pubs.acs.org/Langmuir © 2010 American Chemical Society
Artificial Hairy Surfaces with a Nearly Perfect Hydrophobic Response Shu-Hau Hsu† and Wolfgang M. Sigmund*,†,‡ †
Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611-6400 and ‡ Department of Energy Engineering, Hanyang University, Seoul, South Korea Received October 8, 2009. Revised Manuscript Received December 11, 2009
A nearly perfect hydrophobic interface by dint of mimicking hairs of arthropods was achieved for the first time. These Γ-shape artificial hairs were made via a membrane casting technique on polypropylene substrates. This extreme hydrophobicity merely arises from microstructure modification, and no further chemical treatments are needed. The ultralow adhesion to water droplets was evaluated through video assessment, and it is believed to be attributed to the mechanical response of the artificial hairs. The principle of this fabrication technique is accessible and is expected to be compatible with large-area fabrication of superhydrophobic interfaces.
Owing to remarkable features, such as self-cleaning, antibiofouling, or drag reduction, interest in rendering surfaces waterrepellent has significantly grown within this decade.1-5 Attempts on mimicking the so-called “lotus effect”,6 where a high water contact angle (θc >150°) is accompanied by only a few degrees of roll-off angle, have been extensively demonstrated in the literature through the controlling of surface chemistry and morphology. Recent studies showed that the most crucial criterion mainly relies on roughening the surface into multiple length scales of roughness so that liquid droplets can be retained in the Cassie-Baxter state,7 where air pockets are trapped underneath the liquid, reducing the solid-liquid interface.8 These hierarchically structured surfaces have been fabricated through various routes and demonstrated to have superhydrophobic properties as well.9-11 This amazing water-repellent property is also found in other biological systems comprising a plurality of flexible hairs, and some of them have been recognized for over 100 years.12 Fuzzy leaves, such as the Lady’s Mantle, cause water droplets to form perfect spheres and allow them to roll off easily as a result of being lifted and suspended by coming into contact with the hairs.13 In the animal kingdom, this piliferous exterior plays a more crucial role for numerous living creatures not only to effectively protect their bodies from getting wet but also to provide various functions for their living activity.14-18 These hairs protrude several micrometers from their cuticles, typically inclined at certain angles, with diameters in the micrometer to submicrometer range.18 *Corresponding author. E-mail:
[email protected].
(1) Feng, X. J.; Jiang, L. Adv. Mater. 2006, 18, 3063–3078. (2) Genzer, J.; Efimenko, K. Biofouling 2006, 22, 339–360. (3) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Soft Matter 2008, 4, 224–240. (4) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y. G.; Wang, Z. Q. J. Mater. Chem. 2008, 18, 621–633. (5) Nosonovsky, M.; Bhushan, B. Curr. Opin. Colloid Interface Sci. 2009, 14, 270–280. (6) Neinhuis, C.; Barthlott, W. Ann. Bot. (Oxford, U.K.) 1997, 79, 667–677. (7) Gao, L. C.; McCarthy, T. J. Langmuir 2006, 22, 2966–2967. (8) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546–551. (9) Patankar, N. A. Langmuir 2004, 20, 8209–8213. (10) Bhushan, B.; Jung, Y. C. Nanotechnology 2006, 17, 2758–2772. (11) Nosonovsky, M.; Bhushan, B. Microelectron. Eng. 2007, 84, 382–386. (12) Comstock, J. H. Am. Nat. 1887, 21, 577–578. (13) Otten, A.; Herminghaus, S. Langmuir 2004, 20, 2405–2408. (14) Harpster, H. T. Trans. Am. Microsc. Soc. 1941, 60, 329–358. (15) Thorpe, W. H.; Crisp, D. J. J. Exp. Biol. 1947, 24, 270–303. (16) Crisp, D. J.; Thorpe, W. H. Trans. Faraday Soc. 1948, 44, 210–220. (17) Pal, R. B. J. Entomol. Res. 1950, 41, 121–139. (18) Hinton, H. E. J. Insect Physiol. 1976, 22, 1529–1550.
