Superhydrophobic Cones for Continuous Collection and Directional Transportation of CO2 Microbubbles in CO2 Supersaturated Solutions Xiuzhan Xue,†,⊥ Cunming Yu,§,⊥ Jingming Wang,*,† and Lei Jiang†,‡ †
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, China ‡ Laboratory of Bio-inspired Smart Interface Science, Key Laboratory of Bio-inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, and §Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Green Printing, Instituted of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *
ABSTRACT: Microbubbles are tiny bubbles with diameters below 50 μm. Because of their minute buoyant force, the microbubbles stagnate in aqueous media for a long time, and they sometimes cause serious damage. Most traditional methods chosen for elimination of gas bubbles utilize buoyancy forces including chemical methods and physical methods, and they only have a minor effect on microbubbles. Several approaches have been developed to collect and transport microbubbles in aqueous media. However, the realization of innovative strategies to directly collect and transport microbubbles in aqueous media remains a big challenge. In nature, both spider silk and cactus spines take advantage of their conical-shaped surface to yield the gradient of Laplace pressure and surface free energy for collecting fog droplets from the environment. Inspired by this, we introduce here the gradient of Laplace pressure and surface free energy to the interface of superhydrophobic copper cones (SCCs), which can continuously collect and directionally transport CO2 microbubbles (from tip side to base side) in CO2-supersaturated solution. A gas layer was formed when the microbubbles encounter the SCCs. This offers a channel for microbubble directional transportation. The efficiency of microbubble transport is significantly affected by the apex angle of SCCs and the carbon dioxide concentration. The former provides different gradients of Laplace pressure as the driving force. The latter represents the capacity, which offers the quantity of CO2 microbubbles for collection and transportation. We believe that this approach provides a simple and valid way to remove microbubbles. KEYWORDS: superhydrophobic cone, microbubble, continuous collection, directional transportation, gradient of Laplace pressure
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buoyant forces, the microbubbles stagnate in aqueous media for a long time, and they sometimes cause serious damage.10−12 For example, the carbon dioxide or hydrogen sulfide microbubbles in aqueous media will accelerate the hydrogenation of metal and form hydrogen pores within it. This potentially causes serious corrosion of pipelines and reduces the equipment life. Most traditional methods chosen for gas bubbles’ elimination utilize buoyant forces including chemical methods13−15 and physical methods. They have little impact on
icrobubbles are tiny bubbles with a diameter of less than 50 μm.1 They have demonstrated wide applications in the fields of biomedical engineering,2−4 environmental engineering,5,6 and drag reduction.7−9 In biomedical engineering, microbubble contrast agents opened up a new era in ultrasound, and water with microbubble plays a significant role in the removal of biofilm from the mouths of orthodontic patients. In environmental treatment, the microbubbles can accelerate the generation of hydroxyl radicals, which benefits the treatment of wastewater. In addition, microbubbles on the underwater surface of ships can reduce the skin friction between water and the ship and significantly reduce the drag force of water. However, due to the minute © 2016 American Chemical Society
Received: August 9, 2016 Accepted: November 20, 2016 Published: November 20, 2016 10887
DOI: 10.1021/acsnano.6b05371 ACS Nano 2016, 10, 10887−10893
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ACS Nano microbubbles. Thus, several approaches have been developed to collect and transport microbubbles in aqueous media. Superhydrophobic sponges were used to selectively absorb and steadily store microbubbles.16 The capacity of collection and storage is significantly controlled by the submerged depth of the sponges. In addition, three-dimensional (3D) gradient porous interconnected network surfaces with micronano hierarchical geometries have been used to transport microbubbles directionally.17 Their transportation direction is always restricted by the microbubble positions; i.e., the smaller one transports toward the bigger one. Hence, the realization of innovative strategies to directly collect and transport microbubbles in aqueous media remains a major challenge. It is well-known that shape gradient surfaces can transport water efficiently.18−20 This is attributed to the generation of the Laplace pressure. In nature, both spider silk21 and cactus spines22 take advantage of the conical-shaped surface to collect fog droplets from the environment. Numerous studies have shown that the cone surfaces after a unique modification can yield the gradient of Laplace pressure and surface free energy. This is a cooperative driving forces to collect and transport droplets directionally.23−26 Inspired by these structures, we expect to introduce the gradient of Laplace pressure and surface free energy to the cone surface. This can continuously collect and directionally transport microbubbles in aqueous media. Herein, we report an approach to continuously collect and directionally transport carbon dioxide microbubbles in a CO2supersaturated solution based on the employment of superhydrophobic copper cones (SCCs). The SCCs were obtained by gradient electrochemical corrosion23 and have rough surface structures. They were endowed with gas-absorbing abilities after being modified with a superhydrophobic-nanoparticle solution. A gas layer was formed when the microbubbles encountered the SCCs. It offers a channel for continuous and directional transport of microbubbles. The efficiency of microbubble transport is significantly affected by the apex angle of the SCCs and the carbon dioxide concentration. The former provides different gradients of Laplace pressure as the driving force. The latter represents the capacity, which offers CO2 microbubbles for collection and transportation. We believe that this approach will provide a simple and valid way to remove microbubbles.
