Article pubs.acs.org/Langmuir
Micro−Micro Hierarchy Replacing Micro−Nano Hierarchy: A Precisely Controlled Way To Produce Wear-Resistant Superhydrophobic Polymer Surfaces Eero Huovinen, Janne Hirvi, Mika Suvanto,* and Tapani A. Pakkanen* Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101, Joensuu, Finland ABSTRACT: Superhydrophobic polymer surfaces are typically fabricated by combining hierarchical micro-nanostructures. The surfaces have a great technological potential because of their special water-repellent and selfcleaning properties. However, the poor mechanical robustness of such surfaces has severely limited their use in practical applications. This study presents a simple and swift mass production method for manufacturing hierarchically structured polymer surfaces at micrometer scale. Polypropylene surface structuring was done using injection molding, where the microstructured molds were made with a microworking robot. The effect of the micro− microstructuring on the polymer surface wettability and mechanical robustness was studied and compared to the corresponding properties of micro− nanostructured surfaces. The static contact angles of the micro−microstructured surfaces were greater than 150° and the contact angle hysteresis was low, showing that the effect of hierarchy on the surface wetting properties works equally well at micrometer scale. Hierarchically micro−microstructured polymer surfaces exhibited the same superhydrophobic wetting properties as did the hierarchically micro−nanostructured surfaces. Micro−microstructures had superior mechanical robustness in wear tests as compared to the micro−nanostructured surfaces. The new microstructuring technique offers a precisely controlled way to produce superhydrophobic wetting properties to injection moldable polymers with sufficiently high intrinsic hydrophobicity.
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Cassie−Baxter state (eq 1), where θflat is Young’s angle for a flat surface, fsolid is the area fraction of solid surface in contact with a liquid, and fair is the fraction of air in contact with the liquid.18
INTRODUCTION Organisms have evolved with unique properties allowing them to survive in a diversity of conditions. The surface properties of materials are very important, because surfaces are in continuous interaction with the surroundings. In adapting to their environment, many life forms have developed surface structures with unique functional properties, and characterization of the surfaces has revealed the presence of hierarchical micro− nanoscale structures. Notable examples of functional structures include the microscopic hairs on the legs of water striders, allowing them to float on water;1,2 the hair-like setae on the feet of geckos, allowing them to climb up walls;3,4 the extremely hydrophobic wing surfaces of cicadas2 and butterflies5 that enable them to fly in wet conditions; and the hierarchical micro−nanostructures on the leaves of the lotus plant, which give them self-cleaning properties.2,6−8 The special wetting properties of hierarchical structures can be explained by the wetting theories for a liquid drop on a rough solid surface. When a liquid drop is in contact with a rough surface, two possible states can appear: the Wenzel state,9 where the liquid drop wets the whole surface structure, or the Cassie−Baxter state,10 where the liquid drop lies on the top of the surface structure. Numerous studies8,11−17 have showed that hierarchical structures with superhydrophobic properties (a static contact angle greater than 150° and a contact angle hysteresis that is low) are in the Cassie−Baxter state. A composite surface of air and structure is formed in the © 2012 American Chemical Society
cos θrough = fsolid cos θflat − fair
(1)
The contact angle of a water drop is dependent on the ratio of volume of entrapped air to the volume of the surface structure. For a hierarchical micro−nanostructure, the area fraction of solid surface in contact with a liquid is calculated by multiplying the area fraction of the microstructure and the area fraction of the nanostructure (Figure 1). As a result of multiplying, the area fraction of a two-level structure in contact with a liquid is much smaller than that of a single-level structure, explaining why the superhydrophobic state is best achieved by hierarchical structures. Many different methods have been developed to fabricate hierarchically micro−nanostructured polymer surfaces.11−14,19−35 By combining micro−nanometer-scale structures with low surface energy materials, superhydrophobic wetting properties have been produced.12−14,19−33 Superhydrophobic surfaces have a great potential in commercial applications where self-cleaning, nonwetting, and corrosion resistance properties are needed.36,37 Despite numerous fabrication Received: January 4, 2012 Revised: September 24, 2012 Published: September 25, 2012 14747
