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Bubble dynamics and heat transfer on a wettability patterned surface
International Journal of Heat and Mass Transfer 88 (2015) 544–551
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Bubble dynamics and heat transfer on a wettability patterned surface Xiaodan Chen, Huihe Qiu ⇑ Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Special Administrative Region
a r t i c l e
i n f o
Article history: Received 12 November 2014 Received in revised form 22 April 2015 Accepted 24 April 2015
Keywords: Hydrophilic–hydrophobic patterns Bubble dynamics Controlled nucleation sites
a b s t r a c t In this paper we studied the enhancement of heat transfer on hydrophilic–hydrophobic patterned surface in a transparent mini chamber. Hydrophobic islands on hydrophilic networks were micro fabricated as patterns on a 1 mm 1 mm indium-tin-oxide (ITO) glass heater. The hydrophobic islands with size of 100 lm 100 lm and pitch distance 100 lm are self-assembled monolayers (SAM) of (1H,1H,2H,2H)Perfluorodecyldimethylchlorosilane (FAS) with contact angle of 100°. The input heat flux for the heater ranges up to 300 W/cm2. It is shown that the critical heat flux (CHF) on the wettability patterned surface was enhanced by 90% in comparison with a hydrophilic surface. The visualization results also show that the mechanism for enhancing heat transfer coefficient (HTC) of a wettability patterned surface is caused by increasing the active nucleation sites, decreasing the bubble departure diameter, remaining residual bubbles, and activating more bubble interactions. Patterns increase the CHF by moderating the Helmholtz instabilities, preventing the formation of an insulating vapor layer. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction In electronics industry, as chip performance rapidly improves, the transfer of heat has become more and more critical. Compared to a conventional air cooling system, closed loop liquid cooling may provide a better solution to meet the requirements of high heat flux, small scale and low noise. A vapor chamber applying pool boiling is considered to be a promising solution [1]. Heater surface characteristics are certainly an important determiner of pool boiling efficiency. Much research [2–10] has been done to identify the surface wettability’s effect on nucleate boiling heat transfer. Yang and Kim [2] developed a theoretical model pointing out that the contact angle affects the force equation among vapor momentum, gravity and surface tension, and then determines the vapor’s spread along the heater surface, finally initiating the CHF conditions. Researchers discovered that a super hydrophilic surface and micro/nanoscale modification can significantly enhance the transfer of heat. Ahn et al. [4] figured out that when the contact angle approaches to zero, the liquid spread effect will contributes a lot to higher CHF. The conclusion was confirmed by other researchers [6,7,10], adding that a micro/nanoscale structure also provides numerous nucleation sites that contribute to a larger HTC. But a hydrophilic surface also has its own limitation. Phan et al. [11] figured out that, though HTC can improve when the contact ⇑ Corresponding author. E-mail address: [email protected] (H. Qiu). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.04.086 0017-9310/Ó 2015 Elsevier Ltd. All rights reserved.
angle is very close to 0, a general problem for all hydrophilic surfaces is that, with the enhancement of the surface wettability, the bubble departure diameter increases while the bubble emission frequency decreases. Phan et al. also pointed out the characteristics for a hydrophobic surface causes bubbles to appear at the surface for a low heat flux, however, bubbles are difficult to detach from the surface. Experiment of Nam et al. [12,19] on a Teflon-coated wafer with a contact angle greater than 100° generated similar results; Nam also mentioned that when a bubble departs from the surface, it leaves a small residual bubble nucleus which then grows in the next cycle, leaving no waiting period. It raises the question whether a mixed, or say biphilic surface can perform better than a pure hydrophilic or hydrophobic surface. The answer is yes given by some researches [13–19]. Jo et al. performed a series of studies [15–18] on a surface with hydrophobic Teflon-dots manufactured on a hydrophilic silicon base. They firstly figured out that hydrophobic milli-dots on the silicon surface can enhance HTC but had a negative effect on the CHF [15]. Then they discovered that when the area ratio for a hydrophobic surface increased, the CHF decreased [16]. When the area ratio is low, the CHF will be enhanced by decreasing the dot diameter. Their further investigation showed that heat transfer results seem to be related to the number of dots, dot diameter and pitch distance [17]. Research [20–28] has been conducted on the topic of bubble behavior analysis, which provides references for our study. Generally speaking, bubble behavior includes nucleation, growth, departure and interaction. Basu et al. [24] found out that
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Nomenclature Letters q_ U I R A
q N F
r
heat flux (W/cm2) voltage (V) current (A) electrical resistance (X) area (cm2) nucleation site density number of bubbles force (N) surface tension (N/m)
nucleation density is related to contact angle and wall super heat for sub-cooled flow boiling. The relationship between contact angle and departure diameter has been explained in many other articles [23,27,28]. Bubble interaction has also been intensively investigated [20,22,25]. Gong and Cheng [29] numerically simulated the pool boiling heat transfer of mixed wettability surface, using 2-D lattice Boltzmann method. The simulated bubble behavior and heat transfer results shows that hydrophobic spots on smooth hydrophilic surfaces promotes bubble nucleation, enhancing boiling heat transfer and reducing nucleation time drastically. The mixed wettability surface is also expected to enhance critical heat flux (CHF) by regulating vapor spreading behaviors over the heater surface. Lattice Boltzmann method had been frequently utilized by Cheng’s team because of its capability of simulating the entire ebullition cycle including bubble nucleating process [29–32]. The heater size effects were studied extensively by Kim et al. [33,34]. According to their research, relative small heater size has poorer heat transfer than that of a large size. In this study, we focused on small heater only. Surface treatment applied in this study was expected to enhance the heat transfer even with small heater size. We demonstrated that a wettability patterned surface can control the nucleation cites and significantly enhance heat transfer over hydrophobic and hydrophilic surfaces. The previous research [13] demonstrated that surfaces with networks combining hydrophilic and hydrophobic regions significantly enhance the CHF and the HTC during saturated pool boiling. The best enhancement arises with hydrophilic networks featuring hydrophobic islands. Compared to a hydrophilic surface with 7° wetting angle, the CHF and HTC were 65% and 100% higher, respectively. Wettability patterns can not only raise HTC by promoting more active nucleation sites but also enhanced CHF by moderating Helmholtz instabilities [13,35,36]. To have a better understanding on why and how a biphilic surface can enhance heat transfer in pool boiling, this paper focused on experimental observations from the bubble dynamics point of view.
