Superhydrophobic Films of Electrospun Fibers with Multiple-Scale

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Superhydrophobic Films of Electrospun Fibers with Multiple-Scale Surface Morphology Jong-Min Lim,† Gi-Ra Yi,*,‡ Jun Hyuk Moon,† Chul-Joon Heo,† and Seung-Man Yang*,† National CreatiVe Research InitiatiVe Center for Integrated Optofluidic Systems and Department of Chemical and Biomolecular Engineering, Korea AdVanced Institute of Science and Technology, Daejeon 305-701, Korea, and Nano-Bio System Research Team, Seoul Center, Korea Basic Science Institute, Seoul 136-713, Korea ReceiVed February 10, 2007. In Final Form: April 21, 2007 Superhydrophobic nanofiber films were created from electrospun nanofibers with undulated surfaces at multiple scales in micrometers and nanometers. The electrospun nanofibers were produced out of aqueous solutions which contained water-soluble polymers and different colloids: monodisperse silica or polystyrene microspheres for larger particles and monodisperse silica nanoparticles for smaller particles. Various types of fibrous films were produced depending on the properties of the dispersing medium, the effects of additives, and the compositions of the bidisperse colloids. When polystyrene microspheres were used as sacrificial templates, macropores were left behind in the nanofibers during the removal of polystyrene microspheres by calcination. The nonwoven films of electrospun nanofibers, which were decorated with silica microspheres or macropores, could be continuously produced with considerable ease under a relatively wide range of operating conditions. The surface properties of the films were characterized by contact angle measurement and an X-ray photoelectron spectrometer. Through the surface modification of the electrospun nanofibers with fluorinated silane coupling agents, superhydrophobic surfaces with low sliding angles were successfully prepared.

Introduction Some natural surfaces such as lotus leaves1,2 and butterfly wings3 have self-cleaning properties. Such surfaces have the unusual wetting characteristic of superhydrophobicity with contact angle greater than 150°. Water droplets on the surface readily roll off when the contact angle hysteresis is negligible, and subsequently, surface dust and debris can be removed easily. The wettability of liquid droplets on solid surfaces is governed mainly by two parameters of the chemical composition and the geometrical microstructure. Over the years, considerable efforts have been extended to fabricate such superhydrophobic surfaces by combining these two parameters, including the application of dual sized raspberry-like particles,4 inverse opal surfaces,3 gellike porous coating,5 layer by layer processes,6-8 electrochemical depositions,6,9-12 sol-gel processes,13-15 mechanically assembled * Corresponding authors. E-mail: [email protected] (S.-M.Y.); [email protected] (G.-R.Y.). † Korea Advanced Institute of Science and Technology. ‡ Korea Basic Science Institute. (1) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (2) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 667. (3) Gu, Z. Z.; Uetsuka, H.; Takahashi, K.; Nakajima, R.; Onishi, H.; Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2003, 42, 894. (4) Ming, W.; Wu, D.; van Benthem, R.; de With, G. Nano Lett. 2005, 5, 2298. (5) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377. (6) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q.; Jiang, L.; Li, X. Y. J. Am. Chem. Soc. 2004, 126, 3064. (7) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (8) Jisr, R. M.; Rmaile, H. H.; Schlenoff, J. B. Angew. Chem., Int. Ed. 2005, 44, 782. (9) Wang, S. T.; Feng, L.; Liu, H.; Sun, T. L.; Zhang, X.; Jiang, L.; Zhu, D. B. ChemPhysChem 2005, 6, 1475. (10) Li, Y.; Shi, G. Q. J. Phys. Chem. B 2005, 109, 23787. (11) Yan, H.; Kurogi, K.; Mayama, H.; Tsujii, K. Angew. Chem., Int. Ed. 2005, 44, 3453. (12) Nicolas, M.; Guittard, F.; Geribaldi, S. Angew. Chem., Int. Ed. 2006, 45, 2251. (13) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2003, 19, 5626. (14) Han, J. T.; Lee, D. H.; Ryu, C. Y.; Cho, K. W. J. Am. Chem. Soc. 2004, 126, 4796.