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made of tungsten carbide (Fodesco), and tips were 10 (round), 12 (square), and 190 (square) μm in diameter. Single-level microstructures were fabricated by placing a needle (⌀ 190 μm) in the robot, selecting the parameters of the robot program, and then machining a micropitted structure in aluminum foil. Twolevel microstructures were made by machining a second time, with a needle with a smaller diameter. Proper alignment of the micropits was achieved by first machining a few small micropits at the edge of the micropitted structure and then observing with a microscope the divergence of the smaller micropits from the corner of the larger micropits. The necessary adjustments were then made to the coordinates of the robot program, so that the smaller micropits would lie inside the larger micropits across the whole of the micropitted area. The size of the micro−microstructured area was 4 × 4 mm. Nanostructures were made by an anodic aluminum oxide (AAO) process. An electropolished and microstructured aluminum foil was placed in an acid solution where platinum foil was used as a counterelectrode. The anodization temperature was 3 °C, and the time was 24 h. A detailed description of this process has been reported elsewhere.26 The size of the micro−nanostructured area was 4 × 4 mm. Micro−nanostructured molds were glued onto steel plates (thickness 0.5 mm) with a thermostable epoxy glue (Loctite Hysol 9492 A&B). The structured side of the mold faced upward. Replication of Mold Structures on Polymer Material. The microinjection molding machine was a DSM Midi 2000 extruder. The following processing parameters were selected for polypropylene (PP, HD 120 MO, density 954 kg/m3) on the basis of a previous study:26 mold temperature 70 °C, screw temperature 255 °C, screw rotation speed 80 rpm, and pressure of injection piston 5 bar. Characterization of Surface Structures. Structured polymer samples were imaged with scanning electron microscopy (SEM, Hitachi S4800). Samples were glued onto a stub with carbon and copper tape, and then coated with 5 nm of Au. During the measurements, the acceleration voltages were 3−5 kV, and the working distance was 8 mm. The topography of the surface structures was studied using an atomic force microscope (AFM, Thermo Microscopes EXPLORER 4400-11) in noncontact mode. The water contact angles of the structured surfaces were measured with a KSV Cam 200 contact angle meter. For static contact angle measurements, a drop of ion-exchanged water (5 μL) was placed above the polymer surface at room temperature and photographed once a second for 30 s. The contact angles were defined by fitting a Young−Laplace curve around the drop. Measurements were taken from 6 to 30 s. Six parallel measurements were made for each surface structure, and the average values were used to calculate the contact angles. To conduct the sliding angle measurements, a drop of ionexchanged water (6 μL) was placed a polymer surface on which there was a tiltable plate. The sliding angles reported are the values for the tilt angles at which a drop of water began to roll on the surface. Dynamic angles were also measured for each surface structure. A detailed description of how the advancing and receding contact angles were measured and how the contact angle hysteresis was calculated has been reported elsewhere.51 Mechanical Tests of Surface Structures. The mechanical robustness of the surface structures was tested using a pin-on-disk-type tribometer (CSM+ Instruments Tribometer TRN S/N 18-347). A press test was performed by applying pressure to the surface structure (4 × 4 mm) using different loads against a steel counter face for 30 min. A wear test was conducted by applying pressure to the surface structure with different loads and then wearing it against the steel counter face for 10 m with a speed of 0.50 cm/s. Loads ranging from 1 to 10 N were used on the tests. Following the mechanical tests, the structures were studied by means of the subsequent contact angle measurements, SEM, and optical microscopy (Leica Z16 APO).
Figure 1. The area fraction of a microstructure, a nanostructure, and a hierarchical micro−nanostructure in contact with a liquid.
methods to produce the surfaces, only a few commercial products are currently available.36,38 The costliness and complexity of the production methods have caused some restrictions, but one of the major obstacles has been the generally poor mechanical robustness of such surfaces.36,39 Until now, only a few studies have focused on the mechanical durability of superhydrophobic polymer surfaces.39,40 The fabrication of mechanically robust superhydrophobic surfaces is not, however, straightforward. The coating techniques require multiple procedures, and the hierarchical structure has to be manufactured separately on each surface,40 while in the lamination technique, lengthy cycle times are required to replicate the mold structures on a polymer surface.39 A hierarchy can also be achieved on another scale, by combining micro−microstructures. Micro−microstructures have potential in some applications, such as with microsensors41 and antioil contamination surfaces.42 The microstructure manufacturing methods have not been used in the fabrication of hierarchically micro−microstructured polymer surfaces with superhydrophobic wetting properties.35,41−48 It is reasonable to assume that, as a result of the effect of hierarchy, micro−microstructures would produce highly hydrophobic wetting properties. Moreover, the mechanical robustness of a micro−microstructure should be better than that of a micro− nanostructure.49 Straightforward methods for producing hierarchically micro−microstructured polymer surfaces would provide important alternatives in the field of microfabricating. In the present study, our intention is to find whether a hierarchical micro−microstructure is sufficient for creating superhydrophobic wetting properties. We shall explore possibilities for the fabrication of micro−microstructures permitting the optimization of structures in the Cassie−Baxter state. Finally, we also aim at demonstrating a simple and swift method for the mass production of micro−microstructured polymer surfaces that are mechanically more robust than micro−nanostructured surfaces.
