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Bubble formation on superhydrophobic-micropatterned copper surfaces
Applied Thermal Engineering 35 (2012) 112e119
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Bubble formation on superhydrophobic-micropatterned copper surfacesq Xinwei Wang a, Siwei Zhao b, Hao Wang a, *, Tingrui Pan b, ** a b
Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing, China Department of Biomedical Engineering, University of California, Davis, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 March 2011 Accepted 10 October 2011 Available online 17 October 2011
Surface physicochemical properties, including wettability and micro-nanoscopic roughness, play an important role in boiling heat transfer and interfacial phenomena. In the paper, we report investigation on bubble formation over superhydrophobic-micropatterned copper surfaces. The distinctive nonwetting micropatterns (of 180 180 mm2 squares) were fabricated by our recently reported stereomask lithography process, using a novel superhydrophobic nanocomposite formulation. The superhydrophobic nanocomposite, comprised of polytetrafluoroethylene (PTFE) nanoparticles (of 250 nm in diameter) in a polymeric matrix, presented high degree of hydrophobicity (with water contact angle > 150 ). Standard boiling processes were studied with or without a prior-degassing procedure, experimentally. In addition, experiments on uniform superhydrophobic coating as well as bare copper surfaces were conducted as control. The experimental investigations revealed that the micropatterncoated copper surfaces had low bubble formation temperatures, similar to the uniformly coated superhydrophobic surfaces; and those emerging bubbles were more spherical and less likely to merge into a vapor layer. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Micropatterning Superhydrophobicity Boiling heat transfer Bubble formation Nucleation
1. Introduction Boiling, a highly efficient yet complicated phase transition process, has extensive linkage to various important engineering fields, including thermodynamics, heat transfer, interfacial physicochemistry. Despite numerous investigations since 1930s [1e5], quantitative descriptions and mathematical models on the boiling mechanism are still far from conclusive. Although progresses on single bubble dynamics have been made to reveal the individual processes of bubble nucleation, growth and detachment [6,7], the detailed description of initial nucleation conditions remains unresolved [8,9], which is substantially influenced by solid surface properties (e.g., energy and roughness) and surrounding thermodynamics (e.g., temperature gradient and heat flux). In particular, surface wettability has been shown with considerable impacts on the interfacial phase transition according to recent studies [10e16]. It is generally accepted that hydrophilic surfaces enhance the critical heat flux (CHF), whereas hydrophobic surfaces reduce the initial temperature needed for bubble q Revised for publication in Applied Thermal Engineering, August 2011. * Corresponding author. Tel.: þ86 10 82529060. ** Corresponding author. Tel.: þ1 530 754 9508. E-mail addresses: [email protected], [email protected] (H. Wang), [email protected] (T. Pan). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.10.012
nucleation (i.e. the nucleation temperature) [10,11,16]. For instance, surfaces coated with TiO2 thin films, exhibiting extremely low water contact angles, have been reportedly shown improved CHF [10,11]. Recently, nanostructured surfaces with ultimate topological control have gained increasing interest for enhanced mass and hear transfer. Forrest et al. [12] carried out a boiling analysis on silica nanoparticles-coated nickel wires. The thin-film superhydrophobic coating served as a high-performance heat-conducting interface, which offered a high advancing angle to enhance the nucleate boiling heat transfer while promoting the CHF at a reduced receding angle. In another study, Nam and Ju established [14] a minimum surface superheat of w9 C for bubble formation on a nanostructured copper surface. The authors speculated existence of nanoscale gaseliquid interface on the surface with high contact angle as the primary reason of enhanced heat transfer. A similar observation was also made by Ryan and Hemmingsen [15]. More recently, Phan et al. [16] confirmed the role of surface hydrophobicity in reducing the nucleation temperature. Microfabrication has been initially introduced as a highprecision machining tool to form microscale cavities on solid substrates, allowing localized bubble formation and microscopic observation and analysis [17e26]. For instance, Shoji conducted a series of boiling experiments on micro-cavities with different shapes and dimensions. Conical and cylindrical cavities were
X. Wang et al. / Applied Thermal Engineering 35 (2012) 112e119
113
Fig. 1. Experimental setup.
found having different bubbling frequencies and requiring different superheats [17e19]. In this paper, we have fully utilized microfabrication technique to pattern a superhydrophobic nanocomposite material on copper surfaces to promote bubble formation as well as to facilitate bubble detachment. Characterization of bubble formation and heat transfer on the superhydrophobic-micropatterned surfaces has been thoroughly conducted with or without prior-degassing treatment. In addition, the same experiments have been repeated on a uniform superhydrophobic coating and a bare copper surface for comparisons. 2. Experimental setup The commercially available polytetrafluoroethylene (PTFE) nanoparticles provide a robust superhydrophobic coating once sprayed on the copper surface due to their superb chemical inertness (surface energy of 18.6 mJ/m2) and nanoscale topography (250 nm in diameter). The trapped air bubbles on superhydrophobic coating not only protect the copper surface from being wetted, but more importantly facilitate bubble formation at a lower temperature. 2.1. Fabrication of superhydrophobic micropatterns The superhydrophobic nanocomposite formulation was made from a simple mixture of commercially available polytetrafluoroethylene (PTFE) nanoparticles with a thermally curable polymer matrix [21e23]. In brief, the PTFE-based nanocomposite consisted of 8.5% w/w PTFE nanoparticles of 250 nm in diameter, 16.9% toluene, 47.5% hexane, 22.5% cyclohexane, 4.1% PDMS
Fig. 2. Square superhydrophobic micro sites on the copper surface (a) and the contact angle of water on the superhydrophobic coating (b).
