pubs.acs.org/Langmuir © 2009 American Chemical Society
Superhydrophobic Behaviors of Polymeric Surfaces with Aligned Nanofibers Xianliang Sheng† and Jihua Zhang*,‡ †
College of Science, Inner Mongolia Agriculture University, Hohhot 010018, PR China, and ‡Aerospace Research Institute of Material and Processing Technology, Beijing 100076, PR China Received January 17, 2009. Revised Manuscript Received February 24, 2009
In this article, nanostructured superhydrophobic polymeric surfaces were fabricated by a simple (one-step) reproductive method of anodic aluminum oxide (AAO) template extrusion. By tuning the diameter of the AAO template and the pressure to extrude, high-density polyethylene (HDPE) nanofiber surfaces with different nanometer roughness were prepared, and various sliding angles (SAs) of drops on these surfaces were measured. The results of the impact of drops on the nanostructured HDPE surfaces indicated that SAs were very important for the dynamic wettability of superhydrophobic surfaces. The one-step AAO template extrusion method has the advantage of tailoring the SA values on polymeric surfaces. Therefore, we believe it to be a promising industrial basis for manufacturing functional materials in the fields of agriculture, electronics, and optics.
1. Introduction Surface wettability is an important characteristic of solid materials and is involved in molecular, microscopic surface structures and macroscopic geometrical morphology.1-3 Researchers have paid much attention to tailor surface wettability by chemical composition and microstructures. In particular, surface superhydrophobicity such as the lotus effect (a water contact angle (CA) higher than 150° and a sliding angle (SA) smaller than 10°) has recently attracted great interest and has become a hot issue because of its theoretical values and potential practical applications.4-8 Generally, micrometer- or nanometerorder rough surfaces and low-surface-energy materials are the keys to obtaining superhydrophobic surfaces.4-6 Inspired by this, material scientists have synthesized artificial superhydrophobic surfaces by the solution method, sol-gel method, chemical vapor deposition method, electrospinning method, and others.8-19 *Corresponding author. E-mail:
[email protected]. (1) Adamson, A. M. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: Toronto, 1997; Chapter 1. (2) Chappuis, J. In Multiphase Science and Technology; Hewitt, G. F., Delhaye, J. M., Zuber, N., Eds.; Hemisphere Pub. Corp.: Washington, 1985; Vol. 1, S387. (3) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827. (4) Quere, D. Physica A 2002, 313, 32. (5) McHale, G.; Shirtcliffe, N. J.; Aqil, S.; Perry, C. C.; Newton, M. I. Phys. Rev. Lett. 2004, 93, 036102. (6) (a) Extrand, C. W. Langmuir 2002, 18, 7991. (b) Extrand, C. W. Langmuir 2004, 20, 5013. (7) (a) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2002, 41, 1221. (b) Gao, X.; Jiang, L. Nature (London) 2004, 432, 36. (8) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (9) Shibuchi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512. (10) McCarthy, T. J.; Oner, D. Langmuir 2000, 16, 7777. (11) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818. (12) Blossey, R. Nat. Mater. 2003, 2, 301. (13) Xie, Q.; Xu, J.; Feng, L.; Jiang, L.; Tang, W.; Luo, X.; Han, C. C. Adv. Mater. 2004, 16, 302. (14) Shi, F.; Wang, Z.; Zhang, X. Adv. Mater. 2005, 17, 1005. (15) Miwa, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754. (16) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R. Science 1999, 283, 46. (17) Lam, P.; Wynne, K. J.; Wnek, G. E. Langmuir 2002, 18, 948. (18) Li, H.; Wang, X.; Song, Y.; Lin, Y.; Li, Q.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2001, 40, 1743. (19) Lee, W.; Jin, M-K.; Yoo, W-C.; Lee, J.-K. Langmuir 2004, 20, 7665.
