Nucleate boiling performance on nano/microstructures with different wetting surfaces
© Jo et al.; licensee Springer. 2012
Received: 23 August 2011
Accepted: 6 May 2012
Published: 6 May 2012
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© Jo et al.; licensee Springer. 2012
Received: 23 August 2011
Accepted: 6 May 2012
Published: 6 May 2012
A study of nucleate boiling phenomena on nano/microstructures is a very basic and useful study with a view to the potential application of modified surfaces as heating surfaces in a number of fields. We present a detailed study of boiling experiments on fabricated nano/microstructured surfaces used as heating surfaces under atmospheric conditions, employing identical nanostructures with two different wettabilities (silicon-oxidized and Teflon-coated). Consequently, enhancements of both boiling heat transfer (BHT) and critical heat flux (CHF) are demonstrated in the nano/microstructures, independent of their wettability. However, the increment of BHT and CHF on each of the different wetting surfaces depended on the wetting characteristics of heating surfaces. The effect of water penetration in the surface structures by capillary phenomena is suggested as a plausible mechanism for the enhanced CHF on the nano/microstructures regardless of the wettability of the surfaces in atmospheric condition. This is supported by comparing bubble shapes generated in actual boiling experiments and dynamic contact angles under atmospheric conditions on Teflon-coated nano/microstructured surfaces.
Boiling is a general mechanism in heat transfer systems, such as those used to cool electronic devices and power plant systems. In boiling, the two most important parameters are (1) the boiling heat transfer (BHT), which is directly related to the efficiency of a thermal device, and (2) the critical heat flux (CHF), which requires a safety limitation for the system. Therefore, to transfer or dissipate high heat flux from heat sources in real-world applications, thermal devices and systems should have high BHT and CHF. Over the past century, many techniques for enhancing BHT and CHF have been developed. Most recently, boiling experiments with treated surfaces have been used extensively to study the effect of heating surface characteristics on BHT and CHF.
Of the many surface characteristics, wettability and surface geometry are the key parameters for determining boiling performance. By affecting the dynamics of the phase interface adjacent to the heating surface, wettability and surface geometry influence overall nucleate boiling phenomena, from activated nucleation sites to CHF. In particular, a number of researchers have reported CHF enhancement on well-wetted surfaces [1–3]. The effect of wettability on CHF has also been confirmed by pool boiling experiments with nanofluids, using surfaces modified by nanoparticle deposition, which have recently drawn considerable attention due to the striking CHF enhancement obtained even with very low nanofluid concentrations [4, 5]. Finally, the effect of wettability on CHF is reflected in Kandlikar’s theoretical CHF model, which includes a dynamic contact angle term . Cavity geometry is also significantly related to boiling performance, especially in terms of activated nucleate site density, which is directly related to BHT enhancement. The shape or roughness of the microstructures determines the activated nucleate site density [7–9]. Moreover, microstructures also influence the CHF [10, 11]. However, there are still obstacles to the realization of very high heat generation in thermal devices and systems.
The recent introduction of nanostructured surface modification techniques has opened a new chapter in the study of boiling phenomena and the development of very high heat transfer systems. The fabricated nanostructures provide greatly enhanced boiling performance [12–16]. In particular, reinforced capillary pumping of the working fluid onto the heated surfaces of nanostructures is the main contributor to an abnormally high CHF [13, 17]. Kim et al.  used a ZnO surface to fabricate micro-, nano-, and micro/nanostructured surfaces and reported CHF improvement on these surfaces. Chen et al.  conducted pool boiling experiments on Si and Cu nanowires and reported dramatically enhanced CHF and BHT on the nanostructures. Ahn et al.  used liquid spreading phenomena to analyze the high CHF on zircaloy-4 nanostructured surfaces. However, even with these innovative studies, it has not been possible to draw any general conclusions on the performance of nanostructures in boiling systems, due to the lack of experimental boiling data on nanostructures with various surface characteristics. In particular, previous studies of boiling phenomena on nanostructures have focused exclusively on hydrophilic surfaces. Such biased reports in wettability could cause misunderstanding to the analysis of boiling phenomena because of complicatedly coupled surface factors in one boiling phenomenon: the effect of surface structure and wettability [18, 19].