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EXPERIMENTAL SECTION
Materials and Methods. Fabrication of Micro−Microstructured and Micro−Nanostructured Molds. Aluminum foil (4.0 × 4.0 cm, 0.25 mm thick, 99.997% Al, Puratronic, Alfa Aesar) was electropolished in a mixture of HClO4 and EtOH (1:8) for 135 s with a current of 2.7 A. Platinum foil was used as a counter electrode. The process has been described in detail previously.26,50 Microstructures were made with a microworking robot.26 The microstructured area was tailored with the settings of the robot program, where the controllable parameters were the numbers of lines and columns. The depth of the micropits was controlled by the impact force. The distance between the micropits was also controllable. The size and shape of the micropits were controlled with the diameter of the robot tip needle and the shape of the needle. The needles were 14748
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Figure 2. SEM images of hierarchical micro−microstructures on a PP surface: (A) hierarchical micro−microstructure (square−square), (B) and a close-up, (C) hierarchical micro−microstructure (square−barrel), and (D) and a close-up.
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RESULTS AND DISCUSSION Hierarchical Micro−Microstructures. Two different hierarchically micro−microstructured molds were made with a microworking robot. In the case of the hierarchical micro−
microstructure (square−square), the lower microstructure was fabricated using a square-shape needle tip of diameter 190 μm and the upper microstructure with a square-shape needle 12 μm in diameter. The hierarchical micro−microstructure (square− barrel) was fabricated similarly, the only difference being that the upper microstructure was fabricated using a barrel-shape needle tip 10 μm in diameter. The hierarchical micro−microstructures were tailored to a stable Cassie−Baxter state by using a critical contact angle as the criterion. The Cassie−Baxter10 and Wenzel9 eqs 1 and 2 intersect at one critical angle θcrit, as shown in eq 3, where r is the surface roughness and ϕS is the area fraction of the solid surface in contact with a liquid.52,53
Table 1. Dimensionsa and Surface Roughness of the Hierarchical Micro−Microstructures, Area Fraction of the Hierarchical Micro−Microstructures in Contact With a Liquid, and Criterion θflat > θcrit for Stable Cassie−Baxter State fabricated structure hierarchical micro−micro (square−square) lower microstructure upper microstructure hierarchical micro−micro (square−barrel) lower microstructure upper microstructure
a (μm)
b (μm)
H (μm)
r
ϕs
θflat > θcrit
0.16 190
150
70
1.5
0.31
12
5
11
2.8
0.50
cos θrough = r cos θflat 131 > 127 104 > 102
cos θcrit =
0.08 190
150
70
1.5
0.31
10
8
26
3.5
0.24
(2)
ϕS − 1 r − ϕS
(3)
The Wenzel state is energetically more favorable when θflat < θcrit, but the metastable Cassie−Baxter state can also appear in this regime.54−58 When θflat > θcrit, only the stable Cassie− Baxter state is possible.52,53 The criterion was applied in designing the necessary surface roughness parameters of the hierarchical micro−microstructures to achieve the stable Cassie−Baxter state using the robot technique. The exper-
146 > 127 104 > 103
a
a is the width of the micropillars, b is the distance between the micropillars, and H is the height of the micropillars. 14749
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Figure 3. SEM images of a hierarchical micro−nanostructure on a PP surface.
structures of the molds have been replicated efficiently on the polymer surfaces. With proper injection molding parameters, the micro−microstructures can be replicated on other injectionmolded plastic component surfaces as well. The regularity of the lines and columns reflects the accuracy of the microworking robot. The positioning of the upper micropillars inside the lower ones shows that the alignment process undertaken with the aid of a microscope was successful: the smaller 6400 squareshaped and the 8100 barrel-shaped micropillars on the larger 100 square-shaped micropillars lie exactly where they were positioned during the alignment process. The height, width, and depth of the microstructures can be controlled to within a micrometer by the technique. The dimensions of the micro−microstructures were estimated from the SEM images. The surface roughness and the area fraction of a solid surface in contact with liquid were calculated from the dimensions of the micro−microstructures by using eqs 4−7. For the square-shaped structures, the surface roughness and the area fraction were calculated using eqs 4 and 5, while for the barrel-shaped structures similar calculations were made using eqs 6 and 7, where a is the width of the micropillars, b is the distance between the micropillars, and H is the height of the micropillar. The dimensions and surface roughness of the micro−microstructures and the area fraction of the micro−microstructures in contact with a liquid are presented in Table 1.