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However, these methods are complex, time-consuming, and use expensive and/or noxious hydrophobic materials (e.g., fluorine and/or fluorine-containing compounds), which limits their use in practical applications.20,21 To solve these problems, the template method (nanoimprint), using the nanopatterned metallic sheet or nanoporous anodic metal oxide as a replication template, and tuning the hydrophobic material to the superhydrophobic surface is very attractive.20 The advantage comes from the unique tailoring capability of pattern sizes and heights of replication templates by simply varying the electrochemical parameters and thus easily obtaining large-area superhydrophobic surfaces. However, smooth, flat polymeric surfaces or flowable polymer at ambient temperature (i.e., poly(dimethysiloxane), PDMS) is usually required before it is pressed into nanopores of the template because surface undulation would greatly affect the quality of as-prepared nanostructured polymeric surface. Moreover, to control the height of nanopatterns, the replication templates are not through-hole. Therefore, it was known that the replication template method was more frequently applied in the laboratory. We believe that a technological advance to widen the application of the template method is urgently required. Here, we described a simple, highly reproducible template method for fabricating large-area nanostructured crystalline or amorphous polymer surfaces. In contrast with the conventional method, our method used only granulated or powder polymer before nanoimprinting. Thorough-hole nanotemplates were synthesized and applied to prepare polymeric nanopatterns with various sizes and heights during anodic aluminum oxide (AAO) template extrusion. The surface wettability of nanopatterned high-density polyethylene (HDPE) with various sizes was tuned by the diameter of template nanochannels and applying heat and pressure. The SAs caused by this surface morphology were investigated in detail. Moreover, the phenomena of a drop impacting these surfaces with various SA values were studied. We believed that the nanotechnology of template extrusion would be important in the applications of medicine, sensors, and catalysis because of its simplicity and efficiency. (20) Guo, C.; Feng, L.; Zhai, J.; Wang, G.; Song, Y.; Jiang, L.; Zhu, D. ChemPhysChem 2004, 5, 750. (21) Wang, S.; Song, Y.; Jiang, L. Nanotechnology 2007, 18, 015103.
Published on Web 03/27/2009
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2. Experimental Section Preparation of the AAO Template. AAO templates were prepared according to ref 7, with a thickness of ∼100 μm and a porosity consisting of an array of parallel, straight channels. The average diameter of a pore (channel) was measured with a fieldemission scanning electron microscope (FE-SEM, JEOL JSM 6700F, Japan). Free Image Tool software (v3.00, developed at UTHSCSA by Don Wilcox et al.) was used to treat the FE-SEM pictures. By counting the black and white pixels, we obtained the measured results. The density of porosity, φ, in the template can be estimated by eq 1: φ ¼
4 Aporous 2 πd Atemplate
ð1Þ
where Aporous is the total area of the nanopores, Atemplate is the total area of the templates, and dhis the average diameter of the nanopores in the templates. Preparation of Nanofibers by Template Extrusion. Commercial-grade high-density polyethylene (HDPE) (M w ≈ 140 000 g/mol, density = 0.956 g/cm3, and melting temperature = 129.3 °C) was produced by Yanshan Petrochemical Co., Ltd. (Beijing, China). Figure 1 shows the experimental setup used to prepare the HDPE nanofiber surfaces. A polytetrafluoroethylene (PTFE) sheet was placed on the bottom of a die with a size of Φ = 16 mm 50 mm. Then an as-prepared AAO template was placed on the PTFE sheet. After 0.3 g of granular HDPE materials without any further treatment was piled onto the AAO template, another PTFE sheet was placed on the HDPE granules. The experimental setup can control the experimental pressure and temperature accurately. The setup was heated to above the melting point or flow temperature to make the polymer chain sufficiently mobile, and pressure (90° because the previous studies have shown that a drop in the Cassie state had commonly low adhesion.26 In contrast, a drop in the Wenzel state on superhydrophobic surfaces had large wetting hysteresis (i.e., a large SA value27). The criterion that the transition from the Cassie state to the Wenzel state occurs can be described by some critical conditions such as the critical pressure pw and height hmin. For well-aligned nanofibers, the pressure pw is expressed by eq 428 pw ¼ -2
γ cos θe λ
ð4Þ
where λ is the average space between two adjacent nanofibers and θe is in equilibrium with CA on the smooth surface. Also, the minimum asperity height hmin can be calculated by eq 528 hmin ¼ -
1 -sin θe λ 2 cos θe
ð5Þ
In our cases, the value of θe for the smooth HDPE surface (pressing between two glass sheets at 170 °C (i.e., the S0 surface)) was 96.7 ( 1.7°. Therefore, the critical ratio hmin/λ was estimated to be approximately 0.029. Considering the length of the nanofibers (24) Gogte, S.; Vorobieff, P.; Truesdell, R.; Mammoli, A.; van Swol, F.; Shah, P.; Brinker, C. J. Phys. Fluids 2005, 17, 51701. (25) Sakai, M.; Song, J-H.; Yoshida, N.; Suzuki, S.; Kameshima, Y.; Nakajima, A. Langmuir 2006, 22, 4906. (26) Richard, D.; Quere, D. Europhys. Lett. 2000, 50, 769. (27) Richard, D.; Quere, D. Europhys. Lett. 1999, 48, 286. (28) Carbone, G.; Mangialardi, L. Eur. Phys. J. E 2005, 16, 67.