Figure 1. SEM images of (a) silica microspheres of 700 nm in size and (b) polystyrene latex spheres of 237 nm in size.

monolayers,16 and the like.17 Recently, a few research groups have introduced porous films of beaded electrospun nanofibers to create superhydrophobic surfaces.18-21 The physical properties of these beaded nanofiber films could be controlled depending on the operational parameters of electrospinning, such as applied electric field, feeding rate of polymer solution, net charge density, surface tension, viscosity of the polymer solution (i.e., molecular (15) Shang, H. M.; Wang, Y.; Takahashi, K.; Cao, G. Z.; Li, D.; Xia, Y. N. J. Mater. Sci. 2005, 40, 3587. (16) Genzer, J.; Efimenko, K. Science 2000, 290, 2130. (17) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31. (18) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338. (19) Acatay, K.; Simsek, E.; Ow-Yang, C.; Menceloglu, Y. Z. Angew. Chem., Int. Ed. 2004, 43, 5210. (20) Ma, M. L.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Macromolecules 2005, 38, 9742. (21) Zhu, Y.; Zhang, J. C.; Zheng, Y. M.; Huang, Z. B.; Feng, L.; Jiang, L. AdV. Funct. Mater. 2006, 16, 568.

10.1021/la700392w CCC: $37.00 © 2007 American Chemical Society Published on Web 06/15/2007

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Scheme 1. Schematic Procedure for Multiple-Scale Fibrous Structures from Electrospinning

weight of polymer and concentration of polymer solution), and so on.22 Moreover, the number density and size of the beads on the nanofibers can also be controlled, thus affording subtle control over the surface wetting properties of nanofiber films.20 However, control of the microscopic surface morphology of nanofibers at multiple length scales for modifying more precisely their physicochemical properties still poses a significant challenge. More recently, Ma et al. demonstrated the decoration of the micrometer-scale electrospun fibers with nanometer-scale pores or particles to fabricate the hierarchically roughened nonwoven fabrics.23 They used phase separation during the electrospinning process to induce pores on the electrospun fiber, and subsequent layer-by-layer deposition of particles to introduce nanoscopic roughness. In the present work, we demonstrate a simple method for fabricating fibrous structures of multiple length scales in micrometers and nanometers by electrospinning bidisperse silica particles dispersed in polymer solution. Also, macroporous silica nanofibers were produced by electrospinning silica nanoparticles and polymeric microspheres of which the latter were used as a sacrificial template. On the resulting nanofiber films, micrometerscale roughness was provided by the interfibrillar spacing and decoration of micrometer-scale particles or macropores on electrospun nanofibers, and nanometer-scale roughness originated from silica nanoparticles. The fractional interfacial areas of air contact with a water droplet could be increased by decorating the electrospun nanofibers with macropores or microparticles. As a result, the contact angle of the electrospun binary structured nanofiber films was increased. Notably, a rough surface at multiple-length scales could be continuously produced with considerable ease under a relatively wide range of operating conditions. As shown in Scheme 1, the basic concept of our system was to confine colloidal particles inside electrospun nanofibers and assemble the particles during fiber thinning. In our previous report, we successfully produced well-defined beaded nanofibers from a mixture of various monodisperse silica microspheres and polymers under a wide range of operational conditions.24 In this work, nonwoven films of electrospun silica nanofibers, which were decorated either with silica microspheres or macropores templated from polystyrene particles, were fabricated and then hydrophobically modified with fluorinated silane coupling agents. (22) Fong, H.; Chun, I.; Reneker, D. H. Polymer 1999, 40, 4585. (23) Ma, M.; Gupta, M.; Li, Z.; Zhai, L.; Gleason, K. K.; Cohen, R. E.; Rubner, M. F.; Rutledge, G. C. AdV. Mater. 2007, 19, 255. (24) Lim, J. M.; Moon, J. H.; Yi, G. R.; Heo, C. J.; Yang, S. M. Langmuir 2006, 22, 3445.

