Spontaneous and Directional Bubble Transport on Porous Copper

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Spontaneous and Directional Bubble Transport on Porous Copper Wires with Complex Shapes in Aqueous Media Wenjing Li, Jingjing Zhang, Zhongxin Xue, Jingming Wang, and Lei Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15681 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Applied Materials & Interfaces

Spontaneous and Directional Bubble Transport on Porous Copper Wires with Complex Shapes in Aqueous Media Wenjing Li†, ‡, Jingjing Zhang†,‡, Zhongxin Xue§, 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, P. R. China #

Laboratory of Bio-inspired Smart Interface Science, Technical Institute of Physics and

Chemistry, Chinese Academy of Science, Beijing, 100190, P. R. China §

School of Chemistry and Materials Science, Ludong University, Yantai, 264025, P. R.

China

KEYWORDS：aerophilic copper wire, gas bubble, complex shapes, directional transport, gradient of Laplace pressure

ABSTRACT

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Manipulation of gas bubble behaviors is crucial for gas bubble-related applications. Generally, the manipulation of gas bubble behaviors generally takes advantage of their buoyancy force. It is of great difficulty to control transportation of gas bubbles in a specific direction. Several approaches have been developed to collect and transport bubbles in aqueous media; however, most reliable and effective manipulation of gas bubbles in aqueous media occurs on the interfaces with simple shapes (i.e., cylinder and cone shapes). Reliable strategies for spontaneous and directional transport of gas bubbles on interfaces with complex shapes remain enormously challenging. Herein, a type of 3D gradient porous network was constructed on copper wire interfaces, with rectangle, wave and helix shapes. The superhydrophobic copper wires were immersed in water, and continuous and stable gas films then formed on the interfaces. With the assistance of the Laplace pressure gradient between two bubbles, gas bubbles (including microscopic gas bubbles) in the aqueous media were subsequently transported, continuously and directionally, on the copper wires with complex shapes. The small gas bubbles always moved to the larger ones.

Gas bubbles are ubiquitous in nature and they have many applications in industry and agriculture.1-3 The behavior of gas bubbles is greatly influenced by the buoyancy force in aqueous media. Because the buoyancy force is vertically upward, gas bubbles will move straight up, finally releasing into the atmosphere.4-6 Thus, the industrial and agricultural processes that involve gas bubbles, such as wastewater treatment7, 8 and the flotation recovery of fine mineral particles, 9, 10 generally take advantage of their buoyancy force. As a result, pollutants and mineral particles merely float upwards. Furthermore, when the presence of gas bubbles causes detrimental effects, the usual method for eliminating gas bubbles involves


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increasing the buoyancy force of the gas bubbles, causing them to rise upward vertically. 11-19 Therefore, it is very difficult to control transportation of gas bubbles in a specific direction.

There have been many attempts to achieve controllable and directional manipulation of gas bubbles. Well-designed gas channels have been fabricated on cylindrical and conical interfaces,20-22 and the gradient of the Laplace pressure and surface energy was then introduced to these gas channels. The gradient of the Laplace pressure spontaneously and directionally transported the gas bubbles in the gas channels. However, the directional and efficient transport of gas bubbles only occurs on interfaces with simple shapes (i.e., cylinder and cone shapes), and developing reliable strategies for spontaneous and directional transport of gas bubbles on interfaces with complex shapes remains extremely challenging.

In the current study, a type of 3D gradient porous network was constructed on copper wire interfaces with rectangle, wave and helix shapes, which can maintain robust superhydrophobicity with dynamic stability. After immersing the superhydrophobic copper wire in water, it achieved aerophilicity. A continuous and stable gas film formed on the interface. The gas film contributed to the capture of gas bubbles and the reduction hysteresis of contact angle. Subsequently, with the assistance of the Laplace pressure gradient between two bubbles, the gas bubbles (including microscopic gas bubbles) in the aqueous medium were continuously and directionally transported on the complex shaped copper wires. The small gas bubbles spontaneously and directionally move to the larger ones. This study may provide information for new design strategies to achieve the spontaneous and directional transport of gas bubbles on complex shaped interfaces in aqueous media.