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and the shape of the AAO templates (the spaces and pore diameters of templates retain the same order of magnitude), h/λ of these extruded nanofibers was far greater than the critical value. Thus, small wetting hysteresis should occur on the S2 and S3 surfaces with aligned nanofibers. Further analysis of Figure 2a showed that the pore created by the self-assembly of bundles of the S1 surface (their diameters were 5-30 μm) greatly reduced the h/λ ratio on the S1 surface. Therefore, the high wetting hysteresis on the S1 surface may be introduced by such a reduction of h/λ; that is, the self-assembly or bundles of nanofibers caused the discrepancies between the SAs and the high nanopore density of the template. Therefore, it was addressed that the microstructures of surfaces may greatly affect their dynamic wetting behaviors. To further explore the effect of surface microstructures with extruded nanofibers on wetting hysteresis or SAs, the impact of drops on the nanostructured surfaces was analyzed. Figure 4 shows typical images of an 8.1 mg drop impacting the smooth and nanostructured surfaces. When a drop encountered a solid surface, its initial spherical shape was forced into a pancake-like form that stretched out over the surface. On the smooth hydrophobic HDPE surface (Figure 4a and video S1), the drop retracted to minimize its exposure after the water was stretched out. Figure 4b shows typical images for a drop (8.1 mg) impacting the S1 surface with a high SA value (details in video S2). Because of the large CA value (CA = 145.8 ( 1.1°), the S1 surface was considered to be a superhydrophobic surface. However, the wetting adhesion was so intense that the water drop could not bounce off the surface after impact. The whole drop was fastened onto the S1 surface although it was elongated to a maximum degree (t = 17.6 ms) and its top was hemispheric. The drop rebounded higher than the DOI: 10.1021/la9002077
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Figure 5. Plots of the relationship between drop mass and some physical parameters: (a) the maximum diameter Dmax of impacting water drops, (b) the volume conservation, (c) the minimum thickness h of impacting water drops, and (d) the contact time τ and restitution coefficient ε on the S2 surface.
