Superamphiphobic Surface by Nanotransfer Molding and Isotropic

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Superamphiphobic surface by nanotransfer molding and isotropic etching Sang Eon Lee, Han-Jung Kim, Su-Han Lee, and Dae-Geun Choi Langmuir, Just Accepted Manuscript • DOI: 10.1021/la4011086 • Publication Date (Web): 23 May 2013 Downloaded from http://pubs.acs.org on June 2, 2013

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Superamphiphobic surface by nanotransfer molding and isotropic etching Sang Eon Lee1, Han-Jung Kim1, Su-Han Lee2 and Dae-Geun Choi1,2,* 1

Nano-Mechanical Systems Research Division, Korea Institute of Machinery & Materials (KIMM), 171 Jang-dong, Yuseong-gu, Daejeon, 305-343, Republic of Korea

2

Department of Nano-Mechatronics, University of Science and Technology, 217 Gajungro, Yuseong-gu, Daejeon, 305-350, Republic of Korea

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ABSTRACT We present a novel method of fabricating superhydrophobic and superoleophobic surfaces with nano-scale reentrant curvature by nanotransfer molding and controlled wet etching of the facile undercut. This method produces completely ordered re-entrant nano-structures and prevents capillary-induced bundling effects. The mushroom-like, re-entrant, overhanging structure demonstrates superhydrophobic and superoleophobic characteristics, as tested by water droplet bouncing and contact angle measurements, and has high transparency on a flexible substrate. Widespread use as self-cleaning surfaces is expected in the near future.

KEYWORDS Superhydrophobic; Superoleophobic; Flexible; Reentrant; Overhang; Nanotransfer molding


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1.

Introduction

Inventions of great merit are often inspired by nature, for example, the lotus leaf, Nelumbo nucifera,1 the legs of water striders,2 cicada wings3 and gecko feet.4 Superhydrophobic surfaces are defined as possessing a water (surface tension γlv = 72.1 mN/m) contact angle (CA) greater than 150°, and low CA hysteresis, meaning water droplets easily roll off the surface.5-7 Enormous interest in superhydrophobic surfaces has been generated by extensive industrial and scientific investigation into applications such as self-cleaning surfaces for clothes or solar-cells, and fog-resistant lenses and mirrors.8-10 Numerous studies have focused on obtaining superhydrophobic surfaces by top-down or bottom-up approaches11-13 based on the Cassie-Baxter model. The model suggests the use of a mixed solid-liquid-vapor interface14, because the homogeneous wetting regime of the fully wetted Wenzel model is applicable for flat but not for patterned surfaces.15-17 Despite these many approaches to achieving superhydrophobic surfaces, however, water is not the sole liquid that such surfaces will likely encounter in practice, and therefore many of the superhydrophobic surfaces described in literature may be challenging to apply in practical situations. For example, lotus leaf is superhydrophobic and repels water droplets from its surface. However, when organic liquids with relatively low surface tension, such as toluene (γla = 28.5 mN/m), ethanol (γla = 22.4 mN/m) and ethylene glycol (γla = 47.3 mN/m), contact the lotus structure, the organic liquids show a much smaller CA than water and can easily wet the surface. Superoleophobic surfaces with highly repellent characteristics for low surface tension liquids — in other words, surfaces which show oil-repellant characteristics — are important to applications such as inkjet printing, anti-fouling ship hulls, and anti-fingerprint films.18,

19

Recently, a

microstructure with re-entrant curvature, in which the meniscus of a liquid droplet has a convex curvature, was suggested by Cohen et al.20-22 as an effective method of realizing a 3 ACS Paragon Plus Environment

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superoleophobic surface in accordance with practical requirements. Various re-entrant structured surfaces have been developed by approaches such as ripples created by a Bosch etching process,23 hierarchically structures developed by cross-linked PDMS,24 trapezoidal microstructures created by 3-D diffuser lithography,25 T-bar structures by combinatorial-mold imprint lithography26 and mushroom-like micropillars by micromolding.27 The re-entrant structure would be helpful in creating both the hydro- and oleo-repellent force because it can generate a net force for lifting a droplet upward, even for low surface-tension liquids.22 Up until now, however, the efficient fabrication of nanostructures with re-entrant curvature for superoleophobic surfaces has not been reported. Previous approaches are limited in that they are only applicable to micro scale structures on a rigid substrate,20-22 or involve multiple complicated processes.25, 27

To solve these challenges, in this report we present a facile

method of fabricating nano-scale re-entrant profiles that exhibit both superhydrophobic and superoleophobic characteristics by nanotransfer molding28, 29 and controlled wet etching of the undercut. The engineered surface shows both superhydrophobic and superoleophobic properties with high transparency and flexibility. 2.