In this research, we conducted pool boiling experiments on hydrophilic and hydrophobic nanostructured surfaces with identical nanostructures to classify the wetting and the surface structural effect on nanostructures. The nanostructures (black silicon) were fabricated via a surface treatment procedure to create different wetting surfaces (super-hydrophilic and super-hydrophobic) with exactly the same surface structure and were used as heating surfaces to study boiling phenomena and performance. The results are expected to provide an important contribution in distinguishing the effects of surface structure and wetting on the basic mechanisms of BHT and CHF enhancement due to nano/microscale structures with totally different wetting characteristics.
The specimens were designed for both micro- and nanoscale structures, with a heater on the backside. To form the microstructures, an anisotropic wet-etching technique was used. The nanostructures were then fabricated on the microstructures by deep reactive-ion etching (DRIE). The presence of microstructures with sloped rather than vertical sidewalls is an important condition to ensure conformal formation of nanostructures. In order to fabricate microstructures with sloped sidewalls, a (100) silicon (Si) wafer was selected, owing to its unique sloped sidewall profile during the wet-etching process. A layer of thermally grown SiO2 was then formed on the wafer, and the top layer was patterned, using a photolithographic technique, as an etching mask for microstructure formation. Tetramethylammonium hydroxide was used to etch the exposed area of the (100) Si wafer for 12 min at 90 °C. After the microstructures were formed, the Si etching mask was removed. A 20-nm Ti layer and a 150-nm Pt layer were then deposited and patterned on the backside of the wafer for Joule heating. Nanograss structures were then fabricated by DRIE (specifically, the black silicon method). Conformal formation of silicon nanograss structures was made possible by the sloped sidewalls.
Additional surface treatments were used to realize different wettabilities on the same surface structure. Two surface treatment techniques were used in this research. The first of these was the O2 plasma technique, which was used to clean up some of the residue from the DRIE procedure and accelerate the growth of the native oxidation layer on the silicon surface. Due to the effect of the plasma, the exposed surfaces became super-hydrophilic for a few days. However, even though the effect of the plasma exposure was eliminated after a few days, the treated surfaces retained super-hydrophilic characteristics because of the mixed effect of natively hydrophilic silicon on surface smoothness and roughness [18, 19]. In this study, we will refer to this type of treated surface as an ‘oxidized silicon nano/microstructured surface’ to indicate the presence of nano/microstructures with a native oxide layer on the silicon surface. The second surface treatment technique was used to create a hydrophobic surface on the nano/microstructures, by applying a Teflon coating to the plasma-exposed nano/microsurfaces. Robust Teflon-coated surfaces were fabricated via a spin coating procedure and a soft baking procedure at 90 °C for 10 min. This type of treated surface is called a ‘Teflon-coated nano/microstructured surface’ in this paper.
The test apparatus was designed to carry out pool boiling experiments under atmospheric pressure via the electrical Joule heating method, using an HP Agilent 6575A DC power supply (Santa Clara, CA, USA). The main test pool was an octagonal aluminum bath (with a capacity of 3 L) and was maintained in a saturated condition by a proportional-integral-derivative temperature controller. A high-speed camera (Redlake MotionXtra HG–100 K, San Diego, CA, USA) was installed on the visualization glass to capture images of the bubble dynamics in the nucleate boiling regime. To facilitate both heating and surface modification, a thin-film heater was embedded on one side of the silicon wafer, and artificial surfaces were created on the other side of the wafer via microelectromechanical system techniques. Taking all instrument errors into account, the maximum uncertainties of the heat flux, wall superheat, and heat transfer coefficient were estimated to be less than 15 kW/m2, 1.5 °C, and 0.56 kW/m2 °C, respectively, over the expected CHF range . A numerical simulation was conducted to incorporate the effects of heat dispersal and heat loss. Based on the measured bottom heat flux and temperature information, the reduction procedure was repeated with several heat transfer coefficients until the calculated values matched the experimental magnitudes. The heat transfer coefficient was determined via numerical simulation, by finding a value that satisfied the experimentally obtained target condition.
