Bioinspired Multifunctional Superhydrophobic Surfaces with Carbon

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Bioinspired Multifunctional Superhydrophobic Surfaces with Carbon-Nanotube-Based Conducting Pastes by Facile and Scalable Printing Joong Tark Han,*,†,‡ Byung Kuk Kim,† Jong Seok Woo,† Jeong In Jang,† Joon Young Cho,‡ Hee Jin Jeong,† Seung Yol Jeong,† Seon Hee Seo,† and Geon-Woong Lee† †

Nano Hybrid Technology Research Center, Korea Electrotechnology Research Institute, Changwon 51543, Republic of Korea Department of Electro-Functionality Material Engineering, University of Science and Technology (UST), Changwon 51543, Republic of Korea

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S Supporting Information *

ABSTRACT: Directly printed superhydrophobic surfaces containing conducting nanomaterials can be used for a wide range of applications in terms of nonwetting, anisotropic wetting, and electrical conductivity. Here, we demonstrated that direct-printable and flexible superhydrophobic surfaces were fabricated on flexible substrates via with an ultrafacile and scalable screen printing with carbon nanotube (CNT)-based conducting pastes. A polydimethylsiloxane (PDMS)-polyethylene glycol (PEG) copolymer was used as an additive for conducting pastes to realize the printability of the conducting paste as well as the hydrophobicity of the printed surface. The screen-printed conducting surfaces showed a high water contact angle (WCA) (>150°) and low contact angle hysteresis (WCA < 5°) at 25 wt % PDMS-PEG copolymer in the paste, and they have an electrical conductivity of over 1000 S m−1. Patterned superhydrophobic surfaces also showed sticky superhydrophobic characteristics and were used to transport water droplets. Moreover, fabricated films on metal meshes were used for an oil/water separation filter, and liquid evaporation behavior was investigated on the superhydrophobic and conductive thin-film heaters by applying direct current voltage to the film. KEYWORDS: carbon nanotubes, direct printing, conducting paste, superhydrophobic, patterning, multifunctionality

1. INTRODUCTION Lotus-leaf-motivated superhydrophobic surfaces have received considerable attention for various applications such as selfcleaning, corrosion resistance, drag reduction, cancer cell capture, anti-icing, and antibacterial coatings, water collecting, etc.1−7 The unusual wetting characteristics of a superhydrophobic surface are governed by both the chemical composition and the geometric structure of that surface.8−12 Deposition of synthetic polymers and nanoparticles or electrochemical deposition has been utilized to control the roughness and hydrophobicity of surfaces with an electrical conductivity.13−29 In particular, carbon materials such as carbon nanotubes (CNTs), graphene, graphite, and carbon fibers have been utilized to fabricate electrically conducting and superhydrophobic surfaces because of their ability to remove the static charges accumulated on surfaces. These materials also have potential applications in electrowetting, electromagnetic interference shielding, static charge dissipation, and electrical circuits.13−19 Accordingly, many authors have been dedicated to the fabrication and understanding of superhydrophobic surfaces, particularly those based on CNTs. To the best of our knowledge, no previous studies have considered directly © 2017 American Chemical Society

printing patterned conductive superhydrophobic surfaces via high-throughput processes such as screen printing. If a conductive and superhydrophobic coating can be patterned by a direct printing process such as screen printing, this technology will meet the needs of a wide range of applications in biotechnology, microfluidics, fog harvesting, anisotropic wetting, etc.30−33 Moreover, the printed conducting superhydrophobic film can be used to fabricate thin-film heaters that can be utilized to investigate the evaporation of a water droplet on a surface at specific temperatures and to remove residual solvent molecules. However, in previous studies, superhydrophobic samples needed to be placed on a heating stage to monitor the evaporation of water droplets on the superhydrophobic surface.34,35 This is a highly desirable application because the evaporation of water droplets has been studied based on the evaporation dynamics of droplets on a surface based on the vapor-diffusion model for evaporation.36 In this regard, formulating a printable paste is a prerequisite for Received: November 29, 2016 Accepted: February 3, 2017 Published: February 3, 2017 7780

DOI: 10.1021/acsami.6b15292 ACS Appl. Mater. Interfaces 2017, 9, 7780−7786

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ACS Applied Materials & Interfaces

obtained by measuring the sheet resistance with a four-point probe and considering the film thickness. The sheet resistance measurements were collected using a four-point probe tester (Loresta, MCP-T610). The water contact angles were measured using a contact angle meter (Surface & Electro-Optics Co., Korea, Phoenix300).

