[
    {
        "id": 1,
        "chunk": "# PAPER 光学的 Optically transparent super-hydrophobic thin film fabricated by reusable polyurethane-acrylate (PUA) mold  \n\nTo cite this article: J-S Park et al 2018 J. Micromech. Microeng. 28 025004  \n\nView the article online for updates and enhancements.",
        "category": " References"
    },
    {
        "id": 2,
        "chunk": "# Related content  \n\n- Polyurethane-acrylate-based hydrophobic   \nfilm: Facile fabrication, characterization, and application   \nJongsung Park, Bui Quoc Huy Nguyen, Ji  \nKwan Kim et al. - Hierarchical surfaces for enhanced selfcleaning applications   \nAriadna Fernández, Achille Francone, Lasse H Thamdrup et al. Surface modified nano-patterned SU-8 pillar array optically transparent superhydrophobic thin film   \nYoungsam Yoon, Dong-Weon Lee and Jeong-Bong Lee",
        "category": " References"
    },
    {
        "id": 3,
        "chunk": "# Recent citations  \n\n- Self-cleaning of glass surface to maximize   \nthe PV cell efficiency   \nAdnan Ayaz et al   \nLarge scale roll-to-roll production of   \npolyurethane-acrylate-based hydrophobic   \nfilm: a next-generation protection layer for   \nsolar devices   \nJongsung Park et al Polyurethane-acrylate-based hydrophobic film: Facile fabrication, characterization, and application   \nJongsung Park et al  \n\n![](images/a34f791530e311910fe7c080b8b2dfa9db85e97e818465884707ffc282b3ab79.jpg)",
        "category": " References"
    },
    {
        "id": 4,
        "chunk": "# lopebooks  \n\nBringing together innovative digital publishing with leading authors from the global scientific community  \n\nStart exploring the collection-download the first chapter of every titlefor free  \n\nhis content was downloaded from IP address 59.71.241.21 on 25/04/2021 at 07:27",
        "category": " References"
    },
    {
        "id": 5,
        "chunk": "# 超疏水 Optically transparent super-hydrophobic 制造 thin film fabricated by reusable polyurethane-acrylate (PUA) mold 模型  \n\nJ-S Park1, J-H Park1 and D-W Lee $1,2\\textcircled{\\circ}$  \n\n1  MEMS and Nanotechnology Laboratory, School of Mechanical Engineering, Chonnam National   \nUniversity, Gwangju, Republic of Korea   \n2  Center for Next-generation Sensor Research and Development, Chonnam National University, Gwangju   \n61186, Republic of Korea  \n\nE-mail: mems@jnu.ac.kr (D W Lee)  \n\nReceived 26 September 2017, revised 16 November 2017   \nAccepted for publication 4 December 2017   \nPublished 4 January 2018  \n\n![](images/c71918b3c1b69d347915a318aa74022c2998fb351b84f5a51bff054df20a2c5d.jpg)",
        "category": " Abstract"
    },
    {
        "id": 6,
        "chunk": "# Abstract  \n\nIn this paper, we describe a simple manufacturing method for producing an optically transparent super-hydrophobic polymer thin film using a reusable photo-curable polymer mold. Soluble photoresist (PR) molds were prepared with under-exposed and under-baked processes, which created unique hierarchical micro/nano structures. The reverse phase of the PR mold was replicated on the surface of polydimethylsiloxane (PDMS) substrates. The unique patterns on the replicated PDMS molds were successfully transferred back to the UV curable polyurethane-acrylate (PUA) using a laboratory-made UV exposure system. Continuous production of the super-hydrophobic PDMS thin film was demonstrated using the reusable PUA mold. In addition, hydrophobic nano-silica powder was sprayed onto the micro/nano structured PDMS surfaces to further improve hydrophobicity. The fabricated PDMS thin films with hierarchical surface texturing showed a water contact angle $\\geqslant150^{\\circ}$ . Excellent optical transmittance within the range of visible light of wavelengths between $400{-}800\\mathrm{nm}$ was experimentally confirmed using a spectrophotometer. High efficiency of the super-hydrophobic PDMS film in optical transparency was also confirmed using solar panels. The fabricated PUA molds are very suitable for use in roll-to-roll or roll-to-plate systems which allow continuous production of super-hydrophobic thin films with an excellent optical transparency.  \n\nKeywords: super-hydrophobic, polydimethylsiloxane, UV curable PUA, artificial lotus leaf, nano-silica  \n\nSupplementary material for this article is available online (Some figures may appear in colour only in the online journal)",