Figure 1. SEM images of the copper cones with an apex angle of 7°. (a) SEM image of the copper cone before superhydrophobic modification. (b−d) Magnified SEM images of microscopic morphology from the copper cone’s tip, middle, and bottom parts. (e) SEM image of the copper cone after superhydrophobic modification. (f−h) Magnified SEM images of microscopic morphology on the superhydrophobic modified copper cone’s tip, middle, and bottom parts.
hydrophobic nanoparticles increased the surface roughness greatly. This induced a roughness gradient from the tip to the base. Before modification, the water contact angle (WCA) was 85.2 ± 3.4° and the gas contact angle (GCA) was 155.4 ± 3.4°. After modification, the water droplets remained spherical (WCA 160.5 ± 2.5°), and they cannot stick on the copper cones, exhibiting superhydrophobicity. The GCA of the modified copper cone approximates 0° (Figure S2). It seems that the roughness gradient did not induce a wettability gradient before and after modification. To further confirm whether the roughness gradient causes a wettability gradient or not, the contact states of water microdroplets at different sides of the copper cones were investigated. We found that the unmodified copper cones did not illustrate a wettability gradient, while the modified one had a small gradient. The WCAs of water microdroplets on the tip, middle and base side of the modified copper cone are 136.9 ± 2.2°, 146.5 ± 1.2°, and 152.8 ± 2.3°, respectively (Figure S3). Besides the apex angle of 7° seen in the superhydrophobic copper cones, we also prepared other superhydrophobic copper cones with apex angles of 5°, 9°, 11°, and 13°. The SEM images of the superhydrophobic copper cones with different apex angles are shown in Figure S4. The microbubble collection and transportation processes on the superhydrophobic copper cones (SCCs) in CO2 supersaturation solution were investigated (Figure 2). The concentration of CO2 supersaturation solution is 0.069−0.055 mol/L (more details can be found in the Methods). At first, to minimize the effect of buoyant force on the microbubble behavior on cone surface, the SCC is placed horizontally in the CO2 supersaturation solution (i.e., the tilt angle is ∼0°) as shown in Figure 2a. We observed initial, simultaneous generation as well as in situ growth of CO2 microbubbles (1−5) on the tip side of the SCC. The growing microbubble (2 and 5) moved toward the base side of the SCC and coalesced with an adjacent microbubble (3 and 4) to form a larger bubble (2+3 and 4+5). With continuous absorption of CO2, some tiny bubbles formed on the SCC. They immediately combined with the adjacent large ones. The large bubbles (2+3, 1, and 4+5)
RESULTS AND DISCUSSION The SEM images of constructed copper cones before and after modification are shown in Figure 1. Figure 1a illustrates the morphology of the copper cone before modification. The apex angle (2α) of the copper cone is about 7°, and its surface is relative smooth. The magnified images on the tip (Figure 1b), middle (Figure 1c), and bottom (Figure 1d) parts of the copper cone showed numerous nanoscale papillae on the surfaces. The roughness of the bottom part is larger than the other two parts. After modification with the superhydrophobic SiO2 nanoparticle solution, the apex angle of the copper cone also remained about 7° (Figure 1e). The magnified image of the tip part of the modified copper cone in Figure 1f illustrates that there are some superhydrophobic SiO2 nanoparticles randomly and loosely heaped up on its surface, leaving some tiny copper regions exposed to the exterior. Compared to the tip part, there were much more superhydrophobic SiO2 nanoparticles densely piled up on the middle and bottom parts (Figure 1g,h). A copper cone with a larger curvature resulted in the thicker nanoparticle layers. Consequently, the coverage of super10888
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Figure 2. Superhydrophobic copper cone (SCC) collects and transports microbubbles from CO2-supersaturated solution directionally and rapidly at varying title angles: (a) 0°, (b) 45°, (c) 90°, (d) −45°, (e) −90°. Even when the SCC was vertically fixed with the tip pointing up, the microbubbles were directionally driven from the tip to the base side. Scale bar, 500 μm.