Experimental sections

2.1 Fabrication of overhang structures Nanotransfer-molding has been widely used as a cost-effective, versatile, and efficient technique for the transfer of functional materials such as metals and polymers. The overall fabrication process is depicted schematically in Figure 1. The LOR 1A resist (Microchem, USA) was spin-coated on a glass slide to form the undercut. Glass can be substituted for any other flexible substrate, such as PET or PP film. The LOR 1A resist was then baked at 200 ˚C for 20 minutes on a hot plate. The resist layer was 200 nm thick. Transfer resin 495 PMMA 4 ACS Paragon Plus Environment

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A6 (Microchem, USA) was spin-coated on using a polyurethane acrylate (PUA) mold. The PUA mold was fabricated using a UV-curable polymer resin (Minuta Tech., Korea) consisting of tri (propylene glycol) diacrylate, trimethylolpropane triacrylate as a monomer, and 2,2-dimethoxy-2-phenylacetophenone as a photoinitiator.30,

31

Acrylate polysiloxane

(TEGO Rad 2200N) was added as a releasing agent (1–4%). Flexible PUA molds with a water contact angle of 85° were replicated from Si masters by UV (λ = 365nm) curing at an intensity of 30 mW/cm2 for a duration of 120 s at a distance of 15 cm. The mold, coated with PMMA resin, was pre-baked at 80 ˚C for 60 seconds. The PMMA thickness was approximately 280 nm. The mold with PMMA resist was contacted onto the LOR-coated glass wafer and maintained at 130 ˚C and 5.0 bar for 120 seconds of transfer molding. After a few minutes of cooling, the pressure was relieved and the PUA mold was removed, leaving behind a nano-patterned polymer layer on the glass wafer. The PMMA residual layer was removed using O2 reactive ion etching (50 W, 50 sccm, 70 s) process. Finally, an isotropic wet etch using a tetramethylammonium hydroxide (TMAH) developer with high etch selectivity for LOR 1A, such as AZ 300MIF (Microchem, USA), was performed by simply dipping the nano-patterned substrate for 5 seconds in order to make nano re-entrant structures with undercut shapes. 2.2. Structural and morphological characterization The static CA was measured by taking images of de-ionized water drops (4 µL) using a drop shape analysis system (DSA 100, Krőss, Germany). Results were averaged over at least three different spots per diameter, and the average deviation was ±2˚. The CA of a flat glass substrate without any hydrophobic coating was measured first, and found to be 45˚. The CA values of the nano-scale overhang structure arrays were then measured. CA measurements were performed for two different states - the initial state following application of a single 5 ACS Paragon Plus Environment

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drop, and after drying out from soaking the sample in DI water. Water, ethylene glycol, ethanol, and toluene were used as CA test solvents. In the case of toluene, the CA measurement was performed immediately after dropping the liquid, because toluene can dissolve PMMA structures. 3.

Results and Discussion

3.1.

Design of overhang structures

If a water repellent surface is not flat, the topography can be defined by the Wenzel and the Cassie-Baxter methods. The Wenzel method assumes that liquid fills in the grooves on the rough surface so that water sticks to the surface. On the other hand, the Cassie-Baxter model is effective on a very slippery surface, and uses the area fraction of the solid surface in contact with the liquid. Figure 2 shows a schematic illustration of the difference between nanorods with a high aspect ratio and re-entrant curvature in contact with liquid following a drying process. For high aspect ratio structures such as nanorods, nanotubes or nanowires, the structures readily gather together after a droplet falls or after soaking in water, turning into similarly sized bundles with irregular shapes because of capillary forces32, 33 (Figure 2a). This phenomenon is also known as the nanocarpet effect.34, 35 The deformation of the pyramid shape is determined by the changing distance between each rod, as well as diameter and height. The decrease in height, ∆h, due to tilting of the closest neighbor to a central rod is34 ∆ 2 α/2

(1)

where α is the tilting angle of the rod closest to vertical. A lower tilting angle α creates a gently sloping pyramid, and has the following relationship: /2 /2

(2) 6

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Let cosφ = h/s, sinφ = d1/2S, and S = (h2+d12/4)1/2 and we can derive α 2 /2 /2

(3)

Equation (3) demonstrates that the tilting angle α increases as L becomes larger, but decreases with increasing h and d1; h decreases as the angle α increases. It is important to note that a lower pillar height may produce a lower CA. Thus, if h decreases sharply, the CA will also decrease.36 The capillary-induced bundling effect can be prevented in structures with re-entrant curvature by the top surface of the cap. As shown in Figure 2b, the diameter of the pillar d2 is much greater than d1 whilst the other elements remain unchanged. The cap of the pillar contacts another cap before the centers of the rods start to congregate together; this limits the tilting angle β to small values. Accordingly, the height of the re-entrant curvature is typically higher than nanorods when the same quantity of water contacts the surface. However, if the rod is too slender to support the cap, it becomes difficult to make a reliable hydrophobic surface since the rods can easily bend and become bundled together. In this regard, we designed thick re-entrant curvatures. As predicted, the re-entrant curvature model results in a higher CA than conventional nanorods.

