Factors Affecting the Spontaneous Motion of Condensate Drops on

Mar 17, 2012 - Factors Affecting the Spontaneous Motion of Condensate Drops on Superhydrophobic Copper Surfaces. Jie Feng*†, Zhaoqian Qin†, and Sh...
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Factors Affecting the Spontaneous Motion of Condensate Drops on Superhydrophobic Copper Surfaces Jie Feng,*,† Zhaoqian Qin,† and Shuhuai Yao*,‡ †

Department of Materials Science & Engineering, Zhejiang University of Technology, Hangzhou 310014, China Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Hong Kong 999077, China

‡

S Supporting Information *

ABSTRACT: The coalescence-induced condensate drop motion on some superhydrophobic surfaces (SHSs) has attracted increasing attention because of its potential applications in sustained dropwise condensation, water collection, anti-icing, and anticorrosion. However, an investigation of the mechanism of such self-propelled motion including the factors for designing such SHSs is still limited. In this article, we fabricated a series of superhydrophobic copper surfaces with nanoribbon structures using wet chemical oxidation followed by fluorization treatment. We then systematically studied the influence of surface roughness and the chemical properties of as-prepared surfaces on the spontaneous motion of condensate drops. We quantified the “frequency” of the condensate drop motion based on microscopic sequential images and showed that the trend of this frequency varied with the nanoribbon structure and extent of fluorination. More obvious spontaneous condensate drop motion was observed on surfaces with a higher extent of fluorization and nanostructures possessing sufficiently narrow spacing and higher perpendicularity. We attribute this enhanced drop mobility to the stable Cassie state of condensate drops in the dynamic dropwise condensation process that is determined by the nanoscale morphology and local surface energy.

1. INTRODUCTION Over the past 15 years, many researchers such as Barthlott et al.,1,2 Jiang et al.,3−8 and Kulinich et al.9−11 have performed extensive research on superhydrophobic surfaces (SHSs). Many kinds of functional SHSs have been fabricated on diverse substrates by different methods3−8,12,13 and further integrated into smart devices.14,15 However, their practical applications have been limited by poor mechanical stability11,16 and low stability under dew condensation.17−22 Recently, the spontaneous motion of condensate drops on some SHSs23−29 has attracted increasing attention because of their potential applications in dropwise condensation,28 water collection,7,30 anti-icing,11,21 and anticorrosion.31 A well-accepted explanation of such phenomena is that the spontaneous motion is powered by the surface energy released upon drop coalescence (10 times larger than the typical energy barrier for the Wenzel to Cassie transition), either in the form of out-of-plane jumping or random sweeping,23−25 whereas such release originates from the considerable reduction of the total liquid−air interfacial area after coalescence. However, factors affecting the spontaneous motion of condensate drops on SHSs and the microscopic mechanism have not been studied thoroughly. For example, droplets forming on selected natural and artificial surfaces adopt nearly spherical shapes and become very mobile; however, most SHSs can still be wet completely following sufficient condensation.32 On the other hand, Chen et al.33 found that a lotus leaf retains water repellency after repeated condensation in nature but © 2012 American Chemical Society

becomes sticky to water drops after condensation on a fixed cold plate. However, they demonstrated that mechanical vibration could be used to overcome the energy barrier for transition from the sticky Wenzel state to the nonsticky Cassie state. These imply that in addition to environmental conditions such as the relative humidity (RH) and extent of subcooling, topographical and chemical properties of the SHS may be two other key factors affecting the spontaneous motion of condensate drops on them. The self-propelled Wenzel-toCassie transition of condensate microdroplets may be the precondition of spontaneous motion of the resulting Cassie drops on SHS. In the present work, we fabricated a series of superhydrophobic copper surfaces with various nanostructures and surface energies by controlling the extents of oxidation and fluorization. The effects of the topographical and chemical properties of the SHS on the spontaneous motion of condensate drops and the relationship between the superhydrophobicity stability under dew condensation and the spontaneous motion of condensate drops were thoroughly investigated. The results show that the spontaneous mobility of condensate drops is related to the extent of surface oxidation and fluorization. The enhanced mobility of condensate drops should be attributed to two factors: the depinning ability and Received: February 10, 2012 Revised: March 16, 2012 Published: March 17, 2012 6067