smooth surface during impact. Figure 4c showed a drop impacting the superhydrophobic S2 surface. In contrast with S0 and S1 surfaces, the drop bounced off the S2 surface after impact (video S3). Moreover, the drop rebounded onto the S2 surface several times until it was still. Thus, the S2 surface is a lowadhesive superhydrophobic surface. Figure 4d shows the pancake-like form of the drop after impact where its diameter reached the maximum Dmax. Figure 4e shows a typical image of an impacting drop breaking into smaller drops at a certain Weber number We. Experiments showed that the Weber number of drops, required by the phenomenon, on the S2 surface (We = Fv2D0/γ = 17.06, where D0 is the drop diameter of 3.16 mm) was larger than that on the S1 surface (We = 13.74 for the drop with a drop diameter of 2.54 mm). That the drop broke into smaller drops reduced the kinetic energy, and thus the rebounding of the drops gradually decreased. Note that the CA value of water on the S1 surface was 131.4 ( 2.6° after impact, which is far smaller than the static CA values (145.8 ( 1.1°). Therefore, the issues concerning the impact phenomena of drops on rough surfaces were very important and were relative to the microstructures of surfaces. Figure 5a shows the plots of maximal deformation Dmax versus the mass of impacting drops (the impacting speed remained at 0.63 m/s, with the detailed data listed in Figure S6). By applying the outer diameters of the needles, there were six sizes of drops in our experiments (the typical mass was between 8 and 30 mg). Considering similar SAs between the S2 and S3 surfaces, we chose only the S2 surface as a typical case. The linear function was observed between maximal deformation and the mass of drops in a log-log plot, regardless of the hydrophobicity of the surfaces. 6920 DOI: 10.1021/la9002077
At the 95% confidence level, the slopes of S0, S1, and S2 were calculated to be 0.44, 0.49, and 0.47, respectively. In the process of impacting, the transfer of kinetic energy to surface tension occurred with the neglect of viscous dissipation (water has a viscosity of 1.0 cP at 20 °C). The energy conservation is simply FD30v2 ∼ γD2max (The symbol ∼ denotes that FD30v2 scales as γD2max. The same usage exists in ref 29 and will be applied throughout the entire article), where Dmax is the maximum diameter of the drop after impact.26-32 This yields Dmax ∼ D0We1/2, in addition of the relationship We ∼ D0. Hence, we can draw the conclusion that Dmax ∼ M1/2. The exponent of 0.5 in the function agreed well with our data in the log-log plots. Figure 5b shows the dependence of hDmax on the drops’ mass of M. Because of the volume conservation of hDmax ∼ D30, hDmax ∼ M holds, and the minimum thickness hmin in the vertical direction is reached when the impacting drops are flattened to the maximum diameter Dmax. The minimum thickness hmin was plotted as a function of the mass of drops M in Figure 5c. A square root relation of hmin ∼ M1/2 was observed. For the S0 and S1 surfaces, after impact, the drop’s surface oscillated as a result of the transformation between the kinetic energy and the surface energy. For the S2 surface, the water drop bounced off and rebounded several times until the energy was lost.
(29) Clanet, C.; Beguin, C.; Richard, D.; Quere, D. J. Fluid Mech. 2004, 517, 199. (30) Okumura, K.; Chevy, F.; Richard, D.; Quere, D.; Clanet, C. Europhys. Lett. 2003, 62, 237. (31) Richard, D.; Clanet, C.; Quere, D. Nature (London) 2002, 417, 811. (32) Clanet, C.; Quere, D. J. Fluid Mech. 2002, 460, 131.
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Figure 6. (a and b) FE-SEM top and cross-section images of the HDPE nanofiber surface, respectively. (c) Optical image of a 3 μL drop on the superhydrophobic HDPE surface. By controlling the extruded pressure and temperature, we obtained an HDPE surface with nanofibers at a height of ∼5.2 μm.
More careful discussions focused on the S2 surface because of its possible industrial uses.26 Figure 5d shows the plot of contact time τ versus the mass of drops simply by balancing inertia (FD0/ 2τ2) with capillarity (4γ/D20), which yielded τ ≈ (FD30/8γ)1/2.31 Together with D30 ∼ M, we deduced τ ∼ M1/2, in good agreement with our results, where the exponential relationship existed (the exponent was evaluated as approximately 0.48 at the 95% confidence level). However, the restitution coefficient ε of drops with various mass (ε = |v0 /v|, where v and v0 are the speeds before and after the drops bounce, respectively) was plotted in Figure 5d. Note that ε decreased with the increase in mass, M, of the drops; however, the dispersibility of the data increased. Nevertheless, the value of ε (approximately 0.5) remained almost constant. The drop reached the maximum deformation at a contact time of 4.4 ms, and it lifted off at 13.2 ms after contact and then it reached the highest position after bouncing at t = 28.6 ms (H = 5.21 mm), which√yielded ε = 0.51 (as deduced from the relationship ε = |v0 / v| = (|H0 /H), Figure 4c). Compared with the work by Quere’s group, our ε data were smaller,26-32 which was due to the sizes of drops (which