Table 2. Theoretical Contact Angles and Experimental Contact Angle Measurements on PP Surfaces contact angles (deg) theoretical surface structure PP flat nano hierarchical micro−micro (square−square) lower microstructure upper microstructure (square) hierarchical micro−micro (square−barrel) lower microstructure upper microstructure (barrel) hierarchical micro−nano
experimental
Cassie− Baxter
static
hysteresis
sliding
177
152
104 ± 1 133 ± 9 156 ± 3
17 ± 2 15 ± 9 4±7
>45 25 ± 9 5±6
111
140
139 ± 2
34 ± 8
30 ± 9
133
129
131 ± 3
13 ± 9
12 ± 6
161
162 ± 2
2±4
1±3
111
140
139 ± 2
34 ± 8
30 ± 9
148
145
146 ± 2
11 ± 7
11 ± 7
154
158 ± 8
5±6
4±4
Wenzel
imental contact angle value of a flat PP surface (104°) was used for the upper microstructures as the θflat value, while the experimental contact angle values of the upper microstructures were used as the θflat values for the lower microstructures in the optimization.26,59 The flat PP reference surface was fabricated by using the same technique as that used in producing the structured surfaces. Figure 2 presents SEM images of hierarchical micro− microstructures replicated on a PP surface. The shape and height of the micropillars are identical, which indicates that the
(ϕs)square =
rsquare = 14750
a2 (a + b)2
[(a + b)2 + 4aH ] (a + b)2
(4)
(5)
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Figure 4. AFM topography images of the fabricated PP surfaces: (A) a flat reference surface, (B) the top of the nanostructure on a micro− nanostructured surface, (C) the top of the upper micropillar on a micro−microstructured (square−square) surface, and (D) the top of the upper micropillar on a micro−microstructured (square−barrel) surface.
(ϕs)barrel =
rbarrel =
πa 2 4(a + b)2
[(a + b)2 + πaH ] (a + b)2
case of the hierarchical micro−microstructures, the upper microstructure can be positioned exactly in selected areas of the surface, so that the controlling of the microstructures will function better. The dimensions of the nanopillars can be controlled by changing the settings of the anodization process, although the diameters of the nanopillars will still vary by between 50 and 150 nm. The theoretical calculation of the surface roughness or the area fraction of a solid surface in contact with a liquid cannot be readily applied across the range of nanostructures because the dimensions of the nanopillars are difficult to estimate exactly from the SEM images. In contrast, the dimensions of the hierarchical micro−microstructures can be easily estimated from the SEM images because all of the micropillars are of the same size. The fabrication of a hierarchically micro−nanostructured mold (4 × 4 mm) takes approximately 2 days, during which the anodization process (24 h) occupies most of the time. A hierarchical micro−microstructured mold of the same size can be fabricated much more quickly, in about an hour. The electropolishing and anodization processes have to be conducted in a reaction vessel because acid solutions and counter-electrodes are required. These stages cause the fabrication of nanostructures to be difficult to apply across large surface areas. The fabrication of hierarchical micro− microstructures employs only a single technique. In addition, the anodization process restricts the fabrication of the nanostructure to a soft aluminum mold. In contrast, the fabrication of hierarchical micro−microstructures is not restricted solely to aluminum, but can be fabricated using a wide variety of mold materials. Contact Angle Measurements. Table 2 shows the static and dynamic contact angles and their standard deviations on PP surfaces as well as the theoretical Wenzel and Cassie−Baxter values.