side of the SCCs. The direction of microbubble transport can be controlled by the position of the SCCs (i.e., the title angles of SCCs). In comparison, the copper cones (CCs) can also collect the microbubbles. The microbubbles can only in situ grew on CCs. They cannot transport to their base sides (Figure S5 or see the supporting video 6). Therefore, the conical shape and superhydrophobicity are both critical to microbubble collection and transport. As is known, the concentrations of CO2 supersaturated solution and the apex angles of cones23,25,27 are crucial to the microbubble collection and transportation. The former represents the capacity or the quantity of CO2 microbubbles available for collection and transportation. The latter provides different gradient Laplace pressure as the driving force. The efficiency of microbubble collection and transportation on the SCCs with different apex angles was investigated in CO2supersaturated solution with various concentrations, as shown in Figure 3. CO2-supersaturated solutions were made by compressing CO2 gas into deionized water at 1.5 atm. The CO2 continued to dissolve from the CO2-supersaturated solution when the pressure dropped to 1 atm, and its concentration remained at a certain range during the study. The title angles of the SCCs are fixed to 0°. The method for calculating the efficiency of microbubble collection and transportation is shown in Figure S6. The SCCs have an apex angle of 5°, and the efficiency of the microbubble collection and transportation is 7.97, 11.96, 28.73, and 87.96 mm3/min when the concentrations of CO2-supersaturated solution increase from
then further coalesced with each other to make the largest bubble (4 + 5+1 + 3+2). The bubble (4 + 5+1 + 3+2) moved toward the base side of SCC (black arrows in Figure 2a). New microbubbles 1′ and 2′+3′ (2′ and 3′ coalesced and formed the bubble 2′+3′) collected on the same location as the microbubbles 1 and 2+3. Another cycle of microbubble collection and transportation then began. The continual cycles guarantee the continuous collection and transportation of microbubbles (see supporting video 1). Furthermore, we placed a single SCC at multiple tilt angles (45°, 90°, −45°, and −90°) to determine the effects of the SCC direction on the directional transportation behavior of the microbubbles (Figure 2b−e). When the SCC was positioned at tilt angles of 45° and 90°, buoyant forces acted as a resistant force to impede the directional movement of CO2 microbubbles (Figure 2b,c). However, we can also observe the directional movement of CO2 microbubbles/bubbles in addition to the absorption, growth, and coalescence processes (see the supporting videos 2 and 3). If the tilt angles are below 0° (−45° and −90°), then the buoyant force acts as the driving force. This is beneficial to the directional movement of microbubbles. The microbubbles grew and coalesced followed by directional transport toward the base side (Figure 2d,e). The speed of the bubble transport can be faster (see the supporting videos 4 and 5). These results indicate that the buoyant force of the gas bubble has only minor effects on the directional CO2 microbubble collection and transportation. Microbubbles are always collected from CO2 supersaturated solution. They are then transported to the base 10889
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microbubble collection and transport rose first and then decreased. The efficiency of the microbubble collection and transport reached the maximum values (18.31, 33.30, and 156.67 mm3/min) when the apex angle of the SCC is 9° with CO2 supersaturated solution concentrations of 0.066−0.060, 0.078−0.074 and 0.104−0.082 mol/L. Next, we propose a potential mechanism to characterize the efficient collection and directional transportation of CO2 microbubbles on the SCCs. Figure 4a overviews the collection and transportation of CO2 microbubbles. It is known that the CO2-supersaturated solution, i.e., carbonated water, contains dissolved carbon dioxide gas in water under high pressure.28 It becomes effervescent when the carbonated water is exposed to common atmosphere,29 and this process provides a continuous supply of microbubbles. If the SCC is immerged into