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School of Physical Sciences and Engineering, Australian National UniVersity, Canberra ACT 0200,. Australia, and Shanghai Institute of Applied Physics,...
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Langmuir 2006, 22, 9238-9243

Removal of Induced Nanobubbles from Water/Graphite Interfaces by Partial Degassing Xue H. Zhang,*,†,‡,,| Gang Li,† Nobuo Maeda,‡ and Jun Hu†,§ Nanobiology Laboratory, Bio-X Life Science Research Center, College of Life Science and Biotechnology, Shanghai Jiaotong UniVersity, Shanghai 200030, China, Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National UniVersity, Canberra ACT 0200, Australia, and Shanghai Institute of Applied Physics, Chinese Academy of Sciences, P.O. Box 800-204, Shanghai 201800, China ReceiVed May 22, 2006. In Final Form: July 17, 2006 Nanobubbles at an interface between a hydrophobic solid and water have a wide range of implications, but the evidence for their existence is still being debated. Here we artificially induced nanobubbles on freshly cleaved HOPG substrates in water using the protocol developed previously and subjected the system to moderate levels of degassing (∼0.1 atm for 0.5 to 3 h). The AFM images after the partial degassing revealed that some nanobubbles had coalesced and detached from the substrate because of buoyancy, whereas others apparently remained unaffected. The size and spatial distributions of the nanobubbles after the partial degassing suggest that there is a critical size for a nanobubble above which it may grow. The contact angle of water next to nanobubbles (∼160°) is much larger than the advancing contact angle of a macroscopic water droplet on the same substrate (∼80°) both before and after the partial degassing and concomitant growth and shrinkage of the nanobubbles. The contact angle of a nanobubble also remained unchanged as the nanobubble was moved along the substrate by the AFM tip. The apparent lack of contact angle hysteresis in the nanobubble systems may suggest that the very large contact angle may correspond to a local minimum of the free-energy landscape.

Introduction The existence of nanobubbles at water/solid interfaces was initially proposed as the explanation for the origin of the longrange hydrophobic attractive force, which has been intensively studied in surface science for the last two decades. In 1994, Parker et al. proposed that the bridging of submicroscopic bubbles or cavities between surfaces causes the long-range hydrophobic attraction.1,2 The stepwise feature in the force curves between hydrophobic surfaces was interpreted as the bridging of preexisting nanobubbles on the surface. In 1998, Carambassis et al. also obtained stepwise force curves between the two hydrophobic surfaces.3 For the first time, nanobubbles were directly imaged by tapping mode atomic force microscopy (TM-AFM) in 2000.4,5 From then on, many AFM images of nanobubbles have been reported.6-11 Meanwhile cryofixation fracture,12 X-rays,13 neutron * Corresponding author. E-mail: [email protected]. † Shanghai Jiaotong University. ‡ Australian National University. | Current address: Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia. § Chinese Academy of Sciences. (1) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994, 98, 8468. (2) Ball, P. Nature 2003, 423, 25. (3) Carambassis, A.; Jonker, L.; Attard, P.; Rutland, M. Phys. ReV. Lett. 1998, 80, 5357. (4) Lou, S.; Ouyang, Z.; Zhang, Y.; Li, X.; Hu, J.; Li, M.; Yang, F. J. Vac. Sci. Technol. B 2000, 18, 2573. (5) Ishida, N.; Inoue, T.; Miyahara, M.; Higashitani, K. Langmuir 2000, 16, 6377. (6) Yang, J. W.; Duan, J. M.; Fornasiero, D.; Ralston, J. J. Phys. Chem. B 2003, 107, 6139. (7) Simonsen, A.; Hansen, P.; Klosgen, B. J. Colloid Interface Sci. 2004, 273, 291. (8) Holmberg, M.; Kuhle, A.; Garnaes, J.; Morch, K.; Boisen, A. Langmuir 2003, 19, 10510. (9) Otsuka, I.; Yaoita, M.; Higano, M.; Nagashima, S. Surf. ReV. Lett. 2003, 10, 337. (10) Tyrrell, J.; Attard, P. Phys. ReV. Lett. 2001, 8717, 176104. (11) Agrawal, A.; Park, J.; Ryu, D.; Hammond, P.; Russell, T.; McKinley, G. Nano Lett. 2005, 5, 1751.