RESULTS AND DISCUSSION Figure 1 shows photographs and typical scanning electron microscope (SEM) images on a copper wire with a wavy shape. As shown in Figure 1a, the copper wire has several cycles of wave. The copper wire exhibited superhydrophobic and aerophilic characteristics with a water contact angle of 163.5 ± 4.1° and a gas contact angle of 45.4 ± 3.3°, respectively. Numerous micro-porous structures, which are encircled by dendritic branches, can be recognized on the wavy copper wire surface (Figure 1b). A detailed microstructure analysis (Figure 1c, d) shows that each dendritic branch has been formed by stacked copper nanoparticles, exhibiting a loose structure. The microporous and copper nanoparticles compose the rough multilevel micro-nano hierarchical structures on the wavy copper wire surface. The special 3D porous structures are attributed to the competitive reduction of hydrogen ions and copper ions on the copper substrate in highly acidic media.23 During the electrochemical deposition process, a large number of hydrogen bubbles created on the cathode (i.e., shaped copper wires) move towards the electrolyte–electrode interface during the copper ions’ electro-deposition process. Due to the existence of hydrogen bubbles and the lack of metal ions, there will be no metal deposition actions on the specific point. As the hydrogen bubbles moves upwards because of their lower density (i.e. their buoyancy force), they leave behind a 3D interconnected porous metallic network with dendritic Cu branches. In other words, these unique microporous structures are attributed to the concurrent generation of hydrogen bubbles with extremely rapid metal deposition at high cathodic current densities. The average size and the thickness of the 3D porous structures have been summarized at different deposition times and current density.20 A series of snapshots, demonstrating the movement of the gas bubbles on the copper wires


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with different shapes at different times, is shown in Figure 2. The sizes of the small and large gas bubbles were kept in the same sizes on the shaped wires, respectively. The distances between two gas bubbles on the different shaped wires were kept at two geometry cycles. As depicted in Figure 2a, the axe of the rectangle shaped wire was kept in horizontal level in order to reduce or eliminate the effect of buoyance force on bubble transport process. More analysis about the effect of the title angle on gas bubble transport can be found in Supporting Information S1. Two gas bubbles were steadily trapped on the zenith of their geometry cycles in an aqueous medium at the beginning. Bubble 1 (3 µL) was smaller than bubble 2 (5 µL). As time went on, the volume of bubble 1 decreased and the volume of bubble 2 increased. Finally, bubble 1 coalesced with bubble 2 and a larger bubble 1+2 formed (Figure 2a, Video 1). In order to determine whether the movement of gas bubbles was directional or not, the location of two bubbles was changed (Supporting Information Figure S4a). The small bubble always transported to the large bubble, and the large bubble became increased in size and maintained a nearly spherical shape on the copper wire interface. That is to say, the small bubbles were directionally driven to coalesce with the large ones. Furthermore, gas bubble transport behavior was investigated on the copper wires with wave (Figure 2b, Figure S4b and Video 2) and helix shapes (Figure 2c, Figure S4c and Video 3), respectively. The small bubbles (bubble 1) were also directionally driven to coalesce with the large ones (bubble 2). Even if there is a tiny difference between the two bubbles, the difference of Laplace pressure can drive the small bubble to move slowly toward the big one (Figure S5). Furthermore, we investigated the transport process among three bubbles with different volumes. We found that no matter which position the three bubbles were placed, the smallest one transported firstly,



and then the second large one transported. Finally, the three bubbles coalesce to form one bubble at the position of the largest one. That is to say, the bubble transport direction is always towards to the largest bubble (Supporting Information S4). If introduced a smooth band between the 3D porous structures, the bubble transport is blocked (Figure S7). These results indicate that, no matter the shape of copper wire, the combination of the continuous 3D microporous network with low surface energy allowed the coppers wires to capture and directionally transport gas bubbles and the direction of gas transportation was always towards to the larger/largest bubble.

