Journal of Mechanical Science and Technology 25 (1) (2011) 43~47

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-010-1013-0

Fabrication of semi-transparent super-hydrophobic surface

based on silica hierarchical structures† Ping-Hei Chen1,*, Chin-Chi Hsu1, Pei-Shan Lee1 and Chao-Sung Lin2

1Department of Mechanical Engineering, National Taiwan University, Taiwan, 10617 2Department of Materials Science and Engineering, National Taiwan University, Taiwan, 10617

(Manuscript Received October 5, 2010; Revised November 19, 2010; Accepted November 19, 2010)

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Abstract This study successfully develops a versatile method of producing superhydrophobic surfaces with micro/nano-silica hierarchical struc-

tures on glass surfaces. Optically transparent super hydrophobic silica thin films were prepared by spin-coating silica particles suspended in a precursor solution of silane, ethanol, and H2O with molar ratio of 1:4:4. The resulting super hydrophobic films were characterized by scanning electron microscopy (SEM), optical transmission, and contact angle measurements. The glass substrates in this study were modified with different particles: micro-silica particles, nano-silica particles, and hierarchical structures. This study includes SEM micro-graphs of the modified glass surfaces with hierarchical structures at different magnifications.

Keywords: Superhydrophobic; Transmittance; Hierarchical structure; Sol-gel ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

Superhydrophobic surfaces are of special interest because of their lossless liquid transfer [1], prevention of snow adhesion [2], and dirt-repelling properties. Transparent superhydro-phobic surfaces can be used on the windows of vehicles and buildings to avoid contamination, and are particularly useful for solar collectors. Solar cells are generally viewed as the best option for sustainable electricity. However, solar collectors are typically placed outdoors or on the walls of solar buildings, allowing dust and debris to affect their efficiency. The contact angle is the angle at which a liquid or vapor interface meets a solid surface. For water, a wetting surface is hydrophilic and a de-wetting surface is hydrophobic. Superhydrophobic surfaces have contact angles greater than 150° [3], showing almost no contact between the liquid drop and the surface. Nature is a source of inspiration for research on superhydrophobic sur-faces, providing examples such as Namib desert beetles [4], water striders [5] and the lotus leaf [6], which is the most ele-gant example of a naturally occurring superhydrophobic sur-face. As water droplets roll off the leaf, they take the dirt par-ticles away, creating a self-cleaning effect. Lotus leave have a combination of low surface energy and dual size roughness:

one in the micro scale and another at the nano scale. Many recent studies attempt to prepare superhydrophobic surfaces by mimicking, in one way or another, the lotus leaf surface structure [7]. Researchers have used a number of techniques, including the sol-gel method [8], electro spinning, chemical vapor deposition [9], template etching [10] and lithography [11], to fabricate superhydrophobic surfaces. To develop this functionality effectively, it is important to select both chemical compositions of compounds and roughness of surface [12-14].

2. Experimental procedures

Fig. 1 depicts the three major types of superhydrophobic film. The following discussion describes the procedures for modifying the glass substrate surface with different silica par-ticles. Silane (Methyltriethoxysilane (METES) and Methyltrieth-oxysilane/Decyltrimethoxysilane (METES/DECYS)) and ethanol (EtOH) were purchased from Sigma-Aldrich (USA) and used as received. Unless otherwise noted, all analytical-grade chemicals were used as received. Millipore water was to pre-pare all aqueous solutions. All glass substrates were cleaned with detergent, chromic acid, and alcohol followed by deion-ized water and acetone rinses to remove any surface contami-nation. All glass substrates were dried before coating.

Superhydrophobic silica films were prepared in two steps: (1) Preparation of silica films using a silane (METES and METES/DECYS) precursor with silica particles; (2) Anneal-ing the surface of modified silica films. Super hydrophobic

† This paper was presented at the MECT-10, Seoul, Korea, March 2010. This paper was recommended for publication in revised form by Guest Editors Seung Jin Song, S. Mochizuki

*Corresponding author. Tel.: +886 233662689, Fax.: +886 2 23631755 E-mail address: phchen@ntu.edu.tw

© KSME & Springer 2011

44 P.-H. Chen et al. / Journal of Mechanical Science and Technology 25 (1) (2011) 43~47

silica films were prepared by the sol-gel process using spin coating from an alcoholic solution containing silica precursors of silane, EtOH, and water. The precursor solution was pre-pared from silane, ethanol, and water at the molar ratio of 1:4:4. This mixture was stirred for 2 hours at 60°C. Different micro-silica particle (0.5-10 µm, Sigma-Aldrich, USA) sus-pensions were prepared using various weight percentages of micro-silica particles. The micro-silica particle suspension precursor solution was ultrasonicated for 1 hour and then stored for one day at room temperature to allow condensation. Finally, the suspension of micro-silica particles was spin-coated on clean glass slides at 2500 rpm for 10s and subse-quently annealed at 200 °C for 1.5 hours. Nano silica particles (10-20 nm, 99.5%, Sigma-Aldrich, USA) were used instead of micro-silica particles to prepare nano silica based superhydro-phobic films.