controlling the wettability, printability, and interfacial adhesion on the substrate. Previously, Sekitani et al. also reported the screen-printing formulation of single-walled CNTs by using ionic liquid and a compatible fluorinated copolymer in terms of flexible electronics;37 however, these formulations are not toward the superhydrophobicity of the surface. We developed a direct screen-printing method for realizing conductive and superhydrophobic patterns using CNT-based conducting pastes. We created the paste from multiwalled CNTs (MWCNTs) as a conducting filler and roughness former and poly[dimethylsiloxane-co-methyl(3-hydroxypropyl)siloxane]-graf t-poly(ethylene glycol)methyl ether (PDMSPEG copolymer) as both a rheological modifier for the paste and a hydrophobic binder material. The printed superhydrophobic surface showed good resilience and maintained its performance after multiple adhesive tape test cycles. We demonstrated that the patterned surface showed a sticky superhydrophobicity, and the surface was used to transport liquid droplets to other substrates. Fabricated films were also used for an oil/water separation filter and a superhydrophobic film heater for investigating the evaporation behavior of water droplets on that surface. Not only are direct-patterned superhydrophobic surfaces impressive for their superior performance but also they comprise commercially available materials using a high-throughput process and a scalable printing approach.

3. RESULTS AND DISCUSSION Previously, diverse surface wettability patterning technologies have developed by using lithographic printing, microcontact printing, inkjet printing, and their combination techniques.38−40 In this study, a high-throughput screen-printing process with viscous conducting pastes was applied to fabricate the superhydrophobic pattern on flexible substrates. The conducting pastes were created using conventional dispersion processing for a highly loaded filler system. Paste manufacturing methods involve wetting stages and dispersion stages using a three-roll mill and a planetary centrifugal mixer with ingredients such as MWCNTs (Figure S1), PDMS-PEG copolymer, ethyl cellulose (EC) as a binder, and terpineol as a solvent. Notably, we used a PDMS copolymer composed of a PDMS backbone grafted with 80 wt % of PEG as a low surface energy material for superhydrophobicity considering the printability of the conducting paste (Figure 1a). PEG is commonly used as an

2. EXPERIMENTAL DETAILS Materials. MWCNTs (purchased from Hanwha Chemical, South Korea) were used as received. The hydrophobic PDMS-PEG copolymer was purchased from Sigma-Aldrich and used as an additive for printable conductive pastes. EC and terpineol were bought from Sigma-Aldrich and used as the binder and solvent for the paste formulation, respectively. Fabrication of Conductive and Superhydrophobic Patterns. To prepare printable conducting pastes, the MWCNT powders, PDMS-PEG copolymer, EC, and terpineol were first mixed with a planetary centrifugal mixer (THINKY mixer ARE-310, THINKY Corp., Japan). The mixtures were then treated with a calendar, commonly known as a three-roll mill (model T65, Torrey Hills Technologies, USA). For example, 0.5 g of MWCNT, 2.5 g of PDMSPEG copolymer, 0.2 g of EC, and 6.8 g of terpineol were mixed to prepare the paste. The amount of MWCNTs in all of the pastes was fixed to 5 wt %. This milling cycle was repeated to maximize the dispersion. The gap width between the rollers was adjusted from 500 to 5 μm, resulting in locally high shear forces with a short residence time in order to achieve the desired level of CNT dispersion. The prepared conducting pastes were printed onto the selected substrate using a screen-printing machine with metal mesh screen masks at 25 °C and then annealed at 250 °C to remove the volatile ingredients. Fabrication of Thin-Film Heaters. The thin-film heaters were fabricated in a two-terminal side-contact configuration. The dc voltage was supplied by a power supply to the film heater through a screenprinted silver contact at the film edge. The temperature of the film was measured using an IR thermal imager and a thermocouple located on the film. Characterization. The surfaces morphologies and element mapping of the samples were imaged by field-emission scanning electron microscopy (FESEM) (Hitachi S4800). 3D images of the surfaces were obtained by a 3D laser scanning confocal microscope (VK-9710k, KEYENCE) and atomic force microscopy (AFM) (Veeco NanoScope 8). The thermal degradation behavior of each component in the conducting pastes was confirmed by thermogravimetric analysis (TGA) (TA Instruments, TGA Q500). The chemical composition of prepared films was assessed by XPS using a Multilab2000 (Thermo VG Scientific Inc.) spectrometer with Al Kα radiation as the X-ray excitation source. The electrical conductivities of the films were

