Robust and Stable Transparent Superhydrophobic

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Robust and stable transparent superhydrophobic polydimethylsiloxane films by duplicating via femtosecond laser ablated template Dingwei Gong, Jiangyou Long, Dafa Jiang, Peixun Fan, Hongjun Zhang, Lin Li, and Minlin Zhong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03424 • Publication Date (Web): 20 Jun 2016 Downloaded from http://pubs.acs.org on June 21, 2016

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ACS Applied Materials & Interfaces

Robust and stable transparent superhydrophobic polydimethylsiloxane films by duplicating via femtosecond laser ablated template Dingwei Gong1, Jiangyou Long1, Dafa Jiang1, Peixun Fan1, Hongjun Zhang1, Lin Li2, Minlin Zhong1* 1

Laser Materials Processing Research Centre, School of Materials Science and Engineering,

Tsinghua University, Beijing 100084, PR China 2

Laser Processing Research Centre, School of Mechanical, Aerospace and Civil Engineering,

The University of Manchester, Manchester M13 9PL, England

Abstract: Realizing superhydrophobicity meanwhile high transparency on polydimethylsiloxane (PDMS) surface enlarges its application fields. We applied a femtosecond laser to fabricate well designed structures combining micro-grooves with micro-holes array on mirror finished stainless steel to form a template. Then liquid PDMS was charged for the duplicating process to introduce a particular structure composed of micro-walls array with a certain distance between each other and micro protrusion positioned at the center of plate surrounded by micro-walls. The parameters such as the side length of micro-walls and height of micro-cone were optimized to achieve required superhydrophobicity at the same time high transparency properties. The PDMS surfaces

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show superhydrophobicity with static contact angle up to 154.5 ± 1.7° and sliding angle lower to

6 ± 0.5° , also with a transparency over 91%, a loss less than 1% comparing with plat PDMS by the measured light wavelength in visible light scale. The friction robust over 100 cycles by sandpaper, strong light stability by 8 times density treatment, and thermal stability up to 325℃ of superhydrophobic PDMS surface was investigated. We report here a convenient and efficient duplicating method, being capable to form transparent PDMS surface with superhydrophobicity in mass production, which shows extensive application potentials.

Keywords: Designed micro structure, Duplicating, Superhydrophobicity, Transparency, Robust and stability

1. Introduction Superhydrophobicity properties have been observed on natural plants and animals1-5 such as lotus leaf1-2 for many years, which are the results of millennia’s evolution on their own particular surface structures. Academically, superhydrophobicity follows two standards: the static contact angle (SCA) of over 150° between water droplet and corresponding surface and a relatively small sliding angle (SA) for droplet to roll off is. Since micro-nano dual structure on the upper side of lotus leaf had been observed by W.Barthlott et al1, and the research about direct influence on SCA by surface energy6, it is well known that the two mains factors for superhydrophobic surface are low surface energy and surface structure with appropriate roughness in micro even nano scale7-8. Herein, many attempts in surface superhydrophobicity follow these two principles: generating a suitable micro or nano or micro-nano hierarchical surface structure and then coating


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the low surface energy materials on this surface, or directly creating this structure on a low energy surface 9-13. Realizing transparency on a superhydrophobic surface will evidently enlarge its application fields such as being coated on solar cell panels, which could achieve the self-cleaning or antidust properties with little loss of sunlight energy; it could also be coated on out-wall windows in high buildings for releasing the dangerous job of out-wall windows cleaners and keeping a high transparent view of the outsight. However, according to Mie scattering14, the increase of surface roughness would theoretically result in a decrease of surface transparency, which means, the surface structure in micro even sub-micro scale could no doubt prevent the transparence of visible light. Unfortunately, surface roughness – the structure on the surface is a key point in achieving surface superhydrophobicity regardless how low the surface free energy is. Consequently, the avoidance of the transparency loss by precisely controlling surface roughness - which depends on micro-nano structures on treated surface, and in the meantime making these structures appropriate for achieving superhydrophobicity become an interesting but challengeable issue. Many researches have been taken to present both superhydrophobicity and transparency on treated surfaces. These researches could be divided in two parts by the scale of samples’ structures forming: nano-scale 15-23 and micro-scale24-25. Most of attempts chose sub-micro or nano-structure forming to guarantee transparency thanks to these small sizes. Wei-Heng Huang et al21 used silica nanoparticles and silica acid to form an aimed coating with the SCA exceeding 160° , hysteresis of 7 ± 2° , average transparency of 91%; Hitoshi Ogihara et al22 charged also silica nanoparticles to fabricate a thin film on glass by spraying method to achieve superhydrophobicity and about 80% of transparency, they also researched the influence of surface energy of glass substrates for superhydrophobic and


