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Degassing and Temperature Effects on the Formation of Nanobubbles at the Mica/Water Interface Xue H. Zhang,† Xiao D. Zhang,*,† Shi T. Lou,‡ Zhi X. Zhang,§ Jie L. Sun,† and Jun Hu†,§ Nanobiology Laboratory, Bio-X Life Science Research Center, Shanghai Jiaotong University, Shanghai 200030, China, Physics Department, Rutgers University, Piscataway, New Jersey 08854, and Shanghai Institute of Applied Physics, Chinese Academy of Sciences, P.O. Box 800-204, Shanghai 201800, China Received December 25, 2003. In Final Form: February 12, 2004
Introduction The solid/water interface is of special interest in terms of its ubiquitous presence and its important role in biological and colloidal systems.1 There was an argument for a long time whether bubbles on the nanometer scale could exist at this interface.2,3 The experimental evidence, especially from the studies of direct imaging by atomic force microscopy (AFM), supported strongly the presence of nanobubbles.4-19 Recently the great implication of small bubbles at the interface has been demonstrated in a flurry of papers: for example, the origin of long-range attraction between hydrophobic surfaces immersed in water,20 the slippage of simple fluids near a wall,21,22 the stability of * Corresponding author. E-mail:
[email protected]. † Shanghai Jiaotong University. ‡ Rutgers University. § Chinese Academy of Sciences. (1) Ball, P. Nature 2003, 423, 25-26. (2) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994, 98, 8468-8480. (3) Ljunggren, S.; Eriksson, J. C. Colloids Surf., A 1997, 129-130, 151-155. (4) Carambassis, A.; Jonker, L. C.; Attard, P.; Rutland, M. W. Phys. Rev. Lett. 1998, 80, 5357-5360. (5) Mahnke, J.; Stearnes, J.; Hayes, R. A.; Fornasiero, D.; Ralston, J. Phys. Chem. Chem. Phys. 1999, 1, 2793-2798. (6) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1999, 15, 1562-1569. (7) Considine, R. F.; Hayes, R. A.; Horn, R. G. Langmuir 1999, 15, 1657-1659. (8) Gong, W.; Stearnes, J.; Hayes, R. A.; Fornasiero, D.; Ralston, J. Phys. Chem. Chem. Phys. 1999, 1, 2799-2803. (9) Snoswell, D. R. E.; Duan, J.; Fornasiero, D.; Ralston, J. J. Phys. Chem. B 2003, 107, 2986-2994. (10) Miller, J. D.; Hu, Y. H.; Veeramasuneni, S.; Lu, Y. Q. Colloids Surf., A 1999, 154, 137-147. (11) Ishida, N.; Sakamoto, M.; Miyahara, M.; Higashitani, K. J. Colloid Interface Sci. 2002, 253, 112-116. (12) Tyrrell, J. W. G.; Attard, P. Phys. Rev. Lett. 2001, 87, art. no 176104. (13) Lou, S. T.; Ouyang, Z. Q.; Zhang, Y.; Li, X. J.; Hu, J.; Li, M. Q.; Yang, F. J. J. Vac. Sci. Technol., B 2000, 18, 2573-2575. (14) Lou, S. T.; Gao, J. X.; Xiao, X. D.; Li, X. J.; Li, G. L.; Zhang, Y.; Li, M. Q.; Sun, J. L.; Li, X. H.; Hu, J. Mater. Charact. 2002, 48, 211214. (15) Tyrrell, J. W. G.; Attard, P. Langmuir 2002, 18, 160-167. (16) Steitz, R.; Gutberlet, T.; Hauss, T.; Klo¨sgen, B.; Krastev, R.; Schemmel, S.; Simonsen, A. C.; Findenegg, G. H. Langmuir 2003, 19, 2409-2418. (17) Ishida, N.; Inoue, T.; Miyara, M.; Higashitani, K. Langmuir 2000, 16, 6377-6380. (18) Yang, J. W.; Duan, J. M.; Fornasiero, D.; Ralston, J. J. Phys. Chem. B 2003, 107, 6139-6147. (19) Holmberg, M.; Ku¨hle, A.; Garnæs, J.; Mørch, K. A.; Boisen, A. Langmuir 2003, 19, 10510-10513. (20) Attard, P. Adv. Colloid Interface Sci. 2003, 104, 75-91. (21) Granick, S.; Zhu, Y.; Lee, H. Nat. Mater. 2003, 2, 1-7. (22) de Gennes, P. G. Langmuir 2002, 18, 3413-3414.