(6)
(7)
The hierarchical micro−microstructures fulfill the criterion θflat > θcrit in the stable Cassie−Baxter state. In the case of the square-shaped structures, considerably lower structures (with a height of 11 μm) are required to fulfill the criterion as compared to the case of the barrel-shaped patterns (with a height of 26 μm). The square-shaped structures can also be positioned more densely on a surface than can the barrelshaped structures. When θflat > θcrit, it is energetically favorable for a drop of liquid to stay on top of the structures. It has been shown that in this case the area fraction of the hierarchical micro−nanostructures in contact with the liquid drop can be calculated by multiplying the area fractions of the lower and upper structures.59 The same mathematical model was applied to calculate the area fractions of the hierarchical micro− microstructures in contact with a liquid. Hierarchical Micro−Nanostructure. A hierarchical micro−nanostructure was fabricated for a comparison of the wetting properties, mechanical robustness, and fabrication techniques to those of the hierarchical micro−microstructures. The process of fabricating the hierarchical micro−nanostructure used a previously published technique.26 A microstructure was fabricated with a microworking robot with a square-shaped needle tip with a diameter of 190 μm and using the same parameters as those already used in fabricating the hierarchical micro−microstructure. A nanostructure was added to the microstructured mold by the AAO process. The SEM images of a hierarchical micro−nanostructure on a PP surface are shown in Figure 3. The nanostructure is uniformly replicated across the whole PP surface area. In the 14751
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Figure 5. Contact angle measurements as a function of pressure: (A) static contact angles as a function of pressure in a press test, (B) contact angle hysteresis as a function of pressure in a press test, (C) static contact angles as as a function of pressure in a wear test, and (D) contact angle hysteresis as a function of pressure in a wear test.
were optimized in the stable Cassie−Baxter state, but the sliding angles and contact angle hysteresis were still greater than 10°, suggesting that a single-level structure is not sufficient alone to achieve superhydrophobicity. The effect of hierarchy is evident, showing higher static contact angles and lower hysteresis than in single-level structures. The result is in good agreement with many previous studies,8,11−17 which have shown that hierarchical structures are required for superhydrophobicity to be achieved. A hierarchical structure reduces the contact area between water and microstructure, decreasing adhesion and permitting easy sliding of the water drop.14,15 Theoretical Wenzel and Cassie−Baxter contact angle values for structured surfaces were calculated from eqs 1 and 2. The experimental contact angle of the nanostructure was regarded in eq 1 as Young’s contact angle for a smooth surface.26,59 The same theoretical model was applied to the hierarchical micro− microstructures, by regarding the experimental contact angles of the upper microstructures as Young’s contact angles. The experimental contact angles of the hierarchically micro−micro and micro−nanostructured surfaces correlate well with the theoretical Cassie−Baxter values. This suggests that for the
The static contact angles for water on hierarchically micro− microstructured PP surfaces are higher than 150°, and the contact angle hysteresis is below 5°, which are typical values for superhydrophobic surfaces. The hierarchical micro−microstructure (square−barrel) achieved the highest contact angle value of 163° and the lowest hysteresis at 2°. The contact angles on the hierarchically micro−nanostructured surfaces are almost the same as those on the micro−microstructured surfaces. The hierarchically micro−microstructured surfaces displayed the same superhydrophobic wetting properties as the hierarchically micro−nanostructured polymer surfaces. The results show that the effect of hierarchy on surface wetting properties also works at micrometer scale. The contact angle of a drop of water is dependent on the ratio of the volume of trapped air to the volume of the surface structure and indicates that the same fraction of air is trapped between the micro− microstructures as between the micro−nanostructures. The contact angles were also measured on single-level microstructures. The static contact angle of the square-shaped single-level microstructure is 131°, while that of its barrelshaped equivalent is 146°. The single-level microstructures 14752
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Figure 6. SEM images of structures after mechanical tests: (A) the nanostructure when a pressure of about 250 kPa was applied to the surface, (B) the nanostructure when a load wear pressure below 60 kPa was applied to the surface, (C) the hierarchical micro−microstructure (square−barrel) when a load wear pressure of about 60 kPa was applied to the surface, and (D) the hierarchical micro−microstructure (square−square) when a load wear pressure of about 310 kPa was applied to the surface.