Figure 1. (a) Main ingredients of the conducting paste. (b) Shear viscosity of the CNT-based conducting paste. Inset image shows a photograph of the conducting paste. (c−e) Photographs of screenprinted conducting films (c) on glass with a 200 × 200 μm square pattern, (d) on polyimide film with a 200 μm stripe pattern, and (e) on copper foil without a pattern. (f) FESEM images of the screenprinted CNT pattern on glass corresponding to c. (g and h) Highmagnification images of the film showing microscale bump structures, which is efficient for the superhydrophobicity of the surface and on which the PDMS-PEG-rich area was found, as shown in h.

organic vehicle to modulate the rheological properties of screen-printing pastes.41,42 This water-soluble PEG can be removed by a thermal treatment above 200 °C, as shown in the TGA data (Figure S2). Terpineol can also be removed by heating around 200 °C. Thus, the screen-printed films were thermally treated at 250 °C for 30 min. The prepared paste containing 5 wt % MWCNTs and 20 wt % PDMS-PEG 7781


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Figure 2. (a) Changes in the contact angle (blue) and electrical conductivity (pink) of CNT-based conducting films containing various amounts of the PDMS-PEG copolymer prepared on PI films. Upper inset image in a shows the low contact angle hysteresis (ΔWCA) by dragging the water droplet in the direction of the arrow. Lower inset image in a shows light an LED by connecting it with the superhydrophobic pattern electrode on flexible PI. (b) Photographs of the superhydrophobic mesh-based oil/water separation of water and hexane: (1) brush painting of the conducting paste, (2) superhydrophobicity of the coated mesh after thermal treatment, (3) separated oil/water). (c) Changes in the water contact angle and the electrical resistance change of the film after tape testing to evaluate the durability of the conductive and superhydrophobic film. Upper inset image in c shows photograph before and after the tape test of the coated film. Lower inset image in c shows an FESEM image of the superhydrophobic surface after tape testing.

copolymer showed a high viscosity (>105 cP at a 0.1 s−1 shear rate) that was suitable for screen printing (Figure 1b). The created conducting pastes were screen printed on polyimide (PI) films, copper foils, and glass substrates, which were thermally treated to remove the residual solvent and hydrophilic components. Figure 1c, 1d, and 1e shows optical images of a 200 × 200 μm square pattern, a 200 μm stripe pattern of uncoated regions, and a whole area printed on a copper surface, respectively, and these films confirm the direct printability of the as-fabricated conducting pastes on various substrates. Moreover, paint brushing on irregular metal surfaces was possible, for example, on a spiral copper pipe and a metal mesh (Figure S3). The pattern size could be controlled using silk-screen masks (Figure S4), and diverse patterns were created on plastic substrates (Figure S5). Figure 1f−h shows field-emission scanning electron microscopy (FESEM) images of conducting patterns fabricated by screen printing with CNT pastes containing 20 wt % PDMS-PEG copolymer. By increasing the amount of the PDMS-PEG copolymer, more microstructures were developed due to the lower chemical compatibility of PDMS and MWCNTs (Figures 1g and S6) without sacrificing its printability. The 3D laser confocal microscopy and atomic force microscopy (AFM) images in Figure S7 show microbump and nanostructures on the printed surfaces. The combination of the nanostructure of MWCNTs and the PDMS-induced microstructure of the surface promises superhydrophobicity based on the Wenzel and Cassi-Baxter theories.8,9 In addition, the energy-dispersive spectrometer (EDS) image (Figure S8) and X-ray photoelectron spectroscopy spectra of the films (Figure S9) show that adding more PDMS copolymer caused