reflectivity,14,15 and microbeams16 are widely used to detect the interfacial water and confirm the presence of nanobubbles. However, there are still intense debates on the spontaneous formation of nanobubbles at solid/water interfaces. The controversies are focused on two main aspects. (1) Reproducibility: the measurements from ellipsometry,17,18 the electrochemical quartz crystal microbalance (EQCM),19 evanescent wave atomic force microscopy (EW-AFM),20 and neutron reflectivity21 are inconsistent with the presence of nanobubbles. It is argued that the nanobubbles observed from AFM imaging are due to either the contamination from substrate preparation or the nucleation by the AFM tip.21,22 (2) Thermodynamics: nanobubbles cannot form “spontaneously” on a flat, smooth surface because of the very large water-gas interfacial energy. In addition, pressure inside a nanobubble will be so large that nanobubbles should disappear within a few microseconds.23,24 The nanobubbles that can be observed by AFM imaging can exist at least for hours.25 (12) Switkes, M.; Ruberti, J. Appl. Phys. Lett. 2004, 84, 4759. (13) Jensen, T. R.; Jensen, M. O.; Reitzel, N.; Balashev, K.; Peters, G. H.; Kjaer, K.; Bjornholm, T. Phys. ReV. Lett. 2003, 90, 086101. (14) Steitz, R.; Gutberlet, T.; Hauss, T.; Klosgen, B.; Krastev, R.; Schemmel, S.; Simonsen, A.; Findenegg, G. Langmuir 2003, 19, 2409. (15) Schwendel, D.; Hayashi, T.; Dahint, R.; Pertsin, A.; Grunze, M.; Steitz, R.; Schreiber, F. Langmuir 2003, 19, 2284. (16) Jeon, S. M.; Desikan, R.; Fang, T. A.; Thundat, T. Appl. Phys. Lett. 2006, 88, 103118. (17) Mao, M.; Zhang, J. H.; Yoon, R. H.; Ducker, W. A. Langmuir 2004, 20, 1843. (18) Takata, Y.; Cho, J. H. J.; Law, B. M.; Aratono, M. Langmuir 2006, 22, 1715. (19) Tsionsky, V.; Kaverin, A.; Daikhin, L.; Katz, G.; Gileadi, E. Phys. Chem. Chem. Phys. 2005, 7, 1830. (20) McKee, C. T.; Ducker, W. A. Langmuir 2005, 21, 12153. (21) Doshi, D.; Watkins, E.; Israelachvili, J.; Majewski, J. P. Natl. Acad. Sci. U.S.A. 2005, 102, 9458. (22) Evans, D. R.; Craig, V. S. J.; Senden, T. J. Physica A 2004, 339, 101. (23) Eriksson, J. C.; Ljunggren, S. Colloids Surf., A 1999, 159, 159. (24) Ljunggren, S.; Eriksson, J. C. Colloids Surf., A 1997, 130, 151. (25) Butt, H. J.; Cappella, B.; Kappl, M. Surf. Sci. Rep. 2005, 59, 1.

10.1021/la061432b CCC: $33.50 © 2006 American Chemical Society Published on Web 09/23/2006

