pubs.acs.org/Langmuir © 2010 American Chemical Society
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Superhydrophobic Surfaces: Are They Really Ice-Repellent? S. A. Kulinich,*,†,‡ S. Farhadi,† K. Nose,§ and X. W. Du †
Department of Applied Sciences, University of Quebec, 555 University Blvd., Saguenay, PQ, Canada G7H 2B1, Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada V6T 1Z1, § Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan, and School of Materials Science and Engineering, Tianjin University, Tianjin 300072, P.R. China )
‡
Received October 25, 2010. Revised Manuscript Received November 24, 2010 This work investigates the anti-ice performance of various superhydrophobic surfaces under different conditions. The adhesion strength of glaze ice (similar to that deposited during “freezing rain”) is used as a measure of ice-releasing properties. The results show that the ice-repellent properties of the materials deteriorate during icing/deicing cycles, as surface asperities appear to be gradually damaged. It is also shown that the anti-icing efficiency of superhydrophobic surfaces is significantly lower in a humid atmosphere, as water condensation both on top of and between surface asperities takes place, leading to significantly larger values of ice adhesion strength. This work thus shows that superhydrophobic surfaces are not always ice-repellent and their use as anti-ice materials may therefore be limited.
1. Introduction Superhydrophobic surfaces, which exhibit static contact angles (CAs) with water larger than 150° and low CA hysteresis (CAH), have recently attracted significant attention from both researchers and manufacturers.1-15 It is now widely known that the key elements of surfaces that promote the superhydrophobic state are their chemical composition and micro-nanohierarchical texture.1-5,8,10-18 Among other applications, the ability of artificial superhydrophobic surfaces to delay and reduce the accumulation of wet snow, ice, or frost (and even completely prevent the formation of ice buildups on solid surfaces) has been actively discussed and speculated about for years, as it demonstrates industrial promise *To whom correspondence should be addressed. E-mail: s_kulinich@ yahoo.com. (1) Quere, D. Rep. Prog. Phys. 2005, 68, 2495–2532. (2) Crick, C. R.; Parkin, I. P. Chem.;Eur. J. 2010, 16, 3568–3588. (3) Sakai, M.; Kono, H.; Nakajima, A.; Sakai, H.; Abe, M.; Fujishima, A. Langmuir 2010, 26, 1493–1495. (4) Erbil, H. Y.; Cansoy, C. E. Langmuir 2009, 25, 14135–14145. (5) Yao, X.; Chen, Q. W.; Xu, L.; Li, Q. K.; Song, Y. L.; Gao, X. F.; Quere, D.; Jiang, L. Adv. Funct. Mater. 2010, 20, 656–662. (6) Furuta, T.; Sakai, M.; Isobe, T.; Nakajima, A. Langmuir 2010, 26, 13305– 13309. (7) Tourkine, P.; Le Merrer, M.; Quere, D. Langmuir 2009, 25, 7214–7216. (8) Spori, D. M.; Drobek, T.; Z€urcher, S.; Spencer, N. D. Langmuir 2010, 26, 9465–9473. (9) Kako, T.; Nakajima, A.; Irie, H.; Kato, Z.; Uematsu, K.; Watanabe, T.; Hashimoto, K. J. Mater. Sci. 2004, 39, 547–555. (10) Extrand, C. W. Langmuir 2006, 22, 1711–1714. (11) Kulinich, S. A.; Farzaneh, M. Appl. Surf. Sci. 2009, 255, 4056–4060. (12) Cao, L. L.; Jones, A. K.; Sikka, V. K.; Wu, J. Z.; Gao, D. Langmuir 2009, 25, 12444–12448. (13) Ensikat, H. J.; Schulte, A. J.; Koch, K.; Barthlott, W. Langmuir 2009, 25, 13077–13083. (14) Mockenhaupt, B.; Ensikat, H. J.; Spaeth, M.; Barthlott, W. Langmuir 2008, 24, 13591–13597. (15) Kulinich, S. A.; Farzaneh, M. Surf. Sci. 2004, 573, 379–390. (16) Saito, H.; Takai, K.; Yamauchi, G. Surf. Coat. Int. 1997, 80, 168–171. (17) Kannarpady, G. K.; Sharma, R.; Liu, B.; Trigwell, S.; Ryerson, C.; Biris, A. S. Appl. Surf. Sci. 2010, 256, 1679–1682. (18) Wier, K. A.; McCarthy, T. J. Langmuir 2006, 22, 2433–2436. (19) Mishchenko, L.; Hatton, B.; Bahadur, V.; Taylor, J. A.; Krupenkin, T.; Aizenberg, J. ACS Nano, published online Nov 9, http://dx.doi.org/10.1021/nn102557p. (20) Kulinich, S. A.; Farzaneh, M. Appl. Surf. Sci. 2009, 255, 8153–8157. (21) Wang, H.; Tang, L. M.; Wu, X. M.; Dai, W. T.; Qiu, Y. P. Appl. Surf. Sci. 2007, 253, 8818–8824. (22) Wang, F. C.; Li, C. R.; Lv, Y. Z.; Lv, F. C.; Du, Y. F. Cold Reg. Sci. Technol. 2010, 62, 29–33.
