Article pubs.acs.org/Langmuir
Influence of the Wettability on the Boiling Onset B. Bourdon,† R. Rioboo,*,† M. Marengo,‡ E. Gosselin,† and J. De Coninck† †
Laboratoire de Physique des Surfaces et des Interfaces, Université de Mons, Parc Initialis, Av. Copernic, 1, B-7000 Mons, Belgium Faculty of Engineering, University of Bergamo, Viale Marconi 5, 24044 Dalmine, Italy
‡
ABSTRACT: Experimental investigation of pool boiling is conducted in stationary conditions over very smooth bronze surfaces covered by a very thin layer of gold presenting various surface treatments to isolate the role of wettability. We show that even with surfaces presenting mean roughness amplitudes below 10 nm the role of surface topography is of importance. The study shows also that wettability alone can trigger the boiling and that the boiling position on the surface can be controlled by chemical grafting using for instance alkanethiol. Moreover, boiling curves, that is, heat flux versus the surface superheat (which is the difference between the solid surface temperature and the liquid saturation temperature), are recorded and enabled to quantify, for this case, the significant reduction of the superheat at the onset of incipient boiling due to wettability.
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INTRODUCTION The current trend of miniaturization and of increase of functionality in electronic components leads to strong heating in materials such as conductors and semiconductors. Controlling the heat flux and the temperatures becomes thus crucial. On the other hand, it is known that phase-change or multiphase cooling systems present the highest cooling capacity.1−3 For instance, with the phase-change, electronic cooling is often orders of magnitude more efficient than monophase systems. Correspondingly, the three-phase zone is of primary importance as most of the heat is dissipated in the contact line region4,5 where phase-change is concentrated. In this zone, the liquid−gas interface is highly curved at the microscale region, and both topography and wettability characteristics are at least as important as the solid thermal conductivity.6 When a solid surface in contact with a cooling liquid is heated, before the liquid boils, a so-called superheat, ΔT, appears where ΔT = Tw − Tsat, where Tw is the temperature of the wall and Tsat is the liquid saturation temperature. Thus, it is necessary to reach a surface temperature higher than the equilibrium saturation temperature to activate the boiling phenomenon. For safety and energy-saving reasons, the decrease of the superheat is important in many applications. A simple way to achieve that is to coat the solid surface by some hydrophobic layer. This has been studied before.7−9 Controlling the location of boiling by microscopic treatment down to the nanoscale is still a © 2011 American Chemical Society
challenge. In fact, in microdevices, the surface roughness should be kept as low as possible, because its scale may be near to the refrigerant channel size, to avoid important side-effects such as very high pressure drops. Understanding the complexity of the nucleate pool boiling is still a challenge as all surfaces features such as detailed topography and wettability down to nanometric scales are relevant and influence the heat exchange.10,11 Until recently, surface wettability modification to study boiling has always involved cavities12−16 as nucleation sites. Several papers have shown that the use of nanoparticles could enhance the heat transfer16−19 with uncontrolled cavities resulting from the nanocoating method. Thomas et al.20 and Balss et al.21 showed that on smooth uniform surfaces, low wettability decreased the onset temperature for nucleate boiling in fast transient events. Zhang and co-workers22 investigated the stability of air nanobubbles at the interface between a hydrophobic solid surface and water at ambient temperature. Hibiki and Ishii23 showed that the number of nucleation sites is a function of the wettability of the surface, while Agrawal and co-workers24 proved on isothermal systems that nanobubbles are positioned on hydrophobic patterns. The nucleation theory shows that the free energy to create a vapor nucleus of critical Received: September 16, 2011 Revised: December 2, 2011 Published: December 13, 2011 1618