Before experimenting with fabricated nano/microstructured surfaces, experiments were conducted on bare oxidized silicon and Teflon-coated surfaces (i.e., without any structures), and the results were analyzed to re-establish baselines. The wetting characteristics of a heating surface affect the overall nucleate boiling mechanism, including CHF and BHT, by influencing generated bubble dynamics on the surface. Good wettability leads to a higher CHF than poor wettability [1, 6, 20]. Based on this tendency, Kandlikar postulated a mechanism for CHF that includes the wettability effect . In this study, the CHF values for bare oxidized silicon and Teflon-coated surfaces were found to be 786.23 and 178.98 kW/m2, respectively. A difference of this magnitude between hydrophilic and hydrophobic surfaces is consistent with the experimental tendencies found in the literature for various wetting surfaces.
Such highly enhanced CHF values on nanostructures have been reported in a number of recent studies. Chen et al.  carried out pool boiling experiments on Si and Cu nanowires and observed respective CHF increases of 132.88 % and 139.04 % over a plain Si surface. In the work of Kim et al. , the reported CHF improvement ratios on microstructures, nanostructures, and nano/microstructures ranged from 47.37 % to 107.49 %. The significant CHF improvement on fabricated nanostructured surfaces can be elucidated by considering the effect of capillary phenomena, which carry cooler working fluid into the hot spots to rewet them and delay CHF occurrence [24, 25]. Ahn et al.  observed CHF improvements ranging from 17.63 % to 55.78 % on nanostructures and nano/microstructures and analyzed the different CHF enhancement ratios on surfaces where capillary dispersal occurred, by combining Kandlikar’s wetting consideration and the capillary spreading rate effect. Such variance of CHF increment on nanostructured surfaces is caused by the topological effect of the nanostructures on capillary phenomena . Consequently, it is certain that the liquid dispersal and hot-spot cooling mechanism provided by capillary phenomena is a dominant factor in the higher CHF values observed on nanostructured surfaces (compared to plain surfaces). Here, the dispersal phenomena were confirmed on the oxidized silicon nano/microstructured surface via a surface characterization procedure (the measured contact angle being 0°), together with their beneficial effect on the CHF in this case. However, CHF enhancement on a Teflon-coated nano/microstructured surface cannot be directly explained in terms of capillary wicking phenomena since the dispersal characteristics are absent with a very high contact angle.
We now propose a plausible mechanism to explain the significantly enhanced CHF on the Teflon-coated nano/microstructures as well as the CHF difference between the oxidized silicon and Teflon-coated nano/microstructured surfaces. According to a previous X-ray scattering study, water is able to penetrate 5 to 10 nm into nanoscale hydrophobic cavities, independent of cavity depth, when a hydrophobic nanostructure is immersed in water . This means that the contact angle on a hydrophobic nanostructure under actual boiling conditions can be dramatically reduced from the dynamic contact angle measured under dry conditions since it will be affected by water seeping into the hydrophobic nanostructures. We can confirm this phenomenon by comparing our visualization results, which include the difference between the dynamic contact angle of a water droplet on a dry surface in air (Figure 1) and that of a surface wetted by surrounding water under actual boiling conditions (Figure 3). According to Kandlikar’s prediction, the main mechanism of enhanced CHF with increasing wettability is related to the generated bubble shape under actual boiling conditions. Hence, enhancement of CHF with reduction of the contact angle of generated bubbles on a heating surface is consistent with Kandlikar’s prediction. Nevertheless, even when we include the effect of the generated bubble shape, the dramatically enhanced magnitude on the Teflon-coated nano/microstructured surface cannot be described solely in terms of the wettability effect, as Figure 3 indicates.
