- Email: [email protected]
Analysis of anti-condensation mechanism on superhydrophobic anodic aluminum oxide surface
Applied Thermal Engineering 58 (2013) 664e669
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Analysis of anti-condensation mechanism on superhydrophobic anodic aluminum oxide surface Yanpeng Wu*, Chaoying Zhang School of Mechanical Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Prepare superhydrophobic surface on anodic aluminum oxide surface. Analyze the reason of anticondensation on superhydrophobic surfaces. The density of droplets formed on superhydrophobic surfaces is low. Droplets on superhydrophobic surfaces are easy to detach. This research can solve some problems of equipment using in HVAC systems.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2012 Accepted 26 January 2013 Available online 15 February 2013
Wetting theory about superhydrophobic surfaces reveals that hydrophobicity of surfaces has great relationship with surface roughness and surface free energy. Adopt electrochemical plus fluorine silane modified method to prepare superhydrophobic surface on anodic aluminum oxide surface, which not only enhances surface roughness, but also reduces surface free energy, even the static contact angle can reach 159.2 and anti-condensation is authenticated. Based on the experimental findings, analyze the reason of anti-condensation on superhydrophobic surfaces: one is that the density of droplets formed on superhydrophobic surfaces is low and the number of droplets is little; the other is bigger static contact angle and smaller rolling angle on superhydrophobic surfaces make droplets easy to detach on smaller tilt angle. This research can solve some condensation problems of equipment using in HVAC systems, such as heat exchangers in air conditioning system, cold radiation boards, air supply outlets, and so on. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Superhydrophobic surfaces Contact angle Anti-condensation Detachment
1. Introduction In recent years, people have devoted great attention to superhydrophobic study on the solid surface, and have made significant progress [1e3]. When dripping droplet on the solid surface, forming solideliquidegas three-phase interface and reaching energy
* Corresponding author. Tel.: þ86 13718105898. E-mail address: [email protected] (Y. Wu). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.01.048
balance, the intersection angle from solideliquid interface through internal liquid to gaseliquid interface is called contact angle. The contact angle of droplets on the hydrophobic surface is greater than 90 , contact angle on the superhydrophobic surface is greater than 150 . Superhydrophobic surfaces are extremely common in nature, like the surface of lotus leaf, the feet of water skippers, the wing of butterflies, and the feather of waterfowls [4]. Observing the surface of lotus leaf, a lot of micron small papillae can be seen, between which covered by nano crystalline wax. And this micron-nano quadratic structure made lotus leaf own superhydrophobicity. Hence, inspired by lotus leaf, people realized that the preparation of
Y. Wu, C. Zhang / Applied Thermal Engineering 58 (2013) 664e669
surface micron-nano structure is the key to prepare superhydrophobic surface [5,6]. The features of superhydrophobic surfaces, including anticondensation property, have great contact with surface wettability. Generally speaking, surface wettability is spreading characteristics of droplets on the solid materials surface between solid and liquid interfaces. At present, the basic theory that can explain the surface wettability phenomenon mainly includes: Yong’s equation [7], Wenzel equation [8] and CassieeBaxter equation [9]. Their model diagrams are shown as follows in Fig. 1(aec) (Table 1). Among them, Yong’s equation (Eq. (1)) only applies to the ideal solid surface and indicates that contact angle is related to surface tension. Wenzel equation (Eq. (2)) aims to the wetting sexual behavior whose actual solid surface has surface composition of heterogeneous and structure of rough. It indicates that contact angle is not only related to surface tension, but also surface roughness. Cassie and Baxter found CassieeBaxter equation (Eq. (3)) is suitable for any composite surface contact by analyzing from the viewpoint of thermodynamics. These theories suggested that solid surface energy and surface micro-structure had effects on the surface wettability.
cos q0 ¼ ðgSV gSL Þ=gLV
(1)
cos qCB ¼ rðgSV gSL Þ=gLV ¼ r cos q0
(2)
cos qCB ¼ f1 cos q1 þ f2 cos q2
(3)
Roughness factor is the ratio of the actual contact area of the solideliquid interface and the apparent solideliquid interface contact area. Because r 1, increasing surface roughness could make the hydrophobicity of hydrophobic surface much stronger. These theories interpreted the mechanism of superhydrophobic surface wettability constantly deeply, completely and accurately, but just started from a static point of view. In 1999, Chen and others put forward that the static contact angle was not enough to describe the super-hydrophobic properties of the material surface [10]. It is needed to consider the surface of the dynamic process to determine the surface hydrophobicity. The rolling angle is the product of this theory. It is the critical angle of droplets occurred just rolling on the inclined surface, formed by the inclined surface and horizontal plane. In addition, the relationship between the dynamic contact angle and the velocity of the moving contact line indicates that the macroscopic flow geometry does not influence the advancing dynamic wetting behavior of Newtonian fluids, but does influence the advancing dynamic wetting behavior of non-Newtonian fluids [11]. In summary, the apparent contact angle and rolling angle are two important parameters to describe the surface hydrophobicity status. Surface roughness and surface free energy are two important factors which can affect the performance of surface hydrophobicity. The greater surface roughness is, the lower surface free energy is, and the apparent contact angle will be greater, rolling angle will be smaller, and the surface hydrophobicity will be better.