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Figure 1. Left: Experimental setup. Right: “T”-shape plate (scale bar is 2 cm). monolayers were added to the entirety of the solid surface or only on its central zone presenting thus various patterns. The plate, which has a shape of a “T”, is made in bronze, which is highly polished on its top and then covered with gold by sputtering. Contrary to our previous study26 where silicon was used as the solid material, we here use bronze. This material has been chosen because of its low enough thermal conductivity (64 W/m·K). In fact, with a higher thermal conductivity (like for silicon or copper), the temperature difference between the central zone of the circular plate and its borders is low, and boiling may first appear at the unavoidable heterogeneities along the circumference. The experimental setup is depicted in Figure 1. It can be noticed that the heater that is inserted in the bronze plate and connected to a power supply is only located, looking from above, on the central part of the surface. In a first experiment,32 we noticed that using a cylindrical copper housing around the heater, the onset of boiling appeared more easily on the borders of the heated polished copper surface and prevented one from quantifying the boiling on its smooth central part. This can be explained by the cavities and the high roughness inherently present on the edges of the cylinder. To avoid such a problem, we used a “T”-shape bronze piece presenting a large plate above the heater (diameter of 80 mm) and concentrated our experiments only in the central part of this plate (15 mm diameter) to ensure a high enough temperature to get boiling on this part and not at the edge of the plate.33 Polishing Method. The bronze used, named RG7, presents the following composition: Cu 79−85%; Zn 4−6%; Sn 4−6%; Pb 4−6%; rest 3.75%. The “T”-shape plate is mirror polished on its top part to minimize the surface roughness using a Struers TegraPol 25 polishing system. In two prepolishing steps, Struers SiC papers of grain 220 and then 320 were successively used during 1 min at 150 rpm with a constant load of 160 N each. Next, a series of three polishing steps were performed using MD polishing discs (first MD Largo then MD Mol and finally MD Chem) at the same load (160 N) during 6 min, then 4 min, and finally 1 min using, respectively, Diapro Allegro Largo, then Diapro Mol, and finally OPS suspensions at, respectively, 170 rpm for the first and second steps and 160 rpm for the last step. This last polishing step is continued during 8 min with additional Milli Q water rinsing. After the polishing procedure, the “T”-shape plate is copiously rinsed with Milli-Q water. A gold layer of 24.8 nm ± 3.5 nm is sputtered on its top using a Denton Vacuum Desk V sputter. Every time the surface is polished, the resulting topography is different, and thus the roughness parameters (such as amplitude) are different. Grafting Method. The grafting method was derived from Tsao and co-workers29 and Voué and co-workers30 and is summarized here.
size for the case of heterogeneous nucleation is a function of the wettability.25 The shape of the nanobubbles created on the surface is in fact a function of the contact angle. In our previous study,26 we suggested that the nucleation sites were originated by air nanobubbles. We showed that in flow boiling experiments (but with low volume flows), the boiling zone was following hydrophobic patterns made on atomically smooth surfaces made of OTS grafted silicon wafers. More recently, Forrest and co-workers27 showed the influence of wettability on pool boiling with various different uniform coatings on a wire. Differences of up to 7 °C were noticed on the onset of nucleate boiling, with a minor change of the surface roughness (roughness was not measured directly on the wires, but mean roughness amplitude, Ra, varied from 225 to 290 nm on similarly coated metal plates). Nanofluids and surfactants can also be a possible strategy to decrease the superheat. On an alumina surface (the roughness was not measured) and using different surfactants, Wen and Wang28 showed that the value of the superheat at the onset of nucleate boiling could be lowered from 14 to 7 °C depending on the type and concentration of surfactant in water. These results suggest that either surface tension or wettability or both influence the onset of nucleate boiling. From these results, it is still difficult to acknowledge whether increasing only the surface wettability decreases the onset of nucleate boiling and, quantitatively, which part of the superheat is due to wettability effects, surface tension variation, or even roughness effects. The goal of the present study is to quantify the influence of the wettability in terms of surface temperature and heat fluxes. The boiling curve, that is, the heat flux versus the superheat of the solid surface, is recorded for various surface treatments. We will focus the research in the convection part of the boiling curve up to the boiling onset and the starting of the boiling regime in the boiling curve. This Article is organized as follows: after describing the various surface treatments, we will present the role of the sole roughness and then that of the wettability before concluding this Article.