As a result, superhydrophobic surfaces with low sliding angles were successfully prepared. Various types of fibrous colloidal films were produced depending on the properties of the dispersing medium, the effects of additives, and the composition of the bidisperse colloids. Superhydrophobic surfaces have various applications including self-cleaning surfaces, antifouling coatings, coatings for microfluidic channels and biosensors, and stainresistant textiles.5,17,25-31 Compared to the other superhydrophobic surfaces, superhydrophobic films made of nanofibers are potentially applicable to superhydrophobic free-standing membranes with high pore density for efficient filtration and separation.23 Experimental Section Materials. All solvents and chemicals were of reagent grade and were used without further purification. Tetraethyl orthosilicate (TEOS, 99.999%, Aldrich) was used as a sol-gel precursor to synthesize silica particles. Ammonia (28%) and ethanol (g99.9%) were purchased from Junsei and Merck, respectively. Styrene (99%) and potassium persulfate (98%) from Kanto chemicals were used as a monomer and initiator for the polymerization. For water-soluble polymers, poly(acryl amide) (PAM, Mw: ∼600000-1000000) and poly(ethylene oxide) (PEO, Mw: ∼600000) were purchased from Polysciences and Aldrich, respectively. A dispersion of colloidal silica nanoparticles in water (Ludox TMA) was purchased from Sigma-Aldrich. (Heptadecafluoro-1, 1, 2, 2-tetrahydrodecyl)trichlorosilane (FDTS) was purchased from Gelest. Preparation of Silica and Polystyrene Colloidal Dispersions. Silica particles were synthesized via controlled hydrolysis and condensation of TEOS in ethanol in the presence of water and ammonia following the seeded-growth method developed by Zhang et al.32 Figure 1a shows the SEM image of synthesized silica particles. The particle size was 700 ( 2.7 nm from dynamic light scattering measurement. Polystyrene particles were synthesized by emulsion polymerization in the absence of an emulsifier following the procedures reported in the literature.33 Figure 1b shows the SEM (25) Miwa, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754. (26) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Langmuir 2000, 16, 7044. (27) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. AdV. Mater. 2002, 14, 1857. (28) Blossey, R.; Bosio, A. Langmuir 2002, 18, 2952. (29) Blossey, R. Nat. Mater. 2003, 2, 301. (30) Choi, C. H.; Kim, C. J. Phys. ReV. Lett. 2006, 96, 066001. (31) Choi, C. H.; Ulmanella, U.; Kim, J.; Ho, C. M.; Kim, C. J. Phys. Fluids 2006, 18, 087105. (32) Zhang, J. H.; Zhan, P.; Wang, Z. L.; Zhang, W. Y.; Ming, N. B. J. Mater. Res. 2003, 18, 649. (33) Zou, D.; Derlich, V.; Gandhi, K.; Park, M.; Sun, L.; Kriz, D.; Lee, Y. D.; Kim, G.; Aklonis, J. J.; Salovey, R. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 1909.

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Figure 2. SEM images of electrospun composite nanofibers of PEO and silica particles when the collection time was 5 s: (a, b) with monodisperse silica particles of 50 nm in size before and after calcination, respectively; (c, d) with bidisperse silica particles of 50 and 700 nm in size before and after calcination, respectively; (e, f) with added hexanol in the mixture dispersion of PEO and bidisperse silica particles before and after calcination, respectively. The magnified SEM images were shown in the insets. image of synthesized polystyrene latex spheres. The particle size was 237 ( 18.8 nm from dynamic light scattering measurement. The synthesized particles were washed twice by centrifugation and redispersed by ultrasonication. Finally, the silica and polystyrene particles were dispersed in water. Preparation of Polymer Solution with Colloids. Aqueous solution of PEO or PAM was added into the aqueous silica or polystyrene suspension, and the mixture was used as the source of electrospun nanofibers. For silica nanofiber films, 1.25 g of 6-wt % aqueous PEO or PAM solution and 150 µL of 34-wt % colloidal silica of 50 nm in size were mixed by vigorous stirring. For nanofibers with multiple-scale surface morphology, 0.75 g of 10-wt % aqueous PEO or PAM solution, 0.5 g of 20-wt % aqueous 700 nm silica suspension or 10-wt % aqueous 237 nm polystyrene suspension, and 150 µL of 34-wt % aqueous 50 nm silica suspension were stirred vigorously. The mixture of colloid and polymer in water was agitated again using a vortex generator just prior to the electrospinning process. Fabrication of Colloid-Polymer Nanofibers by Electrospinning. A 1-mL glass syringe connected with a metal-hub needle

