Reversible switch between underwater superaerophilicity and superaerophobicity on the superhydrophobic nanowire-haired mesh for controlling underwater bubble wettability

Controlling the underwater bubble wettability on a solid surface is of great research significance. In this letter, a simple method to achieve reversible switch between underwater superaerophilicity and underwater superaerophobicity on a superhydrophobic nanowire-haired mesh by alternately vacuumizing treatment in water and drying in air is reported. Such reversible switch endows the as-prepared mesh with many functional applications in controlling bubble’s behavior on a solid substrate. The underwater superaerophilic mesh is able to absorb/capture bubbles in water, while the superaerophobic mesh has great anti-bubble ability. The reversible switch between underwater superaerophilicity and superaerophobicity can selectively allow bubbles to go through the resultant mesh; that is, bubbles can pass through the underwater superaerophilic mesh while are fully intercepted by the underwater superaerophobic mesh in a water medium. We believe these meshes will have important applications in removing or capturing underwater bubbles/gas.Controlling the underwater bubble wettability on a solid surface is of great research significance. In this letter, a simple method to achieve reversible switch between underwater superaerophilicity and underwater superaerophobicity on a superhydrophobic nanowire-haired mesh by alternately vacuumizing treatment in water and drying in air is reported. Such reversible switch endows the as-prepared mesh with many functional applications in controlling bubble’s behavior on a solid substrate. The underwater superaerophilic mesh is able to absorb/capture bubbles in water, while the superaerophobic mesh has great anti-bubble ability. The reversible switch between underwater superaerophilicity and superaerophobicity can selectively allow bubbles to go through the resultant mesh; that is, bubbles can pass through the underwater superaerophilic mesh while are fully intercepted by the underwater superaerophobic mesh in a water medium. We believe these meshes will have important applications in removing or capturing ...


I. INTRODUCTION
Controlling the behavior of bubbles on solid materials in a water medium has a wide range of practical applications, such as removing bubbles from water to avoid the bubbles-induced harm, capturing and collecting useful underwater gas bubbles, and so on. 1,22][3][4][5][6] An underwater superaerophobic surface generally displays a contact angle (CA) greater than 150 • to a small bubble in water, while the bubble shows the CA value smaller than 10 • on an underwater superaerophilic surface.Yong et al. found that fish scales show superaerophobicity while lotus leaves show superaerophilicity in water. 7Inspired by these living organisms, they fabricated an artificial underwater superaerophobic silicon surface and an underwater superaerophilic polydimethylsiloxane (PDMS) surface by one-step femtosecond laser ablation.Chen et al. demonstrated that the superhydrophobic sponge has excellent ability for selectively absorbing marine methane bubbles based on the underwater superaerophilicity. 8This approach can potentially restrict marine methane emission and delay global warming.Similarly, Yong et al. also designed a device that has great ability of collecting underwater bubbles/gas by using the femtosecond laser-induced superhydrophobic and underwater a C. Shan and J. Yong contributed equally to this work.b Corresponding author: jlyong@xjtu.edu.cn(J.Y.); chenfeng@mail.xjtu.edu.cn(F.C. superaerophilic porous PDMS sheet as the core component. 9Electrochemical gas evolution reactions play an important role in energy conversion processes and industries.However, the adhesion of generated gas bubbles to the electrode surfaces usually blocks the following catalytic reactions and decreases the efficiency.1][12][13][14] Hydrogen is a sustainable and clean energy source.Yu et al. proposed a new strategy by using an aerophilic electrode with cone shape to timely remove and directionally transport the generated hydrogen bubbles and using a superaerophilic sponge to further collect the generated hydrogen in a hydrogen evolution reaction. 157][18] Those artificial materials usually have single and fixed super-wettability to underwater bubbles, either underwater superaerophobicity or underwater superaerophilicity.To obtain multifunctional applications, a smart surface that can reversibly switch between underwater superaerophobicity and superaerophilicity by a simple way is especially significant but has not been reported until now.Because the underwater superaerophobicity and superaerophilicity are simultaneously integrated into one same surface, such surface can selectively exhibit anti-bubbles or bubbles-capturing property, and can be further used to control the behavior of bubbles in water. In this letter, we report a very simple way to reversibly switch between underwater superaerophilicity and underwater superaerophobicity on a superhydrophobic nanowires-structured copper mesh.The copper mesh was firstly treated by a one-step chemical reaction to coat a layer of nanostructures, and then was modified by fluoroalkylsilane.The resultant mesh showed superhydrophobicity in air and superaerophilicity after the immersion in water.Underwater bubbles were easily absorbed and captured by such mesh.When the mesh (in water) was further suffered from vacuumizing treatment in water, it would switch to underwater superaerophobicity.The superaerophobic mesh had great anti-bubbles ability in water.Surprisingly, the original in-air superhydrophobicity as well as the underwater superaerophilicity can be recovered by just drying the mesh in air.The switch between underwater superaerophilicity and underwater superaerophobicity is reversible and can be repeated many cycles.It is found that the underwater superaerophilic mesh can allow bubbles to pass through in water, while the underwater superaerophobic mesh has bubbles-interception ability.