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Table 1 Parameters in equations of wetting model. Parameters
Nomenclature
Units
Solid-volume surface tension Solideliquid surface tension Liquid-volume surface tension Equilibrium contact angle or material intrinsic contact angle Apparent contact angle Roughness factor Intrinsic contact angle of two mediums Area ratio of two mediums contact with the liquid
gSV gSL gLV q0
mN/m mN/m mN/m
qCB
r
e
q1/q2
f1/f2
e
Chen Xinhua et al. [12] used chemical corrosion method and modification by fluorosilane to obtain superhydrophobic alumina surface and verified the contact angle on micron-nano level alumina surface with fluorosilane modified is bigger than which on smooth surfaces and nano-hole surfaces. Shixiang Lu et al. [13] used solegel method to get superhydrophobic aluminum surface which has strong stability. Shuquan Liang et al. [14] developed a simple method of acid etching and surface passivation to fabricate superhydrophobic surface on polycrystalline aluminum alloy, and analyzed 84% of the superhydrophobic phenomenon was in contact with air cushion. This article adopted electrochemical plus fluorosilane modified method to make anodic aluminum oxide superhydrophobic paint coat which is usually used on heat exchangers in air conditioning system. On one hand, this method used electrochemical method to construct micro structure on surfaces, and on the other hand, used fluorosilane modification to reduce the surface energy. The prepared paint coat made anodic aluminum oxide surface achieve superhydrophobic status, which could improve the condensation on heat exchangers in air conditioning system. Yin Ping et al. [15] proposed superhydrophobic surface could thoroughly prevent the cold surface condensation in air conditioning system, created superhydrophobic polymer point coat on aluminum surface via phase separation technology, applied in air conditioning cold surface, and had good anti-condensation effect. Through temperature control, Youfa Zhang [16] found that superhydrophobic aluminum surface formed dew rarely, and when experimental temperature was cooled to 0 C, it would not freeze later than 30 min. Fochi Wang et al. [17] used experiments to prove that superhydrophobic aluminum surface could prevent freeze. Tian He et al. [18] prepared “Underwater superhydrophobic” surface onto the anodized aluminum which was treated as corrosion protection. Libang Feng [19] used polyethylene absorb plus stearic acid layer modification method to produce alumina surface superhydrophobic paint coat and analyzed its superhydrophobic mechanism. However, these researches mentioned above don’t include the anti-condensation mechanism of anodic aluminum oxide superhydrophobic surface. Hence, based on the production of anodic aluminum oxide superhydrophobic surface, this article analyzes anti-condensation mechanism of superhydrophobic surface.
Fig. 1. (a) Yong’s model diagram. (b) Wenzel model diagram. (c) CassieeBaxter model diagram.
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Y. Wu, C. Zhang / Applied Thermal Engineering 58 (2013) 664e669
Anodic aluminum oxide is widely used in HVAC system, such as large heat exchangers, freezing containers, low temperature facilities, containers, etc. Especially, anodic aluminum oxide owns important theoretical research value and practical application future in the application of heat exchangers in air conditioning system. 2. Preparation of superhydrophobic surface using electrochemical method Superhydrophobic surface can be prepared via two methods, one is to change surface roughness by constructing micron and nanometer level roughness structure, the other is to reduce surface free energy by using low surface energy substance to modify the micron and nanometer level roughness structure. Wherein, the method of preparing suitable micron and nanometer level roughness structure is the key procedure. Classified by preparation method, there are vapor induce phase separation method, stencil printing method, electrospinning method, solegel method, extrusion method, laser and plasma etching method, self-assembly method, stretching method, corrosion method, etc. This article adopted electrochemical method to prepare superhydrophobic surface on anodic aluminum oxide and used fluorine silane to modify parts of the sample. After preparation, measure contact angles and investigate surface hydrophobicity especially the anti-condensation performance. Electrochemical method is using modern chemical method to form rough porous morphology on surface. 2.1. Experimental content 2.1.1. Laser etching Adjust the width of pulse laser light spot to change the width of micro etching groove on surface, adjust laser pulse energy and repeat the etching to change the etching depth on surface, and move the sample to change the etching size. 2.1.2. Fluorine silane modification Clean the samples for 30 min and dry them in 100 C oven. Take 3 pieces of them to soak in 2% fluorine silane CF3(CH2)8CH2Si(OCH3)3 solution for 2 h, and take another 3 pieces without any measures. Finally, dry these 6 samples in 140 C for 1 h.
Fig. 2. Optical microscope image of anodic aluminum oxide using electrochemistry.
The morphology of water-drop on sample surface is shown in Fig. 5. Therefore, the surface of anodic aluminum oxide using electrochemistry has micro nanometer rough structure, which can own hydrophobic property. After anodic aluminum oxide is modified by low surface energy fluorine silane, its surface free energy decreases, and hydrophobic property has promoted to superhydrophobic property. 2.4. Surface condensation experiment Select two experiment samples: one piece is anodic aluminum oxide sample A etched by laser and modified by fluorine silane, the other piece is none processed anodic aluminum oxide sample B. Put two samples in refrigerator (temperature 15 C) for 15 min, use steam humidification method to control the humidity of transparent sealed experimental box, and make the relative humidity reach 80%. Take the sample from the refrigerator, and put them into the environment experimental box. Observe the surface condensation phenomenon of two samples after 10 min. Sample A’s surface almost has no droplets, no condensation, sample B’s surface has variable sizes droplets and obvious condensation phenomenon. Experimental results explicate that anodic aluminum oxide sample via electrochemical and fluorine silane modified has wonderful anti-condensation property.
2.2. Apparent morphology observation The optical microscope image of electrochemical anodic aluminum oxide surface is shown in Fig. 2. The electrochemical method adopted in the experiment constructs micro nanometer level structure in anodic aluminum oxide, and increases the surface roughness and the air cavitation area fraction. According to Cassiee Baxter theory, this method will promote the surface hydrophobic property.
3. Anti-condensation mechanism on superhydrophobic surfaces When the surface temperature of objects is below the dewpoint temperature of air in surroundings, moist air will condense on the low-temperature surfaces, which is called condensation.