As was previously mentioned, capillary phenomena have already been acknowledged as the main reason for CHF enhancement on nano/microstructured surfaces. Thus, the additional CHF increment on the Teflon-coated nano/microstructures might have been caused by capillary phenomena fueled by penetrating water. Based on the earlier report on hydrophobic nanostructures cited above, the water penetration depth should have been 5 to 10 nm, even though the depth of the nanocavities was over 100 nm. However, 5 to 10 nm of penetration seems inadequate to produce such a high CHF on a Teflon-coated nano/microstructured surface since such a value is not much different from the roughness of a cleaned surface. The degassing procedure (for removing dissolved gas from water), which was conducted before the main experiment, appears to play an important role in resolving this issue. This procedure induces some of the nanobubbles to coalesce and be detached from the surface by buoyant forces . Therefore, after the degassing procedure, water will have a better chance of penetrating deeper than 5 to 10 nm into the vacancy left by the detached nanobubbles. (Of course, it cannot completely fill the nanocavities since degassing cannot remove all nanobubbles from the nanostructure.) Accordingly, it is our hypothesis that the combined effects of degassing and water penetration into the nano/microstructures via capillarity contributed to the enhanced CHF on the Teflon-coated nano/microstructures.
Furthermore, it can be hypothesized that differing capillary phenomena on structured surfaces with different wettabilities caused the CHF difference between the oxidized silicon nano/microsurfaces and the Teflon-coated nano/microsurfaces. According to the bubble dynamics visualization results shown in Figure 3, the apparent contact angles of oxidized silicon and Teflon-coated nano/microstructures were almost the same under actual boiling conditions. Hence, capillary phenomena are the only remaining factor. Decisively, the total amount of water absorbed into the nanostructures was clearly different for the two cases. The water absorbed by the hydrophilic nanostructures was able to penetrate into the bottom of the nanocavities, which was not possible in the hydrophobic nanostructures because of the existence of nanobubbles [26, 28]. In conclusion, the CHF difference between the oxidized silicon and Teflon-coated nano/microstructures was apparently caused by the difference in capillary pumping capacity, which in turn is related to the presence of nanobubbles on the hydrophobic nanostructured surfaces.
The important point taken from the present experimental study is that BHT is dramatically enhanced on a nanostructured surface (compared to a plain surface), regardless of wettability, and more enhanced on a Teflon-coated nano/microstructured surface than on an oxidized silicon nano/microstructured surface. The superior enhancement of BHT on Teflon-coated nano/microstructures is related to the occurrence of the ONB. As was previously mentioned, the ONB occurs earlier on a hydrophobic surface than on a hydrophilic surface. In this study, the ONB for the bare Teflon-coated surface was 4.23 K, earlier than the ONB for the bare oxidized silicon surface (10.16 K). Interestingly enough, this tendency also appeared on the fabricated nano/microstructured surfaces. The ONB values of the Teflon-coated and oxidized nano/microstructures were 3.16 and 10.05 K, respectively, which are virtually the same as the values for the corresponding bare surfaces. This means that the wettability effect, and not the surface structure effect, mainly determined the ONB of each heating surface. As Figure 2 indicates, this ONB variation on surfaces with different wettabilities caused the overall difference in BHT between the Teflon-coated and oxidized nano/microstructured surfaces. Consequently, it can be inferred that nanostructures induce a high BHT, due to their numerous activated nucleate sites, while wettability has a small effect on BHT when the structures are unchanged, owing to the different starting points of the nucleate boiling regime (ONB).
We conducted a pool boiling experimental study on identical nano/microstructures with different wettabilities to examine the effect of wettability and surface structure on BHT and CHF. The results indicated dramatically enhanced BHT and CHF on nano/microstructures, independent of their wettability. However, the BHT and CHF increments were affected by the surface wettability. The Teflon-coated nanostructured surfaces had lower CHF and better BHT than the oxidized silicon nanostructured surfaces. We have conjectured that the combined effect of surface wettability and capillary wicking contributes to CHF improvement, even though the measured contact angle under dry conditions was very high, since the dynamic contact angle of the Teflon-coated surface in an actual boiling situation could be sharply reduced by water penetrating into the nanoscale vacancies left by detached nanobubbles after the degassing procedure. As a result of this study, the effect of surface structure and wettability on high BHT is also confirmed. By analyzing the visualization results for boiling phenomena on the fabricated structures, we identified numerous activated nucleation sites, which were responsible for the BHT improvement. The difference in BHT on the same nanostructures with different wettabilities was caused by the difference in the ONB. Therefore, the activated nucleate site density, resulting from the surface structure, and the variation of the ONB with wettability together determined the point of initiation of overall BHT enhancement on the nanostructures.
This research was supported by WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R31–30005). This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2011–0006359).
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