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Effects of nano-fluorocarbon coating on icing
Applied Surface Science 258 (2012) 7219–7224
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effects of nano-fluorocarbon coating on icing Hong Wang, Guogeng He ∗ , Qiqi Tian School of energy and power engineering, Huazhong University of Science and Technology, Wuhan 430074, China
a r t i c l e
i n f o
Article history: Received 22 September 2011 Received in revised form 29 March 2012 Accepted 7 April 2012 Available online 13 April 2012 Keywords: Ice blocking Anti-icing Superhydrophobic surface Nano-fluorocarbon coating
a b s t r a c t Icing is a common phenomenon in many fields, from aeronautics to power lines. Recently, researchers have paid much attention on the superhydrophobic surface as one of the favorable anti-icing techniques. In the present study, we investigated the performance of water icing on a superhydrophobic surface with a nano-fluorocarbon film in the average thickness around 10 nm. The surface topographies and wettabilities were characterized by a scanning electron microscopy system and a video-based contact angle measurement system respectively. To investigate the effects of this nano-fluorocarbon coating on water icing, the water droplet shape, the starting icing time and the whole icing process were observed on both the coated and uncoated surface. It was found that the coated surface has a good ability to retard the starting time of icing while the whole icing time on the coated surface was longer compared the uncoated one under the experimental conditions. The test results showed that the nano-fluorocarbon coating expresses a good anti-icing performance and can be used as a coating material to avoid ice-blocking in the dynamic ice-making system. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, ice thermal energy storage systems have received increasing attention due to their outstanding features and the ability to flatten electric power loads, and have been successfully employed in many field, such as district heating and cooling system, food storage and medical field, etc. [1–3]. In a dynamic ice storage system, ice slurry used as the thermal storage material has good fluidity, and a large amount of cold energy can be transported with less pumping work. Egolf and Kauffeld [4] and Kauffeld et al. [5] defined ice slurry as ice particles with an average characteristic diameter equal or smaller than 1 mm dispersed in an aqueous solution. One of the most promising ways to make ice slurry, i.e. using supercooled water when concomitant to ice blockage, has been widely concerned by many researchers [6,7] thanks to it is simple and energy-saved. Hence, it is necessary to find an effective method to solve the problem to achieve the continuous ice-making. The nano-fluorocarbon coating considered in the present study is appropriate for this purpose. It is well known that icing occurs when the water changes from liquid to the solid phase. Ice adhesion and excessive accumulation on exposed structures will cause serious problems, severe accidents and large economic losses to materials, telecommunication networks, power transmission, etc., especially in cold regions [8–11]. For this end, a variety of anti-icing techniques and extensive studies
∗ Corresponding author. Tel.: +86 27 87542718. E-mail address: [email protected] (G. He). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.043
were developed over the past years [12–14]. Some studies concentrated on the so-called icephobic coatings [15–17] while others focused on an alternative approach, i.e. superhydrophobic coatings [18–21]. The idea of using superhydrophobic coatings is to take advantage of water-repellency and low adhesion of drops in liquid state to such coatings to reduce or eliminate water accumulation on the surface before water iced. Recently, superhydrophobic surfaces have attracted great interest because of their many practical applications, such as in the areas of anti-icing and anti-frosting [20–24]. Studies by Zou et al. [25] investigated the effects of surface roughness and surface energy on ice adhesion strength. It was found that the ice adhesion strength decreases with the growth of water contact angle for surfaces with similar roughness; nevertheless, the smoother sample surfaces have lower ice adhesion strength than the much rougher sandblasted surfaces. Kraj and Bibeau [26] performed some preliminary tests for airfoils coated with hydrophobic and icephobic coatings in an inner wind tunnel. The objective of the tests was to evaluate the ice adhesion force and ice accumulation using different strategies. In 2008, Ferrick et al. [27] used different coatings of combined binder for the tests with varying filler proportions. In their studies, the contact angles for the surfaces ranged from 81◦ to 143◦ . The results showed that the coating made of a mixture of MP-55 and Rain-X Much has lower ice adhesion compared to plain metals. Another study performed by Farhadi et al. [20] also investigated anti-icing performance of several superhydrophobic coating with different surface chemistry and topography. They found that the anti-icing properties of the tested materials deteriorate, as their surface asperities seem to be gradually broken during icing/de-icing cycles. It was
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also shown that the anti-icing efficiency of the tested superhydrophobic surfaces is significantly lower in a humid atmosphere. Other groups discussed the ice formation changes during icing on superhydrophobic surfaces. Liu et al. [28] reported a frost deposition phenomenon on a superhydrophobic surface with a contact angle of 162◦ under natural convection conditions. They observed the formation of initial frost crystal and the frost layer structures during the freezing. From their observations, it was concluded that the surface has a capability to restrain frosting growth, and the frost deposition on the superhydrophobic surface was delayed for 55 min compared with the plain surface under the tested conditions. Yin et al. [29] also investigated the ice formation on five different surfaces with typical wettabilities. The superhydrophobic surface was found clearly reducing ice accumulation in the initial ice formation stage associated with the lower sliding angle of water droplets. However, the correlation between surface wettability and ice formation was not observed. These studies showed useful antiicing performance and weak ice-adhesion as was observed in Wang et al. [30] and Farhadi et al. [20], and benefit for anti-icing strategies. However, Kulinich and Farzaneh [31] reported that the ice adhesion strength on rough hydrophobic surfaces has no correlation with values of water CA. They pointed out that the previously reported correlation between sample hydrophobicity and ice repellency is only valid for rough hydrophobic surfaces with low wetting hysteresis. Jung et al. [32] observed that surface with nanometer-scale surface roughness and higher wettability displayed unexpectedly long freezing delays, and they suggested that anti-icing design must optimize the influence of both wettability and roughness. Kulinich et al. [33] also reported that superhydrophobic surfaces are not always ice-repellent and their use as anti-icing materials may be limited. In their studies, they observed that the ice-repellent properties of the materials deteriorate during icing/de-icing cycles, as surface asperities appear to be gradually damaged, and the anti-icing efficiency of superhydrophobic surfaces is significantly lower in a humid atmosphere. As mentioned above, many interesting developments have been accomplished in hydrophobic surface for anti-icing/de-icing recently. However, most of the works were subject to severe conditions, multi-step processes, expensive material, poor durability, etc. A superhyphobic surface with a nano-fluorocarbon coating for anti-icing, not yet investigated in literature, was addressed in this work. No report on using a nano-fluorocarbon superhydrophobic surface to prevent ice-blocking in dynamic ice-making system has not been made in past studies. This work investigated the effects of the nano-fluorocarbon coating with unique superhydrophobility on icing due to its chemical and physical properties with purposes: (i) to quantify benefits, that application of the nano-fluorocarbon coating; (ii) to provide an understanding of a positive coating strategy to combat icing on surfaces; and (iii) to find an efficient way to prevent ice-blocking in the dynamic ice-making system. This study fabricated the superhydrophobic surface by means of a facile method as defined in the next experimental details. The surface microstructure and the contact angle were investigated with the Scanning Electron Microscope (SEM) and Contact Angle (CA) respectively. In the icing test, the superhydrophobic surface showed a strong ability to mitigate ice formation under the experimental conditions.
