusions on the surface prepared using the prepolymer (TSE3450, Momentive Performance Materials Inc.) was used. The surface pattern of the PDMS stamp was fabricated by a molding process using patterns formed by electron beam lithography as a starting structure. The PDMS stamp was immersed in a toluene solution containing 0.2 wt% polychloroprene (Nisshin EM Co., Japan) and pulled up at a constant rate of 1 mm s À1 to form a polychloroprene thin lm in the recesses of the stamp surface. In this method, a polychloroprene thin lm is formed on the surface of the PDMS stamp as the dip-coated solution dries and solidies. The thickness of the polychloroprene lm formed on the stamp surface can be controlled by adjusting the concentration of the solution and the pull-up speed of the stamp. By pressing the stamp onto the anodic porous alumina, a resist mask with a micrometer-size aperture pattern corresponding to the surface structure of the stamp was transferred. This is because the polychloroprene lm on the stamp surface adheres better to anodic porous alumina than to PDMS, and the lm can be transferred by pressing the stamp against the anodic porous alumina. The sample with the resist mask was immersed in a mixture of 1.8 wt% chromic acid and 6 wt% phosphoric acid at 50 C for 1 h to dissolve the alumina layer from the resist aperture and obtain a micrometersize cavity array with exposed Al at the bottom of the cavities. Although acids other than a mixture of chromic acid and phosphoric acid can be used for the selective dissolution of the alumina layer, the etchant we used does not dissolve Al; thus, so etching can be stopped when residual Al is exposed. The sample was anodized again for 1 h in 0.1 M phosphoric acid at 195 V to obtain a hierarchical structure with nanohole arrays at the bottom of the microcavities. To increase the hole size of the porous alumina formed at the bottom of the microcavities, etching was performed in 10 wt% phosphoric acid at 30 C for 1 h. Aer etching, the sample was treated with uoroalkylsilane (Optool DSX, Daikin Industries, Ltd., Japan) as a mold release agent. A micro-nano hierarchical pillar array was obtained on a polyethylene terephthalate (PET) sheet by photonanoimprinting using anodic porous alumina as a mold and a photocurable monomer (PAK-02, Toyo Gosei Co., Japan).
The exible molds were fabricated by a template process using the micro-nano hierarchical pillar arrays obtained using anodic porous alumina. A PDMS prepolymer (SIM-260, Shin-Etsu Chemical Co., Japan), which is suitable for ne pattern molding, was poured into the hierarchical polymer pillar array, le to cure at 65 C for 3 h, and then peeled off to obtain a exible mold with micro-nano hierarchical structures formed on the surface. Before nanoimprinting using the exible mold, the mold was treated with uoroalkylsilane (Optool DSX, Daikin Industries, Ltd.) to form a release layer on the surface. The exible mold was used to form micro-nano hierarchical patterns on surfaces with curvature by photo-nanoimprinting. The surfaces of the obtained samples were modied with uoroalkylsilane (Optool DSX, Daikin Industries, Ltd.), and the contact angles with water droplets were evaluated. The geometrical structures of the obtained samples were also observed by scanning electron microscopy (SEM; JSM-7500F, JEOL).  Fig. 2(a) shows a square lattice array of microcavities with a diameter of approximately 2 mm and a period of 5 mm. This shows good correspondence with the surface pattern of the stamp used for resist formation. From the cross-sectional SEM image shown in Fig. 2(b), it can be observed that the porous alumina layer in the resist aperture is completely dissolved to the bottom by selective etching. When anodic porous alumina is immersed in an etchant, the etchant penetrates to the bottom of the holes, causing the uniform dissolution of the hole walls from the top to the bottom of the holes and increasing the hole diameter. As etching continues, the hole walls dissolve and the holes coalesce, further dissolving the residue and exposing the residual Al substrate. If a resist mask is formed on anodic porous alumina prior to etching to cover some of the holes, as in this process, the etchant penetrates only inside the holes of the anodic porous alumina at the resist openings, and the hole walls are dissolved. This makes it possible to dissolve only the alumina layer at the resist opening selectively. This is an anisotropic