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transparent properties; Yuping Wu23 et al realized a superhydrophobic film by carbon nano-tubes and TiO2 composite, showing a transmittance of 79%, SCA about 160°, SA less than 2°, and photocatalytic properties. In micro-scale, Doo-Man Chun et al24-25 presented an embossing method to fabricate superhydrophobic polymer surface by using a metal template ablated by a UV nanosecond laser. The

simple

groove

microstructure

on

the

template

surface

was

replicated

onto

polydimethylsiloxane (PDMS) with a micro-crossed wall structure. The optimal result contains a SCA upper to 171° , hysteresis about 10°, and with an transparency loss from 2% to 4% comparing to plat PDMS surface. The as-reported publications demonstrate that fabricating a superhydrophobic and transparent surface in micro-scale by duplicating process using laser ablated template takes advantages as structural stability and mass production. However, on one hand, how to keep the plate area to be as smooth as possible by the mean time to occupy a largest part of surface area is significant. On the other hand, researchers also take account for the properties of treated polymers. In nano scale, Wei-Heng Huang et al21 and shows reported the stability after treated by ultra-sonication test; Yuping Wu23 et al presented the photocatalytic properties. While in micro scale, Doo-Man Chun et al24-25 showed almost nothing on their properties. Considering the work environment, the robust and the stability in different situation define the application capability of polymers in superhydrophobicity also with transparency, it is also of scientific interest to investigate the structural modification and the changes in properties of this polymer after many cycles of friction, being irradiated by strong light, and being thermal treated in a high temperature.


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Concerning the factors above, in this paper, a femtosecond laser is applied to the finished stainless steel surface to ablation to form well designed micro structures as the mold for duplicating, the femtosecond laser takes advantages in causing little heat-affected zone. Then liquid PDMS was charged for the duplicating process to introduce designed micro structures (Fig.1) which were optimized to achieve required superhydrophobicity at the same time high transparency properties. We also focused on the robustness and stability of micro structures on surface and their properties in various situation as friction robust by over 100 cycles of friction by sandpaper, strong light stability by 8 times density of normal sunlight and thermal stability up to 325℃, that a well performed PDMS were found in various conditions.

Figure 1. Schema for two models of designed surface structure and its contact with liquid (a). Micro groove array structure (b, d); micro groove combined micro protrusion array structures (c,e)


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2. Experimental Sections Finished stainless steel template with size of 40 mm × 40 mm × 1mm and 20mm × 20mm × 1mm were cleaned in ethanol using ultrasonic machine. These stainless templates were ablated in several different ablation ways by a Trumph femtosecond pulse laser source (the wavelength of 1030nm, pulse width about 700 fs). According to 1/2 of the maximum intensity of the Gaussian profile, The focused spot diameter of the laser beam was approximately 33µm. The laser applied on stainless steel surfaces were considered in two modes: 1) Laser ablation in lines: grooving cross lines with a distance varying from 200µm to 400µm, with parameters following: repetition rate of 200kHz, speed of 500mm/s, laser power of 4.68J/cm2 repeats 50 times for the same parameters above. 2) Laser ablation point by point with that the points situated in the center of each square (the square was created by the cross line of first mode’s laser ablation), the parameters following are: repetition rate of 800 kHz, laser power varied from 5µJ to 15µJ, 400 pulses in each point, repeat 15 times for the same parameters above. Then these samples were fluoridated by dodecyl acid in order to be easier to release the PDMS after the curing process. Commercial PDMS raw materials (Sylgard 184) of Dow Corning Company were charged with a silicone elastomer base and a hardening agent, they were mixed by weight ratio about 10:1 by magnetic blender machine, and then air bubbles were removed by ultrasonic vibration for 10 minutes. After being pulled on the template, the PDMS were cured in a bake oven at 80℃ for 3h. Finally, the cured PDMS were released at room temperature. Scanning electron microscopy (SEM) and laser microscope were used for surface morphology structural characterizing. For the SEM measure, the electron beam evaporator was firstly charged to coat a conductive layer on the surfaces of samples. The planar array microstructure images


