Large-Area Preparation of Robust and Transparent Superomniphobic

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Large-Area Preparation of Robust and Transparent Superomniphobic Polymer Films Yi Wu, Jing Zeng, Yinsong Si, Min Chen, and Limin Wu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05600 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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Large-Area Preparation of Robust and Transparent Superomniphobic Polymer Films

Yi Wu, Jing Zeng, Yinsong Si, Min Chen, Limin Wu* Department of Materials Science and State Key Laboratory of Molecular Engineering of Polymers, Advanced Coatings Research Center of Ministry of Education of China, Fudan University, Shanghai 200433, China

*Corresponding

author: [email protected] 1

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ABSTRACT: Transparent superamphiphobic surfaces that repel various liquids have many important applications but remain critical challenges in fabrication, such as expensive or complicated fabrication methods, a pair of contradiction between rough surface for superamphiphobicity and smooth surface for transparency, large-area fabrication, etc. Herein, we report a simple and effective strategy for large-scale fabricating robust, transparent and superomniphobic polymer films by the combined unidirectional rubbing and heating-assisted assembly technology. The obtained polymer films display two kinds of special structures of monolayer ordered re-entrant geometries with either hexagonally triangular protrusions or with hexagonally rectangular micropillars, depending upon the sphere diameters of silica templates, and demonstrate excellent repellence to water and low-surface-tension liquids, as well as high transparency. KEYWORDS: transparent, superamphiphobic, liquid-repellent, re-entrant geometries, durability

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Superamphiphobic surfaces display contact angles greater than 150° and low contact angle hysteresis with water and oil.18 Compared to superhydrophobic surfaces, superamphiphobic surfaces have a much broader range of applications, including self-cleaning,9,10 anti-fouling,11,12 chemical shielding,13 spill-resistance,14,15 anti-icing,16 corrosion

prevention,17,18

fuel

transport,1921

and

drag

reduction,22 etc.

The

superamphiphobic capability of materials is arisen from the combination of their special surface topography (re-entrant, convex curvature or overhang) and low-surface-energy chemistry to obtain the so-called Cassie-Baxter state.23 However, fabrication of such superamphiphobic surfaces with specific topography is rather time-consuming, usually involving expensive lithographic tools, or complicated chemical procedures.2429 In addition, the contradiction of surface roughness with transparency poses another challenge:3033 On the one hand, the superamphiphobic surface requires enough surface roughness to obtain high contact angle (CA) and low contact angle hysteresis (CAH). On the other hand, the surface roughness must be small enough to retain high transmittance of light because of Mie scattering from the rough surface. Although people ever used simple dip coating or spraying methods to successfully fabricate transparent superamphiphobic surfaces, the as-obtained surface topography with re-entrant geometry was irregular and uncontrollable.34,35 Relative to irregular structures, the regular re-entrant structure enable a fundamental analysis of the wettability of the surface based on 3


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geometric models, which can help us to further understand and design liquid-repellent surfaces.36 Accordingly, it is of great significance to create such regular surfaces that simultaneously exhibit high CA, low CAH, and high transparency, as well as chemical and mechanical stability for practical applications. In this work, we present a simple and facile strategy for large-scale fabricating robust, transparent and superomniphobic films by the combined unidirectional rubbing and heating-assisted assembly technology. Two kinds of

unprecedented structures,

monolayer ordered re-entrant geometries with hexagonally triangular protrusions or with hexagonally rectangular micropillars are successfully fabricated, depending upon the diameters of silica colloidal sphere for templates. The special structures endow the surfaces with excellent repellence to water and various low-surface-tension liquids. And the film is highly transparent and stable against concentrated acids, alkalis and salts, as well as mechanical damages.

RESULTS AND DISCUSSION As shown in Scheme 1, transparent superomniphobic films are fabricated using a facile unidirectional rubbing combined with heating-treatment method. Briefly, a monolayer silica array is obtained by unidirectionally rubbing a dry powder of silica spheres on a PDMS-coated substrate with another PDMS substrate.37 After the silica spheres are assembled into a monolayer and then thermally treated at above 220 oC for 72 h, an epoxy based photoresist SU-8 is infiltrated into the silcia template and subsequently cured to form film after exposed to UV radiation for 1 min. Monolayer ordered re-entrant structure with excellent liquid-repellent properties is obtained after the silica template is 4