(Hamilton) was used for electrospinning. The metal needle was 110 µm in inner diameter and had a blunt end. The flow rate of the silica/polymer dispersion was maintained at 0.5 mL/h by a syringe pump (Kd Scientific). High voltage in the range 5-13 kV was supplied between the tip of a metal-hub needle and a collector plate using a dc power supply (Glassman, Inc.). The shape of the droplet deformation was observed using a fiber-optic illuminator equipped with a halogen lamp and a CCD camera connected to a computer. The electrospun nanofibers were collected at the copper plate or a boron doped silicon wafer placed beneath the capillary metal needle. The tip to collector distance (TCD) was fixed at 10 cm. The polymer matrix of electrospun nanofiber could be selectively removed by calcination in a muffle furnace for inorganic colloidal fiber films. Modification of Nanofiber Surface. Structured nanofibers on substrates were exposed to O2 plasma for 3 min, and their surface was then reacted with the vapor of (heptadecafluoro-1,1,2,2tetrahydrodecyl)trichlorosilane (FDTS) in a vacuum chamber. Instruments. The size and polydispersity of the silica particles and polystyrene particles were measured from field emission scanning electron microscope (FE-SEM, XL305FEG, Philips) images and by

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Table 1. Constituting Materials for Electrospun Nanofiber Films system A B C D E F

large particle

small particle

polymer matrix

700 nm silica 700 nm silica

50 nm silica 50 nm silica 50 nm silica

700 nm silica 237 nm polystyrene

50 nm silica 50 nm silica 50 nm silica

poly(ethylene oxide) poly(ethylene oxide) poly(ethylene oxide) with hexanol poly(acryl amide) poly(acryl amide) poly(acryl amide)

dynamic light scattering (DLS, Brookhaven Instruments). The morphology of the colloidal nanofiber films was characterized using a FE-SEM and transmission electron microscope (TEM, F20, Tecnai). An X-ray photoelectron spectrometer (XPS, ESCA 2000, VG Microtech, Ltd.) equipped with a Mg KR photon source was used to analyze the chemical composition at the surface of the nanofiber films before and after calcination and after surface modification with a fluorinated silane coupling agent for the calcined films. The contact angles of water on the films were measured using contact angle goniometry (Phoenix 300, Surface Electro Optics). All of the contact angle data were averaged over a number of contact angle measurements. The radius and volume of a water droplet for static measurements were 0.134 cm and 10 µL, respectively. The sliding angle of the water drop on the nanofiber films was measured by placing a 10 µL droplet on the films and tilting slowly the substrate until the water droplet started rolling off.

Results and Discussion

Figure 3. Water droplets on various FDTS treated substrates: (a) plain bare Si wafer; (b) silica nanofiber film of monodisperse 50 nm silica particles in PEO medium without adding hexanol; (c, d) silica nanofiber films of bidisperse silica particles of 50 and 700 nm in size which were electrospun from the mixture dispersion of PEO and bidisperse silica particles in the absence and presence of hexanol, respectively.