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were viewed from horizontally placed samples. In order to observe the height of microstructures, the samples were vertically placed. The optical contact angle measuring system 15 was used to test the superhydrophobic properties (static contact angle of the droplet (SCA) and the sliding angle (SA)) of the samples. For the SCA measuring, the droplet volume was 4 µL, for SA was 9µL, respectively. Sandpaper and counterweight are used for testing and analyzing the robust, a sun simulator was charged for the strong light stability, and the thermal stability tested by a tube furnace, of optimal treated PDMS in superhydrophobicity and transparency.

3. Results and discussions 3.1 Designed structural of duplicated PDMS film for superhydrophobicity and transparency By charging the as-fabricated finished stainless steel micro matrix template, the duplicating experiments were performed by controlling structure size parameters including square length and the height of the protrusion in each square of the matrix. Fig2 shows a well-designed microsubmicron structure on the surface of the PDMS films after the duplicating process, the square unit with a length of 300µm (the width at the bottom of wall is about 45µm) with also a micro protrusion of about 78µm in height and bottom diameter of about 50µm at the center of each square, which is slightly higher than the “wall” surrounding. The wall height is about 40µm, while the four corners of the wall are with the height of about 75µm. In addition, full of submicron protrusions standing on the wall of the square, which could be attributed to the pulse ablation of ultra-fast pulsed laser on template scanning in a high speed. Thus, with the help of ultra-fast pulsed laser and the template, the duplicating process successfully results in a precisely controlled and well-designed micro structure on the PDMS film surface in a shape of matrix with


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a large part of plane and a protrusion at the center of each plane. Then, as also PDMS possesses a low surface free energy, they lead to a well performed superhydrophobicity and maintains the transparency of PDMS. The as-duplicated PDMS films has a SCA of 154.5° ± 0.8° and a SA of

6° ± 0.5° , they result in a prominent superhydrophobic surface, since the SCA of original flat PDMS film is 100° ± 0.8° . The square “wall” with an appropriate distance also full of submicron protrusions and the micro protrusion in the center both play the important roles in enhancing surface roughness to hold the droplet up, which transformed droplet’s contact model from Wenzel model26 to a Wenzel-Cassie-Baxter mixed model8, 27. There is, herein, a large part of air gap between the liquid and the plane below, this could play the major role for the superhydrophobic property. This particular structure with low surface energy material, results in the small sliding angle for water to roll-off. Moreover, this particular designed structure with the ration of plane area over 70% ensured high transparency (Fig.2 f)) over 91% in the visible light scale, which lost only about 1% comparing to plat PDMS surface. From the results above, the side length of square and the height of micro protrusion in matrix could be considered as the two main factors in influencing the level of surface superhydrophobicity as well as transparency. These two factors could be precisely controlled by changing laser parameters. Moreover, they are two separated part, which would not influence each other. Hence, analyzing then optimizing these two parameters was performed in order to achieve better results.