3.2 Surface wettability of overhang structures Scanning electron microscope (SEM) images of the fabricated nano-scale overhanging structures and static CA analysis of the glass substrates are shown in Figure 3. Nano-scale overhanging re-entrant structures were formed very regularly on the substrate. As shown in Figure 3a, we can acquire fully mushroom-like, re-entrant, and overhanging structures via nanotransfer molding and an isotropic wet etching for the controlled undercut shape. We could also get a different style of re-entrant curvature by varying the wet etching time. All of 7 ACS Paragon Plus Environment

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these fabricated structures have the same height (300 nm). The CA of the re-entrant structure with a relatively thick rod, induced by incomplete wet etching (Figure 3b), was lower than that of the well-developed re-entrant structure (Figure 3a). Regularly ordered nanorods with high aspect ratios, as in Figure 3d, were observed, and had a static water CA of 147˚, which suggests that they can create almost superhydrophobic surfaces. However, the nanorod surface did not exhibit superoleophobic characteristics, as shown in Figure 3g. Well-aligned nanorods were found to form triangular pyramids due to capillary force-induced bundling effects after the substrates were allowed to dry out from the fully wetted state, as shown in Figure 3e. Futhermore, as shown in Figure 3f, nanorods in some regions lie flat on the substrate in some regions dependent upon liquid evaporation. For this reason, the substrate becomes a nearly flat surface, and the static CA shows a sharp decrease. By contrast, the CA (CA = 154˚) of the re-entrant structure is consistently high and the pattern topography is identical to that of Figure 3a after the evaporation of water. The nano-scale overhang structures demonstrate outstanding superhydrophobic and superoleophobic characteristics. As shown in Figure 3h, with various liquid droplets such as ethanol (red), ethylene glycol (green), toluene (blue), and DI water (transparent), repellent characteristics are observed due to the reentrant curvature. The inset shows static CA values of ethylene glycol (151˚), ethanol (146˚), and toluene (141˚). For comparison, the CA of ethylene glycol, ethanol and toluene on flat glass are respectively 71˚, 65˚, and 55˚. In order to find a critical pitch, we varied the spacing of the re-entrant curvature as a function of the pillar diameter and measured the contact angle with a toluene drop. The results are as shown Figure 4. As the spacing decreases, the contact angle increases for all pillar diameters. Again, the contact angle was greatest and the superamphiphobic characteristic were most pronounced at fs = 0.5. These results support our idea that there may be a critical contact area fraction at which a surface can be a superamphiphobic. Dramatic increases in CA from the initial stage, despite the low surface 8 ACS Paragon Plus Environment

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tension of these liquids, suggest that re-entrant curvature is a very promising technique for industrial applications. 3.3 Water droplet bouncing test

A number of studies have appeared in the literature that have investigated the impact of a drop on a solid surface.37, 38 The number of droplet bounces has been shown to increase dramatically on a superhydrophobic surface, and complete rebounds have been observed.39, 40 When a water droplet bumps into a surface, different results such as sticking, oscillation, splash, and rebound are expected depending on the surface hydrophobicity, droplet volume, impact velocity, and liquid characteristics. The behavior of free-falling water incident on the surfaces fabricated in the present study was recorded with a high-speed camera (Phantom v9.1, Vision Research Inc. USA, frame rate 2000 frames/s, resolution 670 × 480 pixels). To verify water repellency, 15 µL of water was dropped from a height of 5 cm onto the structured surface. Water droplets were released on three different substrates: nano-scale overhanging shape, cylindrical nanorods, and bare glass. By investigating sequential images of the droplet impact, we identified three distinct outcomes. (See Supporting Information, Figure S1 and movie clips in the Supporting Information 1-3). Although the glass substrate is different, during the first three steps the water droplets underwent the same processes: impacting, spreading, and contraction. However, during the fourth and final identified step, the droplets on each substrate behaved differently. Figure S1a, with the flat glass substrate, shows retraction after the spreading with negligible contraction, and the droplet quickly reaches a stable state. As seen in Figure S1b, huge oscillations occurred on the nanorod structured glass substrate as the droplet juddered to a halt. In contrast to the flat glass substrate, it takes several seconds to reach a steady-state configuration. Figure S1c, with the nano-scale overhanging structured substrate, shows complete rebound of the incident droplet. 9 ACS Paragon Plus Environment

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These results suggest that the design of the nano-scale overhanging structured substrate induces complete retraction of impacting water droplets. As shown in Figure S1d, toluene droplets also completely rebound on re-entrant structured glass, but to a lower height than water. 3.4 Optical properties of nano-scale overhang structured glass