During the electrospinning process, the pendent drop of colloidpolymer dispersion at the tip of the metallic capillary was elongated and formed a stable Taylor cone due to the competing electric Maxwell stress and interfacial tension.34,35 The electrified jet became thinner due to the stretching and whipping process, as well as due to the evaporation of the solvent. As a result, the electrospun nanofiber acted as a confining geometry for colloids.24 The polymer matrixes and polystyrene particles of electrospun nanofiber, which were collected on a substrate, could be selectively decomposed by calcination at 500 °C. The inorganic nanofiber films consisting of binary silica particles or silica particles with pores were then exposed to O2 plasma and treated with FDTS in order to make the surfaces superhydrophobic. To investigate the final morphology of the fibrous films and surface properties after FDTS treatment, we used six different types of colloid-polymer mixture systems for electrospun nanofiber films, as summarized in Table 1. Fabrication of Silica Nanofiber Films Formed with PEO as a Matrix Medium. First, silica-polymer composite nanofibers were electrospun from the aqueous 50 nm silica colloid and PEO solution (system A in Table 1). As shown in Figure 2a, electrospun silica-PEO composite nanofibers with a narrow size distribution have been fabricated under an electric field in the range 0.9-1.3 kV/cm. After selectively burning out PEO of the composite fibers, the inorganic matrix of silica nanofiber was produced. However, small cracks were formed in the nanofibers, as shown in the inset of Figure 2b. In our previous report, we also observed cracks in calcined nanofibers which were composed of larger silica particles. PEO has relatively low affinity for hydrophilic hydroxyl groups of silica surfaces. Therefore, as the electrospun fibers were thinned, silica particles were pulled out from the hydrophobic PEO matrix, which induced non-close-packing of the silica nanoparticles in the nanofibers.24 As a result, the silica nanofibers after calcination at 500 °C did not have sufficient mechanical strength for stand-alone structures.

It should be noted that although the PEO volume fraction was typically larger than 0.5 for the electrospun composite fibers of PEO and 50 nm silica particles, the diameters of fibers were reduced only slightly after calcination as shown in Figure 2a,b. For example, the average diameters of nanofibers of PEO and 50 nm silica particles were 415 and 362 nm before and after calcination, respectively. This slight reduction of the fiber diameter after calcination was due to the packing structure of the silica particles in the electrospun fibers. The silica particles were packed into random non-close-packing in electrospun fibers due to both the presence of polymer and the rapid solidification. The random non-close-packed structure with cracks can be seen clearly from the inset of Figure 2b. Consequently, a large portion of the polymer resided in the interstices between the silica particles, and the size of nanofibers was reduced only slightly during calcination. For more structural variations, an aqueous binary colloid of 700 and 50 nm silica particles was added into the aqueous PEO solution (system B in Table 1), and the mixture was electrospun for structured nanofibers with multiple-scale undulations at micrometers and nanometers (Figure 2c). For realizing superhydrophobic surfaces which have low sliding angles of water droplets, multiple length-scale surface structures are essential.4,36 However, during thermal decomposition of the polymers, nanofibers were broken into several parts, as shown in Figure 2d. These fractures were also caused by the non-close-packing of the silica nanoparticles which consolidated the silica microspheres. The broken parts formed small clusters of bidisperse colloids, as shown in Figure 2d. This poor mechanical durability should be improved for practical applications.18 To address this problem, silica nanoparticles inside the nanofibers should be packed compactly in order to avoid crack formation during calcination. To this end, we first attempted to decrease the surface tension (or increase the wettability) of the suspension medium without changing the viscosity of solution by adding 5-wt % hexanol into the source materials (system C) for the multiplescale structured nanofibers.37 Figure 2e,f shows the SEM images

(34) Taylor, G. Proc. R. Soc. London, Ser. A 1964, 280, 383. (35) Shin, Y. M.; Hohman, M. M.; Brenner, M. P.; Rutledge, G. C. Polymer 2001, 42, 9955.

(36) Gao, L.; McCarthy, T. J. Langmuir 2006, 22, 2966. (37) Morota, K.; Matsumoto, H.; Mizukoshi, T.; Konosu, Y.; Minagawa, M.; Tanioka, A.; Yamagata, Y.; Inoue, K. J. Colloid Interface Sci. 2004, 279, 484.

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Figure 4. SEM images of electrospun composite nanofibers of PAM and silica particles when the collection time was 3 min: (a, b) with monodisperse silica particles of 50 nm in size before and after calcination, respectively; (c, d) with bidisperse silica particles of 50 and 700 nm in size before and after calcination, respectively. The magnified SEM images were shown in the insets.