In order to give more valid analysis of the gas bubble transport phenomena, the volumes of small bubbles at different time on the shaped copper wires were compared. Linear fit of the variation of the small bubble volumes with time demonstrated the average speed of gas bubble transport. The average speed on rectangle-shaped copper wire is larger than that on the wave- and helix-shaped copper wires. The average speed of gas bubble transport seems to have correlation with the shape of the wires (Figure 2d). It is worth noting that the length of the geometry cycles on the shaped copper wires is different. Thus, the length of the channels for gas bubble transport is different, which can influence the bubble transport speed.

In fact, there are too many factors, which can influence the gas bubble transportation speed, including the size difference between the small and large gas bubbles (i.e., the Laplace pressures), the distances between two bubbles and the size of gas layer (i.e., the length and size of the gas transportation channels). We found that the average speed increased with the increase of the Laplace pressure on the rectangle- (Figures S8), wave- (Figure S9), and helix


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-shaped (Figure S10) copper wires. The average transport speed of gas bubbles increased with the decrease of the length of the gas transportation channels (Figure S11). The increase of the channel sizes helped gas bubbles to improve their transport speed (Supporting Information S9).

The directional transport of gas bubbles on copper wire interfaces with different shapes indicates the existence of gas transport channels between these gas bubbles. The contact state between the immersed interface and water is the key element involved in producing gas transport channels.24 Thus, the contact state between the immersed interface and water (three-phase contact state of solid/liquid/gas) was examined by measuring and calculating the forces acting on the copper wire interfaces during immersion into water. Three types of copper wires with rectangle (I), wave (II) and helix (III) shapes were used, as shown in Figure 3aI-aIII. Variations in the force(Fz) of copper wires with rectangle (I), wave (II) and helix (III) shapes, with respect to the moving distance, are illustrated in Figure 3bI, bII and bIII, respectively. We used z to indicate a direction of force. +z is the direction of the vertical downward force. Fz on the copper wires had been set at zero before the copper wires contacted water. Therefore, the weight of the copper wires need not to be considered.

When the copper wire with the rectangle I shape was immersed into an aqueous medium, the force abruptly decreased to below zero. The force curve decreased sharply with increasing immersion depth (Figure 3bI, A-B). When the force dropped to -463.8 µN, the force then directly jumped to 174.1 µN (Figure 3bI, B-B’). The force stayed at 174.1 µN for several millimeters of immersion depth (Figure 3bI, B’-C). If the immersion depth of the copper wire



continued to increase, the force curve abruptly decreased again (Figure 3bI, C-D), and similar cycle were repeated as part A-C in force curve. During immersion process, the horizontal edge of the copper wire with a rectangle (I) shape prevented water from wetting the upper surface, and generated a water dimple above it (Figure 3cI). The air in the dimple resulted in considerable buoyancy force. 25, 26

The buoyancy force (Fb) can be deduced from the integration of the water pressure below the water surface against the immersed area of horizontal edge of the copper wire, which is equal to the displacement of the region inside the water dimple. Fb is related to the square of the wire radius R2 and the radius R. 25 Fb can be calculated as follows, = sin 2 + 2 sin

(1)

Here, d is the immersion depth of the copper wires from water/air interface; α is the angle between the vertical direction and the radius.

The force analysis is shown in Figure S15, which is according to the front and side projection of the rectangle-shaped copper wire during immersion process, as shown in Figure S15a and S15b. The surface tension (Fσ) is along the contact perimeter. Thus, the vertical component of 2Fσ (2Fσsinθ) is upwards direction.

Associated with the surface tension, the buoyancy force make the force curve abruptly decreased below zero. The depth of the dimple is largest at the valley of the force curve. At this point the copper wire pierced the surface and the horizontal edge plunged into water, as well as part of vertical edge (Figure 3cI, d=1.63 mm), resulting in a decrease of buoyancy


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force and significantly increase of Fz of the copper wire. The significant increase of Fz is due to additional water displacement caused by the deformation of the copper wire. The redeformation force (Fp) is related to wR, R–3 and R–4 (where w is the deflection of the copper wire). 25 The force reached to 180 µN and maintained this value (Figure 3cI, d=1.76 mm and d=2.80 mm) until the second horizontal edge generated a new water dimple (Figure 3cI, d=4.38 mm).