etching process based on the hole array structure of anodic porous alumina. Any acid can be used for etching the alumina layer, but etching can be stopped when the Al substrate is exposed by using an etchant containing chromic acid. This is because when the Al substrate is exposed by etching, a passive lm is formed on the Al surface by the effect of the chromic acid, and no further etching occurs. From Fig. 2(b), it can also be observed that a nanohole array structure is formed at the bottom of the microcavity by re-anodization. The depths of the microcavities and nanoholes in the obtained sample were 8 mm and 220 nm, respectively. The thickness of the resist mask formed on the stamp surface by dipcoating depends on the concentration of the polychloroprene solution and the pull-up speed. If a thick resist is formed by dip coating on the entire micropattern on the surface of the stamp, the resist pattern transferred to the porous alumina surface has no openings. On the other hand, if the resist lm formed on the stamp surface is very thin, the holes of anodic porous alumina are not covered by the resist, and the etchant penetrates into all the holes during etching, resulting in dissolution over the entire sample surface. Therefore, to selectively dissolve anodic porous alumina by this process, it is important to control the thickness of the resist lm formed on the stamp surface by dip coating.

Results and discussion
This process can produce anodic porous alumina with a hierarchical structure even when the interhole distance of anodic porous alumina is varied. Fig. 3 shows the results for micro-nano hierarchical structures fabricated from anodic porous alumina with interhole distances of 100, 200, and 300 nm as starting structures. The SEM images in Fig. 3 show that nanohole arrays with hole periods of 100, 200, and 300 nm are formed at the bottom of microholes with a diameter of 2 mm. Fig. 4 shows SEM images of a micro-nano hierarchical pillar array formed on a at substrate using the anodic porous alumina shown in Fig. 2. The low-and high-magnication SEM images of the cross section in Fig. 4(a) and (b) show vertical hierarchical pillars of uniform diameter and height on the  substrate. The height of the obtained hierarchical pillars was 8 mm, which corresponded to the depth of the microcavities in the anodic porous alumina used as the starting structure. The high-magnication SEM image of the upper part of the hierarchical pillars in Fig. 4(c) shows the nanopillar array formed at the tip of a micropillar. When nanoimprinting was performed using the anodic porous alumina master with a releasing layer on its surface, a defect-free hierarchical pillar array was formed over the entire surface of the sample. Fig. 5 shows the observation results of a exible mold fabricated using the hierarchical pillar array shown in Fig. 4 as a template. The photograph in Fig. 5(a) conrms that a exible mold with a size of 2 cm square was successfully formed. The surface SEM image in Fig. 5(b) shows a square lattice of microholes with a diameter of approximately 2 mm and a spacing of 5 mm. In addition, a nanohole array with an interhole distance of 500 nm was observed to be transferred at the bottom of the microcavities. The cross-sectional SEM image in Fig. 5(c) shows that the depth of the microcavities is 8 mm, conrming that the structure of porous alumina used as the starting structure is retained even aer two transfer processes. Fig. 6 shows the results of hierarchical pillar arrays formed on the surface of a glass tube with a diameter of 7 mm by photonanoimprinting using the exible mold. Fig. 6(a) shows photographs of the glass tube aer photo-nanoimprinting. It can be seen that the nanoimprinted area has turned white. This is due to light scattering in areas with hierarchical pillar arrays formed by nanoimprinting using the exible mold. The exible mold fabricated in this study deforms along with the shape of the substrate, making it possible to form a pattern on the entire surface of the glass tube. The optical microscopy image of the imprinted sample shown in Fig. 6(b) conrms that a square lattice of micropillars is formed over a large area. From the SEM images in Fig. 6(c) and (d), vertical micropillars with a nanopillar array at the tip were observed to form on the curved  surface. The exible mold could be detached from the substrate while deforming even during peeling, making it possible to form hierarchical pillars that