Abbreviations ITO indium-tin-oxide 0 standard resistance i number of frame n nucleation site phi hydrophilic surface pho hydrophobic surface CHT critical heat flux HTC heat transfer coefficient SAM self-assembled monolayer FAS (1H,1H,2H,2H)-Perfluorodecyldimethylchlorosilane
2. Experimental investigation 2.1. Materials and fabrication In this study, Pyrex glass with an indium-tin-oxide (ITO) layer was used as the substrate. 1 mm 1 mm ITO layer was etched as the heater having a resistance of around 70 X after annealing (Fig. 1(a)). Hydrophobic islands with size of 100 lm 100 lm and pitch distance 100 lm on hydrophilic networks were micro fabricated as patterns on the heater (Fig. 1(b)). Self-assembled monolayer (SAM) of (1H,1H,2H,2H)-Perfluorodecyldimethylchloro silane (FAS) with a contact angle of 100° were coated on the substrate as the hydrophobic islands. Droplets that condensed on the hydrophilic networks were obviously larger (around 400% larger by volume) than those which condensed on the hydrophobic islands (Fig. 1(c)). A schematic of the heating sample is shown in Fig. 2. 2.2. Experimental setup A schematic of the experimental setup is shown in Fig. 3. A high speed camera (Motion Xtra HG-100K, Redlake Co.) was used to capture bubble behaviors such as bubble departure diameter, bubble departure period and the active nucleation site density. A signal generator and an amplifier were used to supply power with a square wave with frequency of 1 kHz for the experiments. A standard resistor whose resistance was around 70 X was used in the series circuit. The heat flux could be derived from the voltage of both heater and the standard resistor (refer to Eq. (1)). Voltages were recorded by a Data Acquisition Switch Unit (Agilent 34970A).
q_ ¼
U ITO IITO U ITO U 0 ¼ Aheater Aheater R0
ð1Þ
Degassed DI water (boiled for more than 2 h) at room temperature (25 °C) was used as the working fluid in the mini pool. 50 mL volume of water was used in each experiment for repeatability.
Fig. 1. (a) ITO heater with size of 1 mm 1 mm, (b) Hydrophobic islands with size of 100 lm 100 lm, (c) Droplets condensing on the pattern surface.
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Fig. 2. Schematic of the test surface.
Fig. 4. The calibration curves between ITO heater resistance and temperature.
Since the heater size (1 mm 1 mm) is much smaller than the chamber size (with diameter of 60 mm), the subcooling temperature (bulk water temperature) is around room temperature (up to 30 °C for heating 2 h according to the thermal couple in the bulk water). 3. Results and discussion Three types of surfaces (wholly hydrophilic, wholly hydrophobic, and hydrophilic–hydrophobic patterned/biphilic) were examined to assess the influence of wettability on nucleate boiling heat transfer performance. According to the experimental observation, the heterogeneous surface performed best compared to the other two homogeneous surfaces by CHF and HTC. We attribute the effect of heat transfer enhancement to several particular bubble behaviors on a hydrophilic–hydrophobic patterned surface. They are summarized as: confined nucleate sites, bubble interaction and residual nucleus. And furthermore, effects of liquid subcooling and dissolved gas were discussed.
Hydrophilic Surface Wettability Patterned Surface (Figure 1) 300
Heat Flux (W/cm2)
Fig. 3. Schematic of the experimental setup.
temperature/superheat of the heater surface. Fig. 5 showed the subcooled pool boiling curve with heat flux as a function of superheat on different heater surface. It was obviously that the heat transfer of the patterned surface was much higher than that of the hydrophilic surface. For the hydrophobic surface, after onset nucleation boiling (ONB), the bubble is trapped on the surface and then stretching on the whole heated surface. In that case, the ITO heater is over heated quickly and became malfunction. Therefore, the boiling condition for hydrophobic surface is always under unstable condition and we can only record the maximum heat flux for hydrophobic surface before the malfunction occurred. The CHF of three type surfaces are listed in Table 1. It is clear that the CHF for the wettability patterned surface is also the largest which is about 90% greater than that of the hydrophilic surface.
250 200 150 100
3.1. Heat transfer on different wettability surfaces 50
ITO was used as heater because of its electrical conductivity and optical transparency. The calibration curves between ITO heater resistance and temperature was shown in Fig. 4. It implied linear relationship between ITO resistance and the average of the surface temperature. In this case, we could derive the average of the
0
10
20
30
Superheat (K)
40
50
Fig. 5. Pool boiling curve with heat flux as a function of superheat on different heater surface.
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3.3. Bubble behavior on hydrophilic surfaces
Table 1 CHF for different surfaces. Chamber type 2
CHF [W/cm ]
Hydrophilic
Hydrophobic
Patterned
147
>52
>280
3.2. Bubble behavior on hydrophobic surfaces It is noticed that on the hydrophobic surface was very difficult for the bubble to detach, which could explain why the hydrophobic surface reached its CHF very soon when the water started to boil. As reported by Takata et al. [9] very fast film boiling on super-water-repellent surfaces was observed. In our experiments, a bubble appeared when the heat flux reached 52 W/cm2 (ONB) and then grew up until covering the whole size of the heater (1 mm2) without departure (Fig. 6). The bubble was trapped and stretching bigger and bigger on the hydrophobic surface. Surface drying out was supposed to happen thus the surface temperature of the heater rose rapidly. Even with more than one nucleation site activated simultaneously (see Fig. 7), bubbles intended to merge into a big bubble and finally dried out on near its ONB. It looks like that hydrophobic surface promotes the merged bubbles to grow on the heater surface until drying out, just the same as with a single bubble growth process. As a result, it limited the CHF for hydrophobic surface.
t
t+6s
Bubbles on the hydrophilic surface were more likely to grow one by one with a relatively large size (Fig. 8(a)–(d)). It was rare to find more bubbles appearing on the heater surface so that there was almost no bubble interaction for the hydrophilic surface. Because the bubble contact diameter is smaller than that under a hydrophobic surface, the dry out will be delayed which makes the CHF heat flux larger than hydrophobic surface. In the early stage of bubble growth, it was noted that the shapes of single bubbles on the hydrophilic surface and hydrophobic surface were totally different being a sphere for high wettability and a stretched blanket for low wettability. 3.4. Bubble behavior on wettability patterned surfaces 3.4.1. Confined nucleate sites When nucleation occurs on a wettability patterned surface, contact diameter of the bubble will grow in the hydrophobic island and eventually be confined by the size of the hydrophobic island. Because of the confined shape and size of the hydrophobic islands fabricated on the substrate, nucleate sites of bubbles can be precisely controlled on the islands during boiling (Fig. 9). When the heat flux increased, more confined hydrophobic islands were activated increasing the number of nucleate sites. To have a better
t+12s
t+18s
t+24s
Fig. 6. Bubble growth on hydrophobic surface with heat flux of 52 W/cm2.
t
t+0.8s
t+1.6s
t+2.4s
t+3.2s
Fig. 7. Bubbles merged on hydrophobic surface with heat flux of 52 W/cm2.
t
t+1ms
t+2ms
t+3ms
Fig. 8. (a)–(d) Bubble growth on monophilic surface with heat flux of 139 W/cm2.