        "category": " Abstract"
    },
    {
        "id": 7,
        "chunk": "# Introduction  \n\nA super-hydrophobic surface has the characteristic of being highly hydrophobic, i.e. very difficult to wet. Generally, the contact angle of water droplets is ${>}150^{\\circ}$ at the hydrophobic surface and the roll-off angle is ${<}10^{\\circ}$ [1]. There is tremendous effort to create artificial super-hydrophobic surfaces with water contact angles $\\geqslant150^{\\circ}$ that provides various advantages such as better drag reduction, self-cleaning, and anti-sticking effects. Most proposals for super-hydrophobic surfaces have been inspired by the natural world. A common feature of many proposals is the utilization of a dual scale (micro and nano) structure, which have proven to be an effective way for creating super-hydrophobic surfaces [2–4]. A variety of materials, such as black silicon, carbon nanotubes, and polymers, has been widely studied to realize the hierarchical micro-/nano-structured surface similar to a lotus leaf [5–7]. However, these proposals are still struggling with technical issues, such as transparency, scalability, manufacturing cost, and flexibility.  \n\nFor optically transparent surfaces, the material of films should be inherently transparent and the surface roughness should be smaller than the wavelength of visible light. Polymers, such as polymethyl methacrylate (PMMA), polystyrene (PS), and polydimethyl-siloxane (PDMS), have been widely studied in fields requiring high optical transparency. In recent years, studies using water-repellent functional groups, such as fluorine-silane on nano-silica particles, and chemical coating methods, such as titanium dioxide $\\left(\\mathrm{TiO}_{2}\\right)$ and silicon dioxide $(\\mathrm{SiO}_{2})$ , have been conducted. Such a chemical coating method can easily improve water repellency and it has an advantage of not being limited by the shape of the object being coated. However, the long-term durability of the superhydrophobicity coating is poor and discoloration of the film due to chemical reactions with the coating material often occurs. One-step fabrication of optically transparent film based on a PDMS material was reported in our previous report [8]. The unique PDMS structure greatly improves hydrophobicity, but it is difficult to reuse because thick photoresists (PR), more than $40\\ \\mu\\mathrm{m}$ , are used as the mold structure. Further, multiple coatings of the PR are desirable to generate better hierarchical micro-/nano-structures with reverse structures. This method is not easy to reproduce in fabricating the PR mold and requires a lot of processing time.  \n\nThe primary goal of this study is to report a new fabrication method for optically transparent super-hydrophobic thin films that overcomes the drawbacks of currently available manufacturing methods. Thick PR molds with reversed hierarchical structures were prepared using a modified photolithography processes. Next, the unique structure of the PR molds is replicated on the PDMS surface, which is similar to the surface of lotus leaves. The structure is then re-replicated to the photocurable polymer layer for the purpose of reusing the mold. The reusability of the photo-curable polymer is well suited for high volume production processes, such as a roll-to-roll or roll-to-plate process. Optically transparent PDMS thin films prepared from reusable molds have significantly improved hydrophobicity compared to general PDMS thin films without surface structures. In addition, the excellent elasticity and adhesion of the PDMS material can be extended to various curved surfaces without using additional adhesives. The new manufacturing process and successful experimental results reveal that the proposed idea has great potential for diverse uses of the super-hydrophobic transparent PDMS thin film.",
        "category": " Introduction"
    },
    {
        "id": 8,