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in the anti-ice coating sector.7,9,12,5-17,19-24 Though both reduced ice adhesion strength and delayed ice accretion on superhydrophobic surfaces have been reported,7,12,16,19,20,22-25 their anti-ice performance under different conditions has not yet been adequately examined. Superhydrophobic materials were first tested by Saito and coworkers16 and more recently by several other groups,12,19,20,22,24,25 and demonstrated promising anti-icing performance. Reduced ice adhesion was reported,16,20,24,25 while the effect of wetting hysteresis (CAH) on ice adhesion strength was also stressed.20,24 Tourkine and co-workers7 and He et al.23 reported delayed water freezing on rough superhydrophobic surfaces, which is believed to be favorable for reduced ice accumulation, whereas Cao et al.12 and Wang et al.22 reported low ice accumulation on superhydrophobic surfaces exposed to natural outdoor (or artificial) “freezing rain” conditions. However, the systematic study of the ice-repellent properties of superhydrophobic surfaces in different conditions (and over time) has not been reported thus far. In this work, glaze ice was prepared by spraying water microdroplets at subzero temperature, that is, under conditions very close to outdoor ice accretion during “freezing rain”. Ice adhesion was tested on several superhydrophobic samples with different surface topographies and chemistry. The anti-ice properties were examined over more than 20 icing/deicing cycles to evaluate the sample durability. Both “dry” and “wet” superhydrophobic surfaces were iced to simulate their performance in a humid atmosphere. All the samples examined demonstrated both gradually degrading icereleasing properties over repetitive icing/deicing events and a significantly larger adhesion of ice when it was accreted on “wet” superhydrophobic materials.
2. Experimental Section All the samples tested in this work were prepared on aluminum substrates (AA6061, 32 52 1.5 mm3 ). The first group of (23) He, M.; Wang, J. X.; Li, H. L.; Jin, X. L.; Wang, J. J.; Liu, B. Q.; Song, Y. L. Soft Matter 2010, 6, 2396–2399. (24) Kulinich, S. A.; Farzaneh, M. Langmuir 2009, 25, 8854–8856. (25) Dotan, A.; Dodiuk, H.; Laforte, C.; Kenig, S. J. Adhes. Sci. Technol. 2009, 23, 1907–1915.
Published on Web 12/08/2010
DOI: 10.1021/la104277q
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Kulinich et al.