To prepare hierarchical structures with micro/nano-silica particles, a known amount nano silica particles was added to the suspension of micro-silica particles with stirring. The sus-pension was ultrasonicated for 2 hours and stored for one day at room temperature. The final suspension was spin-coated on glass substrates at 2500rpm for 10s and annealed at 200 °C for 1.5 hours. Different ratios of micro-silica particles and nano-silica particles were tested to achieve a high contact angle with high optical transmittance.

The water contact angle was measured with an optical con-tact angle meter (FTA 125, First Ten Angstroms). The sam-ples were examined by SEM (JSM-6300, JEOL) to determine their morphology. The transmittance of visible light was evaluated by a UV–vis scanning spectrometer (U3310, HITACHI) with a cleaned glass as a reference. 2.1 Contact angle system

This study uses FTA-125 photographs to produce schematic drawings of the contact angle meter. The surface free energy of the structure was determined by measuring contact angles as a function of surface tension in a series of liquids. A contact angle meter generally consists of a sample stage to hold the

substrate, a syringe to apply droplets of liquid, a light source to illuminate the droplet, and a set of optics to magnify the image. A computer connected to the camera calculates the contact angles using FTA-125 measurement software Matrix Inspector 2.2. This study uses the tangent leaning method to calculate the contact angles for all measurements. A drop vol-ume of 4 micro liters was chosen for the measurements. The dynamic contact angles were measured by increasing and decreasing the drop volume of a sessile drop, respectively. A syringe was used to control the volume of droplets. When the volume of a droplet increases to a certain value, the contact line starts to expand on the surface. The contact angle of the droplet at the instant is the advancing angle. When the liquid recedes from the surface to a certain value, the contact line begins recede. At this instant, the contact angle of the droplet is the receding angle. The experiments in this study examined each sample using more than 10 values.

2.2 Ultraviolet-visible spectrophotometer (UV/Vis)

Ultraviolet visible near-infra red (UV-Vis-NIR) spectros-copy can characterize the absorption, transmission, and reflec-tivity of a variety of technologically important materials, in-cluding pigments, coatings, windows, and filters. The qualita-tive application of UV/Vis spectroscopy usually requires re-cording at least a portion of the spectrum for characterizing the optical or electronic properties of materials. The instrument used in ultraviolet-visible spectroscopy is called a UV/vis spectrophotometer. This device measures the intensity of light passing through a sample (I), and compares it to the intensity of light before it passes through the sample (Io). The ratio I / Io represents transmittance, and is usually expressed as a percentage (%T). The absorbance, A, is based on the trans-mittance:

A = − log (T %) = − log (I / Io) The experiments in this study measured the transmittance of

superhydrophobic structures on glass surfaces using an ultra-violet-visible spectrometer with clean glass serving as a refer-ence. An ultraviolet-visible spectrum is essentially a graph of light absorbance versus wavelength in a range of ultraviolet or visible regions (400nm-800nm).

3. Results

Super-hydrophobic surfaces were prepared on glass sub-strates with micro-silica particles, nano-silica particles, and hierarchical structures with micro/nano-silica particles. METES and METES /DECYS were used in the precursor solution.

Because coating particles can create a rough surface, the weight of particles becomes is a significant factor in de-wetting surfaces. Fig. 2 shows that weight of particles affects the surface CA. The CA is increases as more particles are

Fig. 1. Glass substrate modified with different particles: micro-silica particles at the top, nano-silica particles in the middle, and hierarchicalstructures at the bottom.

P.-H. Chen et al. / Journal of Mechanical Science and Technology 25 (1) (2011) 43~47 45

added to the precursor. However, the trend of CA remains steady when the particle weight exceeds a particular value. Nanoparticle coating also increases the CA. For example, in METES cases, the glass substrate surfaces modified with micro-silica particles (2g) and nano-particles (2g) achieved superhydrophobic characteristics with static contact angles around 155° and 162°.