        "chunk": "# Experimental methods  \n\nFigure 1 shows a photograph and schematic of a real and artificial lotus leaf, respectively. The hierarchical structure of the lotus leaf is made of a characteristic epidermis and covering wax layer. The epidermis of the lotus plant contains papillae that are $10{-}20\\ \\mu\\mathrm{m}$ tall and $10{-}15\\ \\mu\\mathrm{m}$ wide. As shown in figure 1(a), the epidermis surface with microstructures is covered with epicuticular waxes. These superimposed waxes are hydrophobic and form the second layer of the hierarchically double-layered structure. As shown in figure  1(b), the dual structures of the lotus leaf are successfully mimicked using a unique PR mold. In conventional photolithography, a vertical sidewall is strongly desirable during the exposure and development processes. Here, we hypothesize that a combination of soft-baking and UV exposure conditions created unexpected patterns, which is undesirable in conventional photolithography. However, the unique negative patterns of the PR are relatively useful for super-hydrophobic applications once the shapes are transferred to the PDMS. The pure PDMS has a water contact angle between about $90^{\\circ}$ and $100^{\\circ}$ when there are no structures on the surface. This contact angle can be significantly improved by adding surface structures. The super-hydrophobic water-repellent hierarchical structures of the PDMS surface also provide self-cleaning effects, a behavior that is very attractive in engineering applications. However, one of the drawbacks of this method is the utilization of the PR as a mold material. An organic solvent used to separate the super-hydrophobic PDMS film from the PR mold completely dissolves the PR mold and makes it impossible to reuse.  \n\nPDMS is known as optically clear, non-toxic, and nonflammable material. It is also known as dimethicone, which is a type of silicone oil. The water contact angle is close to $100^{\\circ}$ on a flat PDMS surface and can be increased up to $150^{\\circ}$ when additional processes are performed on the flat PDMS. The improved method for fabricating optically transparent surface-textured PDMS thin films is shown in figure 2. The thick AZ4620 PR is multi-coated on a 4 in silicon wafer (figure 2(b)). After coating the first PR, the soft-baking process is performed in a convection oven at $55~^{\\circ}\\mathrm{C}$ for $3\\mathrm{min}$ . The second PR coating is then repeated using the same process condition as for the first layer. After the third PR coating, the multi-layered PR is soft-baked again in a convention oven at $55~^{\\circ}\\mathrm{C}$ for $45\\mathrm{min}$ . A significant under soft-baking process condition allows the thick PR to maintain a certain amount of solvents, which enhances the dissolution rate of the PR. In general, the thick PR layer requires an increased soft-baking time when the convection oven is employed for the baking process. When the PR is cured in ovens, the solvent starts to evaporate from the PR surface. The cured PR surface layer disturbs continuous evaporation of solvent from the PR; the solvents remaining inside the PR are evaporated through the cured PR surface layer. The underexposed and under-baked positive PR technique is optimized to make the desired reverse image of the hierarchical micro/nano structures (figure 2(c)). In this way, it is possible to mimic the surface of Lotus leaf with negative patterns.  \n\nThe PDMS monomer (base) and curing agent are mixed with a known ratio (10:1) and the mixture is placed in a desiccator to de-gas in a vacuum. The degassed PDMS solution is gently poured on the severely under-baked PR mold and then placed in a vacuum oven again. The spin-coated PDMS sample is degassed until trapped bubbles are completely removed from interface between the PDMS and the PR mold (figure 2(d)). The thickness of the cured PDMS layer is about $2\\mathrm{mm}$ , which is controllable using a changing spin speed and number of PDMS coating layers. Next, the PDMS layer is cured at ${>}80^{\\circ}\\mathrm{C}$ for $\\mathtt{4h}$ . Finally, the super-hydrophobic PDMS film is replicated by removing the PR mold with an acetone solution (figure 2(e)). To optimize the photolithography processes, various temperatures and exposure times were tested during the preliminary experiments. When the temperature for soft-bake was increased, the depth of developed area was decreased and the head of pillar structures was increased.  \n\n![](images/c0f16f76700fe3c1d671b7f0e540463473b2dd8e0660e74d3e7670a444f6ddaf.jpg)  \nFigure 1.  Optical image for (a) natural Lotus leaf and (b) process flow for fabrication of artificial lotus leaf based on PDMS.  \n\n![](images/cbf51f397597910b6f2a2959a9cd1ce36439808c39ea14bdfdd68b16ea9bb89e.jpg)  \nFigure 2.  (a)–(i) Schematic of improved fabrication process employing reusable molds, (j) scanning electron microscope images showing the fabricated micro/nano structured PDMS structures before and after nano silica coating and (k) optical image of water droplet on surfacepatterned PDMS film.  \n\nThe maximum temperature for the fabrication of desired mold structures was about $70~^{\\circ}\\mathrm{C}$ . Subsequently, the liquid phase UV-curable PUA is poured on the replicated PDMS film with unique structures on the surface (figure 2(f)). Next, the UV-curable PUA is solidified for 20 s using a laboratorymade UV system (figure $2(\\mathrm{g)}$ ) with an optical power of $5\\mathrm{mW}$ The cured PUA is then separated from the replicated PDMS mold (figure $2(\\mathrm{h})_{,}^{\\cdot}$ ). The UV-curable PUA is prepared using 4-hydroxybutyl acrylate $(85\\%)$ , acrylic acid $(11\\%)$ , ethylene glycol dimethacrylate $(1\\%)$ , and $2^{\\prime}$ -dimethoxy-2- phenylacetone phenone $(3\\%)$ . The fabricated PUA mold is very useful for repeatable production of the super-hydrophobic PDMS thin film. This is very important because reliable and continuous production of PDMS thin films with the surface texturing can be possible by using the same mold. Further, nano-silica is sprayed on the PDMS surface (figure 2(i)) with a certain distance and angle. These nano-materials slightly increase the water contact angle. Figure 2(j) shows SEM images of replicated optically transparent super-hydrophobic PDMS films with and without additional nano-silica coating processes. The measured water contact angle on the thin PDMS film is shown in figure 2(k). Figure 3 shows atomic-force microscopy (AFM) images of a pure PDMS and nano-silica coated PDMS substrates. AFM images indicate that the average roughness (Ra) value increased from $0.529\\mathrm{nm}$ to $44.5\\mathrm{nm}$ after coating the nano-silica on the PDMS surface.  \n\n![](images/a0ae52a10123ceefbc5400a5ed32e776d043346f683c21ac0873f50d3dc33065.jpg)  \nFigure 3.  Atomic force microscope images (a) before and (b) after nano-silica coating on PDMS films with flat surfaces.  \n\n![](images/b9deea1a9d52ddd6e0817ced233f0f4cb087dceaa1e0db46b6129f5b463b99db.jpg)  \nFigure 4.  Optical images of (a) hydrophobic PDMS positive mold (b) replicated UV-curable polymer mold (c) re-replicated super-hydrophobic PDMS film fabricated from the UV-curable polymer mold and (d) repeated fabrication of PDMS thin films with an ultra-hydrophobic surface.  \n\n![](images/08f94e857ff9fa545a9a55df44eaee651393d3ef1f2dfd288e62afaef78f8310.jpg)  \nFigure 5.  (a) and (b) Water contact angle with increasing number of water droplets on super-hydrophobic PDMS films and the pitch distance of the pillar structure. (c) and (d) Dynamic contact angle and sliding angle as a function of volume of water droplets.  \n\nOne of the great advantages of the proposed fabrication method is the use of the reusable and scalable PUA mold. The method does not require additional chemical treatment during the separation of the cured PDMS film from the UV-cured polymer mold with negative structures. The cured PDMS film can be easily and sufficiently detached from the PUA mold even with a small mechanical force. In addition, the hydrophobic characteristics are further improved after employing an additional nanoparticle (e.g. nano-silica) coating on the replicated PDMS thin film. Figure 4(a) show a SEM image of the fabricated PDMS mold with surface textures. The surface textures of the PDMS were successfully replicated to the PUA thin film as shown in figure  4(b). Figure  4(c) show a SEM image of the surface-textured PDMS re-replicated by using the replicated PUA mold. Figure 4(d) show optical images of continuously produced super-hydrophobic PDMS thin films using the reusable PUA mold. The size of the super waterrepellent PDMS film is about $15\\times15\\mathrm{cm}^{2}$ . The water contact angle, light transmittance, and self-cleaning effect were systematically investigated using various techniques.",