of the multiple-scale nanofibers of 50 and 700 nm silica particles before and after calcination, respectively. Indeed, due to the added hexanol, the crack formation was avoided during selective removal of polymer matrix by calcination. The bidisperse silica particles were embedded in the PEO matrix, and the multiplescale structured silica nanofiber films were connected without any fractures after calcination. The surface of the structured nanofiber film was modified for hydrophobicity by a vapor-phase reaction of FDTS in a vacuum chamber. The wettability of the structured surface was measured by the contact angle of a water droplet. For a control experiment, the surface of a bare silicon wafer was treated with FDTS by the same procedures, of which the contact angle was 117° as shown in Figure 3a. However, for the surface-modified fibrous films of 50 nm silica nanoparticles (system A), the contact angle of a water droplet was increased substantially to 154°, as shown in Figure 3b. As expected, the contact angle was increased further to 160° when a water droplet was placed on the surface-modified multiple-scale nanofiber film of bidisperse silica particles (system B) as shown in Figure 3c. Remarkably, the sliding angle of the multiple-scale structured nanofiber film was less than 2°, which was low enough to realize self-cleaning capability. The nanofiber films in Figure 3b,c were formed without using hexanol, and the silica particles were non-close-packed in the nanofibers. As noted previously, the nanofiber films with non-close-packed silica particles were fragile and easily broken up. Indeed, water droplets could not roll off anymore after several fallen droplets were sliding down, because small fragments of the silica nanofiber films were adsorbed and washed into the water droplets, as shown in Figure 3b,c. The contaminant of small fragments could be avoided by forming close-packed particle structure using hexanol (system C). The multiple-scale structured silica nanofiber films of close packed structures were then treated with FDTS by the

same procedure. As noted from Figure 3d, the water droplet on the crack-free multiple-scale nanofiber film was free from silica fragments. The average contact angle was determined as 160° by the sessile drop method with a low sliding (roll-off) angle ( 55/100 µm2. It means that the uniformity of the film was also improved when ns was large enough. The contact angle of stationary water droplets on the multiscale nanofiber film was 156° with an extremely low sliding angle ( 138/100 µm2. Indeed, the contact angle of the sessile water droplet approached the upper limit when ns > 138/100 µm2. The weight fraction of 50 nm silica particles in calcined fibrous film affected the water contact angle as shown in Figure 5b. The weight fraction of 50 nm silica particles in calcined fibrous film was controlled by changing the volume of 34-wt % aqueous 50 nm silica suspension in the range 150-600 µL, which was added to the polymer solution together with 700 nm silica microspheres for electrospinning. As noted, the water contact angle could be controlled by changing the weight fraction of 50

Figure 6. X-ray photoelectron spectroscopy (XPS) data from (a) before and (b) after calcined film of electrospun multiple-scale structured fibers and (c) surface treated with FDTS for calcined film of electrospun multiple-scale structured fibers formed in PAM medium.

nm silica particles in calcined fibrous film for various number densities of nanofiber segments. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical composition on the surface of multiple-scale nanofiber films. Figure 6 shows the XPS spectra of electrospun nanofiber film before calcination, after calcination and fluorination. An as-prepared electrospun fibrous film before calcination has a nitrogen peak (398 eV) originating from the amide group in PAM as shown in Figure 6a. Since PAM was removed by calcination, the nitrogen peak (398 eV) was not observed for the calcined multiple-scale nanofiber films and the carbon peak (285 eV) reduced drastically, see Figure 6b. Although the bulk carbon

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Figure 7. (a, b) SEM images of electrospun composite nanofibers of PAM and 50 nm silica particles with 237 nm polystyrene particles before and after calcination, respectively. The collection time was 3 min. (c) TEM image of macroporous nanofiber film. (d) Contact angles of water droplets on FDTS treated silica porous nanofiber films in PAM medium as a function of the segment density ns.