2.3. Surface contact angle test Contact angles of 6 samples were measured by angle meter OCA15 (Seiko, Instruments Inc., Japan). As shown in Fig. 3, pick 3 points on each sample surface, and take the average value as the final contact angle. Test results refer to Fig. 4. As the test results shown in Fig. 4, the contact angles of 1, 2, 3 samples modified by fluorine silane are all greater than the contact angles of 4, 5, 6 samples which did not modify by fluorine silane. The CA average value of sample 1, 2, 3 is 159.2 , which reach the superhydrophobic status. The CA average value of sample 4, 5, 6 is 141.6 , which reach the hydrophobic status but not satisfy the superhydrophobic status.
Fig. 3. Selection of sample contact angle test points.
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grow. The change of free energy DGC caused by forming critical liquid core across critical radius is called thermodynamics potential barrier or energy barrier. Therefore, droplets must surmount the energy barrier. However, the value of DGC is closely related to surface contact angle, which is shown below Eqs. (4) and (5) [25]:
DGC ¼ ð4p=3Þf2svl =½rl Rw TlnðP=Psl Þg2 svl f ðqÞ
(4)
f ðqÞ ¼ ð2 þ cos qÞð1 cos qÞ =4
(5)
2
In Eq. (4), rl is the density of the droplets, T is the temperature of the system, P is the pressure of water vapor in the system, Psl is water vapor’s saturated partial pressure consisted with the temperature T when gaseliquid two phases are plane. Because these testings are under the same condition, and the above parameters can be regarded as constants, so Eq. (5) can be simplified as following: Fig. 4. Contact angle test of 6 anodic aluminum oxide samples.
DGC ¼ Cf ðqÞ Condensation commonly exists on some cold areas, such as indoor natatorium, buildings’ envelope, air-conditioning system and so on. In HVAC system, condensation mainly occurs on the surfaces of the fins and pipes of heat exchangers, cold radiation boards, cooling roof, air supply outlets, air ducts and water ducts. The wetting theories of liquid on solid surface are spreading theory and detachment theory. Wang had done many research on spreading theory, such as spreading dynamics and dynamic contact angle of non-Newtonian [20], spreading of completely wetting or partially wetting power-law fluid on solid surface, spreading of completely wetting [21], non-Newtonian fluids with non-powerlaw rheology [22]. However, surface condensation is about detachment theory. Moist air forms lots of tiny initial droplets first, and these droplets grow bigger directly or merge to grow. When these droplets grow to a certain size, the balance between the external force and surface adhesion suffered by droplets will be destroyed, and then the droplets detach away. There are two theories about the formation of initial droplets: one is membrane rupture hypothesis supplied by Jakob [23], which considered that a layer of liquid film was formed on the surface at first, and when the thickness of liquid film came to critical thickness, liquid film cracks, and the fragments of liquid film contract to initial tiny droplets; the other is fixed nucleating center hypothesis supplied by Tammann and Boehme [24], which considered that there were condensation nucleus where initial tiny droplets formed firstly existed randomly on surfaces. Only the size of initial droplets is greater than the critical size, initial droplets can form, exist, and
(6)
Known by Eq. (6), DGC is in proportion to f(q). Make C ¼ 1 (as Fig. 6), DGC increases as q increasing. For example, q ¼ 0 , DGC ¼ 0; q ¼ 70 , DGC ¼ 0.23C; q ¼ 110 , DGC ¼ 0.23C; q ¼ 150 , DGC ¼ 0.98C. As q increasing, the nucleation potential barrier of droplets increases, and the energy barrier increases which should be overcome in droplets forming. The contact angle on superhydrophobic surface is greater than 150 which is much bigger than that on common surfaces, so the energy barrier in droplets forming is greater, and droplets are difficult in forming process. As a result, the droplets density of superhydrophobic is smaller than that of common surfaces, which is one reason of anti-condensation of superhydrophobic surface. When droplets grow to a certain critical size, under external force, droplets will detach from surfaces. The detachment diameter of droplets in vertical surfaces is the least. In HVAC systems, the condensing surfaces of equipment are almost vertical, slant or placed down, and these situations are of benefit to the detachment of droplets. The detachment way of droplets on superhydrophobic is rolling off, which is the best and fastest detachment way. Establish the droplet detachment model on slant superhydrophobic surfaces, as shown in Fig. 7. The slant angle of superhydrophobic surface is a, gravity acting on droplets detachment is FG, the width of contact area of droplets and surfaces is w, advancing angle and receding angle of droplets are qA and qR, gLV is tension between solid and liquid, r is droplets density, V is droplets volume, g is gravity acceleration, and their relationship can be described as following:
Fig. 5. Morphology of water-drop on anodic aluminum oxide hydrophobic surface (left: modified by fluorine silane, right: unmodified).
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Y. Wu, C. Zhang / Applied Thermal Engineering 58 (2013) 664e669
Fig. 8. Droplet’s shape on horizontal superhydrophobic surface.
As Eq. (13) indicates that on the condition of the same parameters of droplets, surface critical slant angle aC is relevant with q0, qA and qB. Consider the influence on aC by q0, take q0 as the single variable, assume constant term of other parameters as 0.3, and Eq. (14) will be received.
Fig. 6. Relationship of DGC and q.