the CO2supersaturated solution, then air is trapped in its microstructures to form many cavities30,31 on the interface of SCCs due to superhydrophobic SiO2 nanoparticles heaped on it. A rougher SCC interface traps more air. Thus, at the base side of the SCC, quasicontinuous or continuous air layers can be formed. Concurrently, the microbubbles effervescence from the carbonated water, burst, and spread on the SCC interface.32 The entrapped air and CO2 microbubble bursting process forms a mixture gas layer on the SCC interface33,34 (more evidence is shown in Supporting Information S7). The thickness of the gas layer exhibits a gradient because of the gradient of wettability and diameter of the SCC. That is to say, the thickness of the gas layer on the base side is larger than that on the tip part (Figures S6 and S7). Therefore, the SCCs can be divided into two parts: the tip part without a continuous gas layer and the remaining part with a continuous gas layer. The tip part without a continuous gas layer forms a microbubble (microbubble 1) generated around the tip point of SCC. This is because heterogeneous nucleation is much easier than homogeneous nucleation35−38 as shown in the inset of Figure 4a. The generated microbubble sticks on the tip point of SCC as a sphere (microbubble 1) and then grows in situ. If the diameter of the microbubble is sufficiently large, the microbubble (microbubble 1′) moves a bit to the side with the larger radius due to the increase in buoyant force. The microbubble continues growing and moving as a sphere until it arrives at the place where the SCC is sufficiently rough. The microbubble spreads, and its shape changes to spherical caps (bubble 2). The resistance force along the taper is greater because the contact lines of the microbubbles are greater further along the
Figure 3. Efficiency of microbubble collection and transportation on the SCCs with different apex angles was investigated in CO2supersaturated solution with various concentrations. The efficiency of microbubble collection and transport on the SCCs increased as the concentrations of CO2 supersaturated solution increased. With the increase in apex angle from 5° to 13°, the efficiency of the microbubble collection and transportation first rose and then decreased. These values were highest when the apex angle of the SCC is 9°.
0.056 to 0.054 and 0.066−0.060 and 0.078−0.074, to 0.104− 0.082 mol/L. If the apex angle of the SCCs changed to 9°, then the efficiency of the microbubble collection and transportation increased from 8.27 to 156.67 mm3/min in the CO2supersaturated solution with low and high concentrations (i.e., 0.056−0.054 and 0.104−0.082 mol/L). Furthermore, if the apex angle of the SCCs reach 13°, then the efficiency of the microbubble collection and transportation increased to 102.16 mm3/min in the 0.104−0.082 mol/L CO2-supersaturated solution. Thus, the efficiency of microbubble collection and transportation on the SCCs increased when the concentrations of CO2 supersaturated solution increased. The effect of the apex angle on the efficiency of the microbubble collection and transportation was studied. In the CO2-supersaturated solution with low concentration (0.056− 0.054 mol/L), the efficiency of the microbubble collection and transportation on the SCCs with the various apex angles (i.e., 5°, 7°, 9°, 11°, and 13°) is about 9 mm3/min. Nevertheless, in the CO2-supersaturated solution with relatively high concentrations, the apex angle of the SCCs significantly influenced the efficiency of the microbubble collection and transportation. As the apex angles increased from 5° to 13°, the efficiency of the
Figure 4. (a) Mechanism of the continuous collection and directional transportation of microbubbles on SCCs from the CO2-supersaturated solution. (b) Curves of the gas growth velocity (v1) and the gas transportation velocity (v2) with different apex angle of SCCs. 10890
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bubble, liquid inertia, viscosity, and surface tension. According to the Scriven solution42 for bubble growth from a CO2 supersaturated solution, the gas growth velocity on SCCs can be calculated as follows (for more details, see Supporting Information S9).