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X. Wang et al. / Applied Thermal Engineering 35 (2012) 112e119
subsequent coating (the detailed protocol of dry-film fabrication can be found in the previous publications [24e26]). In the following step, spray coating technique was applied to establish unique coating over the entire surface area, followed by thermally curing the nanocomposite formula at 120 C for 1 h. Finally, the shadow mask was removed, leaving PTFE micropatterns in the through-hole area on the copper surface. Overall, the superhydrophobic micropatterns were established on a rectangular copper sheet of 60 mm in length, 50 mm in width and 0.6 mm in thickness. 2.2. Measurement of boiling heat transfer As shown in Fig. 1, the experimental evaluation system of bubble formation was comprised of a testing surface, a heating apparatus and a temperature measurement setup. A hot plate was utilized to heat a copper block as a homogenous heat source, which was directly connected to the evaluation surface. A K-type thermocouple was attached to the bottom of the evaluation surface through a shallow trench drilled through the copper block, which continuously monitored the surface temperature by connecting to a multi-channel data logger (Omega OM-DAQPRO-5300). A high-speed CCD camera (XMotion, AOS Technologies) was mounted on a microscope (Ti-U Inverted, Nikon) for continuous imaging of bubble formation. De-ionized (DI) water was used as the working fluid, and degassed by boiling for selected experiments for 1e2 h prior to the measurement. The degassing of the surface was accomplished by storing the sample in a vacuum desiccator for 45 min (The patterned surface is shown in Fig. 2). 3. Results and discussion Table 1 summarized a series of boiling experiments conducted on the superhydrophobic-micropatterned copper surfaces (Tests 1e4) and on the controls (Test 5 on uniform superhydrophobic coating, and Tests 6 and 7 on bare copper surfaces). 3.1. Boiling on the micropatterned surface
Fig. 3. Bubbles on the hydrophobic sites in test 1 (without degassing) when the wall temperature was 41.5 C (a), 52.5 C (b), and 67.3 C (c).
base polymer and 0.5% PDMS curing agent. The PDMS matrix provides excellent adhesion among nanoparticles and substrates [23]. A microfabricated dry-film shadow mask with throughholes was first laminated on the copper substrate for the
In Test 1, neither the working fluid nor the surface was degassed. When the surface temperature reached 41.5 C, bubbles started forming on the superhydrophobic patterns as shown in Fig. 3a. The bubbles were initially well positioned on the micropatterns and no bubbles appeared on the bare copper surface. As the surface temperature further increased, the bubbles grew over the micropatterned region, as shown in Fig. 3b. Therefore, the micropatterns served as effective nucleation sites. The bubble growth on micropatterns can be divided into three stages. First of all, the triple line of the embryo gas bubble expanded till meeting with the wetting boundary of the
Table 1 Summarization of the tests. Test No.
Water degas
Surface degas
Surface
Bubble formation temperature ( C)
1 2 3 4 5 6 7
No No Yes Yes Yes No Yes
No Yes No Yes Yes No Yes
Patterned surface Patterned surface Patterned surface Patterned surface Uniform superhydrophobic Bare copper Bare copper
40 62 82 85 85 62 124
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115
Fig. 4. Bubbles growth from hydrophobic sites in test 4 (with degassing on both the surface and water).