Nanobubble RemoVal from H2O/Graphite Interfaces

In our previous work,4,26-30 we established methods to induce nanobubbles deliberately by the exchange of ethanol and water with high reproducibility. The methods enabled us to study nanobubbles systematically. We also found that predegassing ethanol and water before exchange dramatically decreased the number of nanobubbles that can be produced by the exchange method.30 Other evidence also supported the fact that the entity induced by the exchange of ethanol and water was indeed made of gas, such as the consistency in the formation between micrometer-sized bubbles (observed from an optical microscope) and nanobubbles and the stability of nanobubbles in surfactant solutions.31 Given that there is much controversy as to whether “spontaneously” formed (preexisting) nanobubbles are truly made of gas, our findings elucidating physical properties of these artificially induced nanobubbles provide useful standards to which the spontaneously formed entities could be compared. In this article, we consider the degassing effect on the nanobubbles that have already been produced by the ethanolwater exchange procedure (as opposed to the predegassing studied in previous work30). It has been shown that degassing drastically alter the nature of interactions involving hydrophobic entities.32-34 Experimental Section HOPG Substrates. Highly ordered pyrolytic graphite (HOPG) (ZYB quality from MikroMasch, Madrid, Spain) was used as the substrate. HOPG was freshly cleaved immediately before each experiment by peeling off the outermost layers with adhesive tape. The advancing contact angle of a water droplet on freshly cleaved HOPG is 81 ( 3°, and the receding angle is 63 ( 3°. HOPG had a layered structure that did not change in air, water, or ethanol and thus could be used as the reference when we needed to relocate the area for AFM imaging. It could provide a renewable, smooth (within each layer) surface for AFM imaging and only carbon as the elemental background. Tapping Mode-Atomic Force Microscope (TM-AFM) Imaging. A MultiMode Nanoscope IIIa SPM (Digital Instruments Veeco Metrology Group, New York) with a fluid cell was used to image substrates and/or nanobubbles. V-shaped NP cantilevers (Digital Instruments Veeco Metrology Group, New York) with nominal spring constants of 0.32 or 0.58 N/m were used for imaging in tapping mode in water. The cantilevers were cleaned by immersion in acetone, ethanol, and then water for half an hour. The drive frequency for the TM-AFM imaging was typically 6-12 kHz in water. CCD Camera. Our AFM instrumentation included a long-focallength optical microscope and a charge coupled device (CCD) that allow the recording of (low-resolution) optical images of the substrate. Features on the HOPG substrates, such as the cleavage steps, can be seen from the CCD, which helped identify the locations of areas imaged by the AFM. Formation of Nanobubbles. Nanobubbles were deliberately produced by displacing (predistilled) ethanol with water (purified using a Milli-Q system, Millipore Corporation, Boston), as described previously.4,30 In short, we injected water into the fluid cell after mounting the freshly cleaved HOPG substrate onto the AFM head and imaged several areas to confirm the absence of any leakage of (26) Lou, S.; Gao, J.; Xiao, X.; Li, X.; Li, G.; Zhang, Y.; Li, M.; Sun, J.; Hu, J. Chin. Phys. 2001, 10, S108. (27) Lou, S.; Gao, J.; Xiao, X.; Li, X.; Li, G.; Zhang, Y.; Li, M.; Sun, J.; Li, X.; Hu, J. Mater. Charact. 2002, 48, 211. (28) Zhang, X. H.; Li, G.; Wu, Z.; Zhang, X.; Hu, J. Chin. Phys. 2005, 14, 1774. (29) Zhang, X. H.; Wu, Z.; Zhang, X.; Li, G.; Hu, J. Int. J. Nanosci. 2005, 4, 399. (30) Zhang, X. H.; Zhang, X.; Lou, S.; Zhang, Z.; Sun, J.; Hu, J. Langmuir 2004, 20, 3813. (31) Zhang, X. H.; Maeda, N.; Craig, V. S. J. Langmuir 2006, 22, 5025. (32) Alfridsson, M.; Ninham, B.; Wall, S. Langmuir 2000, 16, 10087. (33) Pashley, R. M. J. Phys. Chem. B 2003, 107, 1714. (34) Maeda, N.; Rosenberg, K.; Israelachvili, J.; Pashley, R. Langmuir 2004, 20, 3129.