taper (father away from the apex). When small bubbles near the apex coalesce, the final position is dominated by the position of the bubble with a larger contact line because this bubble offers more resistance to movement. As a result, small bubbles near the apex are forced away from the apex (bubble 2′). When the gas bubbles move the position with gas layer, the microbubbles coalesced with the gas layer and then wrapped around the SCCs (bubble 2″). The resulting force (F) acts on the gas bubbles at a position with gas layers. It can be deduced as follows (Figure S9): F = FL + FH + FB sin β
⎡ π (r 2 − r 2) ⎤1/2 1 ⎥ v1 = 4 π β k ⎢ 2 ⎣ sin α ⎦ 2
Here, β is a dimensionless growth parameter, k is a diffusion term (the diffusivity), r2 and r1 are the tip and bottom radius of cone tube, and 2α is the apex angle of copper cone. The cone tube is the gas layer covered around the SCCs. According to eq 6, the gas growth velocity (v1) on the interfaces of the SCCs decrease with increasing 2α. The gas transportation on the interfaces of SCCs can be considered as the incompressible fluid flowing through a long cone tube with gradient cross sections. According to Poiseuille’s law,43,44 the transportation velocity can be derived as follows (also see Supporting Information S10).
(1)
Here, FL is the axis-direction component of Laplace pressure driving the bubbles spontaneously and directionally move toward the base side of cone. The gradient of the Laplace pressure (ΔP) arises on the bubble due to the conical shape of the SCC.22−24,27 The gradient of the Laplace pressure is the main force that drives the directional movement of the gas bubbles.39 ΔP = −
∫R
R2
1
2γα dz (r + R 0)2
(2)
v2 =
Here, γ is the surface tension of water, r is the local radius of the SCC (R1 and R2 are the local radii at the two opposite sides of the gas bubble), R0 is the radius of the gas bubble, 2α is the apex angle of the SCC, and dz is the integrating variable along the diameter of the SCC. In addition ⎛1 1 ⎞ FL ∼ 2γ ⎜ − ⎟Sα R2 ⎠ ⎝ R1
(3)
(4)
where R0 is the radius of the bubble and θr and θa are the receding and advancing contact angles, respectively. FB sin β is the axis-direction component of buoyant force FB sin β = ρgV sin β
γ dV (sin α)2 = Q = π (r24 − r14) dt 4μr1r2
(7)
Here, Q is the volumetric flow rate, V is the volume of the liquid transferred as a function of time (t), γ is the surface tension of water, μ is the dynamic viscosity, r1 and r2 are the radius of cone tube on the tip and base side of SCC, and 2α is the apex angle of SCC. The transportation velocity (v2) becomes larger with an increasing apex angle of SCC according to eq 7. The efficiency of the gas bubble collection and transportation is determined by the minimum between the gas growth velocity (v1) and the gas transportation velocity (v2). According to the experimental results, the highest efficiency on the SCCs happens at an apex angle of 9°. Here, we confirmed the curve of gas growth velocity (v1) intersects with the curve of gas transportation velocity (v2) at an apex angle of 9°. The curves of the gas growth velocity (v1) and the gas transportation velocity (v2) with different SCC apex angles can be fitted in Figure 4b. The fitted curve describes nicely the impact that gas growth velocity (v1) and gas transportation velocity (v2) on the efficiency of the gas bubble collection and transportation. The efficiency of the gas bubble collection and transportation is controlled by the gas transportation velocity (v2) on the SCCs when the apex angle is smaller than 9°. It is controlled by gas growth velocity (v1) on the SCCs when the apex angle is larger than 9°. The features described here for the fitted curve agrees well with the experimental results (Supporting Information S10).