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 9. Number of confined nucleate sites increased with increasing of heat flux: (a) 78 W/cm2, (b) 102 W/cm2, (c) 120 W/cm2, (d) 139 W/cm2, (e) 160 W/cm2, (f) 206 W/cm2.
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Two, three, four and more identical bubbles were observed to merge into one bigger bubble in the experiments (Fig. 12). Because of wettability patterns, the total contact area was confined on the hydrophobic islands unlike a big circle on the homogeneous surface. Therefore, the total length of the contact line on the biphilic surface was limited by the hydrophobic islands. The merged bubble was connected by several hydrophobic islands surrounded by liquid film on the hydrophilic regions which implied that the interfacial film was unstable for perturbations. According to Zuber [35], the dryout responsible for CHF is caused by Helmoltz instability that merge individual bubble columns. On a plain surface, the most dangers wavelength, kd, for initiating this instability of bubble columns can be calculated by [36] Fig. 10. Schematic of forces on the bubble with instability shape.
understanding on the bubble dynamics on a biphilic surface, a hypothesis was used to analyze how the bubbles tend to be confined on the hydrophobic islands. We firstly assumed that a bubble sits across hydrophilic and hydrophobic surfaces with particular contact angles as shown in Fig. 10. Because of the unbalanced contact angels on hydrophobic and hydrophilic surfaces, there’s a net lateral force pushing the bubble to the hydrophobic side, which will force the bubble siting on the hydrophobic island. This hypothesis was observed by the experimental results which confirmed that the hydrophobic islands act as nucleation sites during pool boiling. 3.4.2. Bubble interaction and unique shapes When there was more than one bubble on the surface, the neighboring bubbles would interact with each other (Fig. 11).
t
t+33ms
kd ¼ 2p
3r gðql qv Þ
12 ¼
pffiffiffi 3kc
ð2Þ
where kc is the critical wave length. Substituting the interfacial tension r between water and vapor, and ql and qv, we can get kd = 27 mm. However, on a wettability patterned surface, the liquid film is modulated by the surface wetting/non-wetting energy which modify the instability wavelength. According to Sinha et al. [37], a change in the system wettability causes a perturbation in the system’s flow pattern to destabilize any percolating and trapped immobile clusters appeared in the steady state. This implied that the perturbation of wettability patterns may affect the bubble departure diameter and frequency. Because the pattern dimensions (hydrophobic pads and pitches) are much smaller than kd, it is anticipated that the bubble departure diameter can be smaller and the departure frequency can be higher than those on a homogeneous surface. In our experiments (see Fig. 11), it is seldom that the diameter of
t+5ms
t+7ms
t+50ms
t+52ms
t+12ms
t+54ms 2
Fig. 11. Bubble interaction on patterned surface with heat flux of 206 W/cm .
Fig. 12. Bubble merger with increasing numbers of identical bubbles with heat flux of 139 W/cm2.
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Fig. 13. Residual nucleus remain after part of bubble depart from pattern surface.
merged bubbles could go beyond 0.5 mm, which means the bubble departure diameter is controlled by the perturbation of wettability. In our experiments, it was observed that some bubbles broke up frequently after being merged from small bubbles. However, this kind of phenomenon could not be found from the smooth surfaces. This further confirmed that the perturbation caused by the wettability pattern that moderated the critical wave length of the Helmholtz instability [13]. Another important phenomenon was that when the bubble departed from the biphilic surface, some residual bubbles may remain on the hydrophobic islands acting as nucleation sites. Similar results were also reported by Jo et al. [17] from section view of pool boiling with one small bubble remained on the surface when the mother bubble departed from the substrate. In our case, the departure of a merged bubble would leave more than one daughter bubble remaining on the hydrophobic islands compared to a wholly hydrophobic surface (Fig. 13). When more than one bubble merged into a larger one on the patterned surface, the shape of the merged bubble was quite different from bubbles that merged on a homogenous surface. The
contact line under the bubble was more likely to shift from the hydrophilic network to the edge of the hydrophobic islands which was also the reason why identical bubbles on a patterned surface were always confined on islands. In the experiments, there were mainly three shapes of merged bubbles: bridge-shaped bubbles, peanut-shaped bubbles and bowl-shaped bubbles (Fig. 14). A bowl-shaped bubble was generally formed when more than two bubbles merged while others two shapes were normally formed when only two bubbles merged. The bridge-shaped bubbles usually formed at the beginning when two identical bubbles merged into one with two stems attached to the solid surface. The contact line connected along the periphery of the hydrophobic islands or partially shifted to the hydrophilic network, with some water remaining under the ‘‘bridge’’. Some previous work showed a similar result photographed from the side view, calling this kind of shape a mushroom type bubble [4]. The peanut shape was usually the next step of the bridge shape. Evaporation of water under the ‘‘bridge’’ resulted in affecting the surrounding liquid and bubble motion and changing in bubble dynamics caused by the liquid–vapor interaction phenomena by
Fig. 14. Schematic and photographs of unique shapes of merged bubbles: bridge-shaped bubble, peanut-shaped bubble and bowl-shaped bubble.
