Research Article www.acsami.org
Transparent and Superamphiphobic Surfaces from Mushroom-Like Micropillar Arrays Su Yeon Lee,† Yudi Rahmawan, and Shu Yang* Department of Materials Science and Engineerng, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104, United States S Supporting Information *
ABSTRACT: Transparent, superamphiphobic surfaces that repel both water and oils are prepared from mushroom-like micropillar arrays consisting of nanoparticles only at the top of the pillars by controlled compartment filling of silica nanoparticles into the bottom of the poly(dimethylsiloxane) (PDMS) mold, followed by infiltration of epoxy and UV curing. Because silica nanoparticle decorated pillar heads are more resistant to O2 plasma than the polymer pillars, we can precisely control the head size of micropillars and nanoroughness on top of the pillar heads by varying the O2 plasma time. The combination of nanoroughness and mushroom-like micropillars leads to superhydrophobicity and oil repellency to different organic solvents. High transparency is achieved by increasing the spacing ratio of micropillars. Last, we demonstrate anisotropic wetting on the hierarchical surface can be achieved by combining photolithography, replica molding, and self-assembly techniques. KEYWORDS: re-entrant, mushroom-like structures, micropillars, nanoparticles, superamphiphobicity, transparency
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INTRODUCTION 1−3
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areas; however, it is difficult to precisely control the morphology that could repel oil. It will be intriguing to combine the benefits of the two fabrication methods, specifically to address the rising interests of high transparency of the antifouling coatings for surfaces, such as windows, solar panels, safety goggles, and optical devices. Superamphiphobicity requires high surface roughness with re-entrant texture, which could lead to opaqueness in the visible spectrum due to scattering.20,25−27 Transparent, superamphiphobic surfaces have been demonstrated from nanoparticle assemblies13,22 and hierarchical micropillar arrays by flatten the pillar top,17 and by spray coating of nanoparticles onto and between micropillars.28 In the case of nanoparticle assemblies, fractal structures with high porosity are preferred,13 which also makes the surface vulnerable to external mechanical forces. Advances have been made to enhance coating durability by using copolymer grafted nanoparticles that can be crosslinked and bond to the substrate29 or by creating a multilayered, thick film of hierarchical particle assembly,30 which remains superhydrophobic (not necessarily superoleophoboic) even when the top surface layers are removed. Nevertheless, the former approach involves multistep synthesis, while the latter suffers loss of transparency. Compared to nanoparticle assemblies, micropillar arrays offer a more robust model system that is easier to fabricate than microhoodoos and have well-defined roughness to study wettability. The pillars with low aspect ratio (height/diameter)
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In nature, surfaces of lotus leaves, insect legs and wings, and feathers6 show extremely high water-repellency, so-called superhydrophobicity, with apparent water contact angles (WCAs) greater than 150° and small contact angle hysteresis (CAH, typically 30 mN m−1). By comparing the experimental data with that estimated from
nano- and microroughess to achieve high transparency and superamphiphobicity. First, we partially infiltrated silica nanoparticles into a PDMS mold, followed by infiltration of epoxy and UV curing. Micropillars with nanoparticles decorating only on top of the pillars were obtained. After exposure to O2 plasma, the micropillars beneath the nanoparticles were etched, leading to reduction of pillar diameter while increasing the etching duration. The silica nanoparticle layers acted as etch mask; while nanoroughness increased with etching time, the 24201
DOI: 10.1021/acsami.5b07551 ACS Appl. Mater. Interfaces 2015, 7, 24197−24203
Research Article
ACS Applied Materials & Interfaces
SNM no. CBET-1449337. The Laboratory for Research on the Structure of Matter (LRSM), Penn NSF MRSEC (DMR1120901), and Nanoscale Characterization Facility (NCF) are acknowledged for access to SEM.
Cassie−Baxter equation, we concluded that the superamphiphobic surfaces observed in our system were in Cassie−Baxter state. High transparency (>90% transmittance in the visible wavelength) was obtained from micropillars with SR ≥ 3. Furthermore, we demonstrated anisotropic wetting on photopatterned mushroom-like pillars; a water droplet was elongated in the direction parallel to the microlines. The unique hierarchical structures with precisely controlled micropillar structures and nanoparticle assemblies presented in our system offers new insights of the interplay of micro- and nanoroughness and curvature effect to superomniphobicity and transparency. The photopatternability will open doors to design hierarchical surfaces that direct water and oil repellency.
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METHODS
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ASSOCIATED CONTENT
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Fabrication of Mushroom-like Micropillar Arrays. An ethanol suspension containing 5 wt % silica particles (500 nm in diameter, Alfa Aesar) was dropped onto PDMS mold with cylindrical holes, followed by blading the silica suspension across the PDMS mold using a glass slide. After complete evaporation of ethanol, the PDMS mold was placed on diglycidyl ether of bisphenol A-based epoxy resin (DER 354, Dow Chemical) containing 3 wt % photoinitiator (Cyracure UVI 6976, Dow Chemical)-coated glass substrate to infiltrate the resin into the cavities of mold by capillary force. The epoxy resin with silica particles was photopolymerized in the mold by exposure to UV light at 365 nm at a dosage of 17 J/cm2, followed by peeling off the PDMS mold to obtain the micropillars decorated with silica particles. An isotropic bombardments of reactive ions from O2 gas etch the polymerized epoxy resin, resulting in structural evolution to mushroom-like micropillar arrays with re-entrant geometry (70 mTorr, 40 sccm, 50 W). Because as-prepared mushroom-like micropillar microstructures are hydrophilic, their surfaces were hydrophobized by chemical vapor deposition of (tridecafluoro-1,1,2,2tetrahydrooctyl)trichlorosilane (SIT8174.0, Gelest Inc.) in a sample desiccator. Measurements. Contact angles were measured by Ramé−Hart standard automated goniometer (Model 290). The static water CA was measured from a 5 μL deionized water droplet and averaged over three different fresh spots on each sample. Advancing and receding water CAs were measured by automatically adding and removing water in steps of 0.5 μL from the substrate, respectively. SEM images were taken using a FE-SEM (JSM-7500F, JEOL) at an acceleration voltage of 15.0 kV and an average working distance of 8.0 mm. The optical transparency of the micropillar arrays coated glass substrates was measured using a Varian UV−Vis−NIR Cary 5000 spectrophotometer. Epoxy resin coated glass substrate was used as reference. S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07551. (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
* E-mail:
[email protected]. Present Address †
Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), Daejeon 305−600, Korea. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research is supported in part by the National Science Foundation NSF/CBET, award no, CBET-1264808 and NSF/ 24202
DOI: 10.1021/acsami.5b07551 ACS Appl. Mater. Interfaces 2015, 7, 24197−24203
Research Article
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DOI: 10.1021/acsami.5b07551 ACS Appl. Mater. Interfaces 2015, 7, 24197−24203