FG ¼ rVg sin a
(7)
V ¼ ðp=3Þ½w=ð2 sin q0 Þ
FG =w ¼ gLV ðcos qR cos qA Þ
(8)
sin ac ¼ 2gLV sin q0 ðcos qR cos qA Þ io1=3 n h =rg 3= pV 2 2 3cos q0 þ cos3 q0
The shape of droplets on horizontal superhydrophobic surfaces is similar to segment (Fig. 8). Approximately regard the volume of droplets on horizontal surfaces as the same on slant surfaces. In hence, the volume of droplets V is:
V ¼ ph ðR h=3Þ 2
(9)
h ¼ R þ R cos b ¼ Rð1 þ cos bÞ ¼ Rð1 cos q0 Þ
(10)
w ¼ 2R$sin q0
(11)
Eq. (12) is derived from Eqs. (9)e(11). When droplets detach, the relationship (Eq. (13)) between critical slant angle and contact angle is deduced by Eqs. (7), (8) and (12).
Fig. 7. Droplet detachment model on slant superhydrophobic surface.
3
2 3 cos q0 þ cos3 q0
1=3 sin ac ¼ 0:3 sin q0 2 3 cos q0 þ cos3 q0
(12)
(13)
(14)
In Fig. 9, as static contact angle q0 increasing, the sine of surface tilt angle sin ac is decreasing, that is surface tilt angle decreasing (0 < ac < 90 ). Therefore, as for hydrophobic surfaces, static contact angles much greater, droplets much easier to drop down. Advancing angle and receding angle are related factors, and Dq ¼ qA qR expresses contact angle hysteresis, which has relation with rolling angle (Eqs. (7) and (8)). However, rolling angle is surface tilt angle when droplets rolling on slant surfaces. Hence, the weaker contact angle hysteresis is, the smaller rolling angle is, and the speed of droplets rolling from slant surfaces will be much faster. The static contact angle of droplets on superhydrophobic surfaces is greater than 150 , rolling angle is smaller than 10 , so the
Fig. 9. Relationship of surface tilt angle aC and static contact angle q0.
Y. Wu, C. Zhang / Applied Thermal Engineering 58 (2013) 664e669
surface tilt angle which droplets falling off needed is much less than that on common surfaces. That is to say, in the same slant angle, droplets on superhydrophobic are easier to drop, and this is another reason of anti-condensation on superhydrophobic surfaces. 4. Conclusions Enhancing surface roughness and reducing surface free energy can improve hydrophobicity of surfaces. This paper adopts electrochemical plus fluorine silane modified method to prepare superhydrophobic surface on which the static contact angle can reach 159.2 , and anti-condensation is authenticated. Based on the experimental results, anti-condensation on superhydrophobic surfaces is affected by low density and hard existence. One reason is that static contact angle on superhydrophobic surfaces is greater, the energy barrier that droplets forming need to overcome is larger, and droplets are difficult to form and the density is low; the other reason is bigger static contact angle and smaller rolling angle on superhydrophobic surfaces make droplets hard to exist and easy to detach on smaller tilt angle. On the basis of analyzing anti-condensation mechanism on superhydrophobic surfaces, we will focus efforts on the appliance of anti-condensation to HVAC systems, continue inspecting the durability, anti-corrosion and simulation effect of anticondensation on superhydrophobic surfaces, improve the durability and anti-corrosion of superhydrophobic surfaces, and try our best to fulfill the appliance value of superhydrophobic surfaces. Acknowledgements This research was financially supported by National Natural Science Foundation of China (No. 51076011). References [1] M.L. Ma, R.M. Hill, Superhydrophobic surfaces, Current Opinion in Colloid & Interface Science 11 (2006) 193e202. [2] Y. Ken, S. Ogata, 3-D thermodynamic analysis of superhydrophobic surfaces, Journal of Colloid and Interface Science 326 (2008) 471e477. [3] A.M. Samaha, H.V. Tafreshi, M. GadelHak, Superhydrophobic surfaces: from the lotus leaf to the submarine, Comptes Rendus Mecanique 340 (2012) 18e34. [4] L. Jiang, Superhydrophobic surface materials: from natural to biomimetic, Science & Technology Review 23 (2005) 4e8.
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[5] Z.G. Guo, W.M. Liu, B.L. Su, Superhydrophobic surfaces: from natural to biomimetic to functional, Journal of Colloid and Interface Science 353 (2011) 335e355. [6] Z.P. Fan, W.L. Liu, Z.J. Wei, J.S. Yao, X.L. Sun, M. Li, X.Q. Wang, Fabrication of two biomimetic superhydrophobic polymeric surfaces, Applied Surface Science 257 (2011) 4296e4301. [7] T. Young, An essay on the cohesion of fluids, Philosophical Transactions of the Royal Society of London 95 (1805) 65e87. [8] R.N. Wenzel, Resistance of solid surfaces to wetting by water, Industrial & Engineering Chemistry 28 (1936) 988e994. [9] A. Cassie, S. Baxter, Wettability of porous surfaces, Transactions of the Faraday Society 40 (1944) 546e551. [10] C. Wei, Y. Alexander Fadeev, M.C. Hsieh, et al., Ultra hydrophobic and ultra lyophobic surfaces: some comments and examples, Langmuir 15 (1999) 3395e3399. [11] M. Qi, Y.Y. Duan, X.D. Wang, et al., Does macroscopic flow geometry influence wetting dynamic? Journal of Colloid and Interface Science 362 (2011) 221e227. [12] X.H. Chen, Y.M. Ma, X.H. Li, et al., The influence of nano/micro-structure on superhydrophobicity of alumina surface modified by fluorine silane, Chemical Journal of Chinese Universities 12 (2004) 2303e2307. [13] S.X. Lu, Y.L. Chen, W.G. Xu, et al., Controlled growth of superhydrophobic films by solegel method on aluminum substrate, Applied Surface Science 256 (2010) 6072e6075. [14] S.Q. Liang, R.M. Wu, Z.Q. Yuan, C. Hong, Y. Ya, Preparation of superhydrophobic aluminum by a novel immersion method using potassium permanganate solutions, Procedia Engineering 27 (2012) 1752e1757. [15] P. Yin, G.L. Shen, H. Wang, Superhydrophobic surface technique for preventing condensation, HVAC, Hunan University 36 (2006). [16] Y.F. Zhang, X.Q. Yu, H. Wu, et al., Facile fabrication of superhydrophobic nanostructures on aluminum