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Figure 1. SEM images of copper surfaces after immersion in 2.5 M NaOH and 0.1 M (NH4)2S2O8 at 4 °C for different durations: (a, b) 5 min; (c, d) 10 min; (e, f) 30 min; (g, h) 60 min. (a, c, e, g) Low magnification; (b, d, f, h) high magnification. The insets show the profiles of sessile water droplets (0.5 μL) with CAs of (b) 159.8, (d) 161.1, (f) 163.5, and (h) 165.0°. 6068

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Table 1. Structural Parameters, CAs, SAs, and Spontaneous Motion Frequencies of Condensate Drops on Surfaces with Different Oxidation Timesa oxidation time/min

5

10

30

60

length of nanoribbon/μm space of gap/μm α (angle formed by two adjacent nanoribbons)/deg CA/deg SA/deg spontaneous motion frequency/(drops/s)

3.8 2.6 >40 150.4 ± 1.5 1.9 9

4.3 1.9 25.9 151.7 ± 0.6 1.5 22

5.3 1.4 18.2 151.8 ± 0.5 1.0 50

6.5 0.5 4.7 152.7 ± 0.8 1.0 72

The copper foils were oxidized by immersion in 2.5 M NaOH and 0.1 M (NH4)2S2O8 at 4 to 5 °C for different times and were then uniformly fluorinated for 1 h.

a

in water collection, anti-icing, heat-transfer enhancement, and so forth. In fact, this is also a simple, inexpensive, and effective way to observe the spontaneous motion phenomenon, though the quality of images obtained was not as high as that in ESEM. We then used a chargecoupled device (CCD) camera coupled with a microscope to capture the dynamics of spontaneous motion of condensate drops on various SHSs. Condensation experiments were performed in a closed room with an area of 25 m2 and a height of 3 m. The ambient temperature was controlled to 29 ± 1 °C, and the RH was adjusted to 80 ± 3%. The 3 cm × 3 cm × 0.5 mm surface superhydrophobilized copper foils were placed on a horizontally orientated aluminum block that was almost completely immersed in a mixture of ice and water (holding the temperature at 0 to 1 °C). This ensured that the copper foils were stable, thus the following video could be focused on the same area. The spontaneous motion of condensate drops was observed and visualized with an optical microscope (Nikon LV150) with a 10× objective and a charge-coupled device camera at 25 fps. The spontaneous motion phenomenon was quantified by analyzing a 10 min representative video. Five short periods of time (only 1 s each) at 0, 2, 4, 6, and 8 min were selected. Altogether, 5 × 25 pieces of snapshots were used to quantify the average numbers of drop location changes (emergence or disappearance in sequential microscopy images) for 1 s in the captured video (here named the spontaneous motion frequency). In addition to frequency, the relationship between the residue time of condensate droplets at their nuclei or original locations and the drop departure size was also analyzed from the same video to demonstrate further the difference in spontaneous motion phenomena. The residue time defined here is the time duration for the same drop being observed in the video.

the Cassie state of condensate droplets, both of which are determined by the nanostructure morphology and local surface energy. This study would be helpful in designing new surfaces for sustained dropwise condensation, collecting condensed water, and anti-icing.