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Figure 2. Micro structure of optimal designed PDMS in a) laser microscope and in SEM, b) 500x, c) 2000x. The SCA of this optimal surface is 154.5 ± 1.7° SA is 6 ± 0.5° . d) The sample of optimal designed PDMS coated on the paper and e) it was held up to keep a certain distance (10cm) from the paper. f) The transparency of the optimal designed surface is over 91% in the visible light wavelength. 3.1.1 Optimization of the side length of square zone (SLSZ) Two groups of samples were investigated: group one of samples with SLSZ from 200µm to 400µm, and group two with SLSZ from 200µm to 400µm, but also a micro protrusion with height of about 78µm and bottom diameter of about 45µm at the center of each square. Fig.3 shows the micro structures on PDMS surface at different side length. As the width of wall is about 45µm, the area ratio of center plate in each matrix from a) to e) in Fig.3 could be calculated in brief: a) 0.6, b) 0.67, c) 0.72 d) 0.76 e)0.79, this means the surface roughness


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decreases along the increased ratio, and the wall will insert deeper in droplet. According to Wenzel-Cassie Baxter mixed model, this will increase the resistance for droplet to roll-off. As a result, it leads to the increase of SA for the droplet. Meanwhile, the increase of plane part could help light be more easily to go through PDMS, leading to a higher transparency. In adding, a micro protrusion at the center of plate, which led only to about 1% loss of plate area in calculate, could significantly enhance the surface superhydrophobicity. Fig3 c) and f) illustrates the two designed model (without and with the micro protrusion in 300µm side length). Fig4 presents the results of SCA as well as SA for the group one (Fig.4 a)) and group two (Fig4. b)) with the contact of droplet, by viewing the decrease of SA for the same side length in two groups, the SA of 300µm SLSZ samples results in 14 ±1° and 6 ± 0.5° , the positive influence of micro protrusion in superhydrophobicity could be demonstrated. Moreover, by comparing the transparency of treated surfaces with a micro protrusion for the side length of 200µm, 300µm, and 400µm, Fig.4c figures that the 300µm and 400µm own the similar transparency and are closed to the PDMS with no structure on it, while the transparency of 200µm side length is the lowest, which could be understood that the micro protrusion in this plane occupied a much large percentage of area than that of 300µm and 400µm and led a larger loss of transparency. Herein, to take into account both superhydrophobicity and high transparency, the sample with 300µm side length combined a micro protrusion at the center of each plate could be considered as the optimal one.


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Figure 3. Micro structures of duplicated PDMS with size length a) 200µm b) 250µm c) 300µm d) 350µm e) 400µm and f) size length of 300µm with a micro protrusion about 78µm height in the center of each square.

Figure 4. Changes of SCA&SA by the variation of side length of square without protrusions (a); Changes of SCA&SA by the variation of side length of square with a micro protrusions about 78µm in height (b); Changes of transparency by the variation of side length of square with a


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micro protrusions about 78µm in height (plat means the PDMS with not any structure on its surface) (c) 3.1.2 Optimization of the height of micro protrusions In this part, the SLSZ was fixed to 300µm, the single pulse energy applied on micro hole on the mold surface was set to 10µJ, 20µJ, 30µJ, and 40µJ. Fig5 shows the micro structure of the protrusion with increased height by increasing the pulse energy, respectively 48µm (10µJ), 65µm (20µJ), 78µm (30µJ), 98µm (40µJ). Fig6.a) presents the influence of superhydrophobicity by varying the height of micro protrusion. For all these 4 samples, the SCA on their surface are all above 150° and show little difference, while SA differs from each other. The protrusion with the height of 78µm possess the lowest SA for droplet to roll-off. It is found that protrusion height of 78µm is the most proximate to the height of four corner on the wall of square zone. Thus, when the droplet come to contact firstly with the four corner, it would touch the protrusion at the same time, they function together to hold up the droplet to make it cover only the top part of the wall to rest a large ratio of air gap area. As the cross section of the wall is triangle, for the protrusions shorter than 78µm, the droplet are likely to submerge a larger part of wall leading to a smaller ratio of air gap area resulting in a higher SA. As to the protrusion higher than 78µm, probably its “sharp head” would insert into the droplet before the touch of droplet and the wall, this would also raise resistance for droplet to roll-off. In addition, the transparency test Fig6. b) figures out that the difference in height for protrusions could hardly make difference in transparency, since they are almost the same in occupying the area of the plane, it is common to understand the phenomenon in effecting surface transparency. Herein, we could conclude that a protrusion of about 78µm (approximation to the height of 4 corner on the wall) results in an optimal condition for superhydrophobicity and transparency.