is nitric derivate of perfluorpolyoxyalkyl carbonate. The organic solvent is a mixture of fluorinated hydrocarbon solvent and ethanol or isopropyl alcohol, which may guarantee a good dissolvability of the binder and a firmed coating of modifier on the surface. In reaction of fluorine to polymers with hydrogen, a fluorocarbon with special properties is produced by replacing the hydrocarbon chain with fluorocarbon one, and the modifier forms uniformly distributed protrusions on the surface when coated on the solid surface (see Section 3.1, Fig. 1). In the present study, the protrusion depth was also investigated by an atomic force microscope. Based on AFM measurements (not shown here), the largest depth and smallest depth were about 12.5 nm and 7.6 nm, respectively. These micro-protrusions allow the fluorocarbon film in the average thickness around 10 nm on the solid surface, showing a strong superhydrophobicity as discussed in Section 3.2. 2.2. Preparation of samples Two 15 mm × 10 mm × 1.0 mm copper samples were prepared for the test. The superhydrophobic and plain surfaces were prepared on the above samples. It is easy to obtain a plain surface. After immersed in carbon tetrachloride solution for 5 min to remove any pollutant, then samples were rinsed with pure ethanol for several times and dried for 30 min at the room temperature. As a superhydrophobic surface is more complicated to prepare, it was prepared using the same process as above firstly, and then the following procedure: (1) dripped a droplet of the modification coating on the sample surface; (2) rubbed the surface with a soft nylon cloth till the liquid totally etches on the surface; (3) dried for 4 h in a clean chamber at the room temperature. Finally, the superhydrophobic surface with a nano-fluorocarbon film of less than 10 nm in thickness was obtained. 2.3. Sample characterization A scanning electron microscopic system (HITACHI S-4800, Japan) was used to characterize the surface topographies of all samples. SEM micrographs at different magnifications over 4 locations across each sample were examined in the micro-scale uniformity of the sample surfaces. The water contact angles (WCAs) were measured using a videobased contact angle measurement system (DCA, SL200B). The static WCAs were measured at least 3 times across the sample surface using the sessile drop by dispensing 2 L drops of de-ionized water on the surfaces. All WCAs were measured at an ambient temperature 25 ◦ C and relative humidity 50%. 2.4. Icing test The icing tests were performed in a climatic chamber with a constant working temperature of −8 ◦ C. In order to maintain a constant natural convection condition, the samples were placed in a large plexiglass enclosure. The temperature was regulated at the set value by a refrigerating unit. The droplets on the superhydrophobic surface and plain copper surface were dropped by an injector needle tube. During the test, a high-speed digital camera was utilized to capture the shape change of the water droplets. 3. Results and discussion
2. Experimental details 3.1. Surface topography 2.1. Modifier The modifier used in this study, supplied by Wuhan Futejia Co. Ltd (China), is a kind of high polymer that contains a fluoric binder and an organic solvent. The chemical name of the modifier
Fig. 1 shows some typical SEM micrographs of the copper samples with and without nano-fluorocarbon modifier. It can be seen from Fig. 1(a) that there are many protrusions and voids on the nano-fluorocarbon coated surface, while the plain copper surface
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Fig. 1. SEM image comparison: (a) magnification: 20,000×; (b) magnification: 200,000× (left: coated surface and right: plain copper surface).
only has original shadow grooves. Moreover, as seen in Fig. 1(b) with a larger magnification, the protrusions are composed of randomly oriented nanoparticles, having a homogeneous distribution on the superhydrophobic rough surface; however, the uncoated surface is relatively smooth with no any protrusion. Compared the SEM images of the superhydrophobic surface with that of the plain surface, one can see that the nano-fluorocarbon modification coating has notably changed the topography of the copper surface, which indicates that the micronanoscale structure of the coated surface plays an important role in the surperhydrophobic property. 3.2. Surface wettability Wettability, one basic property of solid surface, is dependent on both the topographical structure and the chemical compositions. The typical parameter used to characterize a solid surface wettability is the contact angle (CA). However, when a drop is placed on a surface, not only a single value (CA), but multiple values of
the contact angle can be observed, i.e. contact angle hysteresis, . Contact angle hysteresis is usually quantified as the difference between the advancing and receding contact angles, A and R . Presently, a surface is usually labeled superhydrophobic when both the contact angle is high ( > 150◦ ), and contact angle hysteresis is low ( < 10◦ ). In the present study, the contact angles on the coated and uncoated surface were measured by a video-based contact angle measurement system. As shown in Fig. 2, the static CAs on the two samples were 163.01◦ and 84.24◦ , respectively. Moreover, in this work, the advancing and receding contact angles on the coated surface were 164.62◦ and 158.45◦ , respectively, which shows that the coated surface has a low contact angle hystersis ( = 6.17◦ ). Hence, the coated surface with a nano-fluorocarbon film in the average thickness around 10 nm has a good superhydrophobicity. A superhydrophobic surface is determined mainly by the characteristics of the solid surface. This phenomenon can be explained by the Cassie–Baxter model (see Fig. 3).
Fig. 2. Contact angles of water on different surfaces: (a) water droplet on coated surface, = 163.01◦ ; (b) water droplet on plain copper surface, = 84.24◦ .
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Fig. 3. Cassie–Baxter model.