2. EXPERIMENTAL SECTION 2.1. Preparation. The surface superhydrophobilization of copper foils was performed by the procedures described by Zhang et al.34,35 Briefly, 30 mm × 60 mm × 0.5 mm copper foils (purity 99.99%, Aldrich) were first immersed in a 4 M HCl aqueous solution for 5 s to remove surface oxide and then ultrasonically washed in ethanol and deionized water for 5 min. After that, the copper foils were incubated in an aqueous solution of 2.5 M NaOH and 0.1 M (NH4)2S2O8 at 4 to 5 °C for 5, 10, 30, and 60 min. Then the blue copper foils were thoroughly washed with deionized water and dried at 180 °C for 2 h to make Cu(OH)2 into stable CuO by completing the dehydration reaction. Afterward, the black copper foils were incubated in a 0.5 wt % hexane solution of 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS17, Sigma) at room temperature for 1 h, followed by drying at 120 °C for 1 h. For superhydrophobic copper surfaces with gradually changing surface energy, the copper foils were oxidized for 1 h and then fluoridized for 1, 2, 5, 10, and 30 min, all followed by drying at 120 °C for 1 h. 2.2. Characterization. The morphology of the resulting copper surfaces was characterized by field emission scanning electron microscopy (FE-SEM, S4700, Hitachi, Japan). The nanoribbons scraped off of the as-prepared surfaces were imaged under FE-SEM to obtain their length. For each surface, its structural parameters such as the nanoribbon length, the gap space, and the angles formed by two adjacent nanoribbons were measured and calculated statistically from the SEM images. Among them, the angles were calculated approximately by using the trigonometric functions hypothesizing a triangle formed between two adjacent nanoribbons. X-ray fluorescence spectrometry (XRF, ARL ADVANT’X, ThermoFisher, USA) was used to measure the fluorine content on the as-prepared surfaces. The water contact angles (CAs) and slide angles (SAs) were measured by using a Dataphysics OCA35 contact-angle system with a temperaturecontrolled stage. This stage can precisely maintain the temperature of superhydrophobic surface from −30 to 160 °C. The RH of the ambient air was 60 ± 3% (25 °C) but can be adjusted to 40 ± 3% (22 °C) and 90 ± 3% (30 °C) by an air conditioner and a small steam boiler, respectively. The stability of the superhydrophobicity of the copper surface under dew condensation was checked by measuring static CAs of sessile water droplets placed on the surface at different cooling stages and recording the dynamic changes in CAs of each sessile droplet during the whole cooling procedure at different ambient RH values. The static CAs were measured and averaged over six measurements. 2.3. Condensation Experiments. Because the nuclei of condensate microdroplets may be very small (on the nanoscale), the initial stage of condensation is difficult to visualize via modern microscopy, including ESEM.32,36,37 Compared with ESEM, directly observing vapor condensation by optical microscopy under ambient conditions may be more valuable, especially for practical applications

3. RESULTS AND DISCUSSION 3.1. Morphology and CAs of As-Fabricated Surfaces. Similar to the nanostructures reported by Yang et al.,38,39 Zhang et al.,34,35 and Jiang et al.,40 a series of CuO nanoribbon structures with different densities and perpendicularities were synthesized on our copper surfaces by oxidizing for different durations (Figure 1). When the reaction time was short (5 min), the CuO nanoribbons were sparse and oblique on the substrate. Their length and diameter were 2−4 μm and 100− 500 nm (depending on the location of each nanoribbon), respectively. The CA of a 0.5 μL drop of water on such a structured surface was 159.8° (Figure 1a,b). As the reaction time was increased to 10 min, the nanoribbons covered the substrate more completely and uniformly. Moreover, some small nanoflowers appeared among these nanoribbons. However, the length and diameter of the nanoribbons did not increase significantly. The oblique nanoribbon assemblies were still dominant. The water CA of such a structured surface was 161.1° (Figure 1c,d). When the reaction time was increased to 30 min, there were no significant changes in the length and diameter of the nanoribbons, but some nanoflowers became larger. Moreover, the nanoribbons seemed more vertical and 6069