superhydrophobic micropatterns. During the second stage, the bubble was observed to grow vertically with the contact line pinned at the edge. At the last stage, the expanded bubbles merged together in the adjacent micropatterns and eventually detached from the micropatterned surface. In comparison, the bubble growth patterns were completely different when the degassing was performed prior to the boiling tests, as illustrated in Fig. 4. As can be seen, bubbles initially formed at 85 C, which was a much higher nucleation temperature than that of Test 1 (41.5 C). In the following stage, most bubbles randomly appeared with different sizes along the borders of the micropatterned regions, instead of attaching to the center area. The unexpected phenomena indicated the importance of edge effects of the micropatterns, which facilitated bubble formation, and could be possibly explained by variation in the coating thickness, which was typically thinner along the edge and thus led to higher nucleation temperature (with less thermal conductive resistance). In addition, the border region approached to cavitation formation, which further reduced bubble formation temperature. Furthermore, the topology variation of the micropatterned superhydrophobic surface was confirmed by the atomic force microscopic images (MFP-3D-BIO, AR) as shown in Fig. 5. It showed that the micropattern boundaries presented higher roughness than that of the central region, which could be used to possibly explain the preferred bubble formation along the edges. More detailed discussion will be included in Section 3.4. One corner of the superhydrophobic micropattern is shown in Fig. 5a. The scan size is 40 40 mm within which the area of the scanned corner is about 25 25 mm and the rest is the bare copper surface. Fig. 5c is the corresponding 3D view which shows how the structures are rooted to the copper surface. Fig. 5b shows the height information along line a-a in Fig. 5a. Fig. 6 shows the scanning image of the superhydrophobic
micropattern in the central area. By comparing Fig. 5 with Fig. 6, it implies that the border region has more valleys than the central area. In addition, low thermal conductivity of the micropattern could be an alternative reason. The PTFE material possesses more than 1600 times lower thermal conductivity than that of copper (kPTFE ¼ 0.25 W/(m K), kCu ¼ 400 W/(m K)). That is to say, given the same surface area, the thermal conductance of the 5 mm PTFE coating is on the same order of that of an 8 mm thick copper block. Therefore, the border region may experience higher temperature. 3.2. Boiling on the uniform superhydrophobic surface On the surface coated with uniform superhydrophobic coating, bubbles appeared randomly on the surface and spread out with arbitrary shapes, as shown in Fig. 7. The bubbles tended to coalesce and form a continuous vapor film above the surface, which further impeded mass and heat transfer, similar as the previously described [11,15]. Overall, the surface coating posed a strong impact on the bubble formation and growth. 3.3. Bubble formation temperature Bubble formation temperatures were recorded on different boiling surfaces with or without degassing treatment. Table 1 summarized the measurement data from seven test sets. The bubble formation temperatures were measured to be 40 C, 62 C, 82 C, 85 C, 85 C, 62 C and 124 C for Tests 1e7, respectively. In the tests, three potential factors could influence the bubble formation temperature, i.e., the surface wettability, the degassing of the working fluid, and the degassing of the surface. Test 1 had the lowest bubble formation temperature since neither the
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Fig. 5. AFM scanning result of a site corner and the neighboring copper surface: (a) top view, (b) the height curve along the red line a-a in (a); (c) 3D view. (Scan size 40.00 mm; Scan rate 0.10 Hz; Scan points 256; Scan lines 256; Imaging mode AC). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
superhydrophobic coating nor the working fluid was degassed. While in Test 2 the degassing of the surface elevated the nucleation temperature to 62 C. In comparison, the degassing of water showed a stronger impact on the nucleation temperature (increasing to 82 C). This could be explained by the fact that even though the surface was not completely degassed, the trapped gases still tended to dissolve into the degassed bulk fluid, as shown in Fig. 8. Furthermore, in Test 5 when the surface and the working fluid were both degassed, using the surface uniformly covered by the superhydrophobic PTFE coating, the resulted nucleation
temperature was founded consistently at 85 C. This result was found to be highly consistent with that on the micropatterned surface in Test 4, indicating that in the present work the micropatterned coating was as good as the uniform superhydrophobic coating in lowering the bubble formation temperature. For the bare copper surface used in Test 6, no degassing treatment was applied, which led to the bubble formation at 62 C, the same as that of Test 2. However, with degassing treatment in Test 7, the nucleation temperature rose sharply to 124 C, with a superheat of 24 C. Comparing Tests 4 and 7, it was found that the
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Fig. 6. AFM scanning result of a site center region: (a) top view, (b) height curve along the red line in (a); (c) 3D view. (Scan size 20.00 mm; Scan rate 0.10 Hz; Scan points 256; Scan lines 64; Imaging mode AC). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
hydrophobic micropatterns could considerably lower the bubble formation temperature by about 40 C. 3.4. Discussion As an embryo bubble grows and reaches the cavity opening, the gaseliquid interface becomes convex as shown in Fig. 9a, the mechanical equilibrium can be described by the following equation:
Pv þ Pg ¼ Pl þ
2s r
(1)
where Pg and Pv indicate the partial pressures of gas and vapor, respectively. Pl stands for the liquid pressure, s represents the surface tension, and r is the interfacial radius. Combining Eq. (1) with ClausiuseClapeyron equation gives rise to the necessary superheat of bubble formation as:
Tsat nlv 2s Tl Tsat ðPl Þ> Pg hlv rmin
(2)
where Tsat(Pl) is the saturation temperature, vlv indicates the difference between the specific volume of the saturated vapor and
liquid, and hlv is the latent heat. The maximum value of 1/rmin occurs when the embryo bubble just reaches the cavity opening with an infinite radius of curvature and can be approximated by sin q/R, where R is the radius of cavity mouth, as shown in Fig. 9. Therefore, only if the gas pressure Pg is greater than 2s/Rsinq the superheat for bubble formation can be a negative number. In Test 4, the superheat was measured negative 15 C. Given q ¼ 150 , s ¼ 61.7 103 N/m and R is about 1 mm, Eq. (2) predicts the partial pressure inside the gas bubble would be greater than 61.7 kPa, which is comparable to the saturation vapor pressure Pv at 85 C (62.7 kPa). Thus, the non-condensable nature of gas may play a critical role in the bubble formation on the superhydrophobic surface. Furthermore, two mechanisms could result in the presence of non-condensable gas. First of all, gas can be physically trapped/adsorbed to the superhydrophobic surface [15,16]. Phan et al. [17] recently confirmed decreased bubble formation temperatures on a smooth hydrophobic surface. In addition, the concave boundary region may effectively trap gas, which can be theoretically predicted by the following equation:
Pv þ Pg ¼ Pl
2s r
(3)
Therefore, the capillary pressure could prevent working fluid from spreading/wetting into the bottom of the cavity, since the internal gas pressure rises sharply when the interfacial radius approaches to zero. Test 4 implies that the boundary of
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Fig. 8. Dissolving of the gas bubbles on the hydrophobic sites into the degassed bulk in test 3.