If the copper wire with wave (II) shape was immersed into an aqueous medium, the force experienced similar circulation as copper wire with rectangle shape (Figure 3bII). During the immersion process, there is an obvious water dimple can be recognized (Figure 3cII). Because the water dimple generated above the horizontal edge of the wave shaped copper wire was always smaller than that of the rectangle shaped copper wire, the minimal valley of force curve on the wave shaped copper wire was much larger than that on the rectangle shaped copper wire. Correspondingly, after the water surface was immediately penetrated, Fp on the wave shaped copper wire much smaller than that on the rectangle shaped copper wire because the real displacement area of the whole curvature that the wave shaped wire renders on the water surface is smaller.

Variations in the force (Fz) of copper wires with helix (III) shapes, with respect to the moving distance in Figure 3bIII demonstrated that when the copper wire was immersed into aqueous media, Fz abruptly decreased, and then kept below zero. During immersion process of helix shaped copper wire, copper wire almost vertically enters into water (Figure 3cIII). Fp resulting from the deformation can be ignored compared to the other two force components



(i.e., Fb and 2Fσsinθ).

Therefore, the force curves were greatly related to the shapes of the water dimples. During the immersion processes of the copper wires with rectangle (I), wave (II) and helix (III) shapes, water dimples could be found on the upper side of and around the submersed copper wires. Water did not wet the copper wires because of their superhydrophobicity and aerophilicity (Figure S15). Thus, the generated dimples on the copper wires caused them to trap air resulting in the formation of a continuous gas layer, which provided channels for gas transport. 23,27,28,29

In order to further evaluate the influence of directional gas transport on the prepared copper wires, a potential model was proposed, as shown in Figure 4. Due to the 3D microporous hierarchical structures and the copper wires’ low surface energy, their surfaces exhibited superhydrophobicity wherein the water contact angle was greater than 150°. In a gaseous atmosphere, gas may easily fill the porous structures of a rough surface. When immersed in water, the pore space on the superhydrophobic surface is filled with gas and the multilevel micro-nanostructures direct the surface tension upward, allowing greater resistance to the penetration of water, thus preventing the loss of repellency. 30-33 A continuous gas film around the copper wires is therefore formed under combined action of the hydrostatic pressure, atmospheric pressure, entrapped gas Laplace pressure, and surface tension.34, 35 The gas film around the copper wires is arc-shaped with a particular radius of curvature, determined by the diameter of the copper wire and the thickness of the gas film. Thus, across the liquid–gas interface, there exists a Laplace pressure difference ∆.


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∆P = / (2) where γ and Rw are the tension and the curvature radius of the liquid–gas interface, respectively.

Once the gas bubbles come into contact with the copper wire, they are rapidly captured by the gas film that has been produced by the differences in pressure between the gas film and the gas bubbles. For a gas bubble in an aqueous medium, the Laplace pressure ∆Pb acting on it has a relationship with the radius of curvature Rb as follows: ∆ = 2/ (3) The Laplace pressure decreases as the curvature radius of the bubble increases. If Rb﹥2Rw, according to Equations (2) and (3), the gas film possessed a larger Laplace pressure than a gas bubble, namely ∆Pb＜∆P. When the gas bubble contacts the gas film, the gas film around the copper wire coalesces with the gas bubble to form one linked gas layer. At the same time, the buoyancy force of the gas bubble balances the Laplace force instantaneously. After the formation of a linked gas film, some gas from the film is pushed into the bubble since the gas film has a larger Laplace pressure than the gas bubble. However, the gas film is not exhausted resulting from the special structure and chemical component of the copper wire surface. Since the gas film is extremely thin, the effect of the gas pushed from the gas film can be disregarded. Therefore, the gas bubble can sustain stability as a spherical crown shape on the copper wire interface (Figure 4a). The relationship between the radius of spherical crown R and bubble contact angle θ can be calculated by the Eq. (4) (Supporting Information S16 and Figure S17).36



=

! " (3) # !