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the surfaces oxidized for 30 and 60 min; however, the spontaneous motion frequency on the sample being oxidized for 60 min is 1.44 times higher than that on the sample being oxidized for 30 min and 8 times higher than that on the sample being oxidized for 5 min. This implies that compared with the CA or SA the spontaneous motion phenomenon is more sensitive to the surface roughness of the micro/nanostructures. The detailed nanoribbon morphology (e.g., spacing and perpendicularity) may be the key factor affecting the spontaneous motion of condensate drops on SHS. Figure 3 shows the residue time of condensate drops as a function of the drop departure size. It can be seen that on each

dense (Figure 1e). The corresponding water CA was 163.5° (Figure 1f). For a reaction time of up to 1 h, the density of the nanoribbons was so high that they started to grow vertically and finally covered the copper surface compactly except for a little deformity. The distance between nanoribbons was about 200− 1000 nm. The nanoflowers seemed to be imbedded under the nanoribbon clusters (Figure 1g,h). The CA of such a structure was 165.0°. Detailed nanostructural parameters such as the nanoribbon length, the gap space, and the angles formed by two adjacent nanoribbons are listed in Table 1. 3.2. Effect of Structure on the Spontaneous Motion of Condensate Drops. Figure 2 shows the time-lapse top-view

Figure 2. Time-lapse optical images (top view) of vapor condensation at 80 ± 3% RH (ambient temperature was 29 ± 1 °C) on the horizontally placed copper superhydrophobic surfaces fabricated by oxidation for (A) 5, (B) 30, and (C) 60 min. The coalescence and spontaneous motion of condensate droplets are obvious when the oxidation time exceeds 30 min. The scale bar is 60 μm. Video S1 is available in the Supporting Information.

Figure 3. Residue time of condensate droplets on superhydrophobic copper surfaces as a function of the drop departure size. The copper foils were oxidized by immersion in 2.5 M NaOH and 0.1 M (NH4)2S2O8 at 4 to 5 °C for different periods of time and were uniformly fluorinated for 1 h.

optical images of dropwise condensation on copper surfaces prepared by oxidation for 5, 30, and 60 min. It clearly demonstrates that although all three surfaces are superhydrophobic only the latter two surfaces show the spontaneous motion of condensate drops. The longer the oxidation period, the more obvious the spontaneous motion would be. On the surface being oxidized only for 5 min, three condensate drops coalesced together in 0.08 s, but the merged drops (∼100 μm) no longer had any mobility though their shapes were always spherical (Figure 2A). On the surface being oxidized for 30 min, the rapid removal of large condensate drops was found (Figure 2B). From the coalescence of 60 and 100 μm drops to the disappearance of merged drop, only 0.12 s was used. On the surface being oxidized for 60 min, the same process consumed only 0.08 s (Figure 2C), demonstrating that the spontaneous motion of condensate drops on such a surface was even faster. Although the image sequences demonstrate the condensate drop motion on various SHSs straightforwardly, the quantitative analysis of the condensate drop mobility (e.g., the statistics of the temporal evolution of the condensate drops) is still an arduous task that has to be completed. Because most condensate drops were smaller than 5 μm in diameter, the spontaneous motion frequency (i.e., the number of emerging/ disappearing distinguishable drops in 1 s) was calculated on the basis of drops smaller than 5 μm. Table 1 indicates the relationship among the structural parameters, CAs, SAs, and spontaneous motion frequencies of condensate drops on copper surfaces with different oxidation times. The four samples have different structures but the same fluorination time. It can be seen clearly that although the static CAs and SAs are not significantly different the spontaneous motion frequencies are remarkably different. The SAs are equal for

surface, smaller droplets are more easily self-propelled. Moreover, condensate droplets disappear more quickly on SHS with a longer oxidation time. For the surface being oxidized for 60 min, droplets with an average diameter of less than 80 μm make a significant contribution to spontaneous motion. Droplets with diameters of between 80 and 150 μm can still depart from the surface. For the surface being oxidized for 30 min, droplets with an average diameter of less than 50 μm make a significant contribution to spontaneous motion. Droplets with diameters of between 50 and 150 μm can still depart from the surface; however, the residue time increased greatly. For the surface being oxidized for 5 min, droplets with diameters of 10−70 μm showed very limited mobility. (Larger droplets were not included because they showed no spontaneous motion tendency but continuous growth.) It must be noted that because the small droplets with diameters of less than 10 μm disappeared so quickly their residue time was symbolically estimated by the extrapolation method in the plot shown in Figure 3. 3.3. Effect of the Extent of Fluorination on the Spontaneous Motion of Condensate Drops. Because superhydrophobicity is dependent on the surface roughness and surface energy, on the basis of the optimized oxidation period that we have investigated from the previous section we varied the intrinsic surface energy by applying different fluorination times to the copper surfaces with the same oxidation. Similar condensation experiments were carried out on the resulting surfaces. Figure 4 shows the time-lapse topview optical images of dropwise condensation on copper surfaces prepared by uniformly oxidizing for 1 h and then fluorinating for 1, 10, and 30 min, respectively. This clearly 6070