Fig. 7. Bubble growth on the uniform superhydrophobic surface in test 5 (with degassing of both the surface and the water).
micropatterns facilitates the bubble formation. Comparison of the AFM result in Fig. 5b to that in Fig. 6b shows that the edge region could present more gas-trapping cavities than those of the central area. However, it is still unclear how to quantitatively evaluate the amount of cavitation effects on either a smooth or nanostructured hydrophobic surface [15e17]. Further investigation using highpower high-speed microscopic analysis is currently under way. 4. Conclusions In summary, micropatterned superhydrophobic coating on copper surfaces was able to facilitate boiling heat transfer. Boiling
Fig. 9. Diagrams showing an embryo bubble inside a hydrophobic cavity: (a) inside, (b) at the mouth.
X. Wang et al. / Applied Thermal Engineering 35 (2012) 112e119
process on a uniform superhydrophobic-coated surface and a bare copper surface were also studied under the same conditions for comparison. The micropatterned superhydrophobic surface effectively lowered the bubble formation temperature to 85 C whereas that of the bare copper surface stuck to around 124 C with considerable superheat. Moreover, the superhydrophobic micropatterns controlled the nucleation sites (bubbles tended to align up along the superhydrophobic boundaries), bubble shapes and growth rates, and prevented formation of a continuous vapor film which were typically developed on a uniform superhydrophobic coating. In addition, the degassing of the working fluid was found to be more effective than the degassing of the heat-conducting surface. Acknowledgements This work was supported in part by Natural Science Foundation of China (Grant: 50876001) and National Science Foundation (Awards: ECCS-0846502 and CMMI-0944353). The authors thank Ms. Lingfei Hong for providing guidance on preparing the superhydrophobic coating. References [1] M. Jacob, W. Fritz, Forsch. Geb. Ingenieurwes 2 (1931) 437e447. [2] M. Jacob, W. Linke, Heat transfer from a horizontal plate, Forsch. Geb. Ingenieurwes 4 (1933) 434. [3] R. Becker, W. Doring, Ann. Phys. (Leipzig) 24 (1935) 719. [4] S. Nukiyama, The maximum and minimum value of heat Q transmitted from a metal to boiling water under atmospheric pressure, International Journal of Heat and Mass Transfer 9 (1966) 1419e1433. [5] C.Y. Han, P. Griffith, The mechanism of heat transfer in nucleate pool boilingPart I: bubble initiation, growth and departure, International Journal of Heat and Mass Transfer 8 (1965) 887e904. [6] G. Son, V.K. Dhir, N. Ramanujapu, Dynamics and heat transfer associated with a single bubble during nucleate boiling on a horizontal surface, Journal of Heat Transfer 121 (1999) 623. [7] L. Dong, P.S. Lee, S.V. Garimella, Prediction of the onset of nucleate boiling in microchannel flow, International Journal of Heat and Mass Transfer 48 (2005) 5134e5149. [8] M. Blander, J.L. Katz, Bubble nucleation in liquids, AIChE Journal 21 (1975) 833e848. doi:10.1002/aic.690210502. [9] S.F. Jones, G.M. Evans, K.P. Galvin, Bubble nucleation from gas cavities a review, Advances in Colloid and Interface Science 80 (1999) 27e50.
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[10] Y. Wang, L.L. Wang, M.Y. Liu, Antifouling and enhancing pool boiling by TiO2 coating surface in nanometer scale thickness, AIChE Journal 53 (2007) 3062e3076. [11] Y. Takata, S. Hidaka, J.M. Cao, T. Nakamura, H. Yamamoto, M. Masuda, T. Ito, Effect of surface wettability on boiling and evaporation, Energy 30 (2005) 209e220. [12] E. Forrest, E. Williamson, J. Buongiorno, L.W. Hu, M. Rubner, R. Cohen, Augmentation of nucleate boiling heat transfer and critical heat flux using nanoparticle thin-film coatings, International Journal of Heat and Mass Transfer 53 (2010) 58e67. [13] W. Wu, H. Bostanci, L.C. Chowa, Y. Hong, M. Su, J.P. Kizito, Nucleate boiling heat transfer enhancement for water and FC-72 on titanium oxide and silicon oxide surfaces, International Journal of Heat and Mass Transfer 53 (2010) 1773e1777. [14] Y.S. Nam, Y.S. Ju, Bubble nucleation on hydrophobic islands provides evidence to anomalously high contact angles of nanobubbles, Applied Physics Letters 93 (2008) 103115-1e103115-3. [15] W.L. Ryan, E.A. Hemmingsen, Bubble formation in water at smooth hydrophobic surface, Journal of Colloid and Interface Science 157 (1993) 312e317. [16] H.T. Phan, N. Caney, P. Marty, S. Colasson, Surface wettability control by nanocoating: the effects on pool boiling heat transfer and nucleation mechanism, International Journal of Heat and Mass Transfer 52 (2009) 5459e5471. [17] M. Shoji, Y. Takagi, Bubbling features from a single artificial cavity, International Journal of Heat and Mass Transfer 44 (2001) 2763e2776. [18] M. Shoji, Studies of boiling chaos: a review, International Journal of Heat and Mass Transfer 47 (2004) 1105e1128. [19] R. Mosdorf, M. Shoji, Chaos in nucleate boiling-nonlinear analysis and modeling, International Journal of Heat and Mass Transfer 47 (2004) 1515e1524. [20] S. Siedel, S. Cioulachtjian, J. Bonjour, Experimental analysis of bubble growth, departure and interactions during pool boiling on artificial nucleation sites, Experimental Thermal and Fluid Science 32 (2008) 1504e1511. [21] L. Hong, T. Pan, Photopatternable superhydrophobic nanocomposites for microfabrication, Journal of Microelectromech. System 19 (2010) 246e253. [22] L. Hong, T. Pan, Three-dimensional surface microfluidics enabled by spatiotemporal control of unconventional fluidic interface, Lab on a Chip 10 (2010) 3271e3276. [23] L. Hong, T. Pan, Surface microfluidics fabricated by photopatternable superhydrophobic nanocomposite, Microfluidics and Nanofluidics (2010). doi:10.1007/s10404-010-0728-7. [24] S. Zhao, H. Cong, T. Pan, Direct projection on dry-film photoresist (DP2): do-ityourself three-dimensional polymer microfluidics, Lab on a Chip 9 (2009) 1128e1132 (Cover Image). [25] S. Zhao, A. Chen, A. Revzin, T. Pan, Stereomask lithography (SML): universal multi-object micro-patterning technique for biological applications, Lab on a Chip 11 (2010) 224e230 (Cover Image). [26] T. Pan, W. Wei, From cleanroom to desktop: emerging micro-nanofabrication technology for biomedical applications, Annals of Biomedical Engineering 39 (2010) 600e620.