Langmuir, Vol. 22, No. 22, 2006 9239 the fluid cell or contamination of the surface. Then we injected ethanol into the fluid cell to replace water and imaged several areas again. Finally, we injected more (∼10 mL) water to replace the ethanol in the fluid cell (ethanol-water exchange), which resulted in the formation of nanobubbles. Because the potential influence of ethanol residue will be a source of concern, we had analyzed the chemical composition of the water in the fluid cell after the ethanolwater exchange using gas chromatography/mass spectrometry (AutoSystem XL GC/Turbomass MS, Perkin-Elmer). No ethanol was detected. In addition, after the nanobubbles were observed by the AFM, we replaced the water inside the fluid cell with fresh Milli-Q water and found that the nanobubbles did not disappear. These results can rule out the possibility that the nanobubbles may consist of ethanol vapor. Given that ethanol and water are miscible, ethanol vapor in the nanobubbles, if present, would have quickly dissolved into the aqueous phase. Degassing Procedures. After nanobubbles were produced, tapping mode images of nanobubbles were recorded for many different areas of the HOPG substrate. The features (cleavage steps) on the HOPG substrate and the relative position of the AFM cantilever were recorded for each imaging location from the CCD. After the AFM imaging and the CCD recording, the tubes that had been connected to the two ports of the fluid cell were removed from the fluid cell, and then the fluid cell was carefully removed from the AFM head. During this process, the droplet of water must be left on the HOPG substrate at all times. (If the water droplet detached from the substrate at any stage during this procedure, then the experiment had to be restarted.) The HOPG and the water droplet were then put into a desiccator that was connected to a vacuum pump. A sufficient amount of water was put inside the desiccator as a reservoir to prevent the water droplet on the HOPG substrate from drying. The desiccator was disconnected from the pump after ∼4 min. The pressure inside the desiccator during the degassing procedure was around 0.1 atm (1 × 104 Pa). The vapor pressure of water at 25 °C is 3.17 × 103 Pa. After the desiccator was disconnected from the pump, the sample was kept in the sealed (and partially degassed) desiccator for different periods of time ranging from 0.5 to 3 h. The water droplet on the HOPG substrate did not dry out at this (moderate) level of degassing for these waiting periods. Once the desired level of partial degassing was achieved, the desiccator was gently vented back to atmospheric pressure. Then the HOPG with the water droplet was put back for AFM imaging. Special attention was paid when the AFM head and the fluid cell were mounted: a small amount of water was added to the channels and ports of the fluid cell before mounting so as to prevent the water droplet on the HOPG substrate from being sucked away by the (hydrophilic) fluid cell. Then the areas that had been imaged prior to partial degassing were identified from the features of the cleavage steps using the CCD, and the AFM cantilever was placed into each of these areas. To find the previous areas, we imaged a large area (e.g., 100 µm × 100 µm) by tapping mode AFM first, and then identified the exact location. From the height image of the nanobubbles, the contact angle of the nanobubbles can be deduced, as described in previous work in detail.31

Results Nanobubbles at the water/HOPG interface were induced by the exchange of ethanol and water, as reported previously4,26-30. A typical image of nanobubbles on the HOPG substrate after the exchange is shown in Figure 1. We note that the initial distribution of nanobubbles in water could be uniform over a large area of the substrate, even though locally there was a tendency for the nanobubbles to align along the cleavage steps of the HOPG substrate. The number density and size of the nanobubbles varied in a large range from time to time, depending on the temperature of the liquids and other subtle details of the exchange process (such as the flow rate of the liquids). We could get identical AFM images of nanobubbles in a certain region in the first several scans. If a certain region was repeatedly scanned by the AFM many times (∼10-15), then the AFM tip

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Figure 1. Tapping mode AFM images of nanobubbles on an HOPG substrate in water prior to degassing. The nanobubbles were induced by the exchange of ethanol with water. Initially, the distribution of nanobubbles was uniform over a large area even though locally the nanobubbles tend to align along the steps on HOPG. After a small region (A) was scanned many times, the number of nanobubbles in this region was much smaller but the size of each bubble is larger than in the other region, as shown in (B). These larger nanobubbles could be from the merging of different nanobubbles that were brought close to each other by the scanning AFM tip.

Figure 2. Nanobubbles remain on some areas of HOPG (A) before and (B) after 0.5 h of degassing. Scan size: 5 µm × 5 µm. Also see Supporting Information Figure 3.

could relocate some nanobubbles and sometimes facilitate coalescence of neighboring nanobubbles (Figure 1B), and certain nanobubbles could also be moved by the AFM tip deliberately (Figures 1 and 2 of the Supporting Information). We found that the contact angle of these nanobubbles, after corrections for the AFM tip convolutions, lies in the range of 164 ( 6°. There is no significant difference in the measured contact angle between the nanobubbles before and after the relocation and/or coalescence. After the nanobubbles were imaged, the HOPG and the water droplet were partially degassed. We noted that a few macroscopic bubbles (that were visible with the naked eye) could be formed at different locations during the degassing procedure, some of which subsequently disappeared. Comparisons of the AFM images before and after 0.5-3 h of degassing revealed the existence of two distinct areas after the degassing procedure, and the formation of these two types of areas did not seem to depend on the degassing time in the range studied. In area type 1, all of the nanobubbles remain as shown in Figure 2. Both the steps on the HOPG surface and the location of the nanobubbles (highlighted in Figure 3 of the Supporting Information) show that these two images are from the same area, although unfortunately the contrast of these two images is different because of the AFM tip effect. A comparison of these two images shows that there was little change in the location and number of nanobubbles before and after degassing. In area type 2, all of the nanobubbles disappear from the substrate, as shown in Figure