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Similar relationships between the residue period of condensate drops and the drop departure size were found on the surfaces with different fluorination times (Figure 5).

Figure 4. Time-lapse optical images (top view) of vapor condensation at 80 ± 3% RH (ambient temperature was 29 ± 1 °C) on the horizontally placed copper surfaces prepared by oxidizing for 1 h and then fluorinating for (A) 1, (B) 10, and (C) 30 min. The coalescence and spontaneous motion of condensate droplets are obvious when the oxidation time exceeds 10 min. The scale bar is 60 μm. Video S2 is available in the Supporting Information.

demonstrates that no spontaneous motion of condensate drops was found on surfaces fluorinated for 1 min (Figure 4A), but on the surface being fluorinated for 10 min, three growing condensate drops coalesced together and quickly disappeared in 0.08 s (Figure 4B). On the surface being fluorinated for 30 min, the same process consumed only 0.04 s (Figure 4C), demonstrating that the spontaneous motion of condensate drops on such a surface was even faster. Table 2 indicates the relationship among the fluorine content, the intrinsic CAs, CAs, and SAs, and the spontaneous motion frequencies of condensate drops on the as-prepared surfaces. It can be seen that the fluorine content, intrinsic CAs, and CAs all gradually increased as the fluorination time increased. The surface without fluorination treatment has an intrinsic CA and CA at 57.6 ± 3.6° and 10.1 ± 7.6°, respectively. When the fluorination time was 1 min, 0.074 wt % fluorine content and 127.6 °CA were obtained, whereas the SA could not be measured successfully. (The upper limit of SA as measured by the instrument is 60°.) When the fluorination time was 2 min, 148.7° CA and 10.2° SA were obtained. These values approached the SHS standard. However, only a very weak spontaneous motion phenomenon was found. When the fluorination time was increased to 5 and 10 min, the spontaneous motion phenomenon was still not obvious. However, for the surface being fluorinated for 30 min, a significantly obvious spontaneous motion phenomenon with a frequency of up to 86 drops/s was observed, though the CA had no remarkable changes compared with those being fluorinated for less than 30 min. This implies that the spontaneous motion was more sensitive to the changes in fluorine content, intrinsic CAs, and SAs than to the changes in CAs.

Figure 5. Residue time of condensate droplets on hydrophobic copper surfaces as a function of the drop departure size. The copper foils were oxidized by immersion in 2.5 M NaOH and 0.1 M (NH4)2S2O8 at 4 to 5 °C for 1 h and were then fluorinated for different periods of time.

Condensate drops disappear more quickly on SHS with longer fluorination times. Moreover, the performance difference among these surfaces was more obvious than that on surfaces with different oxidation times. For surfaces being fluorinated for 1 min, the largest droplets that could depart spontaneously were 10 μm in diameter. Droplets with diameters exceeding 10 μm showed no spontaneous motion tendency but continuous growth. For the surface being fluorinated for 5 min, the size of the largest departure droplets changed to 70 μm whereas the residue time was delayed until 586 s. For the surface being fluorinated for 10 min, droplets with a diameter of 100 μm can still depart from the surface spontaneously. Moreover, the residue time decreased to 473 s. For the surface being fluorinated for 30 min, droplets with a diameter of 100 μm can quickly depart from the surface in 138 s. 3.4. Microscopic Mechanism of Spontaneous Motion. The spontaneous motion is powered by the surface energy released upon drop mobile coalescence due to the considerable reduction in the total liquid−air interfacial area.23−25 Here the self-propelled droplets (1−150 μm in diameter) are much larger than a single nanoribbon (100−500 nm in diameter). Thus, the main obstacle to drop motion on a solid surface arises from contact angle hysteresis that causes the pinning at the edge of the drops. The less the pinning effect on the nanoribbon structured surface, the more obvious the