was removed by high temperature heat treatment, a small amount of aliphatic carbon or polymer could be retained.38,39 The XPS spectrum of fluorinated multiple-scale nanofibers films showed strong peaks of fluorine (685 eV), which were not shown in Figure 6a,b. These peaks indicated the presence of FDTS anchored on the surface of the multiple-scale silica nanofiber films. In addition, no chlorine peak (199 eV) in the XPS spectrum of the fluorinated multiple-scale nanofibers indicated that there was no physical adsorption of FDTS (Figure 6c). Fabrication of Macroporous Silica Nanofiber Films Formed with PAM as a Matrix Medium. As another approach for hierarchical surface roughness, macropores were introduced on the silica nanofibers, in which 237 nm polystyrene particles were used as templates for macropores instead of 700 nm silica particles in binary colloids used for the previous multiple structured fibers (system F). As shown in Figure 7a, 237 nm polystyrene particles and 50 nm silica particles were confined by PAM during electrospinning process. After removal of 237 nm polystyrene particles and PAM matrixes by calcination, the stand-alone porous silica nanofibers were successfully fabricated as shown in the SEM image of Figure 7b. The TEM image of the nanofibers with macropores is reproduced in Figure 7c. The water repellency of the porous nanofiber films could be also controlled by changing the segment density ns as shown in Figure 7d. When ns < 42/100 µm2, the water droplet was pinned on the porous nanofiber films. For ns > 55/100 µm2, however, the roll-off angle of water droplet was smaller than 5°. Eventually, the roll-off angle became smaller than 2° when ns > 88/100 µm2. The contact angle of water on the porous nanofiber film was 152° with a low sliding angle ( 121/100 µm2. (38) Shukla, S.; Brinley, E.; Cho, H. J.; Seal, S. Polymer 2005, 46, 12130. (39) Ochs, D.; Dieckhoff, S.; Cord, B. Surf. Interface Anal. 2000, 30, 12.

Theoretical Analysis. The effect of the surface microstructure on water repellency can be explained by two well-known distinct models depending on the degree of surface roughness: namely, the Wenzel model40,41 and the Cassie-Baxter model,42 for lower and higher degrees of surface roughness, respectively. According to the Wenzel model, the high contact angle of water originates from the increase in surface area. The large interfacial energy at the water-solid interface induces penetration of water into the surface cavities. Therefore, the water droplet on the films has high contact angle hysteresis. Meanwhile, in the Cassie-Baxter model, water droplets mostly contact air pockets, which exist between water and a rough solid surface, without penetrating into the surface cavities. Therefore, a superhydrophobic surface of the Cassie-Baxter model has a lower sliding angle. There is a critical degree of surface roughness for the transition from a Wenzel to a Cassie-Baxter state.42 We could control the surface roughness of our FDTS treated electrospun multiple-scale nanofiber films by changing the number density of nanofiber segments. Their surface properties on water repellency were consistent with both the models. When the number density of nanofiber segments was relatively low, the multiple-scale nanofiber films exhibited water repellent behavior according to the Wenzel model. The transition between two states could be determined by measuring the water sliding angle as a function of the segment density ns. For example, in the case of multiplescale nanofiber films from PAM solution with bidisperse silica colloids of 700 and 50 nm in size (system E of Table 1), the water droplet was pinned when ns < 33/100 µm2, which corresponded to the Wenzel state. Meanwhile, when the segment density was (40) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (41) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53, 1466. (42) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.

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Lim et al. Table 2. Contact Angles and Fractional Interfacial Areas of Solid and Air Contact with a Water Droplet (f1 and f2) in Various Systems in Table 1 for ns ) 138/100 µm2

Figure 8. Side view still-shot images of a rolling water drop of 10 µL in volume on a wafer of 10.16 cm in diameter coated with superhydrophobic multiple-scale nanofiber films from PAM solution with bidisperse silica colloids of 700 and 50 nm in size. The wafer was tilted at 1.5° relative to horizontal, and the images were captured from the video clip with a time interval of 0.02 s.

42/100 µm2, the water sliding angle became smaller than 5°. Therefore, there is a transition from Wenzel to Cassie-Baxter state when the segment density is between 33/100 µm2 and 42/ 100 µm2. The electrospun fibrous film with higher number density corresponded to the Cassie-Baxter state superhydrophobic surface, which had a high contact angle (>150°) with a small sliding angle. Clearly, the surface roughness of these films exceeded the critical level for transition to the Cassie-Baxter state.4 The Cassie-Baxter model relates the surface wettability to the surface roughness