more Si atoms to be detected in the printed surfaces on the PI substrate. To study the effect of the amount of the PDMS-PEG copolymer on the hydrophobicity and electrical properties of the printed film, we then measured the water contact angles (WCAs) and electrical conductivities in conducting pastes with various amounts of the PDMS copolymer. Figure 2a shows the WCAs and electrical conductivities of the printed conducting films. By adding a small amount (∼20 wt %) of PDMS-PEG copolymer into the pastes containing 5 wt % MWCNTs, the WCA was dramatically increased to over 150°. This is due to the hydrophobic characteristics of the remaining ingredient materials after the thermal treatment at 250 °C, the surface roughness caused by the MWCNTs, and microstructured bumps, as shown in Figure 1h. At a PDMS copolymer content of 30 wt % in the paste, the WCA reached almost 160°. In addition, by dragging a water droplet over the surface, the contact angle hysteresis was characterized. The upper inset image in Figure 2a clearly shows a low contact angle hysteresis of less than 5°. A water droplet easily rolled off a rolled copper foil coated with MWCNTs (Movie S1). Furthermore, the metal mesh film coated with the conducting paste showed excellent superhydrophobicity and efficient oil/water separation ability (Figure 2b, Movie S2). Next, we measured the electrical conductivity of the printed films on PI substrates after annealing at 250 °C as a function of the weight fraction of the PDMS-PEG copolymer. As shown in Figure 2a, the electrical conductivity was decreased by adding more PDMS copolymer from a maximum of ∼2000 to ∼1000 S m−1 at 20 wt % PDMS copolymer, which is high compared to previously reported values.19 The lower inset image in Figure 2a demonstrates lighting a light-emitting diode (LED) by 7782


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Figure 3. (a) Images of a 10 μL water droplet on the patterned superhydrophobic surfaces while dragging from right to left. Green lines indicate the advancing and receding angles. Dotted circle indicates the initial position of the water droplet. (b) Photograph of water droplets on tilted patterned superhydrophobic surfaces demonstrating the sticky characteristics of the superhydrophobic pattern. (c) Selective placement of water droplets on patterned superhydrophobic surfaces. (d) Water droplet images on the stripe pattern surface showing anisotropic wetting behavior. Along the perpendicular direction, the water droplet does not roll off, even at a 90° tilting angle. (e) Transfer of a water droplet from a superhydrophobic surface to a hydrophilic one using the sticky superhydrophobic patterned surface.

anisotropic wetting behavior, as shown in Figure 3d. We measured the perpendicular and parallel roll-off angles of 10 μLof water droplets on the stripe pattern surface. Along the perpendicular direction, the water droplet was pinned, while along the parallel direction, the roll-off angle was 41°. This anisotropic wetting behavior is similar to that of rice leaves.45 Furthermore, we attempted to transport an aqueous droplet using this sticky superhydrophobic surface. Figure 3e shows the transfer of a water droplet from the superhydrophobic surface to the hydrophilic surface. The patterned superhydrophobic surface can be used as a mechanical hand to transfer small water droplets from a superhydrophobic to a hydrophilic one without any loss or contamination for microsample analysis.46−48 As seen in Figure 3e, a water droplet was first placed on the MWCNT/PDMS-copolymer-based superhydrophobic surface with a WCA of about 160° (step 1). Then the sticky patterned superhydrophobic surface was put into contact with this water droplet (step 2). The sticky surface then picked up the water droplet, i.e., it was completely transferred from the superhydrophobic surface to the patterned superhydrophobic surface (step 3). Finally, the water droplet was released onto a hydrophilic surface with a WCA of about 35° (step 4). In addition, to directly demonstrate the wetting behavior of the conductive film at high temperature, we fabricated thin-film heaters on the PI substrates. Films with electrical conductivities of over 1000 S m−1 were used to fabricate the heaters. Figure 4a and 4b shows the heating behavior of the superhydrophobic films as a function of time and the infrared images of the fabricated superhydrophobic film heaters at applied input voltages, respectively. The superhydrophobic film was heated up to 100 °C within 20 s at 20 V, and the temperature was saturated up to 200 °C within 60 s. No temperature drop was observed even at high temperature, which indicates the stable heating behavior of the MWCNT-based conducting film. Importantly, the Joule-heated surface retained its superhydrophobic characteristics, even over 100 °C, as shown in