1504 DOI: 10.1021/la903813g
These structures can resist the impact of raindrops,16 allow locomotion on the surface of water,19 or even trap a layer of air for respiration when submerged.15,16 Some arthropods have been shown to have contact angles above 150°, which allows them to walk on water.20 Covering surfaces with vertically aligned micro- or nanopillar arrays by means of top-down or bottom-up routes is not rare;21-23 however, reports on direct mimicking of the biological hairy surface are still very limited. One of the reasons is making this high-aspect-ratio structure (>10) is relatively difficult by using top-down technique.24 Moreover, obtaining the unique arrangement of these natural hairs is also a challenge. A few studies are published demonstrating attempts to duplicate the structure via a two-step casting technique by using species as the template for the mold.25-27 Though one report of a cast surface showed superhydrophobicity, only samples with small area could effectively be prepared.26 Some improved molding setup with proper resins allowed users to precisely replicate the hierarchical structure with larger area from several plant surfaces; however, a chemical treatment was needed to achieve superhydrophobicity.27 Synthetic templates provide an alternative selection for making patterned array structures.28 They offer some immediate advantages, such as larger surface area and a variety of pore sizes. Dense arrays of flexible polymeric nanopillars mimicking the gecko’s hairy foot were fabricated by a templating method.29-31 (19) Hu, D. L.; Chan, B.; Bush, J. W. M. Nature 2003, 424, 663–666. (20) Gao, X. F.; Jiang, L. Nature 2004, 432, 36–36. (21) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395–3399. (22) Feng, L.; Song, Y. L.; Zhai, J.; Liu, B. Q.; Xu, J.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2003, 42, 800–802. (23) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701–1705. (24) del Campo, A.; Arzt, E. Chem. Rev. 2008, 108, 911–945. (25) Mock, U.; Forster, R.; Menz, W.; Ruhe, J. J. Phys: Condens. Matter 2005, 17, S639–S648. (26) Goodwyn, P. P.; De Souza, E.; Fujisaki, K.; Gorb, S. Acta Biomater. 2008, 4, 766–770. (27) Koch, K.; Schulte, A. J.; Fischer, A.; Gorb, S. N.; Barthlott, W. Bioinspiration Biomimetics 2008, 3, 046002. (28) Liu, T. B.; Burger, C.; Chu, B. Prog. Polym. Sci. 2003, 28, 5–26. (29) Cho, W. K.; Choi, I. S. Adv. Funct. Mater. 2008, 18, 1089–1096. (30) Majidi, C.; Groff, R. E.; Maeno, Y.; Schubert, B.; Baek, S.; Bush, B.; Maboudian, R.; Gravish, N.; Wilkinson, M.; Autumn, K.; Fearing, R. S. Phys. Rev. Lett. 2006, 97, 076103. (31) Jin, M. H.; Feng, X. J.; Feng, L.; Sun, T. L.; Zhai, J.; Li, T. J.; Jiang, L. Adv. Mater. 2005, 17, 1977.
Published on Web 01/07/2010
Langmuir 2010, 26(3), 1504–1506
Hsu and Sigmund
Letter Scheme 1. Formation of Artificial Hairy Surface by Means of Pressurized Membrane Casting
Figure 1. (a-d) SEM images of membrane cast surfaces with
different pore diameters: (a) 3.0 μm, (b) 1.2 μm, and (c) 0.6 μm; (d) closer view on 0.6 μm cast hairy structure. Water contact angles of a native polypropylene (PP) sheet (e), and 0.6 μm cast hairy PP sheet (f).
Although these surfaces usually demonstrate superhydrophobicity (θc >150°), the droplets adhere on the surface because of their high adhesion to water.30,31 We used commercial track-etched 20 μm thick polycarbonate (PC) membrane filters (pore diameter = 0.6-3.0 μm, ISOPORE, Millipore Inc.) as the mold. The artificial hairy surface was made by using a template casting technique, where the patterns are generated by filling the porous membrane. A polypropylene (PP) sheet obtained from general file jackets was used as the substrate. Polypropylene is a widely used commercially available thermoplastic polymer which is considered hydrophobic and has a melting temperature around 170 °C.32 The casting process was carried out in a vacuum oven where a membrane and a substrate were pressurized between two glass slides. The artificial hairy structure was obtained by peeling off the membrane after the casting process (Scheme 1), yielding a superhydrophobic surface every time. The microstructure of cast surfaces is shown in Figure 1. The morphology of the protruded structures varies with the pore size of the membrane. For 3.0 and 1.2 μm, the density of the pillars is close to the original pore density of the membrane used (Table 1). While for 0.6 μm, the density is about 7 105 cm-2, almost 2 orders of magnitude lower than the membrane (∼4 107 cm-2), which indicates most of the pillars were ripped off during the peeling process. The close-up image in Figure 1d reveals that the protruded structure of individual fibers is similar to natural hairs. (32) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377– 1380.