        "category": " Materials and methods"
    },
    {
        "id": 9,
        "chunk": "# Results and discussion  \n\nFigure 5(a) shows the change of water contact angle as a function of pillar height $(30-60~\\mu\\mathrm{m})$ . Although there is a small difference in contact angle depending on the height of the pillar, it is almost negligible within the error ratio. Next, contact angle experiments were performed according to the spacing between pillar structures, as shown in figure  5(b). Experimental results show that the contact angle and transparency have opposite characteristics. When the pillar pitch is between $60~\\mu\\mathrm{m}$ and $100\\ \\mu\\mathrm{{m}}$ , the super-hydrophobicity of the PDMS surface was very good but the optical transparency decreased in comparison with the pure PDMS films. In contrast, the transparency was excellent at a pitch of $120\\ \\mu\\mathrm{m}$ to $190\\ \\mu\\mathrm{{m}}$ , but the super-hydrophobicity drastically decreased. According to the experimental results, a pitch between $100\\mu\\mathrm{{m}}$ and $120\\ \\mu\\mathrm{m}$ was excellent for both transparency and hydrophobicity. Figure  5(c) shows the dynamic test (water contact angel) results for the PDMS film with a height of $30\\mu\\mathrm{m}$ and pitch of $120\\ \\mu\\mathrm{m}$ . The fabricated PDMS thin films with hierarchical surface texturing showed a water contact angle $\\geqslant150^{\\circ}$ at different water volume from 1 to $10\\mathrm{mm}$ in diameter. Experimental results for sliding angle of various water droplets are also shown in figure 5(d). As the volume of water increased, the sliding angle of the water droplet decreased from $14^{\\circ}$ to $6^{\\circ}$ .  \n\n![](images/ae5b1aa9bb6eb814088923d2ca9bf503a4a331b716212a4b25a6dee0ded021c2.jpg)  \nFigure 6.  (a) Optical transmittance as a function of wavelength for two different PDMS films, with and without nano-silica coating, and (b) optical image of super-hydrophobic PDMS film placed on a printed paper.  \n\nIn preliminary studies, the water contact angle value was measured for $5~\\mu1$ of water. However, this is insufficient as a basis for replicating water repellency in the natural world, such as a rain shower with much different water volumes. For this reason, the water contact angle value was measured with continuous increasing water droplet volumes (minimum $1\\mu1$ to maximum $30~\\mu\\mathrm{l}$ ). The replicated super-hydrophobic film maintains a water contact angle of more than $150^{\\circ}$ for all water volumes. This proves that the optically transparent superhydrophobic film has excellent water repellency. Hence, it is expected that water repellency will be maintained even in a real rain shower. To evaluate the transparency of PDMS films fabricated using the reusable photo-curable polymer mold with a pitch distance of $120\\ \\mu\\mathrm{m}$ , the transmittance in the visible region ${(400-800\\mathrm{nm})}$ was measured using a UV–Visible spectrometer. Two different types of PDMS films, with and without nano-silica, were employed for characterization, and the transmittance of the PDMS films were compared, as shown in figure 6(a). The water contact angle increased from about $138^{\\circ}$ to $158^{\\circ}$ after the nanosilica coating (figures S1 and S2 (stacks.iop.org/JMM/28/025004/mmedia)). In the case of PDMS films without nano-silica coating, the average transmittance was close to $98.9\\%$ . The value slightly decreased to $95.7\\%$ when nano-silica was coated on the PDMS films using a spray method. However, both thin films still showed a high light transmittance suitable for practical applications. These results indicate that the hierarchical micro/nano structures formed on the thin PDMS film do not significantly affect the optical transparency of the thin film. Figure 6(b) also shows an optical image of ultra-hydrophobic PDMS film placed on a printed paper. Excellent transparency of the film was confirmed by verifying the visibility of letters printed on the paper in various colors. To confirm the super-hydrophobic behavior of the PDMS thin film, blue dyed water droplets were dropped on the film; the high water contact angle was also confirmed because the water droplet easily slipped.  \n\nPDMS thin films with different pitch distances were fabricated to evaluate the relationship between the pitch distance and the optical transparency. The prepared PDMS films were attached on a black background with white letters printed on it and the optical transparency was evaluated using a microscope. Figure  7(a) shows the optical images of PDMS thin films with different pitch distances from $80~\\mu\\mathrm{m}$ to $190\\ \\mu\\mathrm{{m}}$ . When the distance was more than $140~\\mu\\mathrm{m}$ , the hierarchical micro/nano structures formed on the PDMS film had little effect on the transmittance. Magnified images of pillars from the top and side views are shown in figures 7(b) and (c). As shown in figure  7(c), an array of water-repellent structures with a mushroom shape was exactly duplicated using the reusable PUA mold.  \n\nLotus leaves are always cleaned with only a rain shower, i.e. they exhibit the self-cleaning effect [9, 10]. The duplicated super-hydrophobic PDMS film is also expected to have the self-cleaning behaviors due to their unique surface structures, similar to lotus leaves. A preliminary experiment investigating this issue was conducted. The PDMS thin film was placed on top of a white paper with black letters (figure 8(a)). Next, the PDMS surface was contaminated with gray dust obtained from a vacuum cleaner (figure 8(b)) and water droplets were then continuously applied to the contaminated PDMS surface, as shown in figure 8(c). After a few minutes, the contaminated surface was clearly clean due to the surface’s self-cleaning ability, as shown in figure  8(d). The results were also compared with that from normal PDMS films with a planar surface; water alone did not completely remove the contamination on the flat PDMS surface. An additional cleaning process was required to completely remove the contamination. The basic experimental results indicate that the proposed PDMS thin films with the super-hydrophobicity and self-cleaning ability have great potential for applications in various engineering areas.  \n\n![](images/bd5f45551a57a341bcc33d06ddb39f686c880263b0b9473cc9b24fe5f1032a71.jpg)  \nFigure 7.  (a) Optical images showing the transmittance of the PDMS films when the pitch distance increased from 80 to $190\\mu\\mathrm{m}$ , and SEM images of the fabricated micro/nano structured PDMS, (b) top view and (c) side view of the micro/nano structured PDMS thin film.  \n\n![](images/e70ac88bf85cfd4c14fdb47595b206beca1c1f6a06997958d0cc37447f816daa.jpg)  \nFigure 8.  Optical images showing the self-cleaning ability of the super-hydrophobic PDMS thin film, (a) initial status, (b) contaminated with gray dust, (c) continuous application of water droplets, and (d) surface cleaned with water droplets.  \n\n![](images/c904a32a669b64d780ac64c13c091e01ecf1a6938b9f430c1df598f191f22449.jpg)  \nFigure 9.  Optical images of (a) solar cell without film on the surface and (b) solar cell covered with super-hydrophobic film.  \n\nTable 1.  Experimental results of coated and uncoated solar cells under various light sources.   \n\n\n<html><body><table><tr><td></td><td>A. Solar panel</td><td>B. Solar panel + hydrophobic film</td><td>Efficiency (A/B)× 100%</td></tr><tr><td>Sunlight</td><td>1.52V</td><td>1.52V</td><td>100%</td></tr><tr><td>LED (white)</td><td>0.78V</td><td>0.77V</td><td>98%</td></tr><tr><td>Yellow lamp</td><td>0.71V</td><td>0.71V</td><td>100%</td></tr><tr><td>UV (360nm)</td><td>1.61V</td><td>1.60 V</td><td>99%</td></tr></table></body></html>  \n\nTo verify the further utility of the self-cleaning transparent thin film, the fabricated PDMS thin film was applied as a protection layer to a solar panel, as shown in figures 9(a) and (b). Because solar cells use sunlight as an energy source, they are exposed to the outside environment. Accordingly, solar cell surfaces are often contaminated with environmental dust, resulting in lowered light