where S is the area of the region enclosed by the gas bubble. FH is the axis-direction component of hysteresis resistance force impeding the movement of bubbles, which can be deduced as follows24 FH ∼ πR 0γ(cos θr − cos θa)cos α
(6)
(5)
where ρ is the density of water, g is the acceleration of gravity, V is the volume of bubbles, and β is the tilt angle of cones (−90° ≤ β ≤ 90°). The tilt angle (β) is zero for horizontally placed hydrophobic and superhydrophobic cones, and the axis-direction component of buoyant force (FB sin β) is nearly equal to zero, which is not large enough to facilitate bubble transport. On a superhydrophobic cone, a continuous gas film (plastron) enables the gas bubbles to present a barrel-like shape. Compared to the hemispherical shape, the barrel morphology facilitates the gas bubbles with a larger enclosed area S and a larger difference of R1 and R2. Consequently, FL acting on superhydrophobic copper cone is larger than that on hydrophobic one. Moreover, R0 is larger further along the taper away from the apex. The FH on the superhydrophobic surface is larger further along the taper away from the apex. In fact, the efficiency of the microbubble collection and transportation is controlled by the cooperation of gas growth and transportation on the SCCs interfaces. The solution-phase bubble growth rate40,41 is influenced by a number of factors such as the rate of molecular diffusion to the interface of
CONCLUSIONS A superhydrophobic copper cone was successfully fabricated and used for gas collection and transportation properties in gas−water−solid three-phase systems. The SCCs continuously collect microbubbles from the carbonated water and then directionally transported them to their base sides driven by the gradient of the Laplace pressure and surface free energy. The direction of microbubble transport can be controlled by the position of the SCCs (i.e., the title angle of SCCs). The efficiency of microbubble transport is significantly affected by the apex angle of the SCCs and the carbon dioxide concentration. The former provides a different gradient Laplace 10891
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AUTHOR INFORMATION
pressure as the driving force. The latter represents the capacity, which offers the quantity of CO2 microbubbles for collection and transportation. The efficiency of the microbubbles collection and transportation was highest when the apex angle of the SCC is 9°.
Corresponding Author
*E-mail:
[email protected]. ORCID
Jingming Wang: 0000-0003-1309-7211 Lei Jiang: 0000-0003-4579-728X
METHODS
Author Contributions
Preparation of Superhydrophobic Copper Cones. The SCCs were prepared via gradient electrochemical corrosion and chemical modification. Commercial copper wires with a diameter of 0.5 mm were chosen to fabricate the cone shape (for more details, see Supporting Information S1). First, it was carefully polished with sandpaper (P1500). The polished copper wires were cleaned with ethanol, acetone, and deionized water under ultrasound and then dried with nitrogen. The copper wires were electroetched at 10 V in an aqueous solution of 0.1 M CuSO4. The electroetched solution was raised up and lowered at 0.5 mm/s for tens of cycles. The copper cones (CCs) were then achieved. We immersed the copper cones in the superhydrophobic SiO2 nanoparticle solution several times to make SCCs. Characterization of SCCs. The SEM images were obtained using a field-emission scanning electron microscope (JEOL-4800F, Japan). The water contact angles were measured using an OCA 20 machine (Data-physics, Germany). Measurement of Microbubble Collection and Transportation Ability. SCCs were fixed on a simple stage with changeable title angles and then immersed in a quartz container filled with CO2supersaturated solutions. The CO2-supersaturated solutions were made by compressing CO2 gas into deionized water at 1.5 atm. The process of microbubble collection and transportation was recorded via a high-speed camera (Olympus, i-speed3). The carbon dioxide concentrations dissolved in CO2 supersaturated solutions were measured via a barium hydroxide precipitation reaction. The 100 mL barium hydroxide (1 mol/L) reacted with 50 mL of CO2supersaturated solution. After the reaction, the pH testing paper was used to confirm whether the carbon dioxide dissolved in CO2 supersaturated solutions reacted completely (the reaction solution to be alkaline). The precipitate was obtained by filtering the reaction solution under vacuum. The concentration of carbon dioxide dissolved in CO2-supersaturated solutions is calculated according to the weight of barium hydroxide precipitate. The concentration determined is the total carbon dioxide including HCO3− and H2CO3. The carbon dioxide concentrations dissolved in CO2-supersaturated solutions were controlled by exposing the CO2 supersaturated solutions to the atmosphere at different times.