maintained an upright structure on each curved surface. Fig. 7(a) shows the results of preparing hierarchical pillar arrays of different heights on a curved surface by photonanoimprinting using a exible mold. Similar to the experimental results shown in Fig. 6, a glass tube with a diameter of 7 mm was used as the substrate for nanoimprinting. The surface structure of the exible mold was controlled by varying the thickness of the porous alumina lm used as the starting material. The SEM images in Fig. 7(a) show that the heights of the obtained hierarchical pillars were 5, 8, and 16 mm. Even in the case of the tallest pillar fabricated in this study (16 mm), it was possible to form pillar arrays on a curved surface while maintaining the upright structure. The SEM images in Fig. 7  show the results of controlling the height of the nanopillars at the tip of the micropillar. The height of the nanopillars was varied from 200 nm to 420 nm by varying the anodization time from 1 h to 2 h during the preparation of anodic porous alumina with a hierarchical structure. These results show that the geometrical structure of hierarchical pillar arrays formed on curved surfaces by photo-nanoimprinting using exible molds can be controlled by changing the starting structure. Fig. 8 shows the results of micro-nano hierarchical pillar arrays formed by nanoimprinting using a exible mold on convex lens surfaces with different curvature radii. From the photographs of the specimens, it can be seen that the entire surface of each spechimen is white, for all curvature radii of 52, 26, and 10 mm, indicating that the ne patterns were transferred. SEM observations of each sample also indicated that hierarchical pillar arrays were formed without defects on all of the curved surfaces of different curvature radii. These results indicate that the exible molds fabricated in this study can be used to form hierarchical pillar arrays not only on twodimensional curved surfaces, such as cylinders and tubes, but also on three-dimensional curved surfaces such as lenses. We believe this is because the exible mold made of PDMS can not only deform exibly but also stretch to follow the convex lens shape. Fig. 9(a) shows the observed results of hierarchical pillar arrays obtained by the 1st, 5th, and 10th photo-nanoimprinting using the same exible mold. In all samples, the formation of a 2 mm-diameter, 4 mm-high micropillar array with a nanopillar array at the tip was conrmed. These results show that exible molds made of PDMS can be used repeatedly. Fig. 9(b) shows the measured contact angles with water droplets exhibited by the surfaces obtained by the 1st through the 10th nanoimprinting. The wettability of the ne patterns obtained by nanoimprinting was evaluated using patterns formed on a smooth substrate rather than on a curved substrate. All of the samples exhibited a water-drop contact angle of approximately 155 and were conrmed to have high waterrepellency. Although the static contact angle could not be measured because of the difficulty in placing the water droplets on the surface, the curved surfaces with the hierarchical pillar arrays were also observed to exhibit high water repellency.

Conclusions
Flexible molds with hierarchical patterns were fabricated using a hierarchical pillar array prepared by the anodization of Al as a template. Photo-nanoimprinting using the obtained exible molds enabled the formation of ordered pillar arrays with micro-nano hierarchical structures on the surfaces of tubular substrates and convex lenses. The exible molds obtained in this study deform following the shape of the substrate, indicating that the exible molds are effective for preparing ne patterns on surfaces with various curved shapes. The structure of the hierarchical pillars formed on the substrate could also be controlled by changing the geometrical structures of anodic porous alumina used as the starting material. Contact angle measurements of the surfaces with the hierarchical pillar array obtained by this process conrmed superhydrophobic properties with a water-drop contact angle exceeding 150 . In addition, the molds could be used repeatedly, and the water repellency of the surfaces obtained from the 1st to the 10th photonanoimprinting remained almost the same. This method is promising for imparting superhydrophobic properties based on micro-nano hierarchical patterns on curved surfaces.

Conflicts of interest
There are no conicts to declare.