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removed and the film is modified with 1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane (PFOTS) by chemical vapor deposition. Figure 1 demonstrates the monolayer ordered re-entrant structures with hexagonally triangular protrusions or hexagonally rectangular micropillars using differnt silica microspheres as the building blocks. When 5 μm-silica sphere-based template is used, the hexagonal tribrachia-posts with re-entrant geometry structure are obtained, in which six triangular protrusions are periodically standing around each hole (inset, Figure 1a and Scheme 1a). Interestingly, when 10, 15, and 20 μm-silica microsphere-based templates are used, another special ordered porous structure is obtained, in which six rectangular micropillars instead of triangular protrusions are standing around each hole (insets, Figure 1b-d and Scheme 1b). We can see later that these structures play a critical role in obtaining the so-called Cassie-Baxter state with low surface tension liquids. The reason why the two totally different kinds of topography is formed from various size silica microspheres could be explained as follows: As illustrated in Scheme 1, after heating the silica template, the process of spin-coating includes two steps: one is the infiltration of SU-8 into the voids of silica template; another is the rinsing of the excess SU-8 from the template. In general, when the silica colloidal microspheres are assembled into a highly crystalline arrangement structure, both of the voids formed by adjacent three silica microspheres (denoted as void 1, Scheme 1a) and adjacent two silica microspheres (denoted as void 2, Scheme 1) exist capillary effect. When the silica microspheres are small, e.g., 5 μm in diameter, the void 2 are easily blocked because heat treatment causes closer contact of adjacent silica microspheres. As a result, hexagonally tribrachia-posts with re-entrant geometry structure appears. On the contrary, when large silica 5


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microspheres are used for the templates, the void 2 are hardly blocked completely under the same heat treatment condition. Meanwhile, the void 1 do not show sufficient capillary effect owing to the larger gap compared with the void 1. As a result, the corresponding SU-8 in the void 1 are easily washed away from the templates during the step 2 in spin-coating process, leaving behind SU-8 in the void 2 which is then cured by UV to cause ordered porous structure with six rectangular micropillars standing around each hole. Figures 2a-c show a 5 μL water droplet used as a testing probe to contact, press and retract from the polymer film surface obtained from 5 μm-silica sphere-based template as an example. One can see that the water droplet is maintaining a nearly perfect spherical shape, and no excess water is left on the film, suggesting a superhydrophobicity and negligible water-adhesion of the obtained polymer film. Moreover, the CAs of ethylene glycol, mineral oil, and olive oil on the obtained polymer films are 152±1°, 148±2°, and 150±2°, respectively, indicating an excellent oil-repellency (Figure 2d-f), which can be further confirmed by the optical photograph of water (dyed in blue), ethylene glycol (dyed in yellow), mineral oil (dyed in red), and olive oil (dyed in green) droplets sitting on the polymer surface in Figure 2g. Table 1 summarizes the apparent CAs and CAH of water and three kinds of oil droplets on the polymer films obtained from different silica microsphere-based templates. The water droplets on all the polymer surfaces adapt to a spherical shape with CA 160° and CAH 5°, and the CAs of three oil droplets approach 150° even above with CAH 150°) even after 7 cycles of bending/recovering process (inset in Figure 6a, and Figure 6c). All these results indicate that the prepared polymer film has satisfactory mechanical robustness to ensure long-term liquid-repellency although it is monolayer. Transparent self-cleaning films or coatings are highly desirable for many applications such as mobile phones, windows and optical devices. Herein, we further investigated the optical transparency of the obtained polymer coatings or films by ultraviolet-visible (UV−vis) light transmittance spectroscopy, and found that hydrofluoric acid concentration and etching time have a significant impact on the transparency of the obtained polymer films. When treated in 2 vol% of hydrofluoric acid for 15 min, the obtained polymer film is translucent or opaque (see the inset in Figure 6a). However, After treated by 5 vol% of hydrofluoric acid for 5 min, the optical transparency of the polymer film is greatly improved. As shown in Figure 7a, before PFOTS modification, the polymer film has a comparable transmittance to plain glass slide. Even after modified with PFOTS, the polymer film still remains above 80% of transmittance in the spectral range of 400−800 nm, showing a good transparency except for outstanding superomniphobicity as shown in Figure 7b. And these films are easily peeled off from glass slides (Figure 7c and 7d), and then transferred onto other substrates. All these results suggest the polymer films or coatings we present here may be used in mobile phones, laptops, cameras as anti-fingerprint self-cleaning transparent protective films.