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Figure 5. Micro structures of duplicated PDMS with and size length of 300µm and a protrusion in the center of each square in different height 48µm (a,e), 65µm (b,f) 78µm (c,g) 98µm (d,h)

Figure 6.a) Changes of SCA&SA by the variation of height of protrusion with length of square of 300µm; b) Changes of transparency by the variation of height of protrusion with length of square of 300µm


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4. Robust and stability of optimized superhydrophobic and transparent (OS&T) PDMS films Considering the work environment, the robust and the stability in different situations defines the application capability of polymers in superhydrophobicity also in transparency. The morphological as well as property changes after being in the condition of friction, strong light irradiation, and thermal treatment would also be a scientific interest to analyze. Herein, we prepared several systematical experiments in analyzing robust of friction (Fig7. left), strong light stability (Fig7. right), and thermal stability of OS&T PDMS films.

Figure 7. Schema for The process of friction resistance (left) and strong light stability (right) experiment

4.1 Robust of OS&T PDMS films in condition of friction by sandpaper The robust of the OS&T products in the condition of friction is an important issue for products’ application. Here, we present a convenient experiment charging sandpaper as well as


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counterweight on the robust of friction condition of the OS&T PDMS films. The OS&T PDMS films (with the SLSZ of 300µm, micro protrusion of 78µm in height) surface was covered by sandpaper in the shape of rectangle then the counterweight was placed on them, two length ends of sandpaper were pulled back and forth with a velocity of about 20 cm/s to achieve a cycle of friction. The number of cycles were counted from 5 to 200. Fig8 a) figures the micro structure of OS&T PDMS film surfaces after 200 cycles of the friction. According to these 3D structures after the various cycles of friction, the main structure, which means the wall of each square and the micro protrusion at the center of square, has been little damaged, showing a good quality in resistance of friction. The resistance of friction could be illustrated as following: the flexible property of the PDMS itself allows it to be in the condition of elastic deformation rather than to be damaged in facing the friction. In addition, the regular square micro structure could effectively decrease the stress concentration on a single point and afford the force by the whole area in average, as the force of friction has been averaged and separated, it would be safer for the micro structure on the “wall” from being damaged. Moreover, has the height of protrusion at the center of square is only slightly higher that the height of the “wall”, it could also be efficiently protected in facing the friction. Fig8c) figures the changes of SCA/SA and transparency after various cycle of friction. It could be found that only the 200 cycles of friction led to a SA over 10° and the loss of transparency is controlled in 3% by this special designed structure. Some particle structures have been observed in some of the plates, they may be produced by friction from the sandpaper and may contribute to the loss of transparency as well as the increase of SA.


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Figure 8. Micro structures of OS&T PDMS measured by SEM (a), and height of the protrusion measured laser microscope (b) after 200 cycles of the friction process. Changes of SCA&SA by the variation of cycle of friction with OS&T PDMS films (c); Changes of transparency by the variation of cycle of friction with OS&T PDMS films (d)

4.2 Strong light stability of OS&T PDMS films As to the potential application to the solar cell coating, strong light stability for coating products has become a no-negligible issue. However, the method to appreciatively simulate the strong light stability for treated PDMS has been little reported. Based on the OS&T PDMS films achieved above, we performed a series of experiments on the strong light stability the OS&T PDMS films were placed in a sunlight simulator for 2 hours from 2 times to 8 times of normal sunshine density, respectively. The front view structure of sunlight simulator treated in 8 density times samples are showed in Fig.9a), which indicates nearly no structural change. This could be understood that the high transparency for visible light of treated samples led to little absorption of sunlight energy, even 8 times of strong sunlight could hardly cause structural change of these


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samples. Moreover, the superhydrophobicity as well as transparency of treated samples in Fig9 confirmed their strong light stability in the wavelength scale of visible light.