Based on various kinds of studies Cassie and Baxter [34] proposed a gas cavity model for superhydrophobic surface wettability. According to the Cassie–Baxter model, air is more likely to be trapped between the protrusions of the superhydrophobic surface and the water droplet. The following equation is given for the equilibrium contact angle of a real surface: cos = f (1 + cos ) − 1
(1)
where is the contact angle for a real rough surface, is the contact angle for the corresponding smooth surface, and f(f < 1) is the fraction of solid–liquid contact area. From Eq. (1), the contact angle of a real surface is always larger than that of an ideal surface under the same conditions. In this work, as shown in Fig. 1, there are many numbers of micro-nano-scale protrusions on the coated copper surface, trapping the air readily under the water droplet, so the contact fraction f decreases and the WCA is much larger than that of a flat plain surface. In general, the nano-fluorocarbon coating film on the copper surface increases hydrophobicity of the surface and provides an ideal superhydrophobic surface. 3.3. Icing characteristics To assure the anti-icing performance of this superhydrophobic coating, a further investigation has been done. Droplets were placed on the plain and coated surfaces respectively, and the icing processes were observed. The temperature inside the chamber was maintained at −8 ◦ C. Figs. 4 and 5 illustrate some of the typical different pictures of the water droplets and ice formation on the two surfaces during the icing test. The water droplet on the coated surface likes a ball with a smaller contact surface area thanks to
its large contact angle on the superhydrophobic surface while the water droplet on the plain copper sample likes a hemisphere with too much contact area. At 180 s, as shown in Fig. 4, most of the hemispheric droplet iced in the chamber. However, at 220 s around, the ball-like droplet on the coated surface began to ice (see Fig. 5). The time needed for the whole icing of droplets on the plain and coated surfaces was about 400 s and 520 s respectively. The experimental results revealed that the time needed for the whole icing process and the starting time for icing on the superhydrophobic surface are all much longer than on the plain surface, i.e. the nano-fluorocarbon coating film can restrain the water icing on the surface well. Icing on a cold surface is a typical phase transition and the crystal growth process will certainly be affected by the contact angle. According to the theory, an icing crystal appears, exists in and grows up on a surface only if its size is larger than its critical radius. As we know a phase transition results in a decrease in the Gibbs free energy. A water droplet, to change the liquid phase into a stable solid state, it must overcome the potential barrier, i.e. the Gibbs free energy difference Gc . Assuming that the shape of spherical cap for a water droplet formed on a flat cold surface, the potential barrier is expressed as follows: Gc =
3 16V 1 lv 4 rc 2 lv f () = 2 3 3g
(2)
where f () =
(2 − 3 cos + cos3 ) ≤1 4
(3)
where Gc is the critical potential barrier, J; rc is the critical radius, m; lv is the surface tension between air and liquid phases, N/m; V1 is the volume of a single atom, m3 ; g is the Gibbs free energy difference when a single atom changes from water liquid into crystal, J; is the contact angle between water droplet and solid surface,◦ . Eqs. (2) and (3) show that the value of f() depends on the contact angle, and the value of f() directly determines the Gibbs free energy difference. Fig. 6 shows the relation between f() and . As shown in Fig. 6, the value of function f() monotonously increases with the contact angle, . Therefore, the growing contact angle will increase the potential barrier and restrain the growth of icing crystal nucleation, i.e. droplet icing on the surface. In this study, the contact angle for the coated superhydrophobic surface was 163.01◦ while 84.24◦ for the plain copper surface (see
Fig. 4. The icing process of a water droplet on the plain copper surface.
Fig. 5. The icing process of a water droplet on the coated surface.
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4. Conclusions
Fig. 6. Variation of f() with .
Fig. 2). Hence, the potential barrier was much larger than that of the plain copper surface, and the critical radius was also accordingly bigger for the superhydrophobic surface. Consequently, the coated superhydrophobic surface can delay the droplet icing on the surface, and the starting time of icing on the coated surface is later than that on the plain copper surface (Figs. 4 and 5). The icing of supercooled water on the supercooler is a heterogeneous nucleation phenomenon, which relates to the Gibbs free energy difference of the whole system. The whole Gibbs free energy difference G(r) is expressed as below: G(r) = G1 + G2
(4)
where G1 =
4 3 r · Gv 3
(5)
G2 = 4r 2 ıLS
(6)
where Gv is the free energy difference between the supercooled water and the ice crystal; r is the radius of the ice crystal; ıLS is the surface tension between the ice crystal and the supercooled water. The critical radius is given as follows: rc =
2ıLS Gv
(7)
According to the above equations, the critical radius decreases with the Gibbs free energy increasing. Fig. 7 shows that when r is less than rc , the Gibbs free energy difference increases with the increase of r. However, when r is larger than rc , the Gibbs free energy difference decreases with the increase of r, i.e. if the contact angle is larger, then the value of rc is bigger and the crystal nucleation does not happen more easily.
Fig. 7. Variation of G with r.