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shifting of the contact line so that the two stems (dry spot on islands) enlarged into an elongated shape to form the peanut shape. For the bowl shape, a droplet was trapped inside a large bubble merged by several small bubbles due to quick process of merging. The latent heat of the trapped droplet under these bubbles with unique shapes would extra contribute to the HTC. Instability of the asymmetrical bubble shape resulted in local convective microflow surrounding the bubbles.
the hydrophilic surface. As we have discussed before, this is caused by the wettability perturbation induced insatiability. For the bubble departure period (departure frequency), as previously mentioned, there were residual bubbles left on the substrate after the larger bubble departed and there was no waiting time for the hydrophobic surface and patterned surface when compared to the hydrophilic surface. 3.6. Effects of liquid subcooling and dissolved gas
3.5. Comparison of bubble dynamics for the three surfaces We consider nucleate site density, bubble departure diameter and bubble period to evaluate bubble dynamics in pool boiling. A larger nucleate site density, smaller bubble departure diameter and shorter bubble period (total period of growth, departure and waiting time) resulted in more efficient heat transfer. According to recorded images, nucleate site densities under different heat fluxes on the different heater surfaces were calculated. Around 30–40 thousand images were used to count the average number of bubbles nucleated on the surface as in Eq. (3).
qn ¼
PN
i¼1 ni
ð3Þ
N
The results are shown in Table 2. For the patterned surface, the total number of hydrophobic islands that acted as controlled nucleation sites was 25. It was noted that, with heat flux of 206 W/cm2, there could be 18 small bubbles activated on the islands at the same time. The average nucleation site density is 13.9 (1.39 107 sites/m2) because of bubble coalescence, which is about 6 times higher than that of hydrophilic surface. Bubble departure diameter was also an evaluation parameter for heat transfer in pool boiling. As stated above, there was normally only one big bubble on both hydrophilic and hydrophobic surfaces while more bubbles nucleated on the patterned heater surface. We calculated the diameter of around 25 bubbles as they departed from the surface and obtained an average departure diameter as shown in Table 3. In Table 3, it is clear that the bubble departure diameter on the patterned surface is about 1/3 of that on
Table 2 Active nucleate site densities with increasing heat flux on different surfaces. Heat flux [W/cm2]
52 78 102 120 139 160 206
Nucleate site density [sites/mm2] Hydrophilic
Hydrophobic
Patterned
0 0 1 1 1.1 – –
1 – – – – – –
0 1.2 3.4 5.1 6.5 8.3 13.9
Table 3 Bubble departure diameter with increasing heat flux on different surfaces. Heat flux [W/cm2]
52 78 102 120 139 160 206
Departure diameter [mm] Hydrophilic
Hydrophobic
Patterned
– – 1.4 1.1 0.93 – –
– – – – – – –
– 0.73 0.38 0.38 0.37 0.35 0.31
According to Rainey et al. [38], large subcooling will cause the bubble departure diameters and frequencies decrease which reduces the amount of heat transferred through latent heat and micro convection. However, increased subcooling also decreases the superheated liquid layer thickness which increases natural convection heat transfer and increases Marongoni convection heat transfer. These effects compensate each other and the overall effects show that it is relative insensitivity of the nucleate boiling curve to liquid subcooling. For the effect of dissolved gas, in subcooling, as mentioned by Rainey et al. [38], the dissolved air enhances heat transfer at low heat fluxes just after incipience but not at higher heat fluxes. The CHF values of the gas-saturated cases were almost the same as the degassed subcooled liquid. For small heater size (such as small wire heaters), the CHF values maybe reduced for the subcooling liquid with dissolved gas. However, the effect is diminished as the heater size increased. In our experiments, our purpose is to compare the wettability patterned surface with hydrophilic and hydrophobic surfaces. The size effect on the CHF with dissolved gas subcooling condition was not considered and which may be studied in the future. 4. Conclusions In conclusion, this study demonstrated how hydrophobic islands worked as localized and controlled nucleate sites even with relatively small heater size. The patterned surface performed the best in heat transfer for subcooled pool boiling in comparison with hydrophilic and hydrophobic surfaces. The nucleation site density of biphilic surface was much higher, when compared to that of the homogeneous surface (72% of hydrophobic islands or 18 of the 25 islands, could be activated simultaneously with heat flux of 206 W/cm2 on the patterned surface). The latent heat of the trapped droplet under bubbles with unique shapes on the biphilic surface would extra contribute to the HTC. Wettability perturbation affects the flow instability which induced local convective microflow surrounding the bubbles. This can affect the merged bubble breakup and departure frequency. The departure diameter was about 1/3 of the size on the homogeneous surface. Smaller bubbles grew and departed on the heater quickly, resulting in a higher heat transfer coefficient. Bubble interactions and instability of big bubbles resulted in both a larger critical heat flux and higher heat transfer coefficient. In the meantime, residual bubbles that remained as the boiling nuclei further promote boiling on the biphilic surface which also contributed to the heat transfer efficiency. Compared to the hydrophilic and hydrophobic surfaces, the biphilic surface had the best heat transfer performance in our study although more parameters such as pitch distance, shape and diameter of the hydrophilic island need optimizing. Conflict of interest None declared.
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Acknowledgment This research is supported by the Research Grants Council (RGC) of the Government of the Hong Kong Special Administrative Region (HKSAR) with Project Nos. 618210 and 617812.