Langmuir 2010, 26(3), 1504–1506
They taper to a point from the base with the pore size diameter while the length varies from less than 1 μm to about 10 μm. Long hairs are tilted around 45° with their tips curled, and some also curled toward or parallel to the substrate. This kind of arrangement is easily found in many arthropods that have the capability of water-walking or underwater breathing.18 Table 1 shows a summary of the contact angles and their hysteresis of the polypropylene surfaces cast by different pore sizes. All the cast samples showed superior hydrophobicity: droplets bounced back and then immediately rolled off when they impacted the surface. The sessile drop test showed the contact angles are all above 150°, which has been significantly increased over flat PP (θc = 93 ( 2°). The process can also be simply applied to other thermoplastic polymers. Following the same procedure, a similar hair structure was able to be created on poly(vinylidene difluoride) (PVDF) substrate, which converts the surface from hydrophilic (θc =81°) to surperhydrophobic (θc=152°) (Figure 2). The extensive study on different substrates associating with the cast structure and their hydrophobicity is currently being conducted. The casted surfaces are able to withstand the impact from fluid somewhat well, that is, compressed air blowing or droplets striking. However, since this high aspect ratio structure was made by soft polymers, the mechanical strength of the structure is not robust. The hairy structure may be damaged if the surface is abraded by solid matter and thus lose its superior hydrophobicity. Figure 3b shows the hair structure was demolished after being rubbed by fingers. While the durability is a general challenge among all superhydrophobic surfaces, publications which address this issue are rare.33,34 Moreover, no such study has ever been carried out on superhydrophobic surfaces consisting of high-aspect-ratio structures. It is well-known that a high contact angle can be effectively achieved on these pillarlike structure surfaces. However, 0.6 μm ascast, hairy surfaces specifically caught our attention here. The droplet could not be placed onto this kind of substrate during the measurement due to the low adhesion to water. We also varied the droplet volume and analyzed the contact angle hysteresis via captured images. It was found that the advancing and receding angles were indistinguishable in these images. Gao and McCarthy have pointed out that contact angles may not be measured accurately at this range of extreme hydrophobicity if this conventional imagecaptured analysis is used.7,35,36 Alternatively, they videotaped the response of water droplets being compressed onto and decompressed from the substrate. A similar strategy was adopted here, but instead of using videotaping we directly captured the video from the computer screen during the operation of the goniometer. Two captured videos are presented in the Supporting Information. The compression test of water droplets on the as-cast hairy PP sheet is shown in the first video. During the test, a 2 μL droplet was slightly compressed on the surface with the advancement of the syringe, and then the syringe was slowly retreated from the (33) Wu, Y. Y.; Bekke, M.; Inoue, Y.; Sugimura, H.; Kitaguchi, H.; Liu, C. S.; Takai, O. Thin Solid Films 2004, 457, 122–127. (34) Cui, Z.; Wang, Q. J.; Xiao, Y.; Su, C. H.; Chen, Q. M. Appl. Surf. Sci. 2008, 254, 2911–2916. (35) Gao, L. C.; McCarthy, T. J. J. Am. Chem. Soc. 2006, 128, 9052–9053. (36) Gao, L. C.; McCarthy, T. J. Langmuir 2007, 23, 9125–9127.
DOI: 10.1021/la903813g
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Table 1. Result of the Contact Angle Measurements of Membrane Cast Polypropylene Surfaces and the Theoretical Contact Angles from Cassie-Baxter Theory pore Size
pore density (cm-2)
cast pillar density (cm-2)
contact angle
contact angle hysteresis
estimated contact area fraction
Cassie-Baxter contact angles
0.6 μm 1.2 μm 3.0 μm
3.8 107 1.7 107 3.1 106
7.0 105 1.3 107 2.4 106
>170° 157 ( 3° 152 ( 4°