The coating with good superhydrophobic properties was prepared on the copper sample by a convenient method. The superhydrophobic property is attributed to the special surface microstructure caused by the coating film. In the icing test, it was observed that the starting time for icing is completely different from each other. The results show that the coated surface with a high contact angle and low contact angle hysteresis can not only be effective in delaying the starting icing time but also increase the whole icing process time compared with the plain copper surface under the same experimental conditions. This implies that the nano-fluorocarbon coating on the copper surface has a better antiicing performance, thus providing a good guidance for designing a dynamic ice-making system against ice-blocking in the supercooler. Acknowledgments The authors would like to thank the financial support provided by the National Natural Science Foundation of China (No. 50976036). The authors also thank the analytical testing center of Huazhong University of Science and Technology for providing the measuring support. References [1] A. Saito, Recent advances in research on cold thermal energy storage, International Journal of Refrigeration 25 (2002) 177–189. [2] T.W. Davides, Slurry ice as a heat transfer fluid with a large number of application domains, International Journal of Refrigeration 28 (2005) 108–114. [3] M. Kauffeld, M.J. Wang, V. Goldstein, K.E. Kasza, Ice slurry applications, International Journal of Refrigeration 33 (2010) 1491–1505. [4] P.W. Egolf, M. Kauffeld, From physical properties to ice slurries to industrial ice slurry applications, International Journal of Refrigeration 28 (2005) 4–12. [5] M. Kauffeld, M. Kawaji, P.W. Egolf, Handbook on Ice Slurries: Fundamentals and Engineering, International Institute of Refrigeration Paris, France, 2005. [6] J-P. Bedecarrats, T. David, J. Castaing-Lasvignottes, Ice slurry production using supercooling phenomenon, International Journal of Refrigeration 33 (2010) 196–204. [7] Y. Teraoka, A. Saito, S. Okawa, Ice crystal growth in supercooled solution, International Journal of Refrigeration 25 (2002) 218–225. [8] J.L. Laforte, M.A. Allaire, J. Laflamme, State-of-the-art on power line de-icing, Atmospheric Research 46 (1998) 143–158. [9] M. Huneault, C. Langheit, J. Caron, Combined models for glaze ice accretion and de-icing of current-carrying electrical conductors, IEEE Transactions on Power Delivery 20 (2005) 1611–1616. [10] K. Matsumoto, T. Kobayashi, Fundamental study on adhesion of ice to cooling solid surface, International Journal of Refrigeration 30 (2007) 851–860. [11] J.L. Jiang, J.Z. Lu, H.C. Lei, Analysis of the causes of tower collapses in Hunan during the 2008 ice storm, High Voltage Engineering 34 (2008) 2468–2474. [12] T. Aoyama, M. Ishikawa, T. Hirata, Effect of surface roughness on adhesive shear strength between pure ice and a solid surface, Japan Society of Refrigeration and Air Conditioning Engineers 23 (2006) 273–281. [13] K. Matsumoto, Y. Daikoku, Fundamental study on adhesion of ice to solid surface: discussion on coupling of nano-scale field with macro-scale field, International Journal of Refrigeration 32 (2009) 444–453. [14] S.A. Kulinich, M. Farzaneh, On ice-releasing properties of rough hydrophobic coatings, Cold Regions Science and Technology 65 (2011) 60–64. [15] R. Menini, Z. Ghalmi, M. Farzaneh, Highly resistant icephobic coatings on aluminum alloys, Cold Regions Science and Technology 65 (2011) 65–69. [16] S.Q. Yang, Q. Xia, L. Zhu, J. Xue, Q.J. Wang, Q.M. Chen, Research on the icephobic properties of fluoropolymer-based materials, Applied Surface Science 257 (2011) 4956–4962. [17] R. Menini, M. Farzaneh, Elaboration of Al2 O3 /PTFE icephobic coatings for protecting aluminum surfaces, Surface and Coatings Technology 203 (2009) 1941–1946. [18] P.N. Manoudis, A. Tsakalof, I. Karapanagiontis, I. Zuburtikudis, C. Panayiotou, Fabrication of super-hydrophobic surfaces for enhanced stone protection, Surface and Coatings Technology 203 (2009) 1322–1328. [19] C. Antonini, M. Innocenti, T. Horn, M. Marengo, A. Amirfazli, Understanding the effect of superhydrophobic coatings on energy reduction in anti-icing systems, Cold Regions Science and Technology 67 (2011) 58–67. [20] S. Farhadi, M. Farzaneh, S.A. Kulinich, Anti-icing performance of superhydrophobic surfaces, Applied Surface Science 257 (2011) 6264–6269. [21] H. Wang, L.M. Tang, X.M. Wu, W.T. Dai, Y.P. Qiu, Y.P. Qiu, Fabrication and anti-frosting performance of super hydrophobic coating based on modified
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[22]
[23]
[24] [25]
[26]
[27]
H. Wang et al. / Applied Surface Science 258 (2012) 7219–7224
nano-sized calcium carbonate and ordinary polyacrylate, Applied Surface Science 253 (2007) 8818–8824. Z.G. Zheng, S. Li, L.Q. Bourdo, K.R. Khedir, M.P. Asar, C.C. Ryerson, A.S. Biris, Exceptional superhydrophobicity and low velocity impact icephobicity of acetone-functionalized carbon nanotube films, Langmuir 27 (2011) 9936–9943. M. He, J.J. Wang, H.L. Li, Y.L. Song, Super-hydrophobic surfaces to condensed micro-droplets at temperatures below the freezing point retard ice/frost formation, Soft Matter 7 (2011) 3393–4000. S.A. Kulinich, M. Farzaneh, Ice adhesion on super-hydrophobic surfaces, Applied Surface Science 255 (2009) 8153–8157. M. Zou, S. Beckford, R. Wei, C. Ellis, G. Hatton, M.A. Miller, Effects of surface roughness and energy on ice adhesion strength, Applied Surface Science 257 (2011) 3786–3792. A.G. Kraj, E.L. Bibeau, Icing characteristics and mitigation strategies for wind turbines in cold climates: adhesion force and accumulation amount, in: Proceeding of WREC, August, Florence, Italy, 2006. M.G. Ferrick, N.D. Mulherin, R.B. Haehnel, B.A. Coutermarsh, G.D. Durell, T.J. Tantillo, L.A. Curtis, T.L.St. Clair, E.S. Weiser, R.J. Cano, T.M. Smith,
[28]
[29]
[30]
[31] [32] [33] [34]
E.C. Martinez, Evaluation of ice release coatings at cryogenic temperature for the space shuttle, Cold Regions Science and Technology 52 (2008) 224–243. Z.L. Liu, Y.J. Gou, J.T. Wang, S.Y. Cheng, Frost formation on a super-hydrophobic surface under natural convection conditions, International Journal of Heat and Mass Transfer 51 (2008) 5975–5982. L. Yin, Q. Xia, J. Xue, S.Q. Yang, Q.J. Wang, Q.M. Chen, In situ investigation of ice formation on surfaces with representative wetttability, Applied Surface Science 256 (2010) 6764–6769. F. Wang, C.R. Li, Y.Z. Lv, F.C. Lv, Y.F. Du, Ice accretion on superhydrophobic aluminum surfaces under low-temperature conditions, Cold Regions Science and Technology 62 (2010) 29–33. S.A. Kulinich, M. Farzaneh, How wetting hysteresis influences ice adhesion strength on superhydrophobic surfaces, Langmuir 25 (2009) 8854–8856. S. Jung, M. Dorrestijn, D. Raps, A. Das, C.M. Megaridis, D. Poulikakos, Are superhydrophobic surfaces best for icephobicity? Langmuir 27 (2011) 3059–3066. S.A. Kulinich, S. Farhadi, K. Nose, X.W. Du, Superhydrophobic surfaces: are they really ice-repellent? Langmuir 27 (2011) 25–29. A. Cassie, S. Baxter, Wettability of porous surfaces, Transactions of the Faraday Society 40 (1944) 546–551.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effects of nano-fluorocarbon coating on icing Hong Wang, Guogeng He ∗ , Qiqi Tian School of energy and power engineering, Huazhong University of Science and Technology, Wuhan 430074, China
a r t i c l e
i n f o
Article history: Received 22 September 2011 Received in revised form 29 March 2012 Accepted 7 April 2012 Available online 13 April 2012 Keywords: Ice blocking Anti-icing Superhydrophobic surface Nano-fluorocarbon coating