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[18] H. Jo, S. Kim, H.S. Park, M.H. Kim, Critical heat flux and nucleate boiling on several heterogeneous wetting surfaces: controlled hydrophobic patterns on a hydrophilic substrate, Int. J. Multiphase Flow 62 (2014) 101–109. [19] Y. Nam, Y.S. Ju, Bubble nucleation on hydrophobic islands provides evidence to anomalously high contact angles of nanobubbles, Appl. Phys. Lett. 93 (2008). [20] L. Zhang, M. Shoji, Nucleation site interaction in pool boiling on the artificial surface, Int. J. Heat Mass Transfer 46 (2003) 513–522. [21] C.X. Yang, Y.T. Wu, X.G. Yuan, C.F. MA, Study on bubble dynamics for pool nucleate boiling, Int. J. Heat Mass Transfer 43 (2) (2000) 203–208. [22] S. Siedel, S. Cioulachtjian, J. Bonjour, Experimental analysis of bubble growth, departure and interactions during pool boiling on artificial nucleation sites, J. Exp. Therm. Fluid Sci. 32 (2008). [23] H.T. Phana, N. Caneya, P. Martya, S. Colassonb, J. Gavilletc, A model to predict the effect of contact angle on the bubble departure diameter during heterogeneous boiling, Int. Commun. Heat Mass Transfer 37 (8) (2010) 964– 969. [24] N. Basu, G.R. Warrier, V.K. Dhir, Onset of nucleate boiling and active nucleation site density during subcooled flow boiling, J. Heat Transfer 124 (4) (2002) 717– 728. [25] S. Chatpun, M. Watanabe, M. Shoji, Nucleation site interaction in pool nucleate boiling on a heated surface with triple artificial cavities, Int. J. Heat Mass Transfer 47 (14–16) (2004). [26] G. Son, N. Ramanujapu, V.K. Dhir, Numerical simulation of bubble merger process on a single nucleation site during pool nucleate boiling, J. Heat Transfer 124 (2002) 51–62. [27] Y. Nam, E. Aktinol, V.K. Dhir, Y. Sungtaek Ju, Single bubble dynamics on a superhydrophilic surface with artificial nucleation sites, Int. J. Heat Mass Transfer 54 (7–8) (2011) 1572–1577. [28] G. Son, V.K. Dhir, N. Ramanujapu, Dynamics and heat transfer associated with a single bubble during nucleate boiling on a horizontal surface, J. Heat Transfer 121 (1999) 623–632. [29] S. Gong, P. Cheng, Numerical simulation of pool boiling heat transfer on smooth surfaces with mixed wettability by lattice Boltzmann method, Int. J. Heat Mass Transfer 80 (2015) 206–216. [30] X. Liu, P. Cheng, X. Quan, Lattice Boltzmann simulations for self-propelled jumping of droplets after coalescence on a superhydrophobic surface, Int. J. Heat Mass Transfer 73 (2014) 195–200. [31] S. Gong, P. Cheng, Lattice Boltzmann simulation of periodic bubble nucleation, growth and departure from a heated surface in pool boiling, Int. J. Heat Mass Transfer 64 (2013) 122–132. [32] S. Gong, P. Cheng, On simulation of pool boiling from patterned surfaces, Int. J. Heat Mass Transfer 80 (2015) 206–216. [33] R. Raj, J. Kim, Heater size and gravity based pool boiling regime map: transition criteria between buoyancy and surface tension dominated boiling, J. Heat Transfer 132 (2010). [34] R. Raj, J. Kim, J. McQuillen, Subcooled pool boiling in variable gravity environments, J. Heat Transfer 131 (2009). [35] N. Zuber, Hydrodynamic aspects of boiling heat transfer (Ph.D. thesis), University of California, 1959. [36] V. Carey, Liquid–Vapor Phase Change Phenomena, Taylor & Francis, London, 1992. [37] S. Sinha, M. Grøva, T.B. Ødegarden, E. Skjetne, A. Hansen, Local wettability reversal during steady-state two-phase flow in porous media, Phys. Rev. E 84 (2011) 037303. [38] K.N. Rainey, S.M. You, S. Lee, Effect of pressure, subcooling and dissolved gas on pool boiling heat transfer from microporous surfaces in FC-72, J. Heat Transfer 125 (2003) 75–83.
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Bubble dynamics and heat transfer on a wettability patterned surface Xiaodan Chen, Huihe Qiu ⇑ Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Special Administrative Region
a r t i c l e
i n f o
Article history: Received 12 November 2014 Received in revised form 22 April 2015 Accepted 24 April 2015
Keywords: Hydrophilic–hydrophobic patterns Bubble dynamics Controlled nucleation sites
a b s t r a c t In this paper we studied the enhancement of heat transfer on hydrophilic–hydrophobic patterned surface in a transparent mini chamber. Hydrophobic islands on hydrophilic networks were micro fabricated as patterns on a 1 mm 1 mm indium-tin-oxide (ITO) glass heater. The hydrophobic islands with size of 100 lm 100 lm and pitch distance 100 lm are self-assembled monolayers (SAM) of (1H,1H,2H,2H)Perfluorodecyldimethylchlorosilane (FAS) with contact angle of 100°. The input heat flux for the heater ranges up to 300 W/cm2. It is shown that the critical heat flux (CHF) on the wettability patterned surface was enhanced by 90% in comparison with a hydrophilic surface. The visualization results also show that the mechanism for enhancing heat transfer coefficient (HTC) of a wettability patterned surface is caused by increasing the active nucleation sites, decreasing the bubble departure diameter, remaining residual bubbles, and activating more bubble interactions. Patterns increase the CHF by moderating the Helmholtz instabilities, preventing the formation of an insulating vapor layer. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction In electronics industry, as chip performance rapidly improves, the transfer of heat has become more and more critical. Compared to a conventional air cooling system, closed loop liquid cooling may provide a better solution to meet the requirements of high heat flux, small scale and low noise. A vapor chamber applying pool boiling is considered to be a promising solution [1]. Heater surface characteristics are certainly an important determiner of pool boiling efficiency. Much research [2–10] has been done to identify the surface wettability’s effect on nucleate boiling heat transfer. Yang and Kim [2] developed a theoretical model pointing out that the contact angle affects the force equation among vapor momentum, gravity and surface tension, and then determines the vapor’s spread along the heater surface, finally initiating the CHF conditions. Researchers discovered that a super hydrophilic surface and micro/nanoscale modification can significantly enhance the transfer of heat. Ahn et al. [4] figured out that when the contact angle approaches to zero, the liquid spread effect will contributes a lot to higher CHF. The conclusion was confirmed by other researchers [6,7,10], adding that a micro/nanoscale structure also provides numerous nucleation sites that contribute to a larger HTC. But a hydrophilic surface also has its own limitation. Phan et al. [11] figured out that, though HTC can improve when the contact ⇑ Corresponding author. E-mail address: [email protected] (H. Qiu). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.04.086 0017-9310/Ó 2015 Elsevier Ltd. All rights reserved.