foils with controlled-condensation and delayedicing effects, Applied Surface Science 25 (2012) 8253e8257. [17] F.C. Wang, C.R. Li, Y.Z. Lv, et al., Ice accretion on superhydrophobic aluminum surfaces under low-temperature conditions, Cold Regions Science and Technology 62 (2010) 29e33. [18] T. He, Y.C. Wang, Y.J. Zhang, Q. Lv, T.G. Xu, T. Liu, Super-hydrophobic surface treatment as corrosion protection for aluminum in seawater, Corrosion Science 8 (2009) 1757e1761. [19] L.B. Feng, H.X. Zhang, P.Z. Mao, et al., Superhydrophobic alumina surface based on stearic acid modification, Applied Surface Science 257 (2011) 3959e3963. [20] X.D. Wang, D.J. Lee, X.F. Peng, et al., Spreading dynamics and dynamic contact angle of non-Newtonian fluids, Langmuir 23 (2007) 8042e8047. [21] X.D. Wang, Y. Zhang, D.J. Lee, Spreading of completely wetting or partially wetting power-law fluid on solid surface, Langmuir 23 (2007) 9258e9262. [22] M. Qi, Y.Y. Duan, X.D. Wang, Spreading of completely wetting, non-Newtonian fluids with non-power-law rheology, Journal of Colloid and Interface Science 348 (2010) 250e254. [23] M. Jakob, Heat transfer in evaporation and condensation, Mechanical Engineering 58 (1936) 729e739. [24] G. Tammann, W. Boehme, Die Zahl der waterdropfchen bei der condensation and verschiedenen festen stoffen, Annalen der Physik 5 (1935) 77e80. [25] X.M. Wu, W.C. Wang, Theoretical analysis of initial behavior of frost formed on a cold surface, Journal of Engineering Thermophysics 24 (2003) 286e288.
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Analysis of anti-condensation mechanism on superhydrophobic anodic aluminum oxide surface Yanpeng Wu*, Chaoying Zhang School of Mechanical Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Prepare superhydrophobic surface on anodic aluminum oxide surface. Analyze the reason of anticondensation on superhydrophobic surfaces. The density of droplets formed on superhydrophobic surfaces is low. Droplets on superhydrophobic surfaces are easy to detach. This research can solve some problems of equipment using in HVAC systems.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2012 Accepted 26 January 2013 Available online 15 February 2013
Wetting theory about superhydrophobic surfaces reveals that hydrophobicity of surfaces has great relationship with surface roughness and surface free energy. Adopt electrochemical plus fluorine silane modified method to prepare superhydrophobic surface on anodic aluminum oxide surface, which not only enhances surface roughness, but also reduces surface free energy, even the static contact angle can reach 159.2 and anti-condensation is authenticated. Based on the experimental findings, analyze the reason of anti-condensation on superhydrophobic surfaces: one is that the density of droplets formed on superhydrophobic surfaces is low and the number of droplets is little; the other is bigger static contact angle and smaller rolling angle on superhydrophobic surfaces make droplets easy to detach on smaller tilt angle. This research can solve some condensation problems of equipment using in HVAC systems, such as heat exchangers in air conditioning system, cold radiation boards, air supply outlets, and so on. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Superhydrophobic surfaces Contact angle Anti-condensation Detachment
1. Introduction In recent years, people have devoted great attention to superhydrophobic study on the solid surface, and have made significant progress [1e3]. When dripping droplet on the solid surface, forming solideliquidegas three-phase interface and reaching energy
* Corresponding author. Tel.: þ86 13718105898. E-mail address: [email protected] (Y. Wu). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.01.048
balance, the intersection angle from solideliquid interface through internal liquid to gaseliquid interface is called contact angle. The contact angle of droplets on the hydrophobic surface is greater than 90 , contact angle on the superhydrophobic surface is greater than 150 . Superhydrophobic surfaces are extremely common in nature, like the surface of lotus leaf, the feet of water skippers, the wing of butterflies, and the feather of waterfowls [4]. Observing the surface of lotus leaf, a lot of micron small papillae can be seen, between which covered by nano crystalline wax. And this micron-nano quadratic structure made lotus leaf own superhydrophobicity. Hence, inspired by lotus leaf, people realized that the preparation of
Y. Wu, C. Zhang / Applied Thermal Engineering 58 (2013) 664e669
surface micron-nano structure is the key to prepare superhydrophobic surface [5,6]. The features of superhydrophobic surfaces, including anticondensation property, have great contact with surface wettability. Generally speaking, surface wettability is spreading characteristics of droplets on the solid materials surface between solid and liquid interfaces. At present, the basic theory that can explain the surface wettability phenomenon mainly includes: Yong’s equation [7], Wenzel equation [8] and CassieeBaxter equation [9]. Their model diagrams are shown as follows in Fig. 1(aec) (Table 1). Among them, Yong’s equation (Eq. (1)) only applies to the ideal solid surface and indicates that contact angle is related to surface tension. Wenzel equation (Eq. (2)) aims to the wetting sexual behavior whose actual solid surface has surface composition of heterogeneous and structure of rough. It indicates that contact angle is not only related to surface tension, but also surface roughness. Cassie and Baxter found CassieeBaxter equation (Eq. (3)) is suitable for any composite surface contact by analyzing from the viewpoint of thermodynamics. These theories suggested that solid surface energy and surface micro-structure had effects on the surface wettability.