a b s t r a c t Icing is a common phenomenon in many fields, from aeronautics to power lines. Recently, researchers have paid much attention on the superhydrophobic surface as one of the favorable anti-icing techniques. In the present study, we investigated the performance of water icing on a superhydrophobic surface with a nano-fluorocarbon film in the average thickness around 10 nm. The surface topographies and wettabilities were characterized by a scanning electron microscopy system and a video-based contact angle measurement system respectively. To investigate the effects of this nano-fluorocarbon coating on water icing, the water droplet shape, the starting icing time and the whole icing process were observed on both the coated and uncoated surface. It was found that the coated surface has a good ability to retard the starting time of icing while the whole icing time on the coated surface was longer compared the uncoated one under the experimental conditions. The test results showed that the nano-fluorocarbon coating expresses a good anti-icing performance and can be used as a coating material to avoid ice-blocking in the dynamic ice-making system. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, ice thermal energy storage systems have received increasing attention due to their outstanding features and the ability to flatten electric power loads, and have been successfully employed in many field, such as district heating and cooling system, food storage and medical field, etc. [1–3]. In a dynamic ice storage system, ice slurry used as the thermal storage material has good fluidity, and a large amount of cold energy can be transported with less pumping work. Egolf and Kauffeld [4] and Kauffeld et al. [5] defined ice slurry as ice particles with an average characteristic diameter equal or smaller than 1 mm dispersed in an aqueous solution. One of the most promising ways to make ice slurry, i.e. using supercooled water when concomitant to ice blockage, has been widely concerned by many researchers [6,7] thanks to it is simple and energy-saved. Hence, it is necessary to find an effective method to solve the problem to achieve the continuous ice-making. The nano-fluorocarbon coating considered in the present study is appropriate for this purpose. It is well known that icing occurs when the water changes from liquid to the solid phase. Ice adhesion and excessive accumulation on exposed structures will cause serious problems, severe accidents and large economic losses to materials, telecommunication networks, power transmission, etc., especially in cold regions [8–11]. For this end, a variety of anti-icing techniques and extensive studies
∗ Corresponding author. Tel.: +86 27 87542718. E-mail address: [email protected] (G. He). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.043
were developed over the past years [12–14]. Some studies concentrated on the so-called icephobic coatings [15–17] while others focused on an alternative approach, i.e. superhydrophobic coatings [18–21]. The idea of using superhydrophobic coatings is to take advantage of water-repellency and low adhesion of drops in liquid state to such coatings to reduce or eliminate water accumulation on the surface before water iced. Recently, superhydrophobic surfaces have attracted great interest because of their many practical applications, such as in the areas of anti-icing and anti-frosting [20–24]. Studies by Zou et al. [25] investigated the effects of surface roughness and surface energy on ice adhesion strength. It was found that the ice adhesion strength decreases with the growth of water contact angle for surfaces with similar roughness; nevertheless, the smoother sample surfaces have lower ice adhesion strength than the much rougher sandblasted surfaces. Kraj and Bibeau [26] performed some preliminary tests for airfoils coated with hydrophobic and icephobic coatings in an inner wind tunnel. The objective of the tests was to evaluate the ice adhesion force and ice accumulation using different strategies. In 2008, Ferrick et al. [27] used different coatings of combined binder for the tests with varying filler proportions. In their studies, the contact angles for the surfaces ranged from 81◦ to 143◦ . The results showed that the coating made of a mixture of MP-55 and Rain-X Much has lower ice adhesion compared to plain metals. Another study performed by Farhadi et al. [20] also investigated anti-icing performance of several superhydrophobic coating with different surface chemistry and topography. They found that the anti-icing properties of the tested materials deteriorate, as their surface asperities seem to be gradually broken during icing/de-icing cycles. It was
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also shown that the anti-icing efficiency of the tested superhydrophobic surfaces is significantly lower in a humid atmosphere. Other groups discussed the ice formation changes during icing on superhydrophobic surfaces. Liu et al. [28] reported a frost deposition phenomenon on a superhydrophobic surface with a contact angle of 162◦ under natural convection conditions. They observed the formation of initial frost crystal and the frost layer structures during the freezing. From their observations, it was concluded that the surface has a capability to restrain frosting growth, and the frost deposition on the superhydrophobic surface was delayed for 55 min compared with the plain surface under the tested conditions. Yin et al. [29] also investigated the ice formation on five different surfaces with typical wettabilities. The superhydrophobic surface was found clearly reducing ice accumulation in the initial ice formation stage associated with the lower sliding angle of water droplets. However, the correlation between surface wettability and ice formation was not observed. These studies showed useful antiicing performance and weak ice-adhesion as was observed in Wang et al. [30] and Farhadi et al. [20], and benefit for anti-icing strategies. However, Kulinich and Farzaneh [31] reported that the ice adhesion strength on rough hydrophobic surfaces has no correlation with values of water CA. They pointed out that the previously reported correlation between sample hydrophobicity and ice repellency is only valid for rough hydrophobic surfaces with low wetting hysteresis. Jung et al. [32] observed that surface with nanometer-scale surface roughness and higher wettability displayed unexpectedly long freezing delays, and they suggested that anti-icing design must optimize the influence of both wettability and roughness. Kulinich et al. [33] also reported that superhydrophobic surfaces are not always ice-repellent and their use as anti-icing materials may be limited. In their studies, they observed that the ice-repellent properties of the materials deteriorate during icing/de-icing cycles, as surface asperities appear to be gradually damaged, and the anti-icing efficiency of superhydrophobic surfaces is significantly lower in a humid atmosphere. As mentioned above, many interesting developments have been accomplished in hydrophobic surface for anti-icing/de-icing recently. However, most of the works were subject to severe conditions, multi-step processes, expensive material, poor durability, etc. A superhyphobic surface with a nano-fluorocarbon coating for anti-icing, not yet investigated in literature, was addressed in this work. No report on using a nano-fluorocarbon superhydrophobic surface to prevent ice-blocking in dynamic ice-making system has not been made in past studies. This work investigated the effects of the nano-fluorocarbon coating with unique superhydrophobility on icing due to its chemical and physical properties with purposes: (i) to quantify benefits, that application of the nano-fluorocarbon coating; (ii) to provide an understanding of a positive coating strategy to combat icing on surfaces; and (iii) to find an efficient way to prevent ice-blocking in the dynamic ice-making system. This study fabricated the superhydrophobic surface by means of a facile method as defined in the next experimental details. The surface microstructure and the contact angle were investigated with the Scanning Electron Microscope (SEM) and Contact Angle (CA) respectively. In the icing test, the superhydrophobic surface showed a strong ability to mitigate ice formation under the experimental conditions.