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Bubble formation on superhydrophobic-micropatterned copper surfacesq Xinwei Wang a, Siwei Zhao b, Hao Wang a, *, Tingrui Pan b, ** a b
Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing, China Department of Biomedical Engineering, University of California, Davis, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 March 2011 Accepted 10 October 2011 Available online 17 October 2011
Surface physicochemical properties, including wettability and micro-nanoscopic roughness, play an important role in boiling heat transfer and interfacial phenomena. In the paper, we report investigation on bubble formation over superhydrophobic-micropatterned copper surfaces. The distinctive nonwetting micropatterns (of 180 180 mm2 squares) were fabricated by our recently reported stereomask lithography process, using a novel superhydrophobic nanocomposite formulation. The superhydrophobic nanocomposite, comprised of polytetrafluoroethylene (PTFE) nanoparticles (of 250 nm in diameter) in a polymeric matrix, presented high degree of hydrophobicity (with water contact angle > 150 ). Standard boiling processes were studied with or without a prior-degassing procedure, experimentally. In addition, experiments on uniform superhydrophobic coating as well as bare copper surfaces were conducted as control. The experimental investigations revealed that the micropatterncoated copper surfaces had low bubble formation temperatures, similar to the uniformly coated superhydrophobic surfaces; and those emerging bubbles were more spherical and less likely to merge into a vapor layer. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Micropatterning Superhydrophobicity Boiling heat transfer Bubble formation Nucleation
1. Introduction Boiling, a highly efficient yet complicated phase transition process, has extensive linkage to various important engineering fields, including thermodynamics, heat transfer, interfacial physicochemistry. Despite numerous investigations since 1930s [1e5], quantitative descriptions and mathematical models on the boiling mechanism are still far from conclusive. Although progresses on single bubble dynamics have been made to reveal the individual processes of bubble nucleation, growth and detachment [6,7], the detailed description of initial nucleation conditions remains unresolved [8,9], which is substantially influenced by solid surface properties (e.g., energy and roughness) and surrounding thermodynamics (e.g., temperature gradient and heat flux). In particular, surface wettability has been shown with considerable impacts on the interfacial phase transition according to recent studies [10e16]. It is generally accepted that hydrophilic surfaces enhance the critical heat flux (CHF), whereas hydrophobic surfaces reduce the initial temperature needed for bubble q Revised for publication in Applied Thermal Engineering, August 2011. * Corresponding author. Tel.: þ86 10 82529060. ** Corresponding author. Tel.: þ1 530 754 9508. E-mail addresses: [email protected], [email protected] (H. Wang), [email protected] (T. Pan). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.10.012
nucleation (i.e. the nucleation temperature) [10,11,16]. For instance, surfaces coated with TiO2 thin films, exhibiting extremely low water contact angles, have been reportedly shown improved CHF [10,11]. Recently, nanostructured surfaces with ultimate topological control have gained increasing interest for enhanced mass and hear transfer. Forrest et al. [12] carried out a boiling analysis on silica nanoparticles-coated nickel wires. The thin-film superhydrophobic coating served as a high-performance heat-conducting interface, which offered a high advancing angle to enhance the nucleate boiling heat transfer while promoting the CHF at a reduced receding angle. In another study, Nam and Ju established [14] a minimum surface superheat of w9 C for bubble formation on a nanostructured copper surface. The authors speculated existence of nanoscale gaseliquid interface on the surface with high contact angle as the primary reason of enhanced heat transfer. A similar observation was also made by Ryan and Hemmingsen [15]. More recently, Phan et al. [16] confirmed the role of surface hydrophobicity in reducing the nucleation temperature. Microfabrication has been initially introduced as a highprecision machining tool to form microscale cavities on solid substrates, allowing localized bubble formation and microscopic observation and analysis [17e26]. For instance, Shoji conducted a series of boiling experiments on micro-cavities with different shapes and dimensions. Conical and cylindrical cavities were
X. Wang et al. / Applied Thermal Engineering 35 (2012) 112e119
113
Fig. 1. Experimental setup.