SEM analysis reveals the particles size distributions. Figs. 3 and 4 show the SEM images modify with 2g micro and nano silica particles with METES. Fig. 3(d) clearly shows micro size distributions from 1um to 5 µm. Fig. 4(c) reveals that the nano silica particles exhibit aggregation behavior. The aggre-gate particles were smaller than 1um. A comparison of Fig. 3(d) and 4(c) shows that the structures have many gaps modi-fied by nano particles. Surface modification with nano parti-cles is rougher than that with micro particles.

Superhydrophobic and high transmittance glass surfaces are useful for covering solar panels. However, the transparency of the surface is a significant topic. The transmittance of the original glass was about 95%, but it decreased after the glass was coated with the thin film. Micro particle surface treatment

created a transmittance of less than 85% for visible light (600 nm). When the glass substrate surface was modified with nano silica particles, the surface transmittance fell to 75%, as Fig. 5 shows. This is because fine particles and small gaps between them make it difficult for light to pass through the glass sur-face. This is why transmittance decreased after surface treat-ment. A comparison of Fig. 2 and Fig. 5 shows that adding more particles achieves a higher CA, but decreases the surface transmittance. The results of CA and transparency apparently contradict each other.

Decreasing the weight of silica particles increases transmit-tance. This simultaneously weakens the superhydrophobic effect. To look after both sides that changes the silane pre-scription to METES/DECYS. The functional group of DECYS has a longer carbon chain than others. This long car-bon chain is positively correlated with hydrophobicity. There-fore, in METES /DECYS cases, reducing the particle weight

Weight of particles (g)

Contactangle(o)

0 0.5 1 1.5 2 2.5 380

100

120

140

160

180

Nano scaleMicro scale

Fig. 2. The figure shows the difference between contact angle andweight of particles (micro/nano scale) with METES.

Fig. 3. SEM micrographs show as modified glass surface with hierar-chical structure of 2g micro-silica particles with METES/DECYS atdifferent scale bars: (A) 100 µm (B) 20 µm (C) 10 µm (D)5µm (CA 155°).

Fig. 4. SEM micrographs show as modified glass surface with hierar-chical structure of 2g nano-silica particles with METES/DECYS at different scale bars: (A) 100 µm (B) 20 µm (C) 5µm (D)2 µm (CA 162°).

Weight of particles (g)

Transmittance(T%600nm)

0 0.5 1 1.5 2 2.5 360

65

70

75

80

85

90

95

100

Micro scaleNano scale

Fig. 5. Figure shows the difference between transmittance and weight of particles (micro/nano scale) with METES.

46 P.-H. Chen et al. / Journal of Mechanical Science and Technology 25 (1) (2011) 43~47

increases transmittance (micro/nano-silica particles (0.1g/0.1 g)). Fig. 6 shows that hierarchical structures on the glass sur-face create a high transmittance (86%) and superhydrophobic surface (CA =158.3°).

4. Conclusions

The glass substrate surfaces modified with microsilica par-ticles and nano particles in this study achieved superhydro-phobic characteristics with static contact angles over 150° at the expense of decreased transmittance. A silane mixture (METES/DECYS) was used as an adhesion promoter to pro-vide enhanced hydrophobicity and linking at the bonding site. Future studies should blend silanes into an adhesive formula-tion or use them as primers on substrates. When added to an adhesive formulation, the silane must be free enough to mi-grate to the interphase region between the silica particles and the surface of attachment. In this way, the silane will be on the surface and the interface, where it can enhance adhesion be-tween the silica particles and the substrate. This approach

could form a monolayer silica particle on the glass substrate. Therefore, a monolayer structure can achieve higher transmit-tance, as Fig. 7 shows (CA=158.3°, T% = 86%).

Acknowledgment

The financial support for this study was provided by the KAUST Award with a project number of KUK-C1-014-12.

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Fig. 6. SEM micrographs show as modified glass surface with hierar-chical structure of 0.1g micro/nano-silica particles with METES/DECYS at different scale bars: (A) 300µm (B) 100 µm (C) 30 µm (D)3µm (CA 158.3°).

Fig. 7. Image shows as high transmittance modified glass surface withhierarchical structure of 0.1g micro/nano-silica particles with METES/DECYS.

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Ping-Hei Chen received his master and Ph. D. degree in mechanical engineering from U. of Minnesota in 1984 and 1988, respectively. He was awarded with a distinguished professor by the National Taiwan University in 2008, the distin-guished research award by NSC in 2008, and ASME fellow in 2009. He is serv-

ing as the program director of thermal, fluid, and energy divi-sion of National Science Council in Taiwan. His major re-search areas are in MEMS, Biomedical devices, nanotechnol-ogy, energy harvesting chips, highly efficient energy systems, and sensors. He has published more than 112 journal papers, two books, and 14 patents.