cos θr ) f1 cos θs - f2 where f1 and f2( )1 - f1) are the fractions of solid-water and air-water contact areas, respectively, and θr and θs denote the contact angles of a water droplet on rough and smooth surfaces, respectively. The contact angle 117° of water on the smooth surface (θs) was measured by using a fluorinated silicon wafer. The fractional contact areas, f1 and f2, were calculated using the Cassie-Baxter model in different systems for ns ) 138/100 µm2, and the measured contact angles and calculated fractional contact areas were listed in Table 2. The effect of decorating the electrospun fibers with macropores or microparticles on the increase of f2 could be clearly shown from the calculation. The surface decorations of electrospun fibers increased the superhydrophobicity of the fibrous films. A 10 µL water droplet was rolled off on the 1.5° tilted wafer (of 10.16 cm in diameter) coated with superhydrophobic multiplescale nanofiber films made of PAM solution with bidisperse silica colloids of 700 and 50 nm in size (system E). The video clips were included in the Supporting Information. The captured images with time interval of 0.02 s from the video clip of a magnified side view are shown in Figure 8. We could calculate the impact velocity and the velocity after the shock of a water

system

contact angle (deg)

fractions of air contact with a water droplet (f1)

fractions of air contact with a water droplet (f2)

A B C D E F

154 160 160 149 156 152

0.186 0.110 0.110 0.262 0.158 0.214

0.814 0.890 0.890 0.738 0.842 0.786

droplet of 0.134 cm in radius from the video clips in the Supporting Information and the microscopic images. The impact velocity and the velocity after the shock were 13.2 and 8.4 cm/s, respectively, when the water droplet was fallen from 0.4 cm high. In general, Weber number (We ) FV2R/γ) and Reynolds number (Re ) FVR/η) provide underlying physics about the behavior of impinged water droplets on the superhydrophobic surfaces.43,44 Here, F, η, and γ denote the density, viscosity, and surface tension, respectively, and V and R represent the characteristic velocity and length scale of a water drop, respectively. Since Weber number is the ratio of deforming kinetic energy to restoring surface energy, the droplet impinged on the superhydrophobic surface keeps a quasispherical shape when Weber number is smaller than unity. We could calculate Weber number and Reynolds number from the experimental data. In our system, the calculated Weber number was 0.319 and much smaller than unity. In addition, the calculated Reynolds number, which is the ratio of inertia forces to dissipative viscous forces, was much larger than unity (174.4). It could be concluded from these two dimensionless numbers (We < 1 and Re > 1) together with the video clips in Supporting Information that the impinged droplets on the superhydrophobic surfaces did not spread during the roll-off but kept rolling. In addition, these experimental data clearly indicated that fluorinated multiple-scale silica nanofiber films were superhydrophobic with self-cleaning capability.

Conclusion We demonstrated a facile method for fabricating multiplescale nanofiber films via an electrospinning process using bidisperse colloidal particles and polymers. To fabricate the multiple-scale nanofiber films, silica microspheres or macropores templated from polystyrene particles were decorated on the inorganic nanofibers made of 50 nm silica particles. Selective removal of organic materials by calcination and subsequent treatment with fluorinated silane coupling agent by vapor-phase reactions enabled the creation of superhydrophobic surfaces with extremely low sliding angle. However, in the case of silicaPEO composite nanofibers, not all particles inside the nanofibers were connected. Consequently, severe cracks appeared after calcinations. These cracks produced tiny fragments which inevitably contaminated the water droplets during contact angle measurement. To solve this problem, we employed two different approaches: the addition of hexanol to the bidisperse silica and PEO solution and the use of more hydrophilic PAM instead of PEO as a matrix. The multiple-scale roughness from bidisperse colloids of 700 and 50 nm silica particles or macropores templated from polystyrene particles and 50 nm silica particles was essential to generate higher contact angles and lower water sliding angles compared to the nanofiber films without multiple-scale roughness. In both cases, stable superhydrophobic multiple-scale fibrous (43) Richard, D.; Quere, D. Europhys. Lett. 2000, 50, 769. (44) Aussillous, P.; Quere, D. Nature 2001, 411, 924.

Superhydrophobic Films of Electrospun Fibers

films with large contact angle (>150°) and low sliding angle (