found having different bubbling frequencies and requiring different superheats [17e19]. In this paper, we have fully utilized microfabrication technique to pattern a superhydrophobic nanocomposite material on copper surfaces to promote bubble formation as well as to facilitate bubble detachment. Characterization of bubble formation and heat transfer on the superhydrophobic-micropatterned surfaces has been thoroughly conducted with or without prior-degassing treatment. In addition, the same experiments have been repeated on a uniform superhydrophobic coating and a bare copper surface for comparisons. 2. Experimental setup The commercially available polytetrafluoroethylene (PTFE) nanoparticles provide a robust superhydrophobic coating once sprayed on the copper surface due to their superb chemical inertness (surface energy of 18.6 mJ/m2) and nanoscale topography (250 nm in diameter). The trapped air bubbles on superhydrophobic coating not only protect the copper surface from being wetted, but more importantly facilitate bubble formation at a lower temperature. 2.1. Fabrication of superhydrophobic micropatterns The superhydrophobic nanocomposite formulation was made from a simple mixture of commercially available polytetrafluoroethylene (PTFE) nanoparticles with a thermally curable polymer matrix [21e23]. In brief, the PTFE-based nanocomposite consisted of 8.5% w/w PTFE nanoparticles of 250 nm in diameter, 16.9% toluene, 47.5% hexane, 22.5% cyclohexane, 4.1% PDMS
Fig. 2. Square superhydrophobic micro sites on the copper surface (a) and the contact angle of water on the superhydrophobic coating (b).
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X. Wang et al. / Applied Thermal Engineering 35 (2012) 112e119
subsequent coating (the detailed protocol of dry-film fabrication can be found in the previous publications [24e26]). In the following step, spray coating technique was applied to establish unique coating over the entire surface area, followed by thermally curing the nanocomposite formula at 120 C for 1 h. Finally, the shadow mask was removed, leaving PTFE micropatterns in the through-hole area on the copper surface. Overall, the superhydrophobic micropatterns were established on a rectangular copper sheet of 60 mm in length, 50 mm in width and 0.6 mm in thickness. 2.2. Measurement of boiling heat transfer As shown in Fig. 1, the experimental evaluation system of bubble formation was comprised of a testing surface, a heating apparatus and a temperature measurement setup. A hot plate was utilized to heat a copper block as a homogenous heat source, which was directly connected to the evaluation surface. A K-type thermocouple was attached to the bottom of the evaluation surface through a shallow trench drilled through the copper block, which continuously monitored the surface temperature by connecting to a multi-channel data logger (Omega OM-DAQPRO-5300). A high-speed CCD camera (XMotion, AOS Technologies) was mounted on a microscope (Ti-U Inverted, Nikon) for continuous imaging of bubble formation. De-ionized (DI) water was used as the working fluid, and degassed by boiling for selected experiments for 1e2 h prior to the measurement. The degassing of the surface was accomplished by storing the sample in a vacuum desiccator for 45 min (The patterned surface is shown in Fig. 2). 3. Results and discussion Table 1 summarized a series of boiling experiments conducted on the superhydrophobic-micropatterned copper surfaces (Tests 1e4) and on the controls (Test 5 on uniform superhydrophobic coating, and Tests 6 and 7 on bare copper surfaces). 3.1. Boiling on the micropatterned surface
Fig. 3. Bubbles on the hydrophobic sites in test 1 (without degassing) when the wall temperature was 41.5 C (a), 52.5 C (b), and 67.3 C (c).
base polymer and 0.5% PDMS curing agent. The PDMS matrix provides excellent adhesion among nanoparticles and substrates [23]. A microfabricated dry-film shadow mask with throughholes was first laminated on the copper substrate for the
In Test 1, neither the working fluid nor the surface was degassed. When the surface temperature reached 41.5 C, bubbles started forming on the superhydrophobic patterns as shown in Fig. 3a. The bubbles were initially well positioned on the micropatterns and no bubbles appeared on the bare copper surface. As the surface temperature further increased, the bubbles grew over the micropatterned region, as shown in Fig. 3b. Therefore, the micropatterns served as effective nucleation sites. The bubble growth on micropatterns can be divided into three stages. First of all, the triple line of the embryo gas bubble expanded till meeting with the wetting boundary of the
Table 1 Summarization of the tests. Test No.