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CONCLUSION In summary, we have developed a facile unidirectional rubbing-heating treatment method for fabrication of transparent superomniphobic polymer films or coatings. Compared to the previously reported self-assembly approaches to prepare liquid-repellent surfaces, our strategy has some obvious merits: It can produce two kinds of unprecedented strucutures: monolayer ordered re-entrant structures with hexagonally triangular protrusions or hexagonally rectangular micropillars. Both of them own not only excellent liquid-repelling and self-cleaning properties, but also high transparency and good durability against physical abrasion, strong acidic / alkaline and salt solutions. Moreover, it is convenient to massively fabricate such ordered structures and polymer films on flat and non-flat substrates such as concave and convex surfaces. The outstanding features of the obtained polymer films we present here may find many important applications such as windows, lenses, mobile phones, laptops, cameras, solar panels, optical devices, etc., as anti-fingerprint or self-cleaning transparent protective films.

MATERIALS AND METHODS Materials. Polydimethylsiloxane (PDMS, SYLGARD 184) was received from Dow Corning Corp. Powders of silica particles (5, 10, 15 and 20 μm) were purchased from Zhiyi Microsphere Technology Co., Ltd. Photoresist SU-8 –based epoxy was purchased from MicroChem Corp. Ethyl acetate (99.5%), hydrofluoric acid (40%), PFOTS, three testing liquid including olive oil, ethylene glycol and mineral oil were obtained from Aladdin Chemical Reagent Corp. Absolute ethanol, HCl, NaOH and NaCl were supplied from Sinopharm Chemical Reagent Corp. Deionized water was used for all experiments. 11


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Fabrication of PDMS substrates and monolayers silica colloidal templates. A mixture of PDMS prepolymer and curing agent was degassed in a vacuum chamber and then gently poured on the substrate, followed by curing in an oven at 80 °C for 2 h as follows: For the PDMS substrate used as rubbing, a conventional weight ratio of 10:1 of prepolymer and curing agent was adopted to increase friction; For the PDMS-coated substrates, a weight ratio of 2:1 was used to reduce the particle-substrate adhesion energy, which can facilitate the contact between silica particles during the heating process. The effect of the weight ratio of the prepolymer and curing agent on silica templates and the corresponding porous structures were investigated as shown in the Supporting Information (Figure S3). Monolayer silica templates were obtained by unidirectional rubbing a dry powder of 5, 10, 15 or 20 μm silica particles on PDMS-coated substrates with PDMS rubbing substrate. These templates were then heated at 220 oC for 72 h. Fabrication of hexagonally ordered porous surfaces with liquid-repellent properties. After the monolayer silica templates were subjected 220 oC for 72 h, the photoresist SU-8 based epoxy was then spin-coated (3000 r.p.m., 30 s, acceleration 500) into the voids of the templates. The surface residual SU-8 was rinsed using ethyl acetate. The SU-8 was subsequently cured to form polymer film or coating after exposed to conventional UV (350-400 nm) radiation for 1 min. The obtained polymer film or coating was further cured in an oven at 80 oC for 2 h, and then etched in 2-5 vol% hydrofluoric acid aqueous solution for 5-15 min to remove the silica template, and subsequently washed in ethanol thoroughly before dried at 60 oC for 2 h. PFOTS was used to modify the sample by CVD at reduced pressure and room temperature for 12 h. The polymer sample is denoted as SU-8 (x), where x stands for a diameter of silica microspheres. 12


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Morphology and contact angles. The morphology of the sample was characterized by using a scanning electron microscope (SEM Philips XL 30). Wetting property of the surfaces of the samples was investigated with an OCA15 contact angle analyzer (Data-physics, Germany). Water, ethylene glycol, mineral oil and olive oil were used for testing droplets. Optical measurements. UV-vis spectroscopy measurements were made on a Beckman Coulter DU520 UV−vis spectrophotometer (Fullerton, CA). The sample (1.0× 1.5 cm2) was scanned in the wavelength range of 300-800 nm. Optical photographs were obtained by using a Cannon digital video camera. Stability measurements. To prove the chemical stability, the samples were immersed in strong acid, strong alkaline and concentrated NaCl salt solutions (1 M) at ambient temperature for 96 h, respectively. After the immersion experiment, the change in CA was recorded. To verify the mechanical stability, the falling sand abrasion test and the sandpaper scratch test of films were conducted by a homemade falling sand experimental device and a homemade polish tester, respectively. For the falling sand abrasion test, a through guided tube (50 cm) was vertically fixed above the testing sample, the test surface of the sample was at 45° to the horizontal direction. The distance between the bottom nozzle of guide tube and the sample surface was 1 inch. Thus, the sand entered from the top of the guide tube and impacted the sample surface. After abrasion experiment, the change in CA of the samples was recorded. For the sandpaper scratch test, a piece of 1500-mesh sandpaper (2.25×2.25 cm2) and a 500 g load were dragged on the surface of the sample in one direction with a speed of 1 13