II. EXPERIMENT
NaOH, (NH 4 ) 2 S 2 O 8 and 1H,1H,2H,2H-perfluorodecyltrimethoxysilane were purchased from Sigma-Aldrich.Ethanol and acetone were obtained from Tianjin Kaixin Chemical Industry, Tianjin, China.All chemicals were analytical grade.Copper meshes (99.9%, 200 mesh size) were purchased from a local store.The meshes were ultrasonically cleaned with acetone, ethanol, and deionized water in advance.
The nanowire-haired mesh was prepared by the immersion of copper mesh in the aqueous solution of 2.5M NaOH and 0.1M (NH 4 ) 2 S 2 O 8 at room temperature (∼20 • C) for 5 min.Then, the mesh was taken out and washed with deionized water, and dried under N 2 .Finally, the nanowire-structured mesh was modified with fluoroalkylsilane molecular by immersing into a 0.2% fluoroalkylsilane solution (in ethanol) for 24 h to lower its surface free energy at room temperature (∼20 • C).Immediately afterwards, the mesh was stored in a vacuum oven at 100 • C for 2 h to make the fluoride layer more stable.
The surface microstructure of the copper mesh was obtained by a Quantan 250 FEG scanning electron microscope (SEM, FEI, America).The wettabilities of in-air water droplet and underwater bubble were investigated by a JC2000D contact-angle system (Powereach, China).The typical volume of the used water droplets was ∼7 µL and that of the bubbles was ∼3 µL.Every value of CA and sliding angle (SA) was obtained by measuring five different positions.The crystal structure and phase of the sample were characterized by an Empyrean X-ray diffraction (XRD, PANalytical, Netherlands) with Cu K-alpha1 X-ray source.The process of selective passage of underwater bubbles was captured by a CAMMC1362 high-speed camera (Mikrotron, Germany) with the maximal frame rate of 2000 fps.