Water degas
Surface degas
Surface
Bubble formation temperature ( C)
1 2 3 4 5 6 7
No No Yes Yes Yes No Yes
No Yes No Yes Yes No Yes
Patterned surface Patterned surface Patterned surface Patterned surface Uniform superhydrophobic Bare copper Bare copper
40 62 82 85 85 62 124
X. Wang et al. / Applied Thermal Engineering 35 (2012) 112e119
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Fig. 4. Bubbles growth from hydrophobic sites in test 4 (with degassing on both the surface and water).
superhydrophobic micropatterns. During the second stage, the bubble was observed to grow vertically with the contact line pinned at the edge. At the last stage, the expanded bubbles merged together in the adjacent micropatterns and eventually detached from the micropatterned surface. In comparison, the bubble growth patterns were completely different when the degassing was performed prior to the boiling tests, as illustrated in Fig. 4. As can be seen, bubbles initially formed at 85 C, which was a much higher nucleation temperature than that of Test 1 (41.5 C). In the following stage, most bubbles randomly appeared with different sizes along the borders of the micropatterned regions, instead of attaching to the center area. The unexpected phenomena indicated the importance of edge effects of the micropatterns, which facilitated bubble formation, and could be possibly explained by variation in the coating thickness, which was typically thinner along the edge and thus led to higher nucleation temperature (with less thermal conductive resistance). In addition, the border region approached to cavitation formation, which further reduced bubble formation temperature. Furthermore, the topology variation of the micropatterned superhydrophobic surface was confirmed by the atomic force microscopic images (MFP-3D-BIO, AR) as shown in Fig. 5. It showed that the micropattern boundaries presented higher roughness than that of the central region, which could be used to possibly explain the preferred bubble formation along the edges. More detailed discussion will be included in Section 3.4. One corner of the superhydrophobic micropattern is shown in Fig. 5a. The scan size is 40 40 mm within which the area of the scanned corner is about 25 25 mm and the rest is the bare copper surface. Fig. 5c is the corresponding 3D view which shows how the structures are rooted to the copper surface. Fig. 5b shows the height information along line a-a in Fig. 5a. Fig. 6 shows the scanning image of the superhydrophobic
micropattern in the central area. By comparing Fig. 5 with Fig. 6, it implies that the border region has more valleys than the central area. In addition, low thermal conductivity of the micropattern could be an alternative reason. The PTFE material possesses more than 1600 times lower thermal conductivity than that of copper (kPTFE ¼ 0.25 W/(m K), kCu ¼ 400 W/(m K)). That is to say, given the same surface area, the thermal conductance of the 5 mm PTFE coating is on the same order of that of an 8 mm thick copper block. Therefore, the border region may experience higher temperature. 3.2. Boiling on the uniform superhydrophobic surface On the surface coated with uniform superhydrophobic coating, bubbles appeared randomly on the surface and spread out with arbitrary shapes, as shown in Fig. 7. The bubbles tended to coalesce and form a continuous vapor film above the surface, which further impeded mass and heat transfer, similar as the previously described [11,15]. Overall, the surface coating posed a strong impact on the bubble formation and growth. 3.3. Bubble formation temperature Bubble formation temperatures were recorded on different boiling surfaces with or without degassing treatment. Table 1 summarized the measurement data from seven test sets. The bubble formation temperatures were measured to be 40 C, 62 C, 82 C, 85 C, 85 C, 62 C and 124 C for Tests 1e7, respectively. In the tests, three potential factors could influence the bubble formation temperature, i.e., the surface wettability, the degassing of the working fluid, and the degassing of the surface. Test 1 had the lowest bubble formation temperature since neither the
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Fig. 5. AFM scanning result of a site corner and the neighboring copper surface: (a) top view, (b) the height curve along the red line a-a in (a); (c) 3D view. (Scan size 40.00 mm; Scan rate 0.10 Hz; Scan points 256; Scan lines 256; Imaging mode AC). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
superhydrophobic coating nor the working fluid was degassed. While in Test 2 the degassing of the surface elevated the nucleation temperature to 62 C. In comparison, the degassing of water showed a stronger impact on the nucleation temperature (increasing to 82 C). This could be explained by the fact that even though the surface was not completely degassed, the trapped gases still tended to dissolve into the degassed bulk fluid, as shown in Fig. 8. Furthermore, in Test 5 when the surface and the working fluid were both degassed, using the surface uniformly covered by the superhydrophobic PTFE coating, the resulted nucleation
temperature was founded consistently at 85 C. This result was found to be highly consistent with that on the micropatterned surface in Test 4, indicating that in the present work the micropatterned coating was as good as the uniform superhydrophobic coating in lowering the bubble formation temperature. For the bare copper surface used in Test 6, no degassing treatment was applied, which led to the bubble formation at 62 C, the same as that of Test 2. However, with degassing treatment in Test 7, the nucleation temperature rose sharply to 124 C, with a superheat of 24 C. Comparing Tests 4 and 7, it was found that the