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an emulsion without a surfactant,23 the microboiling behaviors,24 the mineral flotation,25 and the rupture of a wetting film.26 It is clear that the formation of nanobubbles strongly depended on the properties of the substrates.18 So far, most of the studies have been focused on hydrophobic substrates.12,16,17,19 In contrast, the formation of nanobubbles on a hydrophilic surface has been rarely studied partially due to the belief that bubbles can be hardly formed on such a surface from bulk water under normal conditions.27 In our previous works, nanobubbles were prepared on the smooth and hydrophilic mica surface by the exchange of ethanol and water and imaged with tapping mode AFM in fluid.13,14 To well understand this formation process, the effects of two important factors, dissolved gas and liquid temperature, were explored in this note. Our results demonstrated clearly that dissolved gas contributed much to the formation of nanobubbles and the temperature of the liquids during mixing could influence the number of nanobubbles significantly. Experimental Section In our experiments, water was purified with a Milli-Q system (Millipore Corp., Boston, MA). Ethanol and methanol of GR were purchased from Chinese Chemical Reagent Co. Mica was from S&J Trading Co. (NY). The mica surface freshly cleaved in air was used as the substrate. Nanobubbles were prepared according to the method used by Lou et al.13 and were imaged by AFM in fluid tapping mode. The MultiMode Nanoscope IIIa SPM was from Digital Instruments (Veeco Metrology Group, NY) with an extender module capable of imaging in tapping mode. And a liquid cell with an O-ring was used to realize the fluid environment for imaging. A normal NP probe was used, and the spring constant of the cantilever was 0.32 N/m (Digital Instruments Veeco Metrology Group). The controlled experiments with nondegassed and degassed liquids were performed to explore the influence of dissolved gas. In the nondegassed group, ethanol and water were both placed under the natural ambient conditions and equilibrated with air. In the degassed group, tubes of ethanol and water were placed inside a desiccator and degassed by a vacuum pump (Shanghai Two Penguin Refrigeration Instrument Co., Ltd., Shanghai, China) about 30 min under 0.1 atm. The interval between taking the degassed liquid out of the desiccator and injecting it into the liquid cell was as short as possible. In the study of the temperature effect, nanobubbles were formed by the liquids of different temperatures. Ethanol and water were incubated in a water bath of desired temperature about 30 min before use. Imaging in both degassing and temperature experiments was carried out at room temperature of 21-23 °C.
Results and Discussion The substrate conditions prior to the formation of nanobubbles were characterized by imaging the mica surface in ethanol. The RMS of the typical surface image in Figure 1A was around 0.14 nm. After the injection of water, nanobubbles were formed as shown in Figure 1B,C. The large contrast in the phase image indicated that spherical domains and the substrate were different in their mechanical properties. (23) Pashley, R. M. J. Phys. Chem. B 2003, 107, 1714-1720. (24) Thomas, O. C.; Cavicchi, R. E.; Tarlov, M. J. Langmuir 2003, 19, 6168-6177. (25) Zbik, M.; Horn, R. G. Colloids Surf., A 2003, 222, 323-328. (26) Stockelhuber, K. W.; Radoev, B.; Wenger, A.; Schulze, H. J. Langmuir 2004, 20, 164-168. (27) Jones, S. F.; Evans, G. M.; Galvin, K. P. Adv. Colloid Interface Sci. 1999, 80, 27-50.
10.1021/la0364542 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/24/2004
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Notes
Figure 2. Effect of dissolved gas on the formation of nanobubbles. The average density of nanobubbles decreased obviously when the ethanol and water were degassed.