Table 2. Fluoride Content, Intrinsic CAs, CAs, and SAs, and Spontaneous Motion Frequencies of Condensate Drops on Surfaces with Different Fluorination Timesa fluorination time/min

1

2

5

10

30

fluoride content/wt % intrinsic CA* CA/deg SA/deg spontaneous motion frequency/(drops/s)

0.074 91.3 ± 0.7 127.6 ± 3.4

0.104 93 ± 0.9 148.7 ± 0.6 10.2 7

0.189 100.0 ± 3.2 150.7 ± 0.5 6.1 9

0.454 102.4 ± 1.7 151.5 ± 0.7 1.5 24

1.08 106.4 ± 2.3 152.0 ± 0.9 0.9 86

4

The copper foils were all oxidized in 2.5 M NaOH and 0.1 M (NH4)2 S2O8 at 4 to 5 °C for 1 h and thus had a uniform structure. Intrinsic CA was measured on flat copper surfaces with the same fluorination time as those of nanostructured surfaces.

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spontaneous motion would be. The ability to depin from the surface nano/microstructures determines whether the tiny condensate drops within the narrow gap between the nanoribbons can transit from the Wenzel state to the Cassie state or whether the coalesced drops can eventually recede on the surface profiles. At the very beginning of vapor condensation, because the density of vapor (∼0.74 × 103 g/m3) is far less than that of liquid water (106 g/m3), the nuclei are so small, even compared to the nanoscale roughness, that they may have equal opportunities to start the nucleation at the top, side, and bottom of the nanostructures. At a little later stage, droplet coalescence dominates the drop growth until the gaps between the nanoribbons are filled. Then a Wenzel drop covering not only one gap with a relatively flat external surface forms.27 The water−air interfacial free energy (IFE) of such a shaped condensate, however, is high. When the upward surface tension force is larger than the interaction force between the solid surface and liquid water, the Wenzel drop spontaneously transits its shape to reduce the IFE and finally becomes a composite or a Cassie27 drop. The intrinsic CA on flat copper surfaces is an index reflecting the surface energy that determines the above interaction force. The higher the intrinsic CA, the lower the interaction force. When the fluorination time varied from 0 to 30 min, the intrinsic CAs increased gradually. Thus, on the surfaces with nanoribbon structures, spontaneous motion frequencies of condensate drops increased with an increase in the fluorination time (Table 2). In addition to the chemical aspect, the depinning ability of condensate drops or their Wenzel to Cassie transition also depends on the detailed surface nanostructures. On one hand, the nanostructures with higher perpendicularity decrease the intercrossing between them, thus reducing the receding resistance of the Wenzel to Cassie transition. On the other hand, the upward surface tension force of Wenzel drops is factually the vertical force of the Laplace pressure, which increases significantly with the decrease in the radius of curvature of the water meniscus in each gap and the inclination angle of the gap wall.7,41 A narrower gap leads to a smaller meniscus radius, thus generating a larger Laplace pressure. The high perpendicularity of the nanostructures plays the same role (Figure 6). Therefore, nanoribbons with sufficiently narrow spacing and perpendicularity facilitate the Wenzel-to-Cassie transition of condensate drops. As shown in Figure 6, the small nucleating drops within the nanostructures continue to grow and merge to fill the gaps between the nanoribbons, finally transiting to suspended Cassie drops or composite drops or remaining in sticking Wenzel drops at different locations with the nanoribbon morphology varying from dense to sparse. As a result, the spontaneous motion frequency of the resulting drops increases on the surfaces with a narrower space and more perpendicular nanoribbons (a smaller angle formed by two adjacent nanoribbons) because of increasing oxidation time (as shown in Figure 1 and Table 1). Because the droplets during the Wenzel-to-Cassie transition are so tiny, it is very difficult to observe them directly. To confirm the above hypothesis for the final Cassie state of condensate drops, the profile change of a sessile water drop (4 μL volume, ∼2 to 3 mm diameter) under dew condensation at different RH values (e.g., the superhydrophobicity stability in condensation) was checked on the SHS with different oxidation times. It must be noted that according to the study by Zhang et al.42 the smaller the sessile droplets, the