angle is very close to 0, a general problem for all hydrophilic surfaces is that, with the enhancement of the surface wettability, the bubble departure diameter increases while the bubble emission frequency decreases. Phan et al. also pointed out the characteristics for a hydrophobic surface causes bubbles to appear at the surface for a low heat flux, however, bubbles are difficult to detach from the surface. Experiment of Nam et al. [12,19] on a Teflon-coated wafer with a contact angle greater than 100° generated similar results; Nam also mentioned that when a bubble departs from the surface, it leaves a small residual bubble nucleus which then grows in the next cycle, leaving no waiting period. It raises the question whether a mixed, or say biphilic surface can perform better than a pure hydrophilic or hydrophobic surface. The answer is yes given by some researches [13–19]. Jo et al. performed a series of studies [15–18] on a surface with hydrophobic Teflon-dots manufactured on a hydrophilic silicon base. They firstly figured out that hydrophobic milli-dots on the silicon surface can enhance HTC but had a negative effect on the CHF [15]. Then they discovered that when the area ratio for a hydrophobic surface increased, the CHF decreased [16]. When the area ratio is low, the CHF will be enhanced by decreasing the dot diameter. Their further investigation showed that heat transfer results seem to be related to the number of dots, dot diameter and pitch distance [17]. Research [20–28] has been conducted on the topic of bubble behavior analysis, which provides references for our study. Generally speaking, bubble behavior includes nucleation, growth, departure and interaction. Basu et al. [24] found out that
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Nomenclature Letters q_ U I R A
q N F
r
heat flux (W/cm2) voltage (V) current (A) electrical resistance (X) area (cm2) nucleation site density number of bubbles force (N) surface tension (N/m)
nucleation density is related to contact angle and wall super heat for sub-cooled flow boiling. The relationship between contact angle and departure diameter has been explained in many other articles [23,27,28]. Bubble interaction has also been intensively investigated [20,22,25]. Gong and Cheng [29] numerically simulated the pool boiling heat transfer of mixed wettability surface, using 2-D lattice Boltzmann method. The simulated bubble behavior and heat transfer results shows that hydrophobic spots on smooth hydrophilic surfaces promotes bubble nucleation, enhancing boiling heat transfer and reducing nucleation time drastically. The mixed wettability surface is also expected to enhance critical heat flux (CHF) by regulating vapor spreading behaviors over the heater surface. Lattice Boltzmann method had been frequently utilized by Cheng’s team because of its capability of simulating the entire ebullition cycle including bubble nucleating process [29–32]. The heater size effects were studied extensively by Kim et al. [33,34]. According to their research, relative small heater size has poorer heat transfer than that of a large size. In this study, we focused on small heater only. Surface treatment applied in this study was expected to enhance the heat transfer even with small heater size. We demonstrated that a wettability patterned surface can control the nucleation cites and significantly enhance heat transfer over hydrophobic and hydrophilic surfaces. The previous research [13] demonstrated that surfaces with networks combining hydrophilic and hydrophobic regions significantly enhance the CHF and the HTC during saturated pool boiling. The best enhancement arises with hydrophilic networks featuring hydrophobic islands. Compared to a hydrophilic surface with 7° wetting angle, the CHF and HTC were 65% and 100% higher, respectively. Wettability patterns can not only raise HTC by promoting more active nucleation sites but also enhanced CHF by moderating Helmholtz instabilities [13,35,36]. To have a better understanding on why and how a biphilic surface can enhance heat transfer in pool boiling, this paper focused on experimental observations from the bubble dynamics point of view.
Abbreviations ITO indium-tin-oxide 0 standard resistance i number of frame n nucleation site phi hydrophilic surface pho hydrophobic surface CHT critical heat flux HTC heat transfer coefficient SAM self-assembled monolayer FAS (1H,1H,2H,2H)-Perfluorodecyldimethylchlorosilane
2. Experimental investigation 2.1. Materials and fabrication In this study, Pyrex glass with an indium-tin-oxide (ITO) layer was used as the substrate. 1 mm 1 mm ITO layer was etched as the heater having a resistance of around 70 X after annealing (Fig. 1(a)). Hydrophobic islands with size of 100 lm 100 lm and pitch distance 100 lm on hydrophilic networks were micro fabricated as patterns on the heater (Fig. 1(b)). Self-assembled monolayer (SAM) of (1H,1H,2H,2H)-Perfluorodecyldimethylchloro silane (FAS) with a contact angle of 100° were coated on the substrate as the hydrophobic islands. Droplets that condensed on the hydrophilic networks were obviously larger (around 400% larger by volume) than those which condensed on the hydrophobic islands (Fig. 1(c)). A schematic of the heating sample is shown in Fig. 2. 2.2. Experimental setup A schematic of the experimental setup is shown in Fig. 3. A high speed camera (Motion Xtra HG-100K, Redlake Co.) was used to capture bubble behaviors such as bubble departure diameter, bubble departure period and the active nucleation site density. A signal generator and an amplifier were used to supply power with a square wave with frequency of 1 kHz for the experiments. A standard resistor whose resistance was around 70 X was used in the series circuit. The heat flux could be derived from the voltage of both heater and the standard resistor (refer to Eq. (1)). Voltages were recorded by a Data Acquisition Switch Unit (Agilent 34970A).
q_ ¼
U ITO IITO U ITO U 0 ¼ Aheater Aheater R0
ð1Þ
Degassed DI water (boiled for more than 2 h) at room temperature (25 °C) was used as the working fluid in the mini pool. 50 mL volume of water was used in each experiment for repeatability.
Fig. 1. (a) ITO heater with size of 1 mm 1 mm, (b) Hydrophobic islands with size of 100 lm 100 lm, (c) Droplets condensing on the pattern surface.
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Fig. 2. Schematic of the test surface.
Fig. 4. The calibration curves between ITO heater resistance and temperature.
Since the heater size (1 mm 1 mm) is much smaller than the chamber size (with diameter of 60 mm), the subcooling temperature (bulk water temperature) is around room temperature (up to 30 °C for heating 2 h according to the thermal couple in the bulk water). 3. Results and discussion Three types of surfaces (wholly hydrophilic, wholly hydrophobic, and hydrophilic–hydrophobic patterned/biphilic) were examined to assess the influence of wettability on nucleate boiling heat transfer performance. According to the experimental observation, the heterogeneous surface performed best compared to the other two homogeneous surfaces by CHF and HTC. We attribute the effect of heat transfer enhancement to several particular bubble behaviors on a hydrophilic–hydrophobic patterned surface. They are summarized as: confined nucleate sites, bubble interaction and residual nucleus. And furthermore, effects of liquid subcooling and dissolved gas were discussed.
Hydrophilic Surface Wettability Patterned Surface (Figure 1) 300
Heat Flux (W/cm2)
Fig. 3. Schematic of the experimental setup.
temperature/superheat of the heater surface. Fig. 5 showed the subcooled pool boiling curve with heat flux as a function of superheat on different heater surface. It was obviously that the heat transfer of the patterned surface was much higher than that of the hydrophilic surface. For the hydrophobic surface, after onset nucleation boiling (ONB), the bubble is trapped on the surface and then stretching on the whole heated surface. In that case, the ITO heater is over heated quickly and became malfunction. Therefore, the boiling condition for hydrophobic surface is always under unstable condition and we can only record the maximum heat flux for hydrophobic surface before the malfunction occurred. The CHF of three type surfaces are listed in Table 1. It is clear that the CHF for the wettability patterned surface is also the largest which is about 90% greater than that of the hydrophilic surface.
250 200 150 100
3.1. Heat transfer on different wettability surfaces 50
ITO was used as heater because of its electrical conductivity and optical transparency. The calibration curves between ITO heater resistance and temperature was shown in Fig. 4. It implied linear relationship between ITO resistance and the average of the surface temperature. In this case, we could derive the average of the
0
10
20
30
Superheat (K)
40
50
Fig. 5. Pool boiling curve with heat flux as a function of superheat on different heater surface.