III. RESULTS AND DISCUSSION
Figure 1 shows the SEM images of the resultant mesh.The width of every square void (mesh cell) of the mesh is 70-80 µm.The whole surface of the mesh is completely wrapped by uniform hairy nanowires.The nanowires grow vertically on the copper wires and interlace with each other.The length of the nanowires is about 10 µm, while their diameter ranges from 80 nm to 200 nm (Figure 1c,d).The nanowire-haired mesh is a typical hierarchical rough microstructure which composes of both microscale mesh structure and nanoscale wires.As shown in Figure 2, the XRD pattern demonstrates that the as-synthesized nanowire is composed of orthorhombic-phase Cu(OH) 2 crystals, which is consistent with the standard card (JCPDS No. 80-0656).The three extremely strong peaks in the XRD pattern comes from the copper mesh substrate.Many experiments have also demonstrated that the microstructure of the Cu(OH) 2 nanowires can be controlled just through varying the immersion time in the solution of NaOH and (NH 4 ) 2 S 2 O 8 . 19,20fter the formation of the nanoneedles structure by a one-step chemical reaction and the subsequent fluoroalkylsilane surface modification, the resultant rough copper mesh shows excellent superhydrophobicity.As shown in Figure 3a, a small water droplet on such mesh can keep a spherical shape with the water CA of 152.8 • ± 1.8 • .The mesh also has a very low adhesion to water because this water droplet can roll off freely once the mesh is tilted 4 • (Figure 3e, Movie S1 in the supplementary material).7][38][39] The behavior of an underwater small bubble on this mesh was also investigated after the immersion of the mesh in water.When a bubble was released bellow the mesh, it would rise up until reach to the mesh.The bubble would spread out quickly and be absorbed by the mesh within 100 ms once the bubble just touched the mesh (Figure 3f, Movie S2 in the supplementary material).As a result, the CA to the bubble was only about 3 • ± 2 • , revealing that the resultant mesh shows superaerophilicity in water (Figure 3b).The underwater superaerophilic surface can be used to absorb and capture air bubbles in a water medium.Interestingly, the wettability of the as-prepared mesh can be switched to underwater superaerophobicity by a simple treatment.The superhydrophobic mesh was previously dipped into water in a water container.Then, the water container was placed in a vacuum oven and was treated by vacuumizing under the vacuum degree of < 10 4 Pa for about 10-20 min.Vacuumizing treatment is one of the most commonly used ways for removing bubbles from liquid.After that, the superhydrophobic rough mesh was wetted by water even though it was taken out of the water.The treated mesh could repel air bubbles rather than absorb them.Underwater bubble can maintain an approximately spherical shape on the copper mesh, with the CA of 152.5 • ± 2.5 • to this gas bubble (Figure 3c).Such value demonstrates that the resultant mesh shows underwater superaerophobicity.The mesh also shows ultralow adhesion to the bubble because this bubble could roll away on the 3 • tilted mesh easily (Figure 3g, Movie S3 in the supplementary material).The underwater superaerophobicity endows the mesh with strong anti-bubble ability, preventing air bubbles from adhering to the mesh and passing through the mesh.Therefore, it can be applied to removing bubbles in water.
In fact, the switch from underwater superaerophilicity to underwater superaerophobicity on the as-prepared mesh is reversible.In-air superhydrophobic state can be recovered just by taking the mesh out of water and drying in air (Figure 3a).The drying process only needs several minutes with the aid of hot wind.The recovered mesh will exhibit underwater superaerophilicity again when it is immersed in water (Figure 3b).Such a reversible switch between underwater superaerophilicity and underwater superaerophobicity on the superhydrophobic rough copper mesh can be repeated many times by alternately vacuumizing in water and drying in air (Figure 3d) as long as the mesh is not contaminated during repeated operation.The reversible switch allows the resultant surface to selectively exhibit bubble-capturing or anti-bubble function.
The switchable underwater bubble's behavior of the as-prepared superhydrophobic mesh is ascribed to the change of the wetting state of the mesh/water interface in a water medium.The rough mesh surface shows superhydrophobicity and very low water adhesion in air.Therefore, the water droplet on the mesh is at the Cassie wetting state. 22,23,26As shown in Figure 4a, the droplet is just in contact with the top peaks of the nanowires of the rough mesh, without wetting the surface microstructures.The air filled in the surface microstructure forms a trapped air cushion underneath this water droplet.When such superhydrophobic mesh is immersed in water, a silver mirror-like reflectance on the mesh surface can be visually observed (Figure 5a).This mirror-like interface is ascribed to an air layer trapped between water and the rough microstructures of the as-prepared mesh, demonstrating that the mesh is not wetted by water (Figure 4b). 40,41The phenomenon of mirror effect agrees well with the Cassie wetting state.If a bubble is released below the mesh and touches the mesh in water, the air in the bubble is able to enter into the trapped air layer under the driving force of pressure (Figure 4d).The bubble rapidly spreads out along the air gap because the gas in the bubble can merge with the air in the trapped layer.As a result, the bubble can be completely absorbed by the as-prepared mesh, leading to underwater superaerophilicity with a very small bubble CA (Figure 4e).In contrast, if the underwater rough mesh is suffered from vacuumizing treatment for a short time, the trapped air layer surrounding the mesh will collapse and be removed, as shown in Figure 4c. Figure 5b shows the photography of the as-prepared nanowires-structured copper mesh in water.The silver mirror-like reflectance disappears, demonstrating that the mesh is completely wetted by water.During the vacuumizing treatment, the pressure of the environment decreases dramatically, so the trapped air layer in the microstructures of the mesh grows to a big bubble and finally detaches from the mesh.3][44] A bubble that is released onto the mesh is repelled by the mesh at present, because the water filled in the microstructures prevents the bubble from effectively touching the mesh surface (Figure 4f).This is caused by the natural property that liquid generally repels gas.The bubble looks like sitting on the top of the nanowires of the as-prepared mesh and can only touch the peaks of the microstructures.A near-spherical shape is exhibited by the bubble to minimize its free-energy.Therefore, the mesh shows underwater superaerophobicity after vacuumizing treatment (Figure 4f,g).As long as the as-prepared mesh is further taken out of water and dried in air, the water filled in the space of the surface microstructure of the superhydrophobic nanowires-structured mesh can evaporate away completely.By this way, the mesh re-obtains it superhydrophobicity in air (Figure 4a) as well as original underwater superaerophilicity (Figure 4b).The reversible switch between underwater superaerophilicity and superaerophobicity can be used to control the bubbles to selectively pass through the mesh in a water medium.As shown in Figure 6a and Movie S4 (supplementary material), when the as-prepared superhydrophobic mesh is dipped into water and bubbles are continuously released below the mesh, the bubbles will be absorbed by the mesh completely once they are just in contact with the lower surface of the mesh (step 1-3).It is the direct result of the underwater superaerophilicity of the as-prepared mesh.With more and more bubbles being absorbed, the trapped gas layer in the microstructures of the mesh could bulge from the top of the mesh, forming a convex air bulge (Step 4).The gas bulge was continuously growing by absorbing bubbles, with its shape becoming bigger and bigger (Step 4-6).After enough accumulation, the buoyancy force acting on the gas bulge could overcome the adhesion between the gas bulge and mesh, thereby the gas bulge left the mesh and rose up (step 7-8), as a new big bubble.By this way, all the bubbles could pass through the as-prepared mesh.The ability for allowing bubbles to pass through is associated with the superhydrophobicity and underwater superaerophilicity of the mesh.An air layer will be trapped in both the surface microstructure and the microholes of the nanowire-haired mesh as the mesh is lowered into water.If a bubble touches the bottom of the mesh, the air in this bubble will rapidly enter into such trapped air layer.With more and more bubbles being absorbed by the mesh, the pressure in the trapped air layer becomes big enough to make the trapped gas heave from the top surface of the mesh, and the volume of the generated air bugle rises continuously.Finally, the air bubble is able to leave the mesh by the buoyancy.
If the as-prepared superhydrophobic mesh was immersed in water and the trapped air layer was removed by the vacuumizing treatment, the mesh will intercept bubbles rather than allow bubbles to pass through.As shown in Figure 6b and Movie S5 (supplementary material), with bubbles continuously reaching to the bottom of the mesh, all of the bubbles stopped and were unable to pass through the mesh (step 1-4).The intercepting ability is ascribed to the underwater superaerophobicity of the mesh at this moment.These bubbles are inclined to merge with each other to form a bigger one (step 5-8).After the vacuumizing treatment, water was filled in the microholes of the as-prepared mesh.The trapped water generally blocks the bubbles from passing through the microholes, thereby endowing the mesh with bubbles-interception function.In fact, because the bubble's behavior on the resultant superhydrophobic mesh can be reversibly switched between underwater superaerophilicity and underwater superaerophobicity, the mesh can selectively exhibit the ability of allowing bubbles to pass through or intercepting bubbles.
The selective passage of air bubbles through the superhydrophobic nanowire-haired mesh can be potentially used in water/bubbles separation, bubbles removing, gas collection, and filtration in a water medium.

