Article pubs.acs.org/Langmuir
Droplet Nucleation on a Well-Defined Hydrophilic−Hydrophobic Surface of 10 nm Order Resolution Yutaka Yamada,† Tatsuya Ikuta,† Takashi Nishiyama,†,‡ Koji Takahashi,*,†,‡,§ and Yasuyuki Takata‡,§,∥ †
Department of Aeronautics and Astronautics, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395, Japan JST-CREST, Kyushu University, Fukuoka 819-0395, Japan § International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan ∥ Department of Mechanical Engineering, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395, Japan ‡
S Supporting Information *
ABSTRACT: Water condensation on a hybrid hydrophilic− hydrophobic surface was investigated to reveal nucleation mechanisms at the microscale. Focused ion beam (FIB) irradiation was used to change the wettability of the hydrophobic surface with 10 nm order spatial resolution. Condensation experiments were conducted using environmental scanning electron microscopy; droplets, with a minimum diameter of 800 nm, lined up on the FIB-irradiated hydrophilic lines. The heterogeneous nucleation theory was extended to consider the water molecules attracted to the hydrophilic area, thereby enabling explanation of the nucleation mechanism under unsaturated conditions. Our results showed that the effective surface coverage of the water molecules on the hydrophilic region was 0.1−1.1 at 0.0 °C and 560 Pa and was dependent on the width of the FIB-irradiated hydrophilic lines and hydrophobic area. The droplet nucleation mechanism unveiled in this work would enable the design of new surfaces with enhanced dropwise condensation heat transfer.
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INTRODUCTION The development of new synthetic surfaces that support dropwise condensation of water vapor is expected to improve the heat transfer performance of several industrial applications such as looped heat pipes for electrical devices, 1 air conditioning systems,2 and power generation.3 Such new materials may additionally be useful for reducing energy consumption4 because of their ability to reduce thermal resistance to condensation. Dropwise condensation is of great interest in heat transfer applications because it exhibits 1 order of magnitude higher performance relative to that of filmwise condensation.5,6 Unlike the latter condensation process, whereby the liquid film acts as thermal resistance, dropwise condensation proceeds by droplet removal from a surface, thereby generating fresh surfaces for condensation. In the 1970s, Graham and Griffith7 conducted condensation experiments to investigate the relationship between droplet size and heat transfer during dropwise condensation. They reported that droplets with diameters of 10 μm, even in the presence of superhydrophobic surfaces.8−10 For example, Chen et al.8 fabricated a nanostructured micropyramidal © XXXX American Chemical Society
architecture on a silicon surface using microfabrication techniques and a hydrophobic coating. The authors reported that condensed droplets of several tens of micrometers in diameter departed from the surface because of enhanced hydrophobicity of the Cassie−Baxter surface. In particular, Miljkovic et al.9 and Boreyko and Chen10 reported the jumping-droplet condensation mode. As described in the latter paper,10 droplets grown on such surfaces coalesce with neighboring droplets and move on the surface. This motion induces further coalescence. During this process, the excess surface free energy of the droplet is released and induces the out-of-plane droplet-jumping motion. To develop more effective dropwise condensation surfaces, it is vital to understand the condensation mechanism at a smaller scale. However, such understanding is limited at present despite the numerous existing studies on droplet-growth mechanisms.11−19 Recently, Rykaczewski11 investigated micrometer-scale droplet growth mechanisms using a superhydrophobic surface and reported two distinct growth modes: the constant contact angle and the constant base area. Theoretical study of the micrometer-scale droplet growth mechanism is reported therein.12 However, these studies were conducted under the conditions that condensation primarily occurred on previously nucleated droplets.13,14 An important goal of this research is to Received: September 10, 2014 Revised: November 7, 2014
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dx.doi.org/10.1021/la503615a | Langmuir XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic of the sample preparation. (b) AFM topographic image of a FIB-irradiated area (bright) on a FOPA surface. (c) Crosssectional image of line AB in (b). The measured width of the line was 85 ± 15 nm. dwell time, were set at 7.1 nm, 50%, and 1 μs, respectively. We prepared three samples, each containing hydrophilic lines of the same width at intervals of 1.5, 2.0, 2.5, 3.0, and 3.5 μm. Prior to the condensation experiment, the width of hydrophilic lines was measured using atomic force microscopy (AFM; SPM-8000FM, Shimadzu Co., Japan). Figure 1a shows a schematic of the sample preparation and parts b and c of Figure 1 show a topographical image of the irradiated area and the associated height profile, respectively. The width of the fabricated hydrophilic lines on the three samples was defined as the full-width at half-maximum width and measured as 40 ± 5, 85 ± 15, and 170 ± 25 nm, respectively. As shown in Figure 1b, the FIBirradiated area was raised relative to the surrounding surface. It is believed that, owing to ion collision, the silicon substrate under the FOPA−SAM layer is deformed into an amorphous structure,27,28 and the emitted secondary electrons induce deposition of amorphous carbon. Consequently, the ion beam irradiation is expected to enhance the surface wettability of the resulting material. This was evaluated by conducting bulk-scale contact angle measurements. The static contact angle θ of the FOPA−SAM surface was 104 ± 2°, whereas that measured on the FIB-irradiated surface was 85 ± 5° (further details are given in section S1, Supporting Information). We paid attention to ensure that electron beam irradiation did not alter the surface wettability owing to deposition of contaminants.29 Surface roughness was 1 indicate the attraction of further water molecules, achieving a multilayer molecular coverage. In the present study, the attracted molecules were not directly observed, but quantification of effective molecules close to the surface for nucleation was determined experimentally as shown in Figure 5b. According to our results, the experimentally obtained droplet interval showed no correlation with the width of the hydrophilic and hydrophobic areas as shown in Figure 5a, but the surface coverage for nucleation was dependent on both the widths of the hydrophilic line and the hydrophobic region as shown in Figure 5b. This result implies that narrower hydrophobic areas will yield a higher number density of nucleated droplets on hybrid surfaces under a given saturation ratio; thus, heat transfer at the early stages of condensation was expected to increase. Additionally, narrower hydrophobic areas induce more frequent coalescence events, which work as the trigger of droplet detachment; however, such narrow hydrophobic areas are insufficient for droplet removal. Thus, because narrow hydrophilic areas are advantageous for droplet detachment, achieving condensation on narrow hydrophilic areas and designing hybrid surfaces are necessary. The current study thus focused on modeling the nucleation phenomenon on such surfaces that would assist in designing surfaces with enhanced heat transfer.