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cms-1. After scratch test, the changes in CA of the samples was

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recorded.

ASSOCIATED CONTENT Supporting Information Available. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENTS Financial supports of this research from National Key Research and Development Program of China (2017YFA0204600) and the National Natural Science Foundation of China (51721002, 51673045) are appreciated.

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Figures and Scheme Captions

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Scheme 1. Schematic illustration for fabrication of large-area transparent superomniphobic films by unidirectional rubbing combined with heating-treatment technology.

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Figure 1. SEM images of hexagonal porous structures with triangular protrusions or hexagonal rectangular micropillars obtained from different diameters of silica microspheres. a) 5, b) 10, c) 15 and d) 20 μm. The insets show the single non-closely packed hole structure.

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Figure 2. Antiwetting behavior of the obtaiend polymer films. a-c) Photographs of dynamic measurements of water-adhesion on the obtained polymer films.d-f) Contact angles of oil droplets on polymer surfaces. g) Photographs of water (dyed in blue), ethylene glycol (dyed in yellow), mineral oil (dyed in red) and olive oil (dyed in green) droplets sitting on the polymer surface.

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Figure 3. Models of a unit cell of the polymer films used for estimation of apparent CAs by the Cassie-Baxter equation. a, b) Hexagonal tribrachia-posts with re-entrant geometry structure. (c) Top-view and (d) side-view SEM image of an overall array and a unit cell (denoted with dotted box). (e) Magnification SEM image of the dotted box in Figure d. f, g) Hexagonal rectangular micropillars with re-entrant geometry structure. h) The SEM image of an overall array and a unit cell obtained 20 m silica template (denoted with dotted box).

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Figure 4. Optical photographs of dyed liquid droplets on a 4-inch silicon wafer coated with polymer films. Insets: the corresponding contact angles of water and oil droplets.

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Figure 5. Chemical stability and mechanical robustness test of the obtained polymer films. a-c) Polymer films immersed in strong acidic, strong alkaline and concentrated salt solutions at ambient temperature for 96 h. d-f) The corresponding photographs of dyed liquid droplets on the polymer surfaces dealt with the chemical environment above. g) Abrasion resistance of polymer films by sand abrasive test driven by gravity. h) Schematic illustration for the sandpaper scratch test of polymer films. i) The corresponding photograph of dyed liquid droplets on the polymer film surface dealt with 24


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the sandpaper.

Figure 6. a) Variation in the water contact angle of the polymer film upon repeated bending and recovering cycles. b) SEM image of the polymer film showing excellent flexibility. c) The contact angle of the bending and recovering film.

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Figure 7. Preparation of transparent superomniphobic films. a) UV-vis transmittance spectra of polymer films compared to a plain glass slide. b) Side- and top-viewed photographs of water and oil droplets deposited on a 4-inch glass coated with polymer films. c, d) The stripping and the free-standing polymer films, respectively.

Table 1. Wettability of the liquid-repellent polymer films. Apparent CAs and CAH of 5 μL droplets of various liquids on the polymer films obtained from different diameters of silica microspheres. Polymer films

SU-8 (5)

Water

Ethylene glycol

Mineral oil

Olive oil

CA/CAH [°]

CA/CAH [°]

CA/CAH [°]

CA/CAH [°]

163±2/3±1

152±1/4±2

148±2/10±3

150±2/7±4 26


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SU-8 (10)

161±3/2±2

155±2/6±1

150±4/9±1

152±1/8±1

SU-8 (15)

165±2/3±1

153±2/5±3

152±2/11±1

151±3/8±1

SU-8 (20)

160±2/5±1

154±4/5±1

148±6/10±1

147±5/10±1

TOC

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Large-Area Preparation of Robust and Transparent Superomniphobic

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