An Integrated Janus Mesh: Underwater Bubble Antibuoyancy

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An Integrated Janus Mesh: Underwater Bubble Anti-Buoyancy Unidirectional Penetration Chentao Pei, Yun Peng, Yuan Zhang, Dongliang Tian, Kesong Liu, and Lei Jiang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01001 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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An Integrated Janus Mesh: Underwater Bubble AntiBuoyancy Unidirectional Penetration Chentao Pei,† Yun Peng,† Yuan Zhang,† Dongliang Tian,* ,†,‡ Kesong Liu,*,†,‡ and Lei Jiang†,‡ †

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology, School of

Chemistry, Beihang University, Beijing, 100191, P. R. China ‡

Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing,

100191, P. R. China *Corresponding Author: E-mail [email protected], [email protected]

KEYWORDS: Janus mesh, anti-buoyancy, superwetting, underwater bubble, unidirectional penetration

ABSTRACT: Gas bubbles are a powerful tool with applications in particle visualization, spacers, actuation pistons, and pressure sensors. Controlling the transportation of bubbles in the liquid phase is a challenge that needs to be solved in many industrial processes, such as in the pipe transportation of fluids, the corrosion of ocean vessels, and the control of foaming processes. There are few existing materials capable of the anti-buoyancy unidirectional transportation of

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bubbles. Here, a Janus superwetting mesh is fabricated by integrating aerophilic (AL) and superaerophobic (SAB) surfaces. The resulting composite mesh achieves underwater bubble anti-buoyancy unidirectional penetration. In aqueous solution, bubbles pass through the mesh from the superaerophobic side to the aerophilic side, but are blocked from passing through in the opposite direction. This Janus mesh can be considered to be a bubble diode, so is convenient for use in underwater bubble unidirectional transportation. This work may promote the development of advanced materials for gas bubble directional transportation and separation in aqueous media.

Underwater bubbles are important fluid systems. Such systems have spurred great research interest due to their potential in applications such as catalytic reactions,1-3 solar energy harvesting,4,5 wastewater remediation,6,7 and medicine.8,9 The presence of gas bubbles is sometimes beneficial, e.g., improving heat transfer in the ocean10,11 and improving the flotation recovery of fine mineral particles.12 However, gas bubbles may also induce negative impacts. When crude oil is produced, the oil and water mixture contains bubbles of carbon dioxide, hydrogen sulfide, and oxygen, which can corrode pipes,13,14 shorten equipment lifetimes, and increase resource consumption. Realizing the controllable transportation of bubbles in water offers opportunities for solving scientific and practical issues such as electro-catalytic gas evolution reactions.15,16 In nature, some creatures with asymmetric wettability have the ability to transport fluid directionally.17-19 Inspired by nature, researchers have designed a series of materials with asymmetric wettability such as fabrics, polymeric fibers, and metal meshes, in attempt to realize unidirectional fluid transportation.20-25 However, the directional manipulation of bubbles remains


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a challenge, and this hinders the utilization of bubbles in applications such as the pipe transportation of fluids,26,27 the lifetime of ocean vessels,28 and the control of foaming processes.29 Herein, we demonstrate the anti-buoyancy unidirectional penetration of underwater bubbles via a “bubble diode”. This is easily fabricated by integrating superaerophobic and aerophilic single-layer copper meshes, such that the pore size of the mesh can be adjusted. The resulting system is known as a Janus mesh. Bubbles can penetrate through the Janus mesh from the superaerophobic side to the aerophilic side, but not in the reverse direction. This is due to competition between the gas bubble penetration and spread on the mesh. The combined driving forces of the gradients of the Laplace pressure and ambient pressure overcome the buoyancy force of the bubbles, which results in their unidirectional transport. This work demonstrates a promising methodology for guiding the mechanical behavior of bubbles in water, and the methodology has potential in microfluidics and microdetectors. RESULTS AND DISCUSSION The process used to fabricate the underwater bubble unidirectional penetration composite mesh is shown in Figure 1a, b. A superaerophobic mesh was prepared at room temperature, by etching the copper mesh in a mixed solution of NaOH and (NH4)2S2O8. Needle-like Cu(OH)2 nanowires of 300‒400 nm in diameter grew out from the copper mesh surface in this procedure (Figure 1c, d).30 Because of the superhydrophilicity of the Cu(OH)2 crystals, the obtained mesh can retain a water layer on its surface, and act as a superaerophobic layer for underwater bubbles. The water contact angle (WCA) of the mesh is ~ 0° (Figure 1e), and the underwater bubble contact angle (UBCA) is 150 ± 1° (Figure 1f). To make the superaerophobic copper mesh