Figure 9. Front view structure of strong light treated OS&T PDMS surface for 8 times density of sunlight (a). Changes of SCA&SA by the variation of strength of sunshine with OS&T PDMS films (b); Changes of transparency by the variation of strength of sunshine with OS&T PDMS films (c)

4.3Thermal stability of OS&T PDMS films The thermal stability of the micro designed micro and sub-micro structures on the superhydrophobic and transparent thermosetting polymer films is an interesting issue for scientific research and an important property for its applications. In charging the duplicated OS&T PDMS films discussed above, a series of experiments were performed on the thermal stability of OS&T PDMS films. These OS&T PDMS films were kept in tube furnace for 2h at the temperature ranging from 200 ℃ to 400 ℃ . Fig.10 b) shows the evolution of micro morphology after the heating temperature to 325 ℃ , which indicates nearly no changes. Meanwhile, in macro scale, the films after being heated over 300℃ started to harden because of


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its thermosetting property, and it would be harder and began to curling inward to 350℃ . Moreover, the temperature over 400℃ damaged it in creating numerous macro-view cracks, it is understandable in this sustained hardening process. Fig10 figures the variation of SCA/SA and transparency of the after heating samples, the stability up to 350℃ were confirmed. Therefore, the thermal stability for OS&T PDMS could be kept until 325℃.

Figure 10. Micro structures of OS&T PDMS measured by laser microscope without heating (a)and after 325℃ thermal treatment (b) Changes of SCA&SA by the variation of heating temperature with OS&T PDMS films (c); Changes of transparency by the variation of heating temperature with OS&T PDMS films (d). 4.4 Potential application in Micro Fluid Channels One of the potential applications of this OST & T PDMS film can be in the micro-fluid area, which benefit its superhydrophobic and transparent properties. By the above approach, we designed and fabricated a channel like structure for micro fluid capability. Fig11 presents a designed structure with flat surface functioning as the fluid channel and the rest area with structured surface showing superhydrophobicity. Furthermore, the whole surface results in high transparency.


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Figure11. Schema (a) and Real sample (b) of fluid channel surface. 5. Conclusion a) The well-designed micro structural PDMS films with superhydrophobicity and transparency was achieved by simple duplication process. The superhydrophobicity also transparency of treated PDMS films mainly resulted from their particular structure: an array of square by micro “wall” with the side length of 300µm, full of submicron protrusions standing on the “wall” of the square. Moreover, a micro protrusion with a height about 78µm, which is slightly higher than the corner of the “wall”, is positioned at the center of each square. b) The friction robust by sandpaper and counterweight has been analyzed. Treated OS&T PDMS films keeps the properties as friction robust more than 100 cycles of friction. c) The strong light stability of OS&T PDMS films has been researched. A well stability until 8 times density of sunlight has been found. d) The thermal treatment for morphology changes has been investigated. Before the macro damage led by their thermosetting property over 325 ℃ , the morphology as well as


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superhydrophobic and transparent properties of treated PDMS rest unchanged. Herein, the OS&T PDMS possesses the thermal stability until 325℃. e) The method reported to OS&T PDMS fabrication presents various advantages as low cost, large area, mass fabrication etc., attributing to extensive potential application of these OS&T PDMS films in daily life. Considering its superhydrophobicity and transparency with robustness and stability, it could be coated on solar cell panels, achieving the self-cleaning or anti-dust properties with little loss of sunlight energy and functioning well in various environment. It can be potentially used in micro fluid channel fields.

6. Author Information Corresponding Authors *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

7. Acknowledgement The authors greatly thank the funding support by National Natural Science Foundation of China (51210009, 51575309), the National Key Basic Research and Development Program of China (2011CB013000).


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TOC 80x39mm (300 x 300 DPI)


Robust and Stable Transparent Superhydrophobic

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