is nitric derivate of perfluorpolyoxyalkyl carbonate. The organic solvent is a mixture of fluorinated hydrocarbon solvent and ethanol or isopropyl alcohol, which may guarantee a good dissolvability of the binder and a firmed coating of modifier on the surface. In reaction of fluorine to polymers with hydrogen, a fluorocarbon with special properties is produced by replacing the hydrocarbon chain with fluorocarbon one, and the modifier forms uniformly distributed protrusions on the surface when coated on the solid surface (see Section 3.1, Fig. 1). In the present study, the protrusion depth was also investigated by an atomic force microscope. Based on AFM measurements (not shown here), the largest depth and smallest depth were about 12.5 nm and 7.6 nm, respectively. These micro-protrusions allow the fluorocarbon film in the average thickness around 10 nm on the solid surface, showing a strong superhydrophobicity as discussed in Section 3.2. 2.2. Preparation of samples Two 15 mm × 10 mm × 1.0 mm copper samples were prepared for the test. The superhydrophobic and plain surfaces were prepared on the above samples. It is easy to obtain a plain surface. After immersed in carbon tetrachloride solution for 5 min to remove any pollutant, then samples were rinsed with pure ethanol for several times and dried for 30 min at the room temperature. As a superhydrophobic surface is more complicated to prepare, it was prepared using the same process as above firstly, and then the following procedure: (1) dripped a droplet of the modification coating on the sample surface; (2) rubbed the surface with a soft nylon cloth till the liquid totally etches on the surface; (3) dried for 4 h in a clean chamber at the room temperature. Finally, the superhydrophobic surface with a nano-fluorocarbon film of less than 10 nm in thickness was obtained. 2.3. Sample characterization A scanning electron microscopic system (HITACHI S-4800, Japan) was used to characterize the surface topographies of all samples. SEM micrographs at different magnifications over 4 locations across each sample were examined in the micro-scale uniformity of the sample surfaces. The water contact angles (WCAs) were measured using a videobased contact angle measurement system (DCA, SL200B). The static WCAs were measured at least 3 times across the sample surface using the sessile drop by dispensing 2 L drops of de-ionized water on the surfaces. All WCAs were measured at an ambient temperature 25 ◦ C and relative humidity 50%. 2.4. Icing test The icing tests were performed in a climatic chamber with a constant working temperature of −8 ◦ C. In order to maintain a constant natural convection condition, the samples were placed in a large plexiglass enclosure. The temperature was regulated at the set value by a refrigerating unit. The droplets on the superhydrophobic surface and plain copper surface were dropped by an injector needle tube. During the test, a high-speed digital camera was utilized to capture the shape change of the water droplets. 3. Results and discussion
2. Experimental details 3.1. Surface topography 2.1. Modifier The modifier used in this study, supplied by Wuhan Futejia Co. Ltd (China), is a kind of high polymer that contains a fluoric binder and an organic solvent. The chemical name of the modifier
Fig. 1 shows some typical SEM micrographs of the copper samples with and without nano-fluorocarbon modifier. It can be seen from Fig. 1(a) that there are many protrusions and voids on the nano-fluorocarbon coated surface, while the plain copper surface
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Fig. 1. SEM image comparison: (a) magnification: 20,000×; (b) magnification: 200,000× (left: coated surface and right: plain copper surface).
only has original shadow grooves. Moreover, as seen in Fig. 1(b) with a larger magnification, the protrusions are composed of randomly oriented nanoparticles, having a homogeneous distribution on the superhydrophobic rough surface; however, the uncoated surface is relatively smooth with no any protrusion. Compared the SEM images of the superhydrophobic surface with that of the plain surface, one can see that the nano-fluorocarbon modification coating has notably changed the topography of the copper surface, which indicates that the micronanoscale structure of the coated surface plays an important role in the surperhydrophobic property. 3.2. Surface wettability Wettability, one basic property of solid surface, is dependent on both the topographical structure and the chemical compositions. The typical parameter used to characterize a solid surface wettability is the contact angle (CA). However, when a drop is placed on a surface, not only a single value (CA), but multiple values of
the contact angle can be observed, i.e. contact angle hysteresis, . Contact angle hysteresis is usually quantified as the difference between the advancing and receding contact angles, A and R . Presently, a surface is usually labeled superhydrophobic when both the contact angle is high ( > 150◦ ), and contact angle hysteresis is low ( < 10◦ ). In the present study, the contact angles on the coated and uncoated surface were measured by a video-based contact angle measurement system. As shown in Fig. 2, the static CAs on the two samples were 163.01◦ and 84.24◦ , respectively. Moreover, in this work, the advancing and receding contact angles on the coated surface were 164.62◦ and 158.45◦ , respectively, which shows that the coated surface has a low contact angle hystersis ( = 6.17◦ ). Hence, the coated surface with a nano-fluorocarbon film in the average thickness around 10 nm has a good superhydrophobicity. A superhydrophobic surface is determined mainly by the characteristics of the solid surface. This phenomenon can be explained by the Cassie–Baxter model (see Fig. 3).
Fig. 2. Contact angles of water on different surfaces: (a) water droplet on coated surface, = 163.01◦ ; (b) water droplet on plain copper surface, = 84.24◦ .
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Fig. 3. Cassie–Baxter model.