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become aerophilic, the etched copper mesh was soaked in a mixed solution of n-tetradecyl mercaptan and ethanol for 24 h. After the reaction, the surface morphologies of the two meshes show no remarkable differences, as revealed by scanning electron microscopy (SEM) images (Figure 1c, d, g, h). The primary micro/nanostructure is retained on the surface of the resulting copper mesh (Figure 1h). Notably, the surface of the resulting mesh is superhydrophobic, with a WCA of 151 ± 1° (Figure 1i). When underwater, the surface of the mesh is aerophilic, with an UBCA of 16 ± 1° (Figure 1j). Therefore, the etched mesh can act as an underwater aerophilic layer. The WCAs and UBCAs of these two kinds of meshes do not change significantly over an extended duration (i.e. by less than 2°). This indicates that the mesh surfaces have good stability in air and underwater (Figure S1). To realize underwater bubble unidirectional penetration, the underwater superaerophobic mesh and aerophilic mesh were assembled into a double layer composite mesh (Figure 1b). Compared with the Janus membrane demonstrated previously,20-25 the current composite mesh possesses large pores which can enhance the flow rate, which is beneficial for underwater bubble transportation. Based on the Janus mesh, underwater bubble anti-buoyant unidirectional penetration by the current composite mesh is achieved in an aqueous environment. Bubbles are supplied continuously by a syringe needle located above the mesh. Bubbles are ejected from the needle via a pump, then contact the superaerophobic side of the composite mesh, and finally penetrate through it. The tiny bubbles then merge into larger bubbles of nearly spherical shape, and remain on the aerophilic side for a short time. This result in the ‘‘forward direction’’ of this underwater bubble ‘‘diode’’ (Figure 2a‒c). In contrast, if the composite mesh is inverted and bubbles initially contact the aerophilic side, the bubbles collectively spread and grow on the upper aerophilic side instead of penetrating through the mesh. The increasing bubble volume and thus


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increasing buoyancy eventually causes the bubble to detach from the aerophilic surface. This is referred to as the ‘‘reverse direction’’ of the underwater bubble ‘‘diode’’ (Figure 2d‒f). We conduct underwater anti-buoyant unidirectional bubble experiment of different gas bubbles, and there is no significant difference in the underwater unidirectional transport of bubbles of different gas compositions (Table S1). Moreover, the bubble unidirectional penetration results (for both the forward and reverse directions) do not change after 100 consecutive tests, demonstrating the excellent repeatability of the procedure (Figure S1). A mechanism is proposed to explain the underwater bubble unidirectional penetration (Figure 2). During the bubble penetration process, the gas film on the superhydrophobic surface plays a decisive role.31-33 As illustrated in Figure 2a‒c, a bubble contacting the superaerophobic side of the composite mesh gradually deforms and crosses through the water layer caught in the gaps of the superhydrophilic mesh grid. This generates a gas channel between the bubble and under-gas layer. Once the channel forms, a differential Laplace pressure PL is generated in the gas phase, as the radius of curvature of the bubble is significantly smaller than that of the lower aerophilic gas layer. This is described by the Young-Laplace equation (1),

= −

(1)

where γ is the surface tension of water, R1 is the radius of curvature of the bubble, and R2 is the radius of curvature of the gas film. R1 is smaller than R2, so the Laplace pressure of the bubble is greater than that in the lower layer. In the current work, γ is 0.072 N/m, R1 is about 250 µm, and R2 is approaching infinity. According to Equation (1), the PL is 288 Pa. Therefore, the difference in Laplace pressure should promote the movement of the bubble toward the bottom gas layer. The pressure of the syringe pump plays a significant role in the process of bubble migration