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Figure 3. Nanobubbles totally disappear from some areas of HOPG after 2 h of degassing. AFM image of nanobubbles (A) before degassing and (B) after degassing for 2 h in the same area.

3. The step features on the bare HOPG substrate show that these two images are from the same area. Importantly, we did not observe a uniform change in the size of nanobubbles or a gradual decrease in the number density of nanobubbles over a large area of the substrate after degassing, even though the degassing time was varied from 0.5 to 3 h. The formation of a few macroscopically large bubbles during the degassing procedure (that could be seen by naked eye) suggests that the morphology of the substrate where these bubbles had formed should be markedly different from the area elsewhere. We imaged the vicinity of such an area using TM-AFM. Then, a clear boundary was found, as shown in Figure 4. There were many nanobubbles evenly scattered on one side of the boundary and the bare HOPG substrate on the other side. The AFM image over a large area could show that the bare HOPG region could be fitted well with a circle with a radius of about 17 µm, as shown in Figure 4D. The area of the bare region is about 880 µm2. This result suggests that a very large bubble, presumably one of the macroscopic bubbles seen during the degassing procedure, had detached from the substrate because of buoyancy. Importantly, the increase in the external pressure back to 1 atm by gentle venting of the desiccator did not result in the reformation of nanobubbles on the bare substrate that had been left by the detachment of the large bubble. Unfortunately, we do not have sufficient information for a precise calculation of the volume of the bubble that had detached because of the unknown amount of deformation due to AFM imaging prior to the detachment, and we are unable to calculate the buoyancy force on the detaching bubble that can be compared to the surface force that has held the bubble to the substrate. We noticed that some nanobubbles were significantly larger than the other nanobubbles. For example, the nanobubble marked by a dotted circle in Figure 4C is almost 3 times higher than the other remaining nanobubbles. Such a nanobubble also had a circular rim of the bare substrate around it (Figure 4C). This kind of larger nanobubbles surrounded by the bare substrate could be observed in an area in the vicinity of as well as far away from the clear boundary, as shown in Figure 4E. The size of each circular rim provides an estimate as to the extent of the growth of that bubble during the degassing procedure, as discussed in the next section.

Discussion and Analyses One possible explanation for the situation in Figure 4E may be that large bubbles were formed from the coalescence of several nanobubbles caused by the perturbation by an AFM tip. As mentioned above, if an area was scanned many times, then some larger nanobubbles were formed because of the coalescence of different nanobubbles that were brought together by the AFM

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Figure 4. (A)-(C) A clear boundary exists between an area where the presence of nanobubbles remains unaffected and an area of the bare HOPG surface. The boundary could be fit well by a circle with a radius of 17 µm as shown in (D). Some nanobubbles were larger than others after degassing, and each of these larger nanobubbles had a circular rim of bare substrate around it. An example of such a larger (∼3 times) bubble is shown in (C). This kind of larger nanobubbles surrounded by a bare substrate could also be observed far away from the boundary, as shown in (E).