were larger than theirs). Moreover, a pure transfer of kinetic energy to surface tension did not exist between drops and the S2 surface, or the other energy loss formations were included, such as the internal motion, heat, and incomplete inelastic impact (the SA value was not zero). Noticeably, after the drops bounced off of the surface, the oscillations reduced the kinetic energy as a result of the large sizes of the drops, but the impact of the drop did not affect the CA (159.7°), which was almost same as the static state (160.3 ( 1.5°, Figure 4). In addition, there were various phenomena impacting the superhydrophobic surfaces with different SAs. This indicated that the values of SA played an important role in the dynamic behavior of drops on solid surfaces. On other superhydrophobic surfaces with defects (SA ≈ 60°, extruded by the S2 templates), drops even broke into two parts after impact: the larger part bounced off, and the smaller part stayed on the surface. Moreover, the number of drops rebounding was due to more energy loss in such cases. These complex cases limited the details of our studies. Nevertheless, SAs of drops on the solid surfaces were really the key factor affected the wetting behavior after impacting. To better tailor the SAs on the polymer surfaces with nanofibers, the details of the preparations of nanofibers were discussed, and their effects on the wettabilities of nanostructured surfaces were further explored. Figure 6 shows the FE-SEM image of the nanostructured surface extruded by applying 500 Pa of pressure at Langmuir 2009, 25(12), 6916–6922
a temperature of 170 °C for 20 min (the diameter of the nanopores was about 216 nm in the AAO template). It is well known that thermoplastic materials, such as HDPE, will flow in a pipe as a result of the pressure on the template above their melting points, which has been described by the Hagen-Poiseuille equation (eq 6)33 128ηLQ ΔP ¼ ð6Þ πd 4 where ΔP is the pressure drop, L is the length of pipe, η is the dynamic viscosity, Q is the volumetric flow rate, and d is the diameter of the pipe. Because of the flow speed difference of HDPE between the wall and hole of nanopores in the AAO template at a temperature above the melting point, an obvious “skin/core” structure of HDPE was found in the magnified SEM image (Figure 6a). Figure 6b shows the height of nanofibers as about 5.2 μm. Because of the short height, these nanofibers were rather more vertical to the substrate than those in Figure 2. Figure 6c shows that the surface was superhydrophobic with a CA of 159.3°. Figure S7 shows plots of extruded pressure versus the heights of nanofibers at the extrusion temperature of 170 °C for 20 min. In contrast with Figure 2e, it was deduced that the length of nanofibers increased with the increase in pressure. This provided a new method to tune the diameter and height of extruded nanofibers only by adjusting the diameter of the thorough-hole AAO template, which was more simple than conventional technologies for nanoimprints. Figure S8 shows plots of the SAs of a 3 μL drop versus the height of nanofibers extruded at various pressures. Note that the SAs of drops decreased with the increase in height of the nanofibers, which agreed with eq 5. Therefore, we believe that AAO template extrusion was a simple prospective method by which to adjust the wettabilities of polymer surfaces.
4. Conclusions We provided a simple reproducible method of fabricating nanostructured polymeric surfaces with various SAs. Through tuning the diameter of AAO templates and extruded pressure, various superhydrophobic HDPE nanofiber surfaces were prepared. The drops impacting these nanostructured HDPE surfaces showed that one-step AAO template extrusion was an advantage of tuning the dynamic wettability of polymeric surfaces. In (33) Joseph, S.; Aluru, N. R. Nano Lett. 2008, 8, 452.
DOI: 10.1021/la9002077
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addition, this method was not only applied to these crystalline polymers (for example, HDPE, nylon (PA6), and Teflon) but also was effective for amorphous polymers such as polystyrene (PS), polymethyl methacrylate (PMMA), and polycarbonate (PC). Therefore, we believe it to be a promising industrial basis for producing functional materials in the fields of agriculture, electronics, and optics. Acknowledgment. We gratefully thank Professor Lei Jiang (Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China) for free use of the apparatus and helpful
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discussions. This work was partially supported by the Science Foundation for College of the Inner Mongolia Autonomous Region (NJzy08040) and the Science Foundation for Doctor of Inner Mongolia Agriculture University (K41615). Supporting Information Available: Additional information concerning the FE-SEM images of the AAO template, XRD and DSC results of smooth and various HDPE nanofibers, and movies of a drop impacting the nanostructured surfaces with SAs. This material is available free of charge via the Internet at http://pubs.acs.org.
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