IV. CONCLUSIONS
In conclusion, a superhydrophobic nanowires-structured copper mesh was prepared by a one-step chemical reaction and subsequent fluoroalkylsilane modification.The mesh showed superaerophilicity when it was immersed in water.If the mesh (in water) was further suffered from vacuumizing treatment, it would switch to underwater superaerophobicity.Surprisingly, the original in-air superhydrophobicity and underwater superaerophilicity can be recovered by just drying the mesh in air environment.Such reversible switch between underwater superaerophilicity and underwater superaerophobicity on the superhydrophobic mesh can be repeated for many times.Compared to the previously reported underwater superaerophobic or superaerophilic surfaces, both the underwater superaerophobicity and superaerophilicity can be selectively exhibited by the same nanowire-haired mesh surface.This simple method for switching bubble's behavior endows the mesh with many important applications.For instance, the resultant surface is able to selectively exhibit bubbles-absorption or anti-bubbles function.In addition, the as-prepared superhydrophobic nanowire-haired mesh has the function of selective passage of air bubbles; that is, the underwater superaerophilic mesh can allow bubbles to pass through in water while the underwater superaerophobic mesh has bubbles-interception ability.We believe this method can also achieve switchable underwater superaerophobicity and superaerophilicity on other in-air superhydrophobic substrate and the resultant surface will have important applications in well controlling bubbles' behavior on a solid surface in water.The underwater superaerophobic/superaerophilic meshes can be potentially applied in excluding bubbles-induced hazards and the "smart" use of underwater bubbles.