Figure 5. Schematic diagram of the processes of nanobubble removal by partial degassing. (A, B) During degassing, the size of the nanobubbles can increase. (C, D) The growing nanobubble can merge with other surrounding nanobubbles. When the size is large enough, the bubble will detach from the substrate and leave a circular area of bare substrate. (A)-(E) When the degassing stops before nanobubbles have started growing, the number and location of the nanobubbles do not change. (B)-(E) Even after nanobubbles have started growing, the growing nanobubbles may shrink back after the degassing stops if they have not merged with other neighboring nanobubbles. (C)-(F) If the degassing has stopped after nanobubbles have grown and merged with surrounding nanobubbles but have not detached from the substrate, then the nanobubbles may shrink and leave circular rims of bare substrate around them.

tip (Figure 1B). However, the large bubbles in Figure 1B were distributed randomly and did not have the circular areas of the bare substrate around them, which can be seen in Figure 4E. Therefore, the formation of large nanobubbles with the circular area of the bare substrate around them in Figure 4B was not likely to be due to perturbation by the AFM tip. The morphologic characteristics after the degassing procedure suggest that the following sequence of events must have happened during degassing, as shown schematically in Figure 5: Henry’s law suggests that exposing a water droplet to an external pressure lower than atmospheric pressure could reduce the solubility of air and hence result in the local supersaturation of dissolved gas in the droplet. Then the nanobubbles at the water/HOPG interface may grow. (We note that the volume of gases generally increases and cavitation will also be facilitated under reduced pressure especially when preexisting nanobubbles serve as the nucleation sites.) As the bubbles increase in size, they may engulf other nanobubbles nearby, even if they most likely have the same sign of surface charge. Once large enough, the bubble may detach

from the substrate because of buoyancy and leave a circle of bare substrate behind. If the degassing was interrupted before the bubble grew large enough for the detachment due to buoyancy, then the size of the bubble would shrink back as the external pressure is increased to the atmospheric value. Then this type of bubble would remain on the substrate with the circular rim of the bare substrate around it, as observed (Figure 4C and E). This type of bubble after the external pressure is raised back to 1 atm could be larger than the other nanobubbles, in part because they could have merged with neighboring nanobubbles. Interestingly, the majority of nanobubbles apparently remained unaffected by the degassing process. It has been known that there is a critical size of a bubble above which it can grow when exposed to reduced pressure,35 which is given by the following equation.

Pliquid ) Pvapor + Pair -

2γ sin θ Rc

(1)

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Figure 6. (A) The profile of a nanobubble after it has merged with neighboring nanobubbles can still be fitted with a spherical cap. In B, the circles are the cross-section profile of the nanobubble, and the solid line is a spherical-cap fit to the profile. The contact angle of water next to the nanobubble is 158°. The height and the size of a nanobubble before (C) and after (D) relocation by an AFM tip are almost identical: the lateral radius of the nanobubble is 820 nm in both (C) and (D), and the height of the nanobubble is 78.3 nm in (C) and 77.9 nm in (D).

Here Pliquid is the pressure of the aqueous phase (0.1 atm; we neglected the hydrostatic pressure here because of the small height of the water droplet), Pvapor is the vapor pressure of water inside the bubble, Pair is the partial pressure of air inside the bubble (that has been dissolved in the aqueous phase prior to degassing), γ is the surface tension of water, θ is the contact angle of water on the solid substrate (in this case, HOPG), and Rc is the critical radius of a bubble above which it tends to grow spontaneously. There are a few complications. First, Rc depends on θ, but there is a large variation between the macroscopic (∼80°) and submicroscopic (∼160°) contact angles6,31 (see below). Second, we do not know Pair, the partial pressure of air inside the bubble. Because of these complications, we cannot deduce the critical size quantitatively. Here we merely point out that, from Figure 4, smaller nanobubbles did not grow during our moderate levels of degassing and that the threshold for the mean radius of curvature of these bubbles (not the lateral radius but the radius of the spherical cap after image analyses as in Figure 6B) appears to be on the order of a few micrometers. We also note at this stage that the theory predicts that small nanobubbles (smaller than the critical radius) should shrink and disappear immediately, which is contrary to what we observed. The long-term stability of nanobubbles is a well-recognized mystery,23,24 and our results add one more puzzle in that the stability of many nanobubbles does not appear to be disrupted by the moderate level of degassing. At much lower pressures, the entire water droplet evaporates, and we see no evidence of (left-over) contamination on the dry substrate. It is interesting to analyze the contact angle of a nanobubble that has grown and shrunk during degassing. The methods as to how to deduce the contact angel from AFM images are detailed in ref 31. The profile of these nanobubbles can still be fit well (35) Brennen, C. E. CaVitation and Bubble Dynamics; Oxford University Press: Oxford, U.K., 1995.