Figure 1. SEM images of superhydrophobic surfaces tested. (a) Spin-coated ZrO2-incorporated fluoropolymer (sample A), (b) FAS-17 coated etched Al (sample B), and (c) SA coated etched Al (sample C). CA values measured were 153.2 ( 2.1° (a), 152.1 ( 1.9° (b), and 150.2 ( 2.2° (c). samples was based on a fluoropolymer incorporated with ZrO2 nanopowder. They were prepared by varying the amount of a ZrO2 nanopowder-fluoropolymer suspension in water deposited on polished Al plates followed by drying on a hot plate. Prior to coating, aluminum plates were polished with emery paper and cleaned in organic solvents. ZrO2 nanopowder (3.0 g) with an average size of 20-30 nm (from Aldrich) was mixed with 40 mL of deionized water. The suspension was sonicated for 30 min, after which 3.0 mL of a perfluoroalkyl methacrylic copolymer (Zonyl 8740, DuPont) product was added. The final suspension was stirred for another 3 h before coating the substrates. By spin-coating the suspension at different rotational speeds, samples with different wetting properties were obtained. Upon coating, the samples were heat-treated at 120 °C in air for 3 h to remove residual solvents. The preparation of superhydrophobic surfaces via spin-coating nanopowder-Zonyl suspension on various substrates has been previously described by several groups.11,20,24,26,27 For this study, only samples with CA>150° and CAH 150° and found that it resulted in larger CAH values and resultant increased ice adhesion strengths in all cases. The degree of the latter strongly depended on the condensation time, the surface temperature and the surface nature. It can be therefore concluded that in applications where anti-ice materials are subjected to high humidity and temperatures close to the freezing point, their performance can be strongly affected by water condensation on their surface, which leads to a temporary loss of their anti-ice properties and even (possibly) stronger ice adhesion. Moreover, based on Extrand’s conclusions,10 it can also be predicted that under the condition of high-speed “freezing rain” some superhydrophobic surfaces will fail, losing their anti-ice properties, as water droplets can penetrate into the rough structure and, thus, cause larger ice adhesion strength, similar to the case of “wet” iced sample B described above. Therefore, alternatives with smooth surfaces are likely to perform better in such environments. Another alternative, hypothetically, could be superhydrophobic surfaces with both micro- and nanoscale roughness, which were demonstrated to retain their dewetting properties under moisture condensation conditions better than those with only microstructure.14 Better anti-ice performance in humid environments should be expected from such materials. However, whether such micro- and nanostructured surfaces are mechanically durable over icing/deicing events is still an open question that needs experimental investigations. The abrasive resistance of the superhydrophobic surfaces prepared in this study was far from that required for the wide outdoor application of such materials, and it is expected to be even poorer in the case of micro/ nanorough surfaces. This study therefore raises doubts about the use of superhydrophobic surfaces as universal anti-icing materials. Such surfaces are normally relatively costly to prepare, but they are shown not to be always ice-repellent and to have quite poor abrasive resistance against icing/deicing. And even if this weakness is remedied, for instance, by using some superhydrophobic surfaces with
Langmuir 2011, 27(1), 25–29
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complex topographies19 or surfaces made of rigid17 (or elastic8) materials, it is still difficult to see, at this point, how the poor antiice performance in a humid atmosphere can be overcome. So far, all the cases of promising anti-icing performance have been solely reported for “dry” superhydrophobic surfaces.12,16,19,20,22,24,25 Meanwhile, atmospheric moisture condensation has been reported to take place on all superhydrophobic materials tested, that is, with various surface chemistries and topographies.14,18,23,29-31
4. Conclusions In summary, rough superhydrophobic surfaces are shown not to be always effective anti-ice materials, because of their relatively low abrasive resistance and the inability to deice in all climatic conditions. Though ice accretion on such (“dry”) surfaces can be delayed compared to flat hydrophobic surfaces, when it eventually occurs, it causes a gradual damage of the surface microstructure during icing and/or deicing. This leads to a gradual decrease in the anti-icing performance. On the other hand, in a humid atmosphere (when water condenses in the rough structure) the icing of such surfaces may lead to very large values of ice adhesion strength (anchor effect). The results thus point out that the anti-icing performance of superhydrophobic surfaces may be very limited and should be studied in more detail, with more focus on surface microstructure and mechanical properties as well as on icing conditions and mechanisms. Acknowledgment. This work received financial support from the Natural Sciences and Engineering Research Council of Canada. The authors thank Prof. R. N. M. Dole (Universite du Quebec a Chicoutimi) for reading and commenting on the text. Supporting Information Available: Experimental details for measurement of ice adhesion strength as well as sample surface characterization data. This material is available free of charge via the Internet at http://pubs.acs.org
DOI: 10.1021/la104277q
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