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through the water film, and the Laplace pressure difference plays a crucial role in the penetration process when the bubble and gas film are connected. As a result, the PL and the pump force (FP) collectively overcome the buoyancy of the bubble (FB), so the bubble is transported instead of resting on the superaerophobic side. In contrast, the bubble cannot penetrate through the Janus mesh when the superaerophobic surface is fixed facing downwards (Figure 2d‒f). When the bubble contacts the aerophilic side, it spreads on the gas layer retained on the aerophilic mesh, and cannot connect with the lower layer. There are no conditions for the PL to form between the bubble and SAB mesh. As a result, the bubbles spread over the aerophilic side instead of penetrating. When the bubble passes through the composite mesh, the tendency of penetration is larger than that of spreading. When the bubble is obstructed by the composite mesh, the spreading tendency is dominant compared with penetration. The gas film leads to the spreading of bubbles on the aerophilic mesh, and the bubbles need a pressure to overcome the resistance of the water film to penetrate. In other words, the apparent unidirectional bubble penetration is determined by the position of the gas film in the composite mesh. The pore size reportedly plays a significant role in water permeation.34 We now explore how the pore size affects the unidirectional penetration of the current meshes. In the orientation allowing underwater bubble unidirectional penetration, the bubble readily penetrates the superaerophobic mesh with large pore sizes, but cannot pass through mesh with a smaller pore size (Figure 3). When bubbles with a radius of ~250 µm are continuously applied, underwater bubble unidirectional transportation cannot be realized when the superaerophobic pore size is less than or equal to 300 µm. When the superaerophobic size equals or exceeds 425 µm, the unidirectional transportation of bubbles is somewhat effortless, regardless of the pore size of the aerophilic mesh pore (Figure 3). There is presumably a critical pore size value in the 300‒425


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µm range which allows for bubble unidirectional penetration. The water layer between the bubble and aerophilic mesh is the main resistance for underwater unidirectional penetration. The low critical intrusion pressure P can be calculated by the relationship,35 =

| |

(2)

where D is the pore diameter of the superaerophobic mesh, and θ is the UBCA of the superaerophobic surface. In this work, a superaerophobic mesh with a UBCA of 150° and a pore size of 425 µm is used to form the composite mesh. According to Equation (2), the critical intrusion pressure of the superaerophobic mesh is about 587 Pa. An increase in D will result in a decrease in the low critical intrusion pressure. The larger the superaerophobic mesh pore is, the easier the bubbles pass through. The pore size of the superaerophobic mesh plays a conclusive role in controlling bubble penetration. The aerophilic mesh with its gas layer allows the formation of the differential Laplace pressure. The geometric relationship between R1 and D is, = 2 ∙ −

(3)

When D is 425 µm, the theoretic critical value R1 calculated from Equation (3) is 425 µm. The experimental value of R1 is about 250 µm, which is consistent with the theoretical relationship. We now explore underwater bubble unidirectional penetration in circumstances where the composite mesh is orientated vertically in water (Figure 4). With the pump force (FP), the bubble partly passes through the superaerophobic mesh and contacts the aerophilic mesh, realizing the penetration with the assistance of PL. Again, the bubble is blocked from passing through from the aerophilic side. The origin for the PL illustrated in Figure 4e is identical to that in Figure 2e, while the directions of FP and PL in the former are horizontal. In contrast to the scenario in


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Figure 4a‒d, when the composite mesh is inverted, the diode transporting behavior disappears and the bubble floats to the water surface. This phenomenon can be interpreted by the absence of the necessary conditions to form PL between the bubble and SAB mesh. These experimental results indicate that the composite copper mesh can be placed either vertically or horizontally, and that this has little effect on the passage of bubbles. CONCLUSIONS We have designed and prepared an underwater bubble unidirectional penetration film for gas separation. The design is based on the integration of aerophilic and superaerophobic copper meshes in a composite Janus mesh. Gas bubbles readily pass through the composite mesh from the superaerophobic side, but are blocked from penetrating from the aerophilic side. This difference is due to the competition of bubble penetration and spread on the mesh. The mesh can be considered to be an underwater “bubble diode’’. The pore size of the composite mesh can be tuned, leading to the controllable transportation of underwater bubbles. The presented strategy may further the design of advanced materials for applications in pipe transportation, ocean vessels, and foaming control processes. METHODS Preparation of Underwater Superaerophobic Copper Mesh. Copper meshes were cleaned with ethanol, then immersed in 1 M hydrochloric acid solution and subjected to ultrasonication for 10 min to dislodge surface oxides, followed by rinsing with deionized water. The cleaned copper meshes were then immersed in a mixture of 2.5 M NaOH and 0.15 M (NH4)2S2O8 for 8 min at room temperature, followed by rinsing with deionized water.