FIG. 1 .
FIG.1.SEM images of the nanowires-structured copper mesh prepared by a one-step chemical reaction.

FIG. 3 .
FIG. 3. Reversible switch between underwater superaerophilicity and underwater superaerophobicity by alternately vacuumizing in water and drying in air.(a) Image of a water droplet on the as-prepared mesh after the mesh being dried in air.(b) Bubble on the as-prepared mesh surface in water.(c) Bubble on the as-prepared mesh surface that was previously suffered from vacuumizing treatment in water.(d) Repeatability of the bubble's behavior on the as-prepared mesh being switched between underwater superaerophilicity and underwater superaerophobicity.(e) Water droplet rolling on the as-prepared mesh in air.(f) Bubble being absorbed by the as-prepared mesh in water.(g) Bubble rolling on the vacuum-treated mesh in water.

FIG. 4 .
FIG. 4. Schematic illustration of the switch between underwater superaerophilicity and underwater superaerophobicity.(a) Water droplet on the as-prepared mesh (after drying) in air.(b) Immersion of the mesh in water.(c) The wetting of the as-prepared mesh that was immersed in water and further suffered from vacuumizing treatment.(d,e) Releasing a bubble below the as-prepared mesh in (b): (d) just releasing, (e) after a period of time.(f,g) Releasing a bubble below the as-prepared mesh in (c): (f) just releasing, (g) after a period of time.

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FIG. 6. Underwater bubbles selectively passing through the as-prepared mesh.(a) Underwater superaerophilic mesh allowing bubbles to passing through in water.(b) Bubbles being intercepted by the underwater superaerophobic mesh (after vacuumizing treatment) in water.