cos q0 ¼ ðgSV gSL Þ=gLV
(1)
cos qCB ¼ rðgSV gSL Þ=gLV ¼ r cos q0
(2)
cos qCB ¼ f1 cos q1 þ f2 cos q2
(3)
Roughness factor is the ratio of the actual contact area of the solideliquid interface and the apparent solideliquid interface contact area. Because r 1, increasing surface roughness could make the hydrophobicity of hydrophobic surface much stronger. These theories interpreted the mechanism of superhydrophobic surface wettability constantly deeply, completely and accurately, but just started from a static point of view. In 1999, Chen and others put forward that the static contact angle was not enough to describe the super-hydrophobic properties of the material surface [10]. It is needed to consider the surface of the dynamic process to determine the surface hydrophobicity. The rolling angle is the product of this theory. It is the critical angle of droplets occurred just rolling on the inclined surface, formed by the inclined surface and horizontal plane. In addition, the relationship between the dynamic contact angle and the velocity of the moving contact line indicates that the macroscopic flow geometry does not influence the advancing dynamic wetting behavior of Newtonian fluids, but does influence the advancing dynamic wetting behavior of non-Newtonian fluids [11]. In summary, the apparent contact angle and rolling angle are two important parameters to describe the surface hydrophobicity status. Surface roughness and surface free energy are two important factors which can affect the performance of surface hydrophobicity. The greater surface roughness is, the lower surface free energy is, and the apparent contact angle will be greater, rolling angle will be smaller, and the surface hydrophobicity will be better.
665
Table 1 Parameters in equations of wetting model. Parameters
Nomenclature
Units
Solid-volume surface tension Solideliquid surface tension Liquid-volume surface tension Equilibrium contact angle or material intrinsic contact angle Apparent contact angle Roughness factor Intrinsic contact angle of two mediums Area ratio of two mediums contact with the liquid
gSV gSL gLV q0
mN/m mN/m mN/m
qCB
r
e
q1/q2
f1/f2
e
Chen Xinhua et al. [12] used chemical corrosion method and modification by fluorosilane to obtain superhydrophobic alumina surface and verified the contact angle on micron-nano level alumina surface with fluorosilane modified is bigger than which on smooth surfaces and nano-hole surfaces. Shixiang Lu et al. [13] used solegel method to get superhydrophobic aluminum surface which has strong stability. Shuquan Liang et al. [14] developed a simple method of acid etching and surface passivation to fabricate superhydrophobic surface on polycrystalline aluminum alloy, and analyzed 84% of the superhydrophobic phenomenon was in contact with air cushion. This article adopted electrochemical plus fluorosilane modified method to make anodic aluminum oxide superhydrophobic paint coat which is usually used on heat exchangers in air conditioning system. On one hand, this method used electrochemical method to construct micro structure on surfaces, and on the other hand, used fluorosilane modification to reduce the surface energy. The prepared paint coat made anodic aluminum oxide surface achieve superhydrophobic status, which could improve the condensation on heat exchangers in air conditioning system. Yin Ping et al. [15] proposed superhydrophobic surface could thoroughly prevent the cold surface condensation in air conditioning system, created superhydrophobic polymer point coat on aluminum surface via phase separation technology, applied in air conditioning cold surface, and had good anti-condensation effect. Through temperature control, Youfa Zhang [16] found that superhydrophobic aluminum surface formed dew rarely, and when experimental temperature was cooled to 0 C, it would not freeze later than 30 min. Fochi Wang et al. [17] used experiments to prove that superhydrophobic aluminum surface could prevent freeze. Tian He et al. [18] prepared “Underwater superhydrophobic” surface onto the anodized aluminum which was treated as corrosion protection. Libang Feng [19] used polyethylene absorb plus stearic acid layer modification method to produce alumina surface superhydrophobic paint coat and analyzed its superhydrophobic mechanism. However, these researches mentioned above don’t include the anti-condensation mechanism of anodic aluminum oxide superhydrophobic surface. Hence, based on the production of anodic aluminum oxide superhydrophobic surface, this article analyzes anti-condensation mechanism of superhydrophobic surface.
Fig. 1. (a) Yong’s model diagram. (b) Wenzel model diagram. (c) CassieeBaxter model diagram.
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Anodic aluminum oxide is widely used in HVAC system, such as large heat exchangers, freezing containers, low temperature facilities, containers, etc. Especially, anodic aluminum oxide owns important theoretical research value and practical application future in the application of heat exchangers in air conditioning system. 2. Preparation of superhydrophobic surface using electrochemical method Superhydrophobic surface can be prepared via two methods, one is to change surface roughness by constructing micron and nanometer level roughness structure, the other is to reduce surface free energy by using low surface energy substance to modify the micron and nanometer level roughness structure. Wherein, the method of preparing suitable micron and nanometer level roughness structure is the key procedure. Classified by preparation method, there are vapor induce phase separation method, stencil printing method, electrospinning method, solegel method, extrusion method, laser and plasma etching method, self-assembly method, stretching method, corrosion method, etc. This article adopted electrochemical method to prepare superhydrophobic surface on anodic aluminum oxide and used fluorine silane to modify parts of the sample. After preparation, measure contact angles and investigate surface hydrophobicity especially the anti-condensation performance. Electrochemical method is using modern chemical method to form rough porous morphology on surface. 2.1. Experimental content 2.1.1. Laser etching Adjust the width of pulse laser light spot to change the width of micro etching groove on surface, adjust laser pulse energy and repeat the etching to change the etching depth on surface, and move the sample to change the etching size. 2.1.2. Fluorine silane modification Clean the samples for 30 min and dry them in 100 C oven. Take 3 pieces of them to soak in 2% fluorine silane CF3(CH2)8CH2Si(OCH3)3 solution for 2 h, and take another 3 pieces without any measures. Finally, dry these 6 samples in 140 C for 1 h.