given geometry the hierarchical micro−microstructures are in the Cassie−Baxter state, where the water drops stay on the top of the microstructures in contrast to the case with the Wenzel state where the water drop wets the whole surface structure. The results show the mathematical model that predicts that the wetting states of the hierarchical micro−nanostructure can also be applied for the hierarchical structures at micro−micrometer scale. In the case of the hierarchical micro−microstructures, the experimental contact angles are somewhat higher than the theoretical contact angles calculated from the Cassie−Baxter theory. The reason for the difference is that some roughness occurs on the top of the upper microstructures and on the side walls of the micropillars (Figure 2B and D), thus increasing the experimental contact angles. The roughness on the top of the microstructures was characterized in more detail by AFM. A flat PP reference surface was also measured with AFM to demonstrate that the fabrication technique itself does not structure the polymer surface. The AFM topography images of flat and structured PP surfaces are illustrated in Figure 4. There is no notable roughness to be observed on the flat PP reference surface
(Figure 4A). However, the structuring technique causes some roughness to the top of the upper micropillars on micro− microstructured surfaces (Figure 4C and D). The roughness results from the slight irregularity at the robot needle. The contribution of this minor roughness and the roughness on the side walls of the micropillars to the wetting properties of the micro−microstructures is negligible because the theoretical Cassie−Baxter contact angle values are approximately equal to the experimental contact angle values. The nanostructured surface has, in contrast, a relatively high roughness (Figure 4B). It has a prominent effect on the wetting properties either in the Cassie−Baxter state10 or in the Wenzel state.9 Mechanical Robustness of the Surface Structures. The mechanical robustness of the surface structures was studied by means of press and wear tests. The primary aim of the mechanical tests was to observe the durability of superhydrophobicity by measuring the static contact angles and the contact angle hysteresis as a function of pressure. The results of the mechanical tests are shown in Figure 5. The hierarchically micro−nanostructured surface loses its superhydrophobic wetting properties when a pressure of about 250 kPa is applied to the surface. The pressure breaks down the 14753
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(2) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Bioinspired surfaces with special wettability. Acc. Chem. Res. 2005, 38, 644−652. (3) Autumn, K.; Liang, Y. A.; Hsieh, S. T.; Zesch, W.; Chan, W. P.; Kenny, T. W.; Fearing, R.; Full, R. J. Adhesive force of a single gecko foot-hair. Nature 2000, 405, 681−685. (4) Sitti, M.; Fearing, R. S. Synthetic gecko foot-hair micro/nanostructures as dry adhesives. J. Adhes. Sci. Technol. 2003, 17, 1055− 1073. (5) Zheng, Y.; Gao, X.; Jiang, L. Directional adhesion of superhydrophobic butterfly wings. Soft Matter 2007, 3, 178−182. (6) Barthlott, W.; Neinhuis, C. Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 1997, 202, 1−8. (7) Cheng, Y. T.; Rodak, D. E.; Wong, C. A.; Hayden, C. A. Effects of micro- and nano-structures on the self-cleaning behaviour of lotus leaves. Nanotechnology 2006, 17, 1359−1362. (8) Guo, Z.; Liu, W. Biomimic from the superhydrophobic plant leaves in nature: Binary structure and unitary structure. Plant Sci. 2007, 172, 1103−1112. (9) Wenzel, R. N. Resistance of solid surfaces to wetting by water. Ind. Eng. Chem. 1936, 28, 988−994. (10) Cassie, A. B. D.; Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 1944, 40, 546−551. (11) Jung, Y. C.; Bhushan, B. Contact angle, adhesion and friction properties of micro-and nanopatterned polymers for superhydrophobicity. Nanotechnology 2006, 17, 4970−4980. (12) Lee, Y.; Park, S.-H.; Kim, K.-B.; Lee, J.-K. Fabrication of hierarchical structures on a polymer surface to mimic natural superhydrophobic surfaces. Adv. Mater. 