Figure 3. Effect of the liquid temperature on the formation of nanobubbles. The number of nanobubbles per micron increased with the liquid temperature and showed a rapid growth when the temperature was higher than 30 °C.
Figure 1. AFM images of the substrate and nanobubbles. Panel A shows the mica surface in ethanol prior to the formation of nanobubbles. The height image (B) and phase image (C) of nanobubbles were collected simultaneously. The scanning size of all three images was 1 µm × 1 µm.
Figure 2 shows the average densities of nanobubbles formed by nondegassed and degassed liquids. Because of the possible distortion of soft nanobubbles during imaging,13 the density or the number of nanobubbles per square micron rather than the size coverage of nanobubbles was compared in Figure 2. For the nondegassed group, the average density was 2.9 ( 1.7 per square micron, and for the degassed group, near zero. Although the density in each experiment varied in a wide range, the results showed obviously that the nondegassed liquids formed many more nanobubbles than the degassed ones. It could be concluded that the dissolved gas played an essential role in the formation process.
The fact that the density of those bubbles in AFM images decreased significantly after degassing could prove in a more direct way that what we observed were indeed gas nanobubbles. Some researchers reported AFM images of nanobubbles at varied interfaces.12-19 But AFM could not determine directly whether they were gas bubbles because of the well-known intrinsic limitation of AFM.16 Many efforts, such as the large phase shift, force measurements, and the behaviors under the perturbation from the tip, had to be made to verify that they were gas bubbles.12,15,18,19 In our case, the change of the number of those bubbles with the dissolved gas provided the more convincing evidence. As for the formation of nanobubbles, the influence of temperature has to be taken into account. Studies relevant to the effect of temperature, as Ralston said, will “potentially contribute to the understanding of any entropic changes, particularly in relation to the influence of dissolved gas”.28 Additionally, temperature itself is one of the important physical and chemical factors that have been linked to the nucleation process of nanobubbles at the solid/water interface. Figure 3 illustrates the change of the density of nanobubbles with the liquid temperature during mixing. It seems that there were two regions: when the temperature increased from 9 to around 30 °C, the density of the nanobubbles increased very slowly; the density increased dramatically when the temperature increased further. Similar trends were also observed when nanobubbles were formed by methanol and water with different temperatures. The horizontal axis in Figure 3 just reflects exactly the temperature of water, not the ethanol, during the exchange process. There was an interval of about 3 min between the injections of ethanol and water in the experiments. Thus the temperature of the ethanol had been almost (28) Ralston, J.; Fornasiero, D.; Mwashchuk, N. Colloids Surf., A 2001, 192, 39-51.
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equilibrated with the liquid cell. However, this did not impede us to get the basic trends of the density change of nanobubbles with the liquid temperature. Conclusion In summary, the dissolved gas in the liquids was essential for the formation of nanobubbles at the mica/ water interface. The number of nanobubbles formed by degassed ethanol and degassed water decreased obviously as compared with that by the nondegassed liquids. And the effect of liquid temperature was significant. The density of nanobubbles increased with the liquid temperature and showed a rapid growth when the temperature was over 30 °C. Although this exploration was carried out on a hydrophilic substrate, we believe that the conclusion should benefit the studies of nanobubbles on other kinds of substrates formed by other methods.
Acknowledgment. We gratefully acknowledge the financial support of the project by the National Natural Science Foundation of China for Contract Nos. 10335070, 30200051, and 10304011, the support of the Chinese Academy of Sciences for Contract Nos. KJ 951A1-603, KJ 951-A1-409, KJ 952-J1-469, KJCX 1-06, KSCX I-06, and STZ-00-07, and the support of the Science and Technology Commission of Shanghai Municipality for Contract Nos. 0114NM070, 00XD14029, and 0352NM116. Also, we gratefully acknowledge the support from the Ministry of Science and Technology, PRC, for Contract Nos. 2002CCA00600 and 2003BA310A02. Supporting Information Available: Tip perturbation on nanobubbles at the mica/water interface. This material is available free of charge via the Internet at http://pubs.acs.org. LA0364542