Fig. 2. Optical microscope image of anodic aluminum oxide using electrochemistry.
The morphology of water-drop on sample surface is shown in Fig. 5. Therefore, the surface of anodic aluminum oxide using electrochemistry has micro nanometer rough structure, which can own hydrophobic property. After anodic aluminum oxide is modified by low surface energy fluorine silane, its surface free energy decreases, and hydrophobic property has promoted to superhydrophobic property. 2.4. Surface condensation experiment Select two experiment samples: one piece is anodic aluminum oxide sample A etched by laser and modified by fluorine silane, the other piece is none processed anodic aluminum oxide sample B. Put two samples in refrigerator (temperature 15 C) for 15 min, use steam humidification method to control the humidity of transparent sealed experimental box, and make the relative humidity reach 80%. Take the sample from the refrigerator, and put them into the environment experimental box. Observe the surface condensation phenomenon of two samples after 10 min. Sample A’s surface almost has no droplets, no condensation, sample B’s surface has variable sizes droplets and obvious condensation phenomenon. Experimental results explicate that anodic aluminum oxide sample via electrochemical and fluorine silane modified has wonderful anti-condensation property.
2.2. Apparent morphology observation The optical microscope image of electrochemical anodic aluminum oxide surface is shown in Fig. 2. The electrochemical method adopted in the experiment constructs micro nanometer level structure in anodic aluminum oxide, and increases the surface roughness and the air cavitation area fraction. According to Cassiee Baxter theory, this method will promote the surface hydrophobic property.
3. Anti-condensation mechanism on superhydrophobic surfaces When the surface temperature of objects is below the dewpoint temperature of air in surroundings, moist air will condense on the low-temperature surfaces, which is called condensation.
2.3. Surface contact angle test Contact angles of 6 samples were measured by angle meter OCA15 (Seiko, Instruments Inc., Japan). As shown in Fig. 3, pick 3 points on each sample surface, and take the average value as the final contact angle. Test results refer to Fig. 4. As the test results shown in Fig. 4, the contact angles of 1, 2, 3 samples modified by fluorine silane are all greater than the contact angles of 4, 5, 6 samples which did not modify by fluorine silane. The CA average value of sample 1, 2, 3 is 159.2 , which reach the superhydrophobic status. The CA average value of sample 4, 5, 6 is 141.6 , which reach the hydrophobic status but not satisfy the superhydrophobic status.
Fig. 3. Selection of sample contact angle test points.
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grow. The change of free energy DGC caused by forming critical liquid core across critical radius is called thermodynamics potential barrier or energy barrier. Therefore, droplets must surmount the energy barrier. However, the value of DGC is closely related to surface contact angle, which is shown below Eqs. (4) and (5) [25]:
DGC ¼ ð4p=3Þf2svl =½rl Rw TlnðP=Psl Þg2 svl f ðqÞ
(4)
f ðqÞ ¼ ð2 þ cos qÞð1 cos qÞ =4
(5)
2
In Eq. (4), rl is the density of the droplets, T is the temperature of the system, P is the pressure of water vapor in the system, Psl is water vapor’s saturated partial pressure consisted with the temperature T when gaseliquid two phases are plane. Because these testings are under the same condition, and the above parameters can be regarded as constants, so Eq. (5) can be simplified as following: Fig. 4. Contact angle test of 6 anodic aluminum oxide samples.
DGC ¼ Cf ðqÞ Condensation commonly exists on some cold areas, such as indoor natatorium, buildings’ envelope, air-conditioning system and so on. In HVAC system, condensation mainly occurs on the surfaces of the fins and pipes of heat exchangers, cold radiation boards, cooling roof, air supply outlets, air ducts and water ducts. The wetting theories of liquid on solid surface are spreading theory and detachment theory. Wang had done many research on spreading theory, such as spreading dynamics and dynamic contact angle of non-Newtonian [20], spreading of completely wetting or partially wetting power-law fluid on solid surface, spreading of completely wetting [21], non-Newtonian fluids with non-powerlaw rheology [22]. However, surface condensation is about detachment theory. Moist air forms lots of tiny initial droplets first, and these droplets grow bigger directly or merge to grow. When these droplets grow to a certain size, the balance between the external force and surface adhesion suffered by droplets will be destroyed, and then the droplets detach away. There are two theories about the formation of initial droplets: one is membrane rupture hypothesis supplied by Jakob [23], which considered that a layer of liquid film was formed on the surface at first, and when the thickness of liquid film came to critical thickness, liquid film cracks, and the fragments of liquid film contract to initial tiny droplets; the other is fixed nucleating center hypothesis supplied by Tammann and Boehme [24], which considered that there were condensation nucleus where initial tiny droplets formed firstly existed randomly on surfaces. Only the size of initial droplets is greater than the critical size, initial droplets can form, exist, and
(6)
Known by Eq. (6), DGC is in proportion to f(q). Make C ¼ 1 (as Fig. 6), DGC increases as q increasing. For example, q ¼ 0 , DGC ¼ 0; q ¼ 70 , DGC ¼ 0.23C; q ¼ 110 , DGC ¼ 0.23C; q ¼ 150 , DGC ¼ 0.98C. As q increasing, the nucleation potential barrier of droplets increases, and the energy barrier increases which should be overcome in droplets forming. The contact angle on superhydrophobic surface is greater than 150 which is much bigger than that on common surfaces, so the energy barrier in droplets forming is greater, and droplets are difficult in forming process. As a result, the droplets density of superhydrophobic is smaller than that of common surfaces, which is one reason of anti-condensation of superhydrophobic surface. When droplets grow to a certain critical size, under external force, droplets will detach from surfaces. The detachment diameter of droplets in vertical surfaces is the least. In HVAC systems, the condensing surfaces of equipment are almost vertical, slant or placed down, and these situations are of benefit to the detachment of droplets. The detachment way of droplets on superhydrophobic is rolling off, which is the best and fastest detachment way. Establish the droplet detachment model on slant superhydrophobic surfaces, as shown in Fig. 7. The slant angle of superhydrophobic surface is a, gravity acting on droplets detachment is FG, the width of contact area of droplets and surfaces is w, advancing angle and receding angle of droplets are qA and qR, gLV is tension between solid and liquid, r is droplets density, V is droplets volume, g is gravity acceleration, and their relationship can be described as following:
Fig. 5. Morphology of water-drop on anodic aluminum oxide hydrophobic surface (left: modified by fluorine silane, right: unmodified).