Thus, a smaller bubble (Bubble 1, R1) possesses larger Laplace pressure than a larger one (Bubble 2, R2). Consequently, driven by the differential Laplace pressure, the small bubble can be transferred to the large one directionally, forming a larger one (Bubble 1+2, R0, Figure 4b). Furthermore, if ∆Pb﹥∆P, namely Rb﹤2Rw, then the gas bubble is absorbed by the gas film on the surface of copper wire (Figure S18). That is to say, the gas bubble mixed together with the gas film. The absorption process is quite similar to the bubble bursting process on aerophilic plane surfaces underwater, such as lotus leaf surfaces 37 or superaerophilic sponges 35

. The adsorption by the gas film is an additional and critical feature of our observations.

CONCLUSIONS In summary, aerophilic copper wires were successfully fabricated using copper wires with rectangle, wave and helix shapes, via a facile control electro-chemical method. The aerophilic copper wires with a 3D micro-porous network perfectly maintained a constant gas film underwater for several days. By taking advantages of the gas film, gas bubbles were efficiently captured. In addition, the gas film was used as a channel for transportation of gas bubble and caused little resistance force. Driven by the difference of the Laplace pressure, gas bubbles were efficiently directionally transported on copper wires with complex shapes. The small gas bubble spontaneously and directionally moves to the larger bubbles.

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Sample Preparation: Copper wires were bended into different shapes, including rectangles waves and helixes. Copper wires (purity 99.8%) with a diameter of 410.0 µm, were cleaned with dilute hydrochloric acid, dilutes sodium hydroxide, acetone, and finally deionized water. Wires were then used as the substrate (cathode) for copper deposition. A copper plate (99.8%, 2 × 2 cm2) was used as the counter electrode (anode) for copper deposition. The distance between the anode and cathode was kept at 4 cm. A constant DC current was applied to the surface by using a precision DC power supply (Agilent U8002A, USA) in an electrolyte mixture of 1.5 M sulfuric acid containing 0.2 M copper sulfate. Deposition was performed in a stationary electrolyte solution (without stirring or N2 bubbling) at room temperature. The current density and deposition time were 0.6 A and 45 s for maintenance of the substrate microstructures during copper deposition. After electro-deposition, the copper wires were rinsed with alcohol and immersed in a 1.0 wt% ethanol solution of fluoroalkylsilane (1H, 1H, 2H, 2H-heptadecafluorodecyl trimethoxy silane) at room temperature for 12 h. Afterward, the copper wires were kept in a desiccator until further tests were performed.

Characterization of the Microstructures: The morphologies of copper wires were observed using an environmental SEM (Quanta 250 FEG, USA). Characterization of the Distribution of Gas Film: A square quartz cell (50 × 50 × 30 mm3) filled with water was placed on the plate of a micro-electromechanical balance system with high sensitivity (Dataphysics DCAT 11, Germany). A sample segment (a straight with a uniform diameter of ～410 µm from the tip to end) was selected and suspended on the microelectronic balance vertically in a given orientation. The water surface was moved



upwards to the horizontal parts of the copper wire at a very slow rate of 0.05 mm·s-1. The copper wire penetrated the water surface and was finally immersed. The water surface continually moved upward after the copper wire was immersed until an immersion depth of 15 mm was reached. During this process, the relative positions and relative supporting forces of the water surface were plotted automatically as a force curve by the data collection system on the balance. This method enables force differences to be measured at precision of 0.001mN·m-1, and is sufficient for detecting tiny force differences in the sample. Characterization of Gas Bubble Transport: A square quartz cell (50 × 50 × 50 mm3) filled with water was placed on the lifting platform, and copper wires were carefully positioned in the water at a depth of 20 mm. Gas bubbles of different volumes were released from a bent needle (outer-φ, 0.52 mm; inner-φ, 0.26 mm), and placed at different positions on the coppers wires. The movement of the gas bubbles was recorded by a high-speed video camera (Photron Fastcam SA-5), with 250 frames per second.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]. Give contact information for the author(s) to whom correspondence should be addressed.

Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank the financial support by National Research Fund for Fundamental Key Projects (2013CB933000), the Natural Science Foundation of Beijing Municipality (2152018), National Natural Science Foundation (21501087, 21121001 and 91127025), the Key Research Program of the Chinese Academy of Sciences (KJZD-EW-M01), the 111 Project (No.B14009), Beijing Higher Education Young Elite Teacher Project, and the Fundamental Research Funds for the Central Universities (YWF-17-BJ-Y-38). Supporting Information Available: The effect of tilt angles of the whole copper wires on the gas bubble transportation; videos of the directional gas bubble transport on the copper wire interfaces with rectangle, wave and helix shapes (Left: small bubbles; Right: large bubbles); microscopic observations of the directional gas bubbles transported on the copper wire interfaces with rectangle, wave, and helix shapes (Left: large bubbles; Right: small bubbles); the difference of the bubble size needed to initiate the transportation; gas bubble transport process between three bubbles with different volumes; the effect of a continuous gas film on gas transport; the effect of the Laplace pressure, channel length and the thickness of trapped gas layers on the speed of gas bubble transport on wave-shaped copper wire; forces acting on the a rectangle shaped copper wire during immersion; the static contact angle



images of water droplets and air bubbles on the copper wire interfaces with rectangle and helix shapes; and the calculation of the radii R of the gas bubbles with spherical crown shape.

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FIGURE CAPTION

Figure 1. a) Photographs and b-d) SEM images of a copper wire with a wavy shape. a) A copper wire with several circles of wave shape. Contact angles of water and gas bubble are 163.5 ± 4.1° and 45.4 ± 3.3°, respectively. b) Numerous microporous structures on the copper wire surface encircled by dendritic branches. c) and d) Dendritic and loose branches formed by the stacking of copper nanoparticles. (Current density: 0.6 A cm−2; deposition time: 45 s).


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Figure 2. Microscopic observations of the directional gas bubbles transported on the copper wire interfaces with a) rectangle, b) wave, and c) helix shapes and d) the volume variations of gas bubbles with time on the shaped copper wires. Regardless of shape, the coppers wires captured and directionally transported gas bubbles, and the gas transportation direction was always from small bubbles to larger ones. R: rectangle-shaped copper wire; W: wave-shaped copper wire; H: helix-shaped copper wire. Scale bar: 5 mm.



Figure 3. a) Simple diagrams of copper wires with rectangle (I), wave (II) and helix (III) shapes. b) The variations of Fz on the complex shaped copper wires (rectangle (I), wave (II) and helix (III)) with respect to the immersion depth. c) Series of video representative snapshots of contact state between water and the copper wires with rectangle (I), wave (II) and helix (III) shapes during the immersion process. Scale bar: 1 mm; d: immersion depth of the bottom edge from air/water interfaces.


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Figure 4. Mechanism of spontaneous and directional transport of gas bubbles on the copper wire with wave shape. a) A continuous gas film formed around the copper wire. The gas bubbles stably sustain as a spherical crown shape on the copper wire interface. The smaller bubble (Bubble 1, R1) has a larger Laplace pressure than the larger one (Bubble 2, R2). b) Driven by the differential Laplace pressure, the small bubble is consequently transferred to the large one directionally, forming a larger bubble (Bubble 1+2, R0).



TABLE OF CONTENTS

With the assistance of the Laplace pressure gradient between two bubbles, reliable and directional transport of gas bubbles (from small bubbles to larger ones) are achieved on complex shaped interfaces by introducing a type of 3D gradient porous network to trap a continuous and stable gas film.


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Spontaneous and Directional Bubble Transport on Porous Copper

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