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Preparation of Underwater Aerophilic Copper Mesh. The above-prepared underwater superaerophobic mesh was purged three times with water and ethanol, then modified by 1 mM ntetradecyl mercaptan/ethanol solution for 24 h at room temperature. The resulting mesh was washed with ethanol, and then dried in air. Underwater Bubble Unidirectional Penetration Experiment. Fix the prepared underwater superaerophobic and aerophilic meshes by clamps, the composite mesh has been assembled. The composite mesh was fixed horizontally in the middle of a square quartz sink filled with water. Bubbles were continuously provided by an injection pump (LSP01-1C, LONGER, P. R. China) at a speed of 50 µL/min. A digital camera was used to record the progress of the bubbles. Characterization. The surface morphologies of the samples were examined using a fieldemission scanning electron microscope (JEOL JSM-6700F). The contact angles of the meshes were measured using a Dataphysics OCA20 contact-angle system (Dataphysics, German) at ambient temperature. Digital photos were collected using a digital camera (Nikon, Japan). ASSOCIATED CONTENT Supporting Information Underwater anti-buoyant unidirectional bubble penetration of different gas compositions, and the stabilities of the meshes. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Prof. Dongliang Tian: E-mail: [email protected]


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Prof. Kesong Liu: E-mail: [email protected] Author Contributions C. Pei, D. Tian, K. Liu and L. Jiang conceived and designed the experiments. C. Pei and Y. Peng performed the experiments. D. Tian and K. Liu supervised the experiment process. C. Pei, D. Tian, K. Liu and Y. Zhang wrote and revised the manuscript. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors acknowledge the financial support of National Basic Research Program of China (2013CB933003), Beijing Young Talent Support Program, the 111 project, Beijing Nova Program, Beijing Natural Science Foundation (L160003, 2172033), the Chinese National Natural Science Foundation (21671012), and the Fundamental Research Funds for the Central Universities. REFERENCES (1)

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Figure 1. Schematic of the procedure used to fabricate the underwater bubble unidirectional penetration composite mesh, SEM images showing morphologies, and digital camera images showing surface wettabilities (mesh pore size: 425 µm). a, b) Schematic of the method used to prepare the underwater superaerophobic mesh, aerophilic mesh, and composite mesh for underwater bubble unidirectional penetration. c, d) SEM images and e, f) digital camera images showing the wettability of the underwater superaerophobic mesh. The etched mesh is covered with dense nanowires. The mesh is superhydrophilic with a WCA of 0°, and underwater it is superaerophobic with a UBCA of 150°. g, h) SEM images and i, j) digital camera images showing the wettability of the underwater aerophilic mesh. After modification with n-tetradecyl mercaptan, the mesh possesses micro/nano multiscale surface structures. The mesh is superhydrophobic with a WCA of 151°, and underwater it is aerophilic with a UBCA of 16°.


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Figure 2. Underwater bubble anti-buoyant unidirectional penetration and schematic of the mechanism. a‒c) The bubble readily passes through the composite mesh from the superaerophobic (SAB) side to the aerophilic (AL) side. The superaerophobic surface is fixed facing upwards, so the bubble can gradually deform and pass through the water layer caught in the gaps of the superaerophobic mesh grid. This generates a gas channel between the bubble and under-gas layer, jointly driven by the pump force (FP) and Laplace pressure difference (PL). This driving force overcomes the buoyancy (FB), so the bubble passes through the composite mesh. d‒f) The bubble is blocked from the AL side. The aerophilic surface is fixed facing upwards, so there is no Laplace pressure difference (PL) between the bubble and SAB mesh. The bubble fails to overcome the buoyancy, so does not penetrate the composite mesh. Untagged arrows in the schematic indicate the bubble’s moving/spreading direction. Scale bars: 1 mm.


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Figure 3. Effect of mesh pore size on underwater bubble unidirectional penetration (UBUP). Schematic diagrams of a single grid, and a bubble contacting the superaerophobic (SAB) mesh with a different pore size to that of the composite mesh. The bubble cannot penetrate the superaerophobic mesh with the smaller pore size, but can penetrate the mesh with the larger pore size, despite the smaller pore size of the underlying aerophilic (AL) mesh. The radius of curvature of the bubble is about 250 µm.


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Figure 4. Underwater bubble unidirectional penetration on a composite mesh fixed vertically in water. a‒d) The bubble readily passes through the composite mesh from the superaerophobic (SAB) side to aerophilic (AL) side. e) When the superaerophobic surface is fixed facing right, the bubble readily passes through the composite mesh, driven by the pump force (FP) and Laplace pressure difference (PL). f‒i) The bubble is blocked from the aerophilic side. j) When the aerophilic surface is fixed facing right, there is no Laplace pressure difference (PL) between the bubble and SAB mesh, so the bubble cannot penetrate the composite mesh. Untagged arrows in the schematic indicate the bubble’s moving/spreading direction. Scale bars: 1 mm.


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ACS Nano

Table of Contents Graphic and Synopsis

An integrated Janus superwetting mesh film is fabricated for underwater bubble anti-buoyancy unidirectional transport and separation in aqueous solution. This finding hints at approaches for designing smart interface materials.


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An Integrated Janus Mesh: Underwater Bubble Antibuoyancy

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