⊥
X.X. and C.Y. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the National Research Fund for Fundamental Key Projects (2013CB933000), Beijing National Science Foundation (2152018), Beijing Higher Education Young Elite Teacher Project, and the 111 Project (B14009). REFERENCES (1) Agarwal, A.; Ng, W. J.; Liu, Y. Principle and Applications of Microbubble and Nanobubble Technology for Water Treatment. Chemosphere 2011, 84, 1175−1180. (2) Soetanto, K.; Chan, M. Fundamental Studies on Contrast Images from Different Sized Microbubbles: Analytical and Experimental Studies. Ultrasound in Medicine & Biology 2000, 26, 81−91. (3) Blomley, M. J. K.; Cooke, J. C.; Unger, E. C.; Monaghan, M. J.; Cosgrove, D. O. Clinical Review-Microbubble Contrast Agents: A New Era in Ultrasound. BMJ 2001, 322, 1222−1225. (4) Mukumoto, M.; Ohshima, T.; Ozaki, M.; Konishi, H.; Maeda, N.; Nakamura, Y. Effect of Microbubbled Water on the Removal of a Biofilm Attached to Orthodontic Appliances: An in Vitro Study. Dent. Mater. J. 2012, 31, 821−827. (5) Zheng, T.; Wang, Q.; Zhang, T.; Shi, Z.; Tian, Y.; Shi, S.; Smale, N.; Wang, J. Microbubble Enhanced Ozonation Process for Advanced Treatment of Wastewater Produced in Acrylic Fiber Manufacturing Industry. J. Hazard. Mater. 2015, 287, 412−420. (6) Chu, L.; Xing, X.; Yu, A.; Zhou, Y.; Sun, X.; Jurcik, B. Enhanced Ozonation of Simulated Dyestuff Wastewater by Microbubbles. Chemosphere 2007, 68, 1854−1860. (7) Shen, X.; Ceccio, S. L.; Perlin, M. Influence of Bubble Size on Micro-Bubble Drag Reduction. Exp. Fluids 2006, 41, 415−424. (8) Madavan, N. K.; Deutsch, S.; Merkle, C. L. Reduction of Turbulent Skin Friction by Microbubbles. Phys. Fluids 1984, 27, 356− 363. (9) Kodama, Y.; Kakugawa, A.; Takahashi, T.; Kawashima, H. Experimental Study on Microbubbles and Their Applicability to Ships for Skin Friction Reduction. Int. J. Heat Fluid Flow 2000, 21, 582−588. (10) López, D. A.; Pérez, T.; Simison, S. N. The Influence of Microstructure and Chemical Composition of Carbon and Low Alloy Steels in CO2 Corrosion. A State of the Art Appraisal. Mater. Deg. 2003, 24, 561−575. (11) Radkevych, O. I.; Chumalo, H. V. Damage to the Metal of Industrial Pipelines in a Hydrogen Sulfide Environment. Mater. Sci. 2003, 39, 596−600. (12) Liu, Z.; Gao, X.; Du, L.; Li, J.; Kuang, Y.; Wu, B. Corrosion Behavior of Low-Alloy Steel with Martensite/Ferrite Microstructure at Vapor-Saturated CO2 and CO2-Saturated Brine Conditions. Appl. Surf. Sci. 2015, 351, 610−623. (13) Vardar-Sukan, F. Efficiency of Natural Oils as Antifoaming Agents in Bioprocesses. J. Chem. Technol. Biotechnol. 1988, 43, 39−47. (14) Kougias, P. G.; Tsapekos, P.; Boe, K.; Angelidaki, I. Antifoaming Effect of Chemical Compounds in Manure Biogas Reactors. Water Res. 2013, 47, 6280−6288. (15) Kougias, P. G.; Boe, K.; Tsapekos, P.; Angelidaki, I. Foam Suppression in Overloaded Manure-Based Biogas Reactors Using Antifoaming Agents. Bioresour. Technol. 2014, 153, 198−205.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b05371. Selection of the diameter of copper wires; contact angle of water and gas on copper sheet before and after modification; WCAs of water microdroplets; SEM images of SCCs; in situ growth behavior of microbubbles on unmodified CCs; calculation of the efficiency of microbubble collection and transport; evidence of the gas layer; schematic image of forces acting on a gas bubble; calculation of gas growth and transportation velocity on the interfaces of SCCs; experimental curves of microbubble collection and transportation efficiency on SCCs with different apex angles in CO2 supersaturated solution (PDF) Videos 1−5 showing microbubble collection and transportation processes (ZIP) 10892
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DOI: 10.1021/acsnano.6b05371 ACS Nano 2016, 10, 10887−10893