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Fig. 6. AFM scanning result of a site center region: (a) top view, (b) height curve along the red line in (a); (c) 3D view. (Scan size 20.00 mm; Scan rate 0.10 Hz; Scan points 256; Scan lines 64; Imaging mode AC). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
hydrophobic micropatterns could considerably lower the bubble formation temperature by about 40 C. 3.4. Discussion As an embryo bubble grows and reaches the cavity opening, the gaseliquid interface becomes convex as shown in Fig. 9a, the mechanical equilibrium can be described by the following equation:
Pv þ Pg ¼ Pl þ
2s r
(1)
where Pg and Pv indicate the partial pressures of gas and vapor, respectively. Pl stands for the liquid pressure, s represents the surface tension, and r is the interfacial radius. Combining Eq. (1) with ClausiuseClapeyron equation gives rise to the necessary superheat of bubble formation as:
Tsat nlv 2s Tl Tsat ðPl Þ> Pg hlv rmin
(2)
where Tsat(Pl) is the saturation temperature, vlv indicates the difference between the specific volume of the saturated vapor and
liquid, and hlv is the latent heat. The maximum value of 1/rmin occurs when the embryo bubble just reaches the cavity opening with an infinite radius of curvature and can be approximated by sin q/R, where R is the radius of cavity mouth, as shown in Fig. 9. Therefore, only if the gas pressure Pg is greater than 2s/Rsinq the superheat for bubble formation can be a negative number. In Test 4, the superheat was measured negative 15 C. Given q ¼ 150 , s ¼ 61.7 103 N/m and R is about 1 mm, Eq. (2) predicts the partial pressure inside the gas bubble would be greater than 61.7 kPa, which is comparable to the saturation vapor pressure Pv at 85 C (62.7 kPa). Thus, the non-condensable nature of gas may play a critical role in the bubble formation on the superhydrophobic surface. Furthermore, two mechanisms could result in the presence of non-condensable gas. First of all, gas can be physically trapped/adsorbed to the superhydrophobic surface [15,16]. Phan et al. [17] recently confirmed decreased bubble formation temperatures on a smooth hydrophobic surface. In addition, the concave boundary region may effectively trap gas, which can be theoretically predicted by the following equation:
Pv þ Pg ¼ Pl
2s r
(3)
Therefore, the capillary pressure could prevent working fluid from spreading/wetting into the bottom of the cavity, since the internal gas pressure rises sharply when the interfacial radius approaches to zero. Test 4 implies that the boundary of
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Fig. 8. Dissolving of the gas bubbles on the hydrophobic sites into the degassed bulk in test 3.
Fig. 7. Bubble growth on the uniform superhydrophobic surface in test 5 (with degassing of both the surface and the water).
micropatterns facilitates the bubble formation. Comparison of the AFM result in Fig. 5b to that in Fig. 6b shows that the edge region could present more gas-trapping cavities than those of the central area. However, it is still unclear how to quantitatively evaluate the amount of cavitation effects on either a smooth or nanostructured hydrophobic surface [15e17]. Further investigation using highpower high-speed microscopic analysis is currently under way. 4. Conclusions In summary, micropatterned superhydrophobic coating on copper surfaces was able to facilitate boiling heat transfer. Boiling
Fig. 9. Diagrams showing an embryo bubble inside a hydrophobic cavity: (a) inside, (b) at the mouth.
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process on a uniform superhydrophobic-coated surface and a bare copper surface were also studied under the same conditions for comparison. The micropatterned superhydrophobic surface effectively lowered the bubble formation temperature to 85 C whereas that of the bare copper surface stuck to around 124 C with considerable superheat. Moreover, the superhydrophobic micropatterns controlled the nucleation sites (bubbles tended to align up along the superhydrophobic boundaries), bubble shapes and growth rates, and prevented formation of a continuous vapor film which were typically developed on a uniform superhydrophobic coating. In addition, the degassing of the working fluid was found to be more effective than the degassing of the heat-conducting surface. Acknowledgements This work was supported in part by Natural Science Foundation of China (Grant: 50876001) and National Science Foundation (Awards: ECCS-0846502 and CMMI-0944353). The authors thank Ms. Lingfei Hong for providing guidance on preparing the superhydrophobic coating. References [1] M. Jacob, W. Fritz, Forsch. Geb. Ingenieurwes 2 (1931) 437e447. [2] M. Jacob, W. Linke, Heat transfer from a horizontal plate, Forsch. Geb. Ingenieurwes 4 (1933) 434. [3] R. Becker, W. Doring, Ann. Phys. (Leipzig) 24 (1935) 719. [4] S. Nukiyama, The maximum and minimum value of heat Q transmitted from a metal to boiling water under atmospheric pressure, International Journal of Heat and Mass Transfer 9 (1966) 1419e1433. [5] C.Y. Han, P. Griffith, The mechanism of heat transfer in nucleate pool boilingPart I: bubble initiation, growth and departure, International Journal of Heat and Mass Transfer 8 (1965) 887e904. [6] G. Son, V.K. Dhir, N. Ramanujapu, Dynamics and heat transfer associated with a single bubble during nucleate boiling on a horizontal surface, Journal of Heat Transfer 121 (1999) 623. [7] L. Dong, P.S. Lee, S.V. Garimella, Prediction of the onset of nucleate boiling in microchannel flow, International Journal of Heat and Mass Transfer 48 (2005) 5134e5149. [8] M. Blander, J.L. Katz, Bubble nucleation in liquids, AIChE Journal 21 (1975) 833e848. doi:10.1002/aic.690210502. [9] S.F. Jones, G.M. Evans, K.P. Galvin, Bubble nucleation from gas cavities a review, Advances in Colloid and Interface Science 80 (1999) 27e50.
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