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3.3. Bubble behavior on hydrophilic surfaces
Table 1 CHF for different surfaces. Chamber type 2
CHF [W/cm ]
Hydrophilic
Hydrophobic
Patterned
147
>52
>280
3.2. Bubble behavior on hydrophobic surfaces It is noticed that on the hydrophobic surface was very difficult for the bubble to detach, which could explain why the hydrophobic surface reached its CHF very soon when the water started to boil. As reported by Takata et al. [9] very fast film boiling on super-water-repellent surfaces was observed. In our experiments, a bubble appeared when the heat flux reached 52 W/cm2 (ONB) and then grew up until covering the whole size of the heater (1 mm2) without departure (Fig. 6). The bubble was trapped and stretching bigger and bigger on the hydrophobic surface. Surface drying out was supposed to happen thus the surface temperature of the heater rose rapidly. Even with more than one nucleation site activated simultaneously (see Fig. 7), bubbles intended to merge into a big bubble and finally dried out on near its ONB. It looks like that hydrophobic surface promotes the merged bubbles to grow on the heater surface until drying out, just the same as with a single bubble growth process. As a result, it limited the CHF for hydrophobic surface.
t
t+6s
Bubbles on the hydrophilic surface were more likely to grow one by one with a relatively large size (Fig. 8(a)–(d)). It was rare to find more bubbles appearing on the heater surface so that there was almost no bubble interaction for the hydrophilic surface. Because the bubble contact diameter is smaller than that under a hydrophobic surface, the dry out will be delayed which makes the CHF heat flux larger than hydrophobic surface. In the early stage of bubble growth, it was noted that the shapes of single bubbles on the hydrophilic surface and hydrophobic surface were totally different being a sphere for high wettability and a stretched blanket for low wettability. 3.4. Bubble behavior on wettability patterned surfaces 3.4.1. Confined nucleate sites When nucleation occurs on a wettability patterned surface, contact diameter of the bubble will grow in the hydrophobic island and eventually be confined by the size of the hydrophobic island. Because of the confined shape and size of the hydrophobic islands fabricated on the substrate, nucleate sites of bubbles can be precisely controlled on the islands during boiling (Fig. 9). When the heat flux increased, more confined hydrophobic islands were activated increasing the number of nucleate sites. To have a better
t+12s
t+18s
t+24s
Fig. 6. Bubble growth on hydrophobic surface with heat flux of 52 W/cm2.
t
t+0.8s
t+1.6s
t+2.4s
t+3.2s
Fig. 7. Bubbles merged on hydrophobic surface with heat flux of 52 W/cm2.
t
t+1ms
t+2ms
t+3ms
Fig. 8. (a)–(d) Bubble growth on monophilic surface with heat flux of 139 W/cm2.
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 9. Number of confined nucleate sites increased with increasing of heat flux: (a) 78 W/cm2, (b) 102 W/cm2, (c) 120 W/cm2, (d) 139 W/cm2, (e) 160 W/cm2, (f) 206 W/cm2.
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Two, three, four and more identical bubbles were observed to merge into one bigger bubble in the experiments (Fig. 12). Because of wettability patterns, the total contact area was confined on the hydrophobic islands unlike a big circle on the homogeneous surface. Therefore, the total length of the contact line on the biphilic surface was limited by the hydrophobic islands. The merged bubble was connected by several hydrophobic islands surrounded by liquid film on the hydrophilic regions which implied that the interfacial film was unstable for perturbations. According to Zuber [35], the dryout responsible for CHF is caused by Helmoltz instability that merge individual bubble columns. On a plain surface, the most dangers wavelength, kd, for initiating this instability of bubble columns can be calculated by [36] Fig. 10. Schematic of forces on the bubble with instability shape.
understanding on the bubble dynamics on a biphilic surface, a hypothesis was used to analyze how the bubbles tend to be confined on the hydrophobic islands. We firstly assumed that a bubble sits across hydrophilic and hydrophobic surfaces with particular contact angles as shown in Fig. 10. Because of the unbalanced contact angels on hydrophobic and hydrophilic surfaces, there’s a net lateral force pushing the bubble to the hydrophobic side, which will force the bubble siting on the hydrophobic island. This hypothesis was observed by the experimental results which confirmed that the hydrophobic islands act as nucleation sites during pool boiling. 3.4.2. Bubble interaction and unique shapes When there was more than one bubble on the surface, the neighboring bubbles would interact with each other (Fig. 11).
t
t+33ms
kd ¼ 2p
3r gðql qv Þ
12 ¼
pffiffiffi 3kc
ð2Þ
where kc is the critical wave length. Substituting the interfacial tension r between water and vapor, and ql and qv, we can get kd = 27 mm. However, on a wettability patterned surface, the liquid film is modulated by the surface wetting/non-wetting energy which modify the instability wavelength. According to Sinha et al. [37], a change in the system wettability causes a perturbation in the system’s flow pattern to destabilize any percolating and trapped immobile clusters appeared in the steady state. This implied that the perturbation of wettability patterns may affect the bubble departure diameter and frequency. Because the pattern dimensions (hydrophobic pads and pitches) are much smaller than kd, it is anticipated that the bubble departure diameter can be smaller and the departure frequency can be higher than those on a homogeneous surface. In our experiments (see Fig. 11), it is seldom that the diameter of
t+5ms
t+7ms
t+50ms
t+52ms
t+12ms
t+54ms 2
Fig. 11. Bubble interaction on patterned surface with heat flux of 206 W/cm .
Fig. 12. Bubble merger with increasing numbers of identical bubbles with heat flux of 139 W/cm2.
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Fig. 13. Residual nucleus remain after part of bubble depart from pattern surface.