connecting it with the superhydrophobic MWCNT-patterned electrode on flexible PI. Moreover, the electrical conductivity of the printed films can be controlled considering their applications and/or chemical or mechanical stability. The durability of conducting superhydrophobic surfaces, i.e., its resistance against peeling off, is also of great importance in terms of practical applications. Figure 2c shows the WCAs and electrical resistance changes after the tape test with adhesive tape. The WCAs did not change even after performing the tape test 20 times, as shown in the inset image. However, after every tape test, the electrical resistance of the conducting superhydrophobic surface gradually increased due to the detachment of a small amount of MWCNTs from the surface. Vertically aligned MWCNTs were also observed in the FESEM image (lower inset image in Figure 2c), which means that the detaching process enhances the nanoscale surface roughness compared to before taping test (Figure S10). Moreover, the retained superhydrophobicity means that the PDMS copolymer exists uniformly along the out-of-plane direction of the film. Notably, the patterned superhydrophobic surface showed high contact angle hysteresis (>50°) by dragging the water droplet from right to left, which indicates a so-called sticky superhydrophobic surface (Figures 3a and S11). Considering the spacing factor (ratio of the diameter of bumps and the pitch distance between them),43 the patterned superhydrophpbic surface is the wetted Wenzel state because of the relatively thin film thickness (100 μm). Therefore, the wetting behavior of water droplets on printed pattern surfaces is dominated by contact line pinning. These sticky and superhydrophobic characteristics of the patterned film allow easily placing a spherical water droplet even on the curved surface without rolling off, as shown in Figure 3b.44 Spherical water droplets (dyed blue, green, and red to aid visualization) were also placed at the target positions due to the sticky property of the patterned superhydrophobic surface. Moreover, the water droplet on the stripe pattern showed an 7783


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Figure 4. (a) Temperature profile of the conductive and superhydrophobic film at various applied voltages as a function of time. (b and c) Infrared thermal images of the flexible film heater and a water droplet on the superhydrophobic film heater. Inset photograph in c is a water droplet showing superhydrophobicity. (d) Photographs of water droplets on the superhydrophobic film heated to 100 °C by applying a dc voltage as the droplet decreases in size due to evaporation. (e) Change of the water droplet diameter as a function of evaporation time on the superhydrophobic film heater surface. (f) Plot of the total time for evaporation as a function of temperature.

Figure 4c. Thus, we attempted to investigate the evaporation characteristics of sessile water droplets on the superhydrophobic surfaces. Previous works on evaporation from superhydrophobic surfaces were designed by placing the test substrate on a thin-film heater.34,35 In this study, we directly observed the water droplet on a Joule-heated superhydrophobic surface at various temperatures. Figure 4c shows images of 10 μL of water droplets evaporating at 100 °C. The surface continued to show superhydrophobicity until the water completely evaporated. Figure 4d shows the reduction of the droplet diameter on the superhydrophobic surfaces at various temperatures. As expected, increasing the surface temperature dramatically decreased the total time for evaporation on the superhydrophobic. Popov et al. attempted to predict the evaporation dynamics of droplets on a surface based on the vapor-diffusion model for evaporation.36 They reported that the rate of evaporation of a sessile droplet is dependent on the contact radius and contact angle of the droplet. Girard et al. supported the exponential relation between the total evaporation time and the substrate temperature.36 For our superhydrophobic surfaces, the dependence of the total time for evaporation tF was also fitted with a power law, tF = aTbsub, where a = 617 465 and b = −1.803. This result indicates that the vapor-diffusion model overpredicts the rate of evaporation

from a superhydrophobic surface, as reported in a previous report.49

4. CONCLUSION In summary, we demonstrated the use of a conducting paste formulated from MWCNTs and a PDMS-PEG copolymer for direct patterning using a screen-printing process. Superhydrophobic and conductive surfaces printed directly on diverse substrates show that durable superhydrophobic patterning can be utilized as a liquid droplet transportation tool at the microscale and as a test bed for studying the evaporation of liquid droplets on heated surfaces. Moreover, the superhydrophobic metal mesh film coated with the conducting paste showed excellent oil/water separation ability. Printable surfaces with conductivity and superhydrophobicity can be readily implemented for multifunctional surface engineering with an ultrafacile and scalable approach.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15292. Thermogravimetric analysis of a conducting paste, photographs of brush painting, optical images of printed 7784


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patterns, spray coater, FESEM images of coated films, confocal microscopy and AFM images of coated films, EDS profiles, photographs of water droplets while dragging (PDF) Movie for rolling of water droplets on copper foil (AVI) Movie of oil/water separation by the superhydrophobic mesh (AVI)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-55-280-1678. Fax: +82-55-280-1590. E-mail: [email protected]. ORCID

Joong Tark Han: 0000-0002-3351-3975 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Center for Advanced SoftElectronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (2014M3A6A5060953), and by the KERI Primary Research Program of MSIP/NST(1712-N0101-28/32).

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ABBREVIATIONS MWCNTs, multiwalled carbon nanotubes; PDMS-PEG copolymer, poly[dimethylsiloxane-co-methyl(3-hydroxypropyl)siloxane]-graf t-poly(ethylene glycol)methyl ether; PI, polyimide; WCAs, water contact angles; LED, light-emitting diode

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REFERENCES

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