where ρv represents the number density of the vapor at pressure (P + ΔP). The radius of the vapor hemisphere rvapor on the surface is calculated as follows: rvapor =
3
3Vvapor 2π
(12)
(11)
Equation 6 was performed under the assumption that the vapor-supplying regions of adjacent nucleation sites do not overlap. On the basis of this assumption, the sum of two neighboring vapor hemispheres radii (shown in Figure 4c) is expected to coincide with the minimum interval between two droplets (2rvapor) on the same line. Thus, this model can be used to estimate the relationship between surface conditions and droplet nucleation. Data of the droplet intervals were compared with this extended model. The plots shown in Figure 5a indicate the experimentally obtained minimum droplet intervals. The lines shown in Figure 5a represent the droplet intervals predicted from eq 11, which are constant for a given saturation ratio S as defined by S = (P + ΔP)/P0. On the basis of eq 6, the onset of nucleation is dependent on the saturation ratio only. Additionally, the critical nucleation radius estimated by our proposed model became larger with decreasing S values. However, the contact angle of the nucleated droplet became smaller with decreasing S values according to eq 2; the contact angle is more sensitive to S than the critical radius. Consequently, a smaller nucleated droplet volume was obtained, subsequently leading to reduced droplet intervals when nucleation occurred at low S values. As shown in Figure 5a, the experimentally obtained minimum droplet interval agreed with the predicted values when S = 1.0005−1.002. In contrast, erroneous data were obtained at S = 1.0001 because the droplet height under these conditions was smaller than the diameter of a single water molecule (i.e., 0.31 nm). Furthermore, although our proposed model can be used to predict the minimum interval of nucleated droplets, it is possible that the actual droplet interval becomes larger than the predicted value. This phenomenon can be explained as follows. Droplet coalescence occurs at the submicrometer scale that decreases the number of droplets and induces the formation of larger intervals. The second factor relates to the nonuniform wettability on the hydrophilic line. Previously nucleated droplets attract water molecules in the vapor phase, thus interrupting further nucleation in the surrounding region. The agreement between the experimentally determined droplet interval and that predicted when S = 1.0005 was attributed to the presence of sites with relatively larger hydrophilicity. Thus, the results indicate that nucleation starts within an S value range of 1.0005−1.002.
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CONCLUSIONS In the present study, water condensation experiments on a hybrid hydrophilic−hydrophobic surface using ESEM were conducted to investigate nucleation mechanisms. The nanometer-sized hydrophilic areas were fabricated using FIB irradiation on the hydrophobic surface; the surface structure was investigated in detail using AFM. Our findings revealed that droplets nucleated on the hydrophilic areas under unsaturated conditions, and the intervals of the droplets were measured. An extended nucleation model was used to explain the measured droplet intervals; the extended model takes into account attracted water molecules on the hydrophilic surface. Relative to the experimentally obtained results, the effective surface coverage of the attracted water molecules for nucleation was estimated between 0.1 and 1.1, and was dependent on the widths of the hydrophilic line and the hydrophobic area. These findings provide insights into the design of new hydrophilic− hydrophobic surfaces capable of enhancing the heat transfer performance of dropwise condensation. E
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ASSOCIATED CONTENT
S Supporting Information *
Detailed description of the surface property of hydrophobic and hydrophilic areas and droplet formation on the hydrophilic surface. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by the Japan Society for the Promotion of Science (JSPS) Kakenhi (Grant Nos. 23360101, 23656153, 23760191, 24560237, 25289041, and 25420164), JST-CREST, and a Grant-in-Aid for JSPS Fellows (25-4996). We thank Prof. Atsushi Takahara (Kyushu University) for his advice on FOPA.
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