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with a spherical cap, as shown in Figure 6A and B. Importantly, we found that the contact angle of the grown and shrunk nanobubbles is very similar to that of the other unaffected nanobubbles. We also compared the contact angle of nanobubbles before and after they were moved by the AFM tip (but without degassing). Again, we found that the lateral size and the height of these nanobubbles were almost identical before and after the relocation, as shown in Figure 6C and D. As reported previously,31 the contact angle of nanobubbles remained constant, within error, over a large range of size. Given that the time resolution of the AFM imaging is always poor (at least a few minutes per image), we cannot rule out the possibility that the deformed nanobubbles quickly recovered the original shape after they were moved by the AFM tip. Then the results suggest that the nanobubbles may have very flexible three-phase lines that are virtually free of pinning. One potential source of such flexible three-phase lines is the line tension. The precise value of line tension is yet to be established, but mounting evidence suggests that it ought to be on the order of 10-9 to 10-11 N.36,37 Because of its small value, the line tension is expected to influence the contact angle of a droplet or a bubble only when its length scale is submicroscopic. We note that there is a large amount of hysteresis between the advancing (81 ( 3°) and the receding (63 ( 3°) contact angles of a macroscopic water droplet on a freshly cleaved HOPG surface. We also note that the contact angle of water next to nanobubbles, regardless of the partial degassing and/or their relocation by an AFM tip, is much larger (164 ( 6°). This large discrepancy between the macroscopic and submicroscopic contact angles has been a puzzle in this field, and some authors attribute this discrepancy to the effect of line tension.6 However, in our previous study the uncertainty in the contact angle measurements precluded any sensible determination of the line tension, and the underlying physical reason that had caused the difference in the nanoscopic and the macroscopic contact angles remained to be elucidated. The contact angle of the nanobubbles here appears to take a quite well-defined value (164 ( 6°) despite various extents of deformation during relocation and different sizes of nanobubbles after growing and shrinking. This apparent lack of contact angle hysteresis of water next to nanobubbles may suggest that the system is in some sort of a local minimum in the freeenergy landscape and that the line tension may well be a factor that contributes to the formation of the local minimum. We note however that the global minimum should correspond to a state in which all nanobubbles have dissolved into the aqueous phase.

Conclusions Nanobubbles could be removed from the water/HOPG interface by partial degassing. On the basis of the AFM images, several intermediate stages of nanobubble removal were proposed. A moderate level of lower external pressures could result in the growth of some nanobubbles. When the size of the nanobubble increases, it may merge with the surrounding nanobubbles. Once large enough, the bubble may detach from the substrate because of buoyancy. The increase in the external pressure back to 1 atm did not result in the formation of nanobubbles on the bare substrate, which had been left by the detachment of the large bubble. When degassing is interrupted before the detachment, the bubble may shrink and remain on the substrate with a rim of the bare substrate. The size of such a circular rim provides estimate as to the extent of growth of that bubble during degassing. Such a bubble, after shrinkage, was larger than the other nanobubbles, which (36) Amirfazli, A.; Neumann, A. W. AdV. Colloid Interface Sci. 2004, 110, 121. (37) Pompe, T.; Herminghaus, S. Phys. ReV. Lett. 2000, 85, 1930.

Nanobubble RemoVal from H2O/Graphite Interfaces

apparently remained unaffected by the degassing procedure. The contact angles of water next to these nanobubbles and next to a nanobubble after relocation by an AFM tip are similar to each other but much larger than that of a macroscopic water droplet on the same substrate. These results add further evidence that nanobubbles are truly made of gas and, importantly, also suggest that the anomalously high contact angle of water next to nanobubbles may correspond to a local minimum in the freeenergy landscape. Acknowledgment. We thank Dr. Vincent Craig and Dr. Mika Kohonen of the Australian National University for helpful

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discussions. Professor Yamin Dong of Shanghai Normal University is gratefully acknowledged for assistance with the AFM. Supporting Information Available: Processes involving moving and merging nanobubbles via the AFM tip in tapping mode. AFM images of nanobubbles remaining on some areas of HOPG before and after degassing. This material is available free of charge via the Internet at http://pubs.acs.org. LA061432B