Figure 6. Scheme of vapor condensation and the Wenzel-to-Cassie transition of drops on surfaces with different types of nanostructures. The small radius of curvature of the water meniscus in the gap between nanostructures, which brings about a high Laplace pressure of droplets in gaps, is closely related to the gap width and perpendicularity of the nanostructure walls. Cassie, composite, and Wenzel state drops are formed in narrower, moderate, and wider nanogaps, respectively.

higher the CAs would be. Thus in the first part of the Results and Discussion section, we measured the CAs and SAs by splashing an ∼0.5 μL droplet on the as-measured surfaces. The CAs were all approaching or higher than 160°, and the SAs were all less than 2°. However, it was very difficult to splash down such a small droplet onto these samples because the droplets would slip off from them so easily.43 For convenience, 4 μL drops were used in superhydrophobicity stability measurements. Different from the CAs of the 0.5 μL droplet, the CAs of 4 μL drops at room temperature were all less than 155°. Both the static CAs of sessile drops placed on the surfaces at different cooling stages (Figure 7a,c,e,g) and the dynamic changes in CAs of each sessile drop during the whole 10 min cooling procedure (Figure 7b,d,f,h) show a consistent trend in the superhydrophobicity change versus the copper oxidation time. From Figure 7a,c,e−g and Figure 7b,d,f−h, it can be seen that the superhydrophobicity stability under dew condensation of the as-prepared surfaces is enhanced along the increase in oxidation time. Especially for surfaces being oxidized for 30 or 60 min, the surfaces possessed stable superhydrophobicity during condensation. However, for surfaces being oxidized for only 5 or 10 min, the CAs started to decrease in the presence of condensate drops (as the surface temperature decreases). From the microscopic viewpoint, this significant decrease in CAs (Figure 7a−d) should be caused by the coalescence of the larger sessile water drops with the underlying tiny Wenzel-state droplets condensed earlier on the as-prepared surface (Figure 7a,c) or Wenzel-state satellite droplets generated subsequently in the vicinity of the sessile drop17 (Figure 7b,d), whereas the relatively stable CAs (Figure 7e−h) should be caused by the coalescence of the larger sessile drops with the underlying condensate droplets (Figure 7e,g) or satellite droplets (Figure 7f,h), both in the Cassie state. 6072

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Figure 7. Static CAs of water drops placed on the copper superhydrophobic surfaces in different cooling stages (a, c, e, and g) and dynamic CAs of each water drop during the whole cooling procedure (b, d, f, and h). Samples a/b, c/d, e/f, and g/h are surfaces oxidized for 5, 10, 30, and 60 min, respectively.

4. CONCLUSIONS Factors affecting the spontaneous motion of condensate drops on copper superhydrophobic surface were investigated by adjusting the surface roughness and surface energy. The results show that the spontaneous motion is significantly affected by the surface roughness and surface energy as well

as the superhydrophobicity stability under dew condensation. The nanostructures with sufficiently narrow spacing, higher perpendicularity, and lower surface energy cause less pinning of the condensate drops and contribute to the enhanced drop mobility. We envision that the basic findings learned from this study will open a new door to designing 6073

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new functional materials to extend the applications of SHS in the energy and water industries.

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ASSOCIATED CONTENT

S Supporting Information *

Video S1 corresponding to Figure 2 and Video S2 corresponding to Figure 4 recorded the spontaneous motion of condensate drops on SHS with different nanostructures and different fluorine contents, respectively. 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]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Hong Kong RGC General Research Fund (grant no. 621110), the Innovation Technology Commission and Industrials (grant no. ITS/530/09), and the National Natural Science Foundation of China (grant no. 51172206).

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REFERENCES

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