Based on various kinds of studies Cassie and Baxter [34] proposed a gas cavity model for superhydrophobic surface wettability. According to the Cassie–Baxter model, air is more likely to be trapped between the protrusions of the superhydrophobic surface and the water droplet. The following equation is given for the equilibrium contact angle of a real surface: cos = f (1 + cos ) − 1
(1)
where is the contact angle for a real rough surface, is the contact angle for the corresponding smooth surface, and f(f < 1) is the fraction of solid–liquid contact area. From Eq. (1), the contact angle of a real surface is always larger than that of an ideal surface under the same conditions. In this work, as shown in Fig. 1, there are many numbers of micro-nano-scale protrusions on the coated copper surface, trapping the air readily under the water droplet, so the contact fraction f decreases and the WCA is much larger than that of a flat plain surface. In general, the nano-fluorocarbon coating film on the copper surface increases hydrophobicity of the surface and provides an ideal superhydrophobic surface. 3.3. Icing characteristics To assure the anti-icing performance of this superhydrophobic coating, a further investigation has been done. Droplets were placed on the plain and coated surfaces respectively, and the icing processes were observed. The temperature inside the chamber was maintained at −8 ◦ C. Figs. 4 and 5 illustrate some of the typical different pictures of the water droplets and ice formation on the two surfaces during the icing test. The water droplet on the coated surface likes a ball with a smaller contact surface area thanks to
its large contact angle on the superhydrophobic surface while the water droplet on the plain copper sample likes a hemisphere with too much contact area. At 180 s, as shown in Fig. 4, most of the hemispheric droplet iced in the chamber. However, at 220 s around, the ball-like droplet on the coated surface began to ice (see Fig. 5). The time needed for the whole icing of droplets on the plain and coated surfaces was about 400 s and 520 s respectively. The experimental results revealed that the time needed for the whole icing process and the starting time for icing on the superhydrophobic surface are all much longer than on the plain surface, i.e. the nano-fluorocarbon coating film can restrain the water icing on the surface well. Icing on a cold surface is a typical phase transition and the crystal growth process will certainly be affected by the contact angle. According to the theory, an icing crystal appears, exists in and grows up on a surface only if its size is larger than its critical radius. As we know a phase transition results in a decrease in the Gibbs free energy. A water droplet, to change the liquid phase into a stable solid state, it must overcome the potential barrier, i.e. the Gibbs free energy difference Gc . Assuming that the shape of spherical cap for a water droplet formed on a flat cold surface, the potential barrier is expressed as follows: Gc =
3 16V 1 lv 4 rc 2 lv f () = 2 3 3g
(2)
where f () =
(2 − 3 cos + cos3 ) ≤1 4
(3)
where Gc is the critical potential barrier, J; rc is the critical radius, m; lv is the surface tension between air and liquid phases, N/m; V1 is the volume of a single atom, m3 ; g is the Gibbs free energy difference when a single atom changes from water liquid into crystal, J; is the contact angle between water droplet and solid surface,◦ . Eqs. (2) and (3) show that the value of f() depends on the contact angle, and the value of f() directly determines the Gibbs free energy difference. Fig. 6 shows the relation between f() and . As shown in Fig. 6, the value of function f() monotonously increases with the contact angle, . Therefore, the growing contact angle will increase the potential barrier and restrain the growth of icing crystal nucleation, i.e. droplet icing on the surface. In this study, the contact angle for the coated superhydrophobic surface was 163.01◦ while 84.24◦ for the plain copper surface (see
Fig. 4. The icing process of a water droplet on the plain copper surface.
Fig. 5. The icing process of a water droplet on the coated surface.
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4. Conclusions
Fig. 6. Variation of f() with .
Fig. 2). Hence, the potential barrier was much larger than that of the plain copper surface, and the critical radius was also accordingly bigger for the superhydrophobic surface. Consequently, the coated superhydrophobic surface can delay the droplet icing on the surface, and the starting time of icing on the coated surface is later than that on the plain copper surface (Figs. 4 and 5). The icing of supercooled water on the supercooler is a heterogeneous nucleation phenomenon, which relates to the Gibbs free energy difference of the whole system. The whole Gibbs free energy difference G(r) is expressed as below: G(r) = G1 + G2
(4)
where G1 =
4 3 r · Gv 3
(5)
G2 = 4r 2 ıLS
(6)
where Gv is the free energy difference between the supercooled water and the ice crystal; r is the radius of the ice crystal; ıLS is the surface tension between the ice crystal and the supercooled water. The critical radius is given as follows: rc =
2ıLS Gv
(7)
According to the above equations, the critical radius decreases with the Gibbs free energy increasing. Fig. 7 shows that when r is less than rc , the Gibbs free energy difference increases with the increase of r. However, when r is larger than rc , the Gibbs free energy difference decreases with the increase of r, i.e. if the contact angle is larger, then the value of rc is bigger and the crystal nucleation does not happen more easily.
Fig. 7. Variation of G with r.
