Bioinspired Design of Underwater Superaerophobic and

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Bioinspired Design of Underwater Superaerophobic and Superaerophilic Surfaces by Femtosecond Laser Ablation for Anti- or Capturing Bubbles Jiale Yong, Feng Chen, Yao Fang, Jinglan Huo, Qing Yang, Jingzhou Zhang, Hao Bian, and Xun Hou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14819 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 27, 2017

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ACS Applied Materials & Interfaces

Bioinspired

Design

of

Underwater

Superaerophobic

and

Superaerophilic Surfaces by Femtosecond Laser Ablation for Anti- or Capturing Bubbles

Jiale Yong1, Feng Chen1,*, Yao Fang1, Jinglan Huo1, Qing Yang2,*, Jingzhou Zhang1, Hao Bian1, and Xun Hou1 1

State Key Laboratory for Manufacturing System Engineering and Key Laboratory of Photonics Technology

for Information of Shaanxi Province, School of Electronics & Information Engineering, Xi’an Jiaotong University, Xi’an, 710049, PR China 2

School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an, 710049, PR China

*

Corresponding author: [email protected] (F.C.); [email protected] (Q.Y.)

Abstract A micro/nanoscale hierarchical rough structure inspired by the underwater superaerophobicity of fish scales was fabricated by ablation of a silicon surface by a femtosecond laser. The resultant silicon surface showed superhydrophilicity in air and became superaerophobic after immersion in water. Additionally, inspired by the underwater superaerophilicity of lotus leaves, we showed that the polydimethylsiloxane (PDMS) surface after femtosecond laser ablation exhibits superhydrophobicity in air and became superaerophilic in water. The underwater superaerophobic surface showed excellent anti-bubble ability, while the underwater superaerophilic surface could absorb and capture air bubbles in a water medium. The experimental results revealed that the in-air superhydrophilic surface generally shows superaerophobicity in water and that the in-air superhydrophobic surface generally shows underwater superaerophilicity. An underwater superaerophobic porous aluminum sheet with through microholes was prepared, and this sheet was able to intercept underwater bubbles and further remove bubbles from water. By contrast, the underwater superaerophilic porous polytetrafluoroethylene (PTFE) sheet could allow the bubbles to pass through the sheet. We believe that these results are highly significant for providing guidance to researchers and engineers for obtaining excellent control of bubbles’ behavior on a solid surface in a water medium. Keywords: underwater superaerophobicity; underwater superhydrophobicity; superhydrophilicity; femtosecond laser

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superaerophilicity;


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1. Introduction Solid, liquid, and gas are the three basic states of matter. These states do not exist as independent systems but rather are usually interlaced with one another. In the past two decades, extreme wettabilities at the in-air liquid/solid interface (e.g., superhydrophobicity, superhydrophilicity, superoleophobicity, superoleophilicity) and at the underwater oil/solid interface (e.g., underwater superoleophobicity, underwater superoleophilicity) have been extensively studied.1-8 The designed surfaces with special wettability have a wide variety of practical applications in self-cleaning,9,10 anti-fogging/icing,11-13 anti-fouling,14 anti-corrosion,15 lab-on-a-chip,16,17 microdroplets manipulation,18-20 microfluidics,21,22 cell engineering,23,24 drag reduction,25,26 and oil/water separation.27-29 Gas can appear in water in the form of bubbles. In some cases, the presence of the underwater bubbles is inconvenient.30 For example, the bubbles in a microfluidic system may block the micro-channel and then hinder the basic functioning of the micro-device; during vein injection, the entrance of excess bubbles into human blood vessels from the infusion set endangers lives because these bubbles can easily result in an embolism; the generated bubbles that adhere onto the electrode will lower the speed of electrochemical reactions,31,32 and the bubbles adhering onto the window of an underwater camera or a diving goggle will lead to poor visibility. Nevertheless, in some cases, a clever use of bubbles in the water is beneficial. For example, in some seafloor locations, methane gas is continually seeping out of the earth’s crust and rising to the sea surface in the form of bubbles.33 The methane gas is then finally released into the atmosphere, resulting in a waste of significant energy sources. The capture and utilization of such self-escaping methane can contribute to meeting the great demand for energy sources. The bubbles can also be used to propel and to control the location of micro-particles in an aqueous medium.34 Therefore, control of the behavior of a bubble on a solid substrate in a water medium has broad application prospects, especially for the two extreme cases of an underwater superaerophobic surface and an underwater superaerophilic surface. If an air bubble on a surface has a contact angle (CA) greater than 150° in a water medium, the surface is underwater superaerophobic. Correspondingly, the underwater superaerophilic surface is defined as exhibiting a CA less than 10° to a bubble. Establishing the principles for the preparation of underwater superaerophobic and superaerophilic surfaces is highly significant and valuable, and to date, the simple fabrication and application of underwater superaerophobic and superaerophilic surfaces have been extremely challenging. In this work, we showed that fish scales and lotus leaves exhibit underwater superaerophobicity and underwater superaerophilicity, respectively. Inspired by these living organisms, we conjecture that the behavior of the bubble on a substrate in water is highly correlated with the substrate’s in-air water wettability. An artificial superhydrophilic silicon surface and a superhydrophobic polydimethylsiloxane (PDMS) surface were fabricated by simple femtosecond laser ablation. The superhydrophilic silicon surface became superaerophobic after immersion in water, while the superhydrophobic PDMS surface became superaerophilic in water, very