transmission and photoelectric conversion efficiency. In the solar cell, an antifouling film with high transparency that does not affect energy efficiency and maintains its own cleanliness without special management is required. The $10\\times6\\mathrm{cm}^{2}$ solar panel was fully covered by the fabricated super-hydrophobic PDMS thin film as shown in figure  9(b). No chemical treatment and adhesive layer were required for attachment and even when detaching from the panel. The energy loss was compared with the output voltage before and after PDMS film attachment. Four different light sources, sunlight, LED light (white), yellow light, and UV light $360\\mathrm{nm})$ were employed for characterization. Results indicate that there was $1-2\\%$ voltage loss for the LED and UV light experiments (table 1). Under the sunlight and yellow light condition, the solar cell had the highest output voltage, and voltage loss was almost negligible values.",
        "category": " Results and discussion"
    },
    {
        "id": 10,
        "chunk": "# Conclusion  \n\nThis paper proposed a new fabrication method for continuous production of optically transparent super-hydrophobic PDMS thin films. A unique surface of the PDMS thin film fabricated by a PR mold was re-replicated to a reusable PUA mold. Employing the photo-curable PUA mold extremely simplified the production process to generate these PDMS thin films with the unique surface. Various PDMS thin films with different pitch distances were fabricated to obtain the optimized conditions for transparency and hydrophobicity. In addition, nano-silica coating was also applied to further improve the hydrophobicity of the PDMS thin film. Excellent self-cleaning ability and optical transparency were exper­ imentally confirmed using a solar panel. The new fabrication process and successful experimental results reveal that the proposed method has significant potential for many engineering applications.",
        "category": " Conclusions"
    },
    {
        "id": 11,
        "chunk": "# Acknowledgments  \n\nThis work was supported by Commercialization Promotion Agency for R&D Outcomes (COMPA) funded by the Ministry of Science, ICT and Future Planning (MSIP) (No 2016K000209).",
        "category": " References"
    },
    {
        "id": 12,
        "chunk": "# ORCID iDs  \n\nD-W Lee $\\circledcirc$ https://orcid.org/0000-0002-0847-4505",
        "category": " References"
    },
    {
        "id": 13,
        "chunk": "# References  \n\n[1] Yoon Y, Lee D W and Lee J B 2012 Surface modified nanopatterned SU-8 pillar array optically transparent superhydrophobic thin film J. Micromech. Microeng. 22 035012   \n[2] Feng L, Li S, Li Y, Li H, Zhang L, Zhai J, Song Y, Liu B, Jiang L and Zhu D 2001 Super-hydrophobic surfaces: from natural to artificial Adv. Mater. 14 1857–60   \n[3] Lin T S, Wu C F and Hsieh C T 2006 Enhancement of waterrepellent performance on functional coating by using the Taguchi method Surf. Coat. Technol. 200 5253–8   \n[4] Dai S, Ding W, Wang Y, Zhang D and Du Z 2011 Fabrication of hydrophobic inorganic coatings on natural lotus leaves for nanoimprint stamps Thin Solid Films 519 5523–7   \n[5] Xiu Y, Zhu L, Hess D W and Wong C P 2007 Hierarchical silicon etched structures for controlled hydrophobicity/ superhydrophobicity Nano Lett. 7 3388–93   \n[6] Inoue Y, Yoshimira Y, Ikeda Y and Kohno A 2000 Ultrahydrophobic fluorine polymer by Ar-ion bombardment Colloids Surf. B 19 257–61   \n[7] Li S, Li H, Wang X, Song Y, Liu Y, Jiang L and Zhu D 2002 Super-hydrophobicity of large-area honeycomb-like aligned carbon nanotubes J. Phys. Chem. B 106 9274–6   \n[8] Yoon Y, Lee D W and Lee J B 2013 Fabrication of optically transparent PDMS artificial lotus leaf film using underexposed and underbaked photoresist mold J. Microelectromech. Syst. 22 1073–80   \n[9] Patankar N A 2004 Mimicking the lotus effect: influence of double roughness structures and slender pillars Langmuir 20 8209–13   \n[10] Park J and Lee D 2017 Highly reproducible and scalable transparent PDMS thin film with super-hydrophobic surface Int. National Conf. of IEEE MEMS 2017 (Las Vegas, 22–26 January) pp 679–82",
        "category": " References"
    }
]