The optical transmittance of the substrates was investigated by UV-VIS transmittance spectroscopy to elucidate suitability for applications such as solar cell panels and anti-fogging goggles.41 Figure 5 depicts the transmittance spectra of various samples including bare glass, a nano structured glass substrate and nanorod structured glass. In the range of 500 - 600 nm, the transmittance of the bare glass was 90%; this was slightly reduced to 78% for the nanoscale overhang structured glass and nanorod glass. Nevertheless, the visibility was not noticeably impacted, as seen in the Figure 5 insets, which show the actual view through the nano-scale overhang structured glass. We demonstrated nano-scale overhang structures via the nano transfer molding method on a flexible PET substrate, as shown in Figure S2. The good transmittance and flexibility signify that our approach can be readily adapted to many diverse substrates. 4. Conclusions

We have presented a straightforward fabrication technique for re-entrant curvature superhydrophobic and superoleophobic nano-scale overhang structured surfaces. In contrast to nanorod structured superhydrophobic surfaces, nano-scale overhang structured surfaces show highly robust performance. We produced re-entrant profiles that exhibit oleophobicity with low surface tension liquid whilst maintaining relatively high transparency and flexibility. In addition, we also demonstrated that this nano-scale overhang structured surface results in 10 ACS Paragon Plus Environment

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full rebounding of free-falling water after impacting the substrate. This result shows outstanding potential for self-cleaning applications. Acknowledgements This research was supported by KIMM research funds (NK169D and SC0890) and a NRF grant (No.2011-0031563 and 2012-0006201). Supporting Information Videos of the water droplet bouncing tests on nano-scale overhang (re-entrant) structured glass. Sequential images of the dynamic behavior of 15 µL free falling water and toluene droplets (Figure S1). Images of a flexible and highly transparent nano scale overhang structured material on a PET film (Figure S2). FOTS surface treatment effect (Figure S3) and the hysteresis data of various liquids (Table S1) This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *

Dae-Geun

Choi,

Tel.:

+82-42-868-7846;

Fax:

+82-42-868-7123;

E-mail:

[email protected]


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Figure 1. Schematic diagram of the fabrication process for PMMA nano-scale overhang structured arrays on a glass substrate. (a) Nanotransfer molding process (b) Nanorod (c) Reentrant curvature structure with undercut after controlled wet developing.


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Figure 2. Schematic illustration comparing nano rod and re-entrant curvature structures. (a) Bundling of pillar structures due to capillary attraction. This brings about the nano-carpet effect and thus the CA decreases sharply. (b) Decreasing CA is restricted because the cap of the rod touches another cap before the centers of the rods can touch.


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Figure 3. (a) SEM images of the fabricated nano scale overhang structures formed by wet etching for 5 s. Inset: CA values with DI water. (CA = 154˚) (b) A less developed re-entrant curvature with wet etching for 2s. (CA = 146˚) (c) Initial state of a round-shaped pillar. (CA = 131˚) (d) SEM images of the fabricated high-aspect ratio nanorod structures (CA = 138˚) (e, f) Bundled nanorods due to capillary forces: (e) Bundled pyramid shape (f) Stuck to the substrate. (g) Droplets of ethanol (red), ethylene glycol (green), toluene (blue), and DI water (transparent) on nanorod structured glass. Inset: CA values of ethanol, ethylene glycol and toluene. (h) Omniphobicity of nano-scale overhang structured arrays on glass substrate. Droplets of ethanol (red), ethylene glycol (green), toluene (blue), and DI water (transparent) on nano-scale overhang structured glass. Inset shows static CA values of ethanol, ethylene glycol and toluene. All of the structures are the same height (300 nm).


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Figure 4. Contact angle data at the different pattern dimensions. The spacing between overhang structures was varied from 100 nm to 500 nm. a is diameter, b/a is the ratio of spacing/diameter, and * fs is the fraction factor, defined as fs = (πa2) / 4(a+b)2. The heights of all structures are 300 nm.


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Figure 5. Transmittance spectra of various samples over the visible to near-infrared wavelength regime. Inset: Photograph of the bare and nano-scale overhang structure samples on glass.


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TABLE OF CONTENTS ONLY Superamphiphobic surface by nanotransfer molding and isotropic etching

We present a novel method to fabricate superhydrophobic and superoleophobic surfaces with nano-scale reentrant curvature by nanotransfer molding and controlled wet etching of the facile undercut. This method produces completely ordered re-entrant nano-structures and prevents wettability increasing. The mushroom-like, re-entrant, and overhanging structure demonstrates high transparency and flexibility and will be widely used on self-cleaning surfaces in the near future.


Superamphiphobic Surface by Nanotransfer Molding and Isotropic

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