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Fig. 8. Droplet’s shape on horizontal superhydrophobic surface.
As Eq. (13) indicates that on the condition of the same parameters of droplets, surface critical slant angle aC is relevant with q0, qA and qB. Consider the influence on aC by q0, take q0 as the single variable, assume constant term of other parameters as 0.3, and Eq. (14) will be received.
Fig. 6. Relationship of DGC and q.
FG ¼ rVg sin a
(7)
V ¼ ðp=3Þ½w=ð2 sin q0 Þ
FG =w ¼ gLV ðcos qR cos qA Þ
(8)
sin ac ¼ 2gLV sin q0 ðcos qR cos qA Þ io1=3 n h =rg 3= pV 2 2 3cos q0 þ cos3 q0
The shape of droplets on horizontal superhydrophobic surfaces is similar to segment (Fig. 8). Approximately regard the volume of droplets on horizontal surfaces as the same on slant surfaces. In hence, the volume of droplets V is:
V ¼ ph ðR h=3Þ 2
(9)
h ¼ R þ R cos b ¼ Rð1 þ cos bÞ ¼ Rð1 cos q0 Þ
(10)
w ¼ 2R$sin q0
(11)
Eq. (12) is derived from Eqs. (9)e(11). When droplets detach, the relationship (Eq. (13)) between critical slant angle and contact angle is deduced by Eqs. (7), (8) and (12).
Fig. 7. Droplet detachment model on slant superhydrophobic surface.
3
2 3 cos q0 þ cos3 q0
1=3 sin ac ¼ 0:3 sin q0 2 3 cos q0 þ cos3 q0
(12)
(13)
(14)
In Fig. 9, as static contact angle q0 increasing, the sine of surface tilt angle sin ac is decreasing, that is surface tilt angle decreasing (0 < ac < 90 ). Therefore, as for hydrophobic surfaces, static contact angles much greater, droplets much easier to drop down. Advancing angle and receding angle are related factors, and Dq ¼ qA qR expresses contact angle hysteresis, which has relation with rolling angle (Eqs. (7) and (8)). However, rolling angle is surface tilt angle when droplets rolling on slant surfaces. Hence, the weaker contact angle hysteresis is, the smaller rolling angle is, and the speed of droplets rolling from slant surfaces will be much faster. The static contact angle of droplets on superhydrophobic surfaces is greater than 150 , rolling angle is smaller than 10 , so the
Fig. 9. Relationship of surface tilt angle aC and static contact angle q0.
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surface tilt angle which droplets falling off needed is much less than that on common surfaces. That is to say, in the same slant angle, droplets on superhydrophobic are easier to drop, and this is another reason of anti-condensation on superhydrophobic surfaces. 4. Conclusions Enhancing surface roughness and reducing surface free energy can improve hydrophobicity of surfaces. This paper adopts electrochemical plus fluorine silane modified method to prepare superhydrophobic surface on which the static contact angle can reach 159.2 , and anti-condensation is authenticated. Based on the experimental results, anti-condensation on superhydrophobic surfaces is affected by low density and hard existence. One reason is that static contact angle on superhydrophobic surfaces is greater, the energy barrier that droplets forming need to overcome is larger, and droplets are difficult to form and the density is low; the other reason is bigger static contact angle and smaller rolling angle on superhydrophobic surfaces make droplets hard to exist and easy to detach on smaller tilt angle. On the basis of analyzing anti-condensation mechanism on superhydrophobic surfaces, we will focus efforts on the appliance of anti-condensation to HVAC systems, continue inspecting the durability, anti-corrosion and simulation effect of anticondensation on superhydrophobic surfaces, improve the durability and anti-corrosion of superhydrophobic surfaces, and try our best to fulfill the appliance value of superhydrophobic surfaces. Acknowledgements This research was financially supported by National Natural Science Foundation of China (No. 51076011). References [1] M.L. Ma, R.M. Hill, Superhydrophobic surfaces, Current Opinion in Colloid & Interface Science 11 (2006) 193e202. [2] Y. Ken, S. Ogata, 3-D thermodynamic analysis of superhydrophobic surfaces, Journal of Colloid and Interface Science 326 (2008) 471e477. [3] A.M. Samaha, H.V. Tafreshi, M. GadelHak, Superhydrophobic surfaces: from the lotus leaf to the submarine, Comptes Rendus Mecanique 340 (2012) 18e34. [4] L. Jiang, Superhydrophobic surface materials: from natural to biomimetic, Science & Technology Review 23 (2005) 4e8.
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