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EXPERIMENTAL PROCEDURE
A series of pool boiling experiments were performed with water on a solid plate, which was modified in various ways. For some experiments, the solid was chemically grafted, while for others it was not. Grafted 1619
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Table 1. Roughness Parameters for the Surface Polishing (in nm) Used in the Study Measured from Images (955 μm × 1275 μm) Taken with a 10× Magnification Objective and an Optical Profilometer surface polishing
mean roughness amplitude, Sa
rms roughness amplitude, Sq
maximum peak to valley roughness amplitude, Sz
reduced valley depth, Svk
maximum valley height, Sv
maximum peak height, Sp
S1 S2 S3
9.7 ± 1.1 7.6 ± 0.9 7.9 ± 0.9
12.6 ± 1.4 9.7 ± 1 10 ± 1
137.3 ± 11.5 97 ± 8.1 86 ± 7.2
17.2 ± 0.4 12.7 ± 0.3 11.3 ± 0.3
95.5 ± 13.6 67.2 ± 9.5 45.4 ± 6.4
41.8 ± 5.2 29.8 ± 3.7 40.8 ± 5
Figure 2. AFM image of the typical topography of polished surfaces at a scale of 5 × 5 μm. Left: 3D view of the topography. Right: Amplitude of the signal. Ra is 5.8 nm; Rz is 44.9 nm. The surface is first cleaned by an exposure to UV and ozone during 10 min in a UV/O3 oven. The surface is then immersed in 120 μL of tetradecanethiol (Sigma-Aldrich) in 100 mL of ethanol during 2 h.29 After being rinsed with ethanol, the surface is ultrasonicated during 15 min in ethanol and dried with nitrogen. The grafting of the alkanethiol surface was checked using IR spectroscopy. As the alkanethiol SAM grafting on gold has a low durability at ambient conditions,31 we performed the boiling experiments for less than 2 h after each grafting. We have checked that the grafting was present after the boiling experiment by contact angles measurements. In addition, we have used the high sensitivity of Fourier transform infrared spectroscopy (FTIR) associated with a SAGA accessory (Thermo Scientific), which is particularly adapted for grazing incidence analysis of submicrometer graftings on metallic substrates. IR spectra were recorded on an FTIR 6700 infrared spectrophotometer (ThermoFischer) equipped with a mercury−cadmium−telluride (MCT) detector at a resolution of 2 cm−1 with a mirror speed of 0.6329 cm/s. For each measurement, 32 scans were collected and averaged to obtain the final absorbance spectrum. The spectrometer was continuously purged using an air dryer (Parker-Zander, Germany) at a flow rate of 30 standard cubic feet per hour (SCFH). The nongrafted surfaces were cleaned using the UV/O3 procedure to provide a very hydrophilic surface. Advancing and receding static contact angles were measured with a Krüss DSA100 drop shape analyzer. Pool Boiling Procedure. The solid surface temperature was measured by means of three thermocouples (K type) inserted in the bronze piece as close as possible to the surface (see Figure 1). We verified the accuracy of the temperature measurements by also comparing to the measures using an infrared camera in the range from 90 to 120 °C in air. The temperature measured by this infrared camera FLIR D335 (resolution, 320 × 240 pixels; precision, 50 mK) of the solid surface was similar to the values determined by the thermocouples within the precision of the infrared camera. This allows us to consider that the thermocouple measurements in water accurately provide the solid surface temperature. Nevertheless, the precision of the K-type thermocouples is imposed at the standard value of ±0.5 °C and defines our temperature measurement precision. Three additional thermocouples were used to verify the temperature in the water just above the central zone of the
bronze disk, in the Teflon housing (at 3 mm from the heater), and in the ambient atmosphere surrounding the system. Atmospheric pressure was recorded to calculate the saturation temperature, which had an average value of 100.0 ± 0.2 °C. Milli-Q water was used to perform the boiling curve. It was degassed before the experiment by boiling for 1 h prior to experiments.34 In the water pool (Figure 1), two additional heaters enable one to heat the water to the saturation temperature. In this way, the water is kept degassed in the same way all along the experiments. The procedure was carried on always in the same way for all pool boiling experiments, ensuring repeatability on the present results.34−37 The boiling curve (surface temperature versus heat flux) is determined by increasing the heat power by a tiny amount (typically between 1 and 2 W on the power supply), and a time of 3 min is waited before measurement of the temperature to ensure a nontransient behavior. Nevertheless, despite waiting for a stabilization period of 1.5 h before starting the recording of the experimental data, we could notice that the values of the heat flux in the lower half of the convective part of the boiling curve were overestimated because of the temperature variation of the Teflon insulation. The time lags defined above were chosen to avoid any important effect on the onset of nucleate boiling, keeping the total duration of the experiment minimized to avoid surface modifications (chemical or topographical).31,32,38 The heat flux on the following figures is averaged over the total area of the “T”-shape plate top surface. To ensure reproducibility, experiments were carried out three times for each case. The boiling curves presented in this Article show averaged values of these experiments.
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RESULTS AND DISCUSSION
Surface Characterization. Mirror-polished bronze surface roughness values were measured using an optical profilometer Sensofar PLu Neox at a 10× magnification (i.e., images of 955 μm × 1275 μm) on at least three locations of the surface. Values of the surface roughness parameters are summarized in Table 1. Atomic force microscopy has been performed to determine the roughness at submicrometer scale. It can be seen in Figure 2 that the roughness amplitude is diminished on a 5 × 1620
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Figure 3. FTIR spectra of the (A) ungrafted surface, the grafted surface before boiling (B), after the starting of the boiling (C), and after the whole boiling experiment (D).