merged bubbles could go beyond 0.5 mm, which means the bubble departure diameter is controlled by the perturbation of wettability. In our experiments, it was observed that some bubbles broke up frequently after being merged from small bubbles. However, this kind of phenomenon could not be found from the smooth surfaces. This further confirmed that the perturbation caused by the wettability pattern that moderated the critical wave length of the Helmholtz instability [13]. Another important phenomenon was that when the bubble departed from the biphilic surface, some residual bubbles may remain on the hydrophobic islands acting as nucleation sites. Similar results were also reported by Jo et al. [17] from section view of pool boiling with one small bubble remained on the surface when the mother bubble departed from the substrate. In our case, the departure of a merged bubble would leave more than one daughter bubble remaining on the hydrophobic islands compared to a wholly hydrophobic surface (Fig. 13). When more than one bubble merged into a larger one on the patterned surface, the shape of the merged bubble was quite different from bubbles that merged on a homogenous surface. The
contact line under the bubble was more likely to shift from the hydrophilic network to the edge of the hydrophobic islands which was also the reason why identical bubbles on a patterned surface were always confined on islands. In the experiments, there were mainly three shapes of merged bubbles: bridge-shaped bubbles, peanut-shaped bubbles and bowl-shaped bubbles (Fig. 14). A bowl-shaped bubble was generally formed when more than two bubbles merged while others two shapes were normally formed when only two bubbles merged. The bridge-shaped bubbles usually formed at the beginning when two identical bubbles merged into one with two stems attached to the solid surface. The contact line connected along the periphery of the hydrophobic islands or partially shifted to the hydrophilic network, with some water remaining under the ‘‘bridge’’. Some previous work showed a similar result photographed from the side view, calling this kind of shape a mushroom type bubble [4]. The peanut shape was usually the next step of the bridge shape. Evaporation of water under the ‘‘bridge’’ resulted in affecting the surrounding liquid and bubble motion and changing in bubble dynamics caused by the liquid–vapor interaction phenomena by
Fig. 14. Schematic and photographs of unique shapes of merged bubbles: bridge-shaped bubble, peanut-shaped bubble and bowl-shaped bubble.
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shifting of the contact line so that the two stems (dry spot on islands) enlarged into an elongated shape to form the peanut shape. For the bowl shape, a droplet was trapped inside a large bubble merged by several small bubbles due to quick process of merging. The latent heat of the trapped droplet under these bubbles with unique shapes would extra contribute to the HTC. Instability of the asymmetrical bubble shape resulted in local convective microflow surrounding the bubbles.
the hydrophilic surface. As we have discussed before, this is caused by the wettability perturbation induced insatiability. For the bubble departure period (departure frequency), as previously mentioned, there were residual bubbles left on the substrate after the larger bubble departed and there was no waiting time for the hydrophobic surface and patterned surface when compared to the hydrophilic surface. 3.6. Effects of liquid subcooling and dissolved gas
3.5. Comparison of bubble dynamics for the three surfaces We consider nucleate site density, bubble departure diameter and bubble period to evaluate bubble dynamics in pool boiling. A larger nucleate site density, smaller bubble departure diameter and shorter bubble period (total period of growth, departure and waiting time) resulted in more efficient heat transfer. According to recorded images, nucleate site densities under different heat fluxes on the different heater surfaces were calculated. Around 30–40 thousand images were used to count the average number of bubbles nucleated on the surface as in Eq. (3).
qn ¼
PN
i¼1 ni
ð3Þ
N
The results are shown in Table 2. For the patterned surface, the total number of hydrophobic islands that acted as controlled nucleation sites was 25. It was noted that, with heat flux of 206 W/cm2, there could be 18 small bubbles activated on the islands at the same time. The average nucleation site density is 13.9 (1.39 107 sites/m2) because of bubble coalescence, which is about 6 times higher than that of hydrophilic surface. Bubble departure diameter was also an evaluation parameter for heat transfer in pool boiling. As stated above, there was normally only one big bubble on both hydrophilic and hydrophobic surfaces while more bubbles nucleated on the patterned heater surface. We calculated the diameter of around 25 bubbles as they departed from the surface and obtained an average departure diameter as shown in Table 3. In Table 3, it is clear that the bubble departure diameter on the patterned surface is about 1/3 of that on
Table 2 Active nucleate site densities with increasing heat flux on different surfaces. Heat flux [W/cm2]
52 78 102 120 139 160 206
Nucleate site density [sites/mm2] Hydrophilic
Hydrophobic
Patterned
0 0 1 1 1.1 – –
1 – – – – – –
0 1.2 3.4 5.1 6.5 8.3 13.9
Table 3 Bubble departure diameter with increasing heat flux on different surfaces. Heat flux [W/cm2]
52 78 102 120 139 160 206
Departure diameter [mm] Hydrophilic
Hydrophobic
Patterned
– – 1.4 1.1 0.93 – –
– – – – – – –
– 0.73 0.38 0.38 0.37 0.35 0.31
According to Rainey et al. [38], large subcooling will cause the bubble departure diameters and frequencies decrease which reduces the amount of heat transferred through latent heat and micro convection. However, increased subcooling also decreases the superheated liquid layer thickness which increases natural convection heat transfer and increases Marongoni convection heat transfer. These effects compensate each other and the overall effects show that it is relative insensitivity of the nucleate boiling curve to liquid subcooling. For the effect of dissolved gas, in subcooling, as mentioned by Rainey et al. [38], the dissolved air enhances heat transfer at low heat fluxes just after incipience but not at higher heat fluxes. The CHF values of the gas-saturated cases were almost the same as the degassed subcooled liquid. For small heater size (such as small wire heaters), the CHF values maybe reduced for the subcooling liquid with dissolved gas. However, the effect is diminished as the heater size increased. In our experiments, our purpose is to compare the wettability patterned surface with hydrophilic and hydrophobic surfaces. The size effect on the CHF with dissolved gas subcooling condition was not considered and which may be studied in the future. 4. Conclusions In conclusion, this study demonstrated how hydrophobic islands worked as localized and controlled nucleate sites even with relatively small heater size. The patterned surface performed the best in heat transfer for subcooled pool boiling in comparison with hydrophilic and hydrophobic surfaces. The nucleation site density of biphilic surface was much higher, when compared to that of the homogeneous surface (72% of hydrophobic islands or 18 of the 25 islands, could be activated simultaneously with heat flux of 206 W/cm2 on the patterned surface). The latent heat of the trapped droplet under bubbles with unique shapes on the biphilic surface would extra contribute to the HTC. Wettability perturbation affects the flow instability which induced local convective microflow surrounding the bubbles. This can affect the merged bubble breakup and departure frequency. The departure diameter was about 1/3 of the size on the homogeneous surface. Smaller bubbles grew and departed on the heater quickly, resulting in a higher heat transfer coefficient. Bubble interactions and instability of big bubbles resulted in both a larger critical heat flux and higher heat transfer coefficient. In the meantime, residual bubbles that remained as the boiling nuclei further promote boiling on the biphilic surface which also contributed to the heat transfer efficiency. Compared to the hydrophilic and hydrophobic surfaces, the biphilic surface had the best heat transfer performance in our study although more parameters such as pitch distance, shape and diameter of the hydrophilic island need optimizing. Conflict of interest None declared.
X. Chen, H. Qiu / International Journal of Heat and Mass Transfer 88 (2015) 544–551
Acknowledgment This research is supported by the Research Grants Council (RGC) of the Government of the Hong Kong Special Administrative Region (HKSAR) with Project Nos. 618210 and 617812.
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