The coating with good superhydrophobic properties was prepared on the copper sample by a convenient method. The superhydrophobic property is attributed to the special surface microstructure caused by the coating film. In the icing test, it was observed that the starting time for icing is completely different from each other. The results show that the coated surface with a high contact angle and low contact angle hysteresis can not only be effective in delaying the starting icing time but also increase the whole icing process time compared with the plain copper surface under the same experimental conditions. This implies that the nano-fluorocarbon coating on the copper surface has a better antiicing performance, thus providing a good guidance for designing a dynamic ice-making system against ice-blocking in the supercooler. Acknowledgments The authors would like to thank the financial support provided by the National Natural Science Foundation of China (No. 50976036). The authors also thank the analytical testing center of Huazhong University of Science and Technology for providing the measuring support. References [1] A. Saito, Recent advances in research on cold thermal energy storage, International Journal of Refrigeration 25 (2002) 177–189. [2] T.W. Davides, Slurry ice as a heat transfer fluid with a large number of application domains, International Journal of Refrigeration 28 (2005) 108–114. [3] M. Kauffeld, M.J. Wang, V. Goldstein, K.E. Kasza, Ice slurry applications, International Journal of Refrigeration 33 (2010) 1491–1505. [4] P.W. Egolf, M. Kauffeld, From physical properties to ice slurries to industrial ice slurry applications, International Journal of Refrigeration 28 (2005) 4–12. [5] M. Kauffeld, M. Kawaji, P.W. Egolf, Handbook on Ice Slurries: Fundamentals and Engineering, International Institute of Refrigeration Paris, France, 2005. [6] J-P. Bedecarrats, T. David, J. Castaing-Lasvignottes, Ice slurry production using supercooling phenomenon, International Journal of Refrigeration 33 (2010) 196–204. [7] Y. Teraoka, A. Saito, S. Okawa, Ice crystal growth in supercooled solution, International Journal of Refrigeration 25 (2002) 218–225. [8] J.L. Laforte, M.A. Allaire, J. Laflamme, State-of-the-art on power line de-icing, Atmospheric Research 46 (1998) 143–158. [9] M. Huneault, C. Langheit, J. Caron, Combined models for glaze ice accretion and de-icing of current-carrying electrical conductors, IEEE Transactions on Power Delivery 20 (2005) 1611–1616. [10] K. Matsumoto, T. Kobayashi, Fundamental study on adhesion of ice to cooling solid surface, International Journal of Refrigeration 30 (2007) 851–860. [11] J.L. Jiang, J.Z. Lu, H.C. Lei, Analysis of the causes of tower collapses in Hunan during the 2008 ice storm, High Voltage Engineering 34 (2008) 2468–2474. [12] T. Aoyama, M. Ishikawa, T. Hirata, Effect of surface roughness on adhesive shear strength between pure ice and a solid surface, Japan Society of Refrigeration and Air Conditioning Engineers 23 (2006) 273–281. [13] K. Matsumoto, Y. Daikoku, Fundamental study on adhesion of ice to solid surface: discussion on coupling of nano-scale field with macro-scale field, International Journal of Refrigeration 32 (2009) 444–453. [14] S.A. Kulinich, M. Farzaneh, On ice-releasing properties of rough hydrophobic coatings, Cold Regions Science and Technology 65 (2011) 60–64. [15] R. Menini, Z. Ghalmi, M. Farzaneh, Highly resistant icephobic coatings on aluminum alloys, Cold Regions Science and Technology 65 (2011) 65–69. [16] S.Q. Yang, Q. Xia, L. Zhu, J. Xue, Q.J. Wang, Q.M. Chen, Research on the icephobic properties of fluoropolymer-based materials, Applied Surface Science 257 (2011) 4956–4962. [17] R. Menini, M. Farzaneh, Elaboration of Al2 O3 /PTFE icephobic coatings for protecting aluminum surfaces, Surface and Coatings Technology 203 (2009) 1941–1946. [18] P.N. Manoudis, A. Tsakalof, I. Karapanagiontis, I. Zuburtikudis, C. Panayiotou, Fabrication of super-hydrophobic surfaces for enhanced stone protection, Surface and Coatings Technology 203 (2009) 1322–1328. [19] C. Antonini, M. Innocenti, T. Horn, M. Marengo, A. Amirfazli, Understanding the effect of superhydrophobic coatings on energy reduction in anti-icing systems, Cold Regions Science and Technology 67 (2011) 58–67. [20] S. Farhadi, M. Farzaneh, S.A. Kulinich, Anti-icing performance of superhydrophobic surfaces, Applied Surface Science 257 (2011) 6264–6269. [21] H. Wang, L.M. Tang, X.M. Wu, W.T. Dai, Y.P. Qiu, Y.P. Qiu, Fabrication and anti-frosting performance of super hydrophobic coating based on modified
7224
[22]
[23]
[24] [25]
[26]
[27]
H. Wang et al. / Applied Surface Science 258 (2012) 7219–7224
nano-sized calcium carbonate and ordinary polyacrylate, Applied Surface Science 253 (2007) 8818–8824. Z.G. Zheng, S. Li, L.Q. Bourdo, K.R. Khedir, M.P. Asar, C.C. Ryerson, A.S. Biris, Exceptional superhydrophobicity and low velocity impact icephobicity of acetone-functionalized carbon nanotube films, Langmuir 27 (2011) 9936–9943. M. He, J.J. Wang, H.L. Li, Y.L. Song, Super-hydrophobic surfaces to condensed micro-droplets at temperatures below the freezing point retard ice/frost formation, Soft Matter 7 (2011) 3393–4000. S.A. Kulinich, M. Farzaneh, Ice adhesion on super-hydrophobic surfaces, Applied Surface Science 255 (2009) 8153–8157. M. Zou, S. Beckford, R. Wei, C. Ellis, G. Hatton, M.A. Miller, Effects of surface roughness and energy on ice adhesion strength, Applied Surface Science 257 (2011) 3786–3792. A.G. Kraj, E.L. Bibeau, Icing characteristics and mitigation strategies for wind turbines in cold climates: adhesion force and accumulation amount, in: Proceeding of WREC, August, Florence, Italy, 2006. M.G. Ferrick, N.D. Mulherin, R.B. Haehnel, B.A. Coutermarsh, G.D. Durell, T.J. Tantillo, L.A. Curtis, T.L.St. Clair, E.S. Weiser, R.J. Cano, T.M. Smith,
[28]
[29]
[30]
[31] [32] [33] [34]
E.C. Martinez, Evaluation of ice release coatings at cryogenic temperature for the space shuttle, Cold Regions Science and Technology 52 (2008) 224–243. Z.L. Liu, Y.J. Gou, J.T. Wang, S.Y. Cheng, Frost formation on a super-hydrophobic surface under natural convection conditions, International Journal of Heat and Mass Transfer 51 (2008) 5975–5982. L. Yin, Q. Xia, J. Xue, S.Q. Yang, Q.J. Wang, Q.M. Chen, In situ investigation of ice formation on surfaces with representative wetttability, Applied Surface Science 256 (2010) 6764–6769. F. Wang, C.R. Li, Y.Z. Lv, F.C. Lv, Y.F. Du, Ice accretion on superhydrophobic aluminum surfaces under low-temperature conditions, Cold Regions Science and Technology 62 (2010) 29–33. S.A. Kulinich, M. Farzaneh, How wetting hysteresis influences ice adhesion strength on superhydrophobic surfaces, Langmuir 25 (2009) 8854–8856. S. Jung, M. Dorrestijn, D. Raps, A. Das, C.M. Megaridis, D. Poulikakos, Are superhydrophobic surfaces best for icephobicity? Langmuir 27 (2011) 3059–3066. S.A. Kulinich, S. Farhadi, K. Nose, X.W. Du, Superhydrophobic surfaces: are they really ice-repellent? Langmuir 27 (2011) 25–29. A. Cassie, S. Baxter, Wettability of porous surfaces, Transactions of the Faraday Society 40 (1944) 546–551.