Figure 4. Boiling curves (heat flux versus surface superheat) versus the roughness on three different ungrafted surfaces. Details of roughness parameters are given in Table 1.
5 μm area, which is logical. The relative smoothness and regularity of the surface are shown in this figure. Several locations (at least five) have been tested with compatible results. The nongrafted surfaces are very hydrophilic; that is to say that both advancing and receding static contact angles were null prior to boiling. As described in our previous study,26 the contact angles after experiments on the ungrafted part are significantly increased (between 20° and 40° with low repeatability) due to the long duration of the experiment and possible contamination due to open air access of the setup.
For the grafted surface, the values of the advancing and receding static contact angles are, respectively, 108.1° (±1°) and 98.6° (±1°). The value of the wettability hysteresis corresponding to the grafted part is low enough to consider that the grafting is homogeneous and compact. It is indeed known that various alkanethiol grafting will give an hysteresis ranging from 7.5° to 10° depending on the length of the alkyl chain on smooth gold-coated silicon wafer.30 This indicates that from a wettability point of view, the surface is smooth enough to be comparable with a flat surface.39 The wettability was measured after boiling experiments and resulted in 108.2° (±1.8°) for the advancing static contact angle and 89.2° 1621
dx.doi.org/10.1021/la203636a | Langmuir 2012, 28, 1618−1624
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Figure 5. Example of boiling dynamics with a grafting having a half-disk shape in the central part of the plate after the onset of boiling taken at 45° tilt to see the bubble’s departure from the surface at constant heat flux. Black circles are added to visualize the grafted zone and the heated one. Scale bar is 1 cm. Bottom right: Top view.
such as the maximum valley depth height or the maximum peak height because the measurement is taken over a surface instead of a line. Several empirical correlations exist in the literature that determine the onset of nucleate boiling and/or the boiling curve as a function of the various influencing parameters, among which is average roughness.13,28,42 Table 1 presents some of the main surface roughness parameters. While the onset of incipient boiling temperature is increasing from S1 to S3, we can see that several roughness parameters are continually decreasing from S1 to S3 such as the maximum peak to valley roughness amplitude or the maximum valley height, while others such as mean roughness amplitude do not. This suggests that usual parameters as mean or even rms roughness amplitudes may not be the most relevant parameters. A more detailed study of the boiling phenomenon due to surface topography is necessary25,43 and is beyond the scope of this Article. Nevertheless, it imposes us to quantify as much as possible the roughness characteristics44 and work with surface as smooth as possible to isolate the wettability effects. Indeed, in Figure 4, it is possible to see that the onset of nucleate boiling appears at different wall superheat with different roughnesses. It is possible to calculate, using linear correlations in the convective part and the early boiling part of the boiling curve, the moment of incipient boiling. Using the bootstrap technique,45 error bars on these temperatures can be estimated. It varies from 8.8 °C (±0.4 °C) to 12.3 °C (±1.4 °C) from the rougher to the smoother surface polishing S1−S3. Despite the fact that these surfaces are much smoother than previous studies, which quantify the superheat in pool boiling experiments,16,27,38,46,47 we show that roughness is still of importance at values below 10 nm for the mean roughness amplitude (Sa), around 10 nm for the rms roughness amplitude (Sq), and 150 nm for the maximum valley−peak height (Sz).
(±1.8°) for the receding one. The decrease of the receding static contact angle is attributed to the possible degradation by oxidation of the alkanethiol grafting,31 but could also partly be due to the low stability of the alkanethiol grafting with temperature.40 Complementary FTIR analysis has showed that the grafting is still present after the boiling experiments as shown in Figure 3. In the 2820−2980 cm−1 region of the spectra, very weak C− H asymmetric and symmetric stretching vibrations bands are observed around 2920 and 2854 cm−1, which correspond to CH2 vibrations, and around 2966 and 2877 cm−1 for the CH3 vibrations. These infrared values are indicative of weakly organized monolayers.41 Nevertheless, these alkanethiol grafted surfaces characterized here remain stable during the boiling experiments as can be seen by comparing the curves in Figure 3 before (B), during (C), and after (D) boiling. Roughness Influence. In Figure 4, it is possible to see an example of the boiling curve, which is typical of such experiment. On the ungrafted surfaces, as soon as the boiling starts, the slope of the boiling curve changes. In the first part of the curves, convection alone is cooling the surface, and a low linear slope can be seen. When boiling appears, the cooling is more efficient, and the slope of the linear approximation is larger. The heat transfer coefficients extracted from our data are comparable to that of the literature42,43 (typically