2007, 19, 2330−2335. (13) Han, W.; Wu, D.; Ming, W.; Niemantsverdriet, H.; Thüne, P. C. Direct catalytic route to superhydrophobic polyethylene films. Langmuir 2006, 22, 7956−7959. (14) Cortese, B.; D’Amone, S.; Manca, M.; Viola, I.; Cingolani, R.; Gigli, G. Superhydrophobicity due to the hierarchical scale roughness of PDMS surfaces. Langmuir 2008, 24, 2712−2718. (15) Nosonovsky, M.; Bhushan, B. Hierarchical roughness optimization for biomimetic superhydrophobic surfaces. Ultramicroscopy 2007, 107, 969−979. (16) Nosonovsky, M.; Bhushan, B. Hierarchical roughness makes superhydrophobic states stable. Microelectron. Eng. 2007, 84, 382−386. (17) Li, W.; Amirfazli, A. Hierarchical structures for natural superhydrophobic surfaces. Soft Matter 2008, 4, 462−466. (18) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley: New York, 1990; pp 387−389. (19) Fürstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Wetting and self-cleaning properties of artificial superhydrophobic surfaces. Langmuir 2005, 21, 956−961. (20) Sun, M.; Luo, C.; Xu, L.; Ji, H.; Ouyang, Q.; Yu, D.; Chen, Y. Artificial lotus leaf by nanocasting. Langmuir 2005, 21, 8978−8981. (21) Lu, X.; Zhang, C.; Han, Y. Low-density polyethylene superhydrophobic surface by control of its crystallization behavior. Macromol. Rapid Commun. 2004, 25, 1606−1610. (22) Lee, S.-M.; Kwon, T. H. Effects of intrinsic hydrophobicity on wettability of polymer replicas of a superhydrophobic lotus leaf. J. Micromech. Microeng. 2007, 17, 687−692. (23) Feng, L.; Zhang, Y.; Xi, J.; Zhu, Y.; Wang, N.; Xia, F.; Jiang, L. Petal effect: A superhydrophobic state with high adhesive force. Langmuir 2008, 24, 4114−4119. (24) Kwon, Y.; Patankar, N.; Choi, J.; Lee, J. Design of surface hierarchy for extreme hydrophobicity. Langmuir 2009, 25, 6129−6136. (25) Feng, J.; Tuominen, M. T.; Rothstein, J. P. Hierarchical superhydrophobic surfaces fabricated by dual-scale electron-beamlithography with well-ordered secondary nanostructures. Adv. Funct. Mater. 2011, 21, 3715−3722. (26) Puukilainen, E.; Rasilainen, T.; Suvanto, M.; Pakkanen, T. A. Superhydrophobic polyolefin surfaces: Controlled micro- and nanostructures. Langmuir 2007, 23, 7263−7268. (27) Xie, Q.; Fan, G.; Zhao, N.; Guo, X.; Xu, J.; Dong, J.; Zhang, L.; Zhang, Y.; Han, C. C. Facile creation of a bionic super-hydrophobic block copolymer surface. Adv. Mater. 2004, 16, 1830−1833.
nanostructure (Figure 6A), and the Cassie−Baxter state does not exist in the collapsed structure. The micro−microstructured surfaces retain their superhydrophobic behavior when a pressure of about 500 kPa has been applied to the surface. The durability of the superhydrophobic micro−microstructured surfaces under the application of mechanical pressure is thus almost a factor of magnitude better than that of the micro− nanostructured surface. The superhydrophobic micro−microstructured surfaces endure also mechanical wear significantly better than do the micro−nanostructured surfaces. The nanostructure breaks down at a load wear pressure already below 60 kPa (Figure 6B). The micro−microstructure, in contrast, remained intact after the wear with the same load wear pressure (Figure 6C). With higher load pressure of 250 kPa, some roughening occurs on the top of the upper microstructures (Figure 6D). The micro−microstructured surfaces retained their superhydrophobic behavior up to the load wear pressure of about 310 kPa.
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CONCLUSIONS The superhydrophobic wetting properties of hierarchical micro−nanostructures on surfaces are well-known and extensively studied on many surfaces. In this work, we have demonstrated that superhydrophobic polypropylene surfaces can be also produced with hierarchical micro−microstructuring and without chemical surface modification. Polymer surfaces were injection molded having static water contact angles above 150° and low sliding angles. There are two primary advantages to using micro−microstructuring instead of micro−nanostructuring. The first advantage is the simple single-step manufacturing process of the hierarchical injection molding. The developed microrobot technique allows rigorous spatial control of the mold, enabling swift preparation of complicated hierarchical three-dimensional surface architectures. In contrast, standard micro−nanostructures are typically manufactured by multistep processes with limited area control. The second advantage is in the durability and robustness of the produced hierarchical structures. The nanostructures of virtually any hierarchical micro−nanomodifed surfaces have the fragile nanolevel structure with very poor resistance to pressure and wear. The micro−microstructured surfaces have been shown in our work to have up to an order of magnitude better pressure and wear resistance than those of the micro− nanostructured surfaces. The molding technique is also readily applicable to other injection moldable polymers.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +358132513345. E-mail: tapani.pakkanen@uef.fi. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support provided by the Finnish Funding Agency for Technology and Innovation (TEKES MI-project) and the European Regional Development Fund (ERDF) is gratefully acknowledged.
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