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similar to the fish scale and lotus leaf, respectively. The behavior of underwater superaerophobic/superaerophilic sheets with through microholes to bubbles was also investigated. It was found that such porous sheets allowed selective transport of the bubbles.

2. Experimental Section 2.1 Femtosecond laser ablation Hierarchical rough microstructures were formed on the sample surface by femtosecond laser ablation. A Ti:sapphire laser system (Libra-usp-he, Coherent, America) was used. The center wavelength, duration and repetition rate of the laser beam were 800 nm, 50 fs and 1 kHz, respectively. The laser beam was focused on the sample surface through an objective lens. The silicon surface was ablated at the laser power of 15 mw, scanning speed of 2 mm/s, and the shift of scanning lines of 2 µm, by using an objective lens with NA of 0.45. The PDMS sheet was obtained by mixing the pre-polymer and curing agent (DC-184, Dow Corning Corporation) at a ratio of 10:1. After removing bubbles from the mix by vacuuming, the mix was further spread onto a glass slide and cured at 100 ℃ for 2 h. The PDMS surface was ablated at the laser power of 30 mw, scanning speed of 4 mm/s, and the shift of scanning lines of 4 µm, by using an objective lens with NA of 0.30. The aluminum surface and the polytetrafluoroethylene (PTFE) surface were ablated at the laser power of 30 mw, scanning speed of 5 mm/s, and the shift of scanning lines of 5 µm, by using an objective lens with NA of 0.40. 2.2 Generating through-microholes array To prepare rough porous sheets with through microholes, the sheets were pre-drilled by a mini drill prior to femtosecond laser ablation. The diameter of the drill bit was 300 µm. The drill bit was controlled to pass through the aluminum sheet (thickness = 0.1 mm) with the speed of 0.2 mm/s and pass through the PTFE sheet (thickness = 0.3 mm) with the speed of 0.3 mm/s. 2.3 Characterization The microstructure of the sample surfaces was examined using a scanning electron microscope (Quantan 250 FEG, FEI, America). The wettability of the as-prepared surface and the behavior of the underwater bubble on the sample surface were investigated using a contact-angle system (JC2000D, Powereach, China). The typical volume of the used water droplet was ~7 µL and that of the bubble was ~3 µL. The processes of absorption of a water droplet or an air bubble were captured by a high-speed CMOS camera (CAMMC1362, Mikrotron, Germany) with a maximal frame rate of 2000 fps.

3. Results and Discussion Air bubbles were never observed to adhere to the skin of a fish. Such adhesion of an



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air bubble would be very problematic for the fish. For example, if the bubbles mainly adhere to one side of the fish’s body, it will be hard for the fish to balance its body, especially for small fish. The adhered bubbles will also increase the water resistance, so that the fish will not be able to swim as fast and will be easier prey. After millions of years of evolution and natural selection, fish have evolved a perfect multifunctional surface to survive in their harsh environment. Therefore, the above problems are absent because bubbles are almost completely unable to attach to fish scales. The skin of most fish such as carp is generally covered by fan-like scales (Figure 1a). The scale is made up of a protein and hydrophilic calcium phosphate skeleton. The outside surface of the fish scale is also coated with a thin layer of mucus. Figures 1b,c show the scanning electronic microscopy (SEM) images of the surface of a fish scale. It can be seen that many micro-papillae are distributed in an orderly fashion on the surface along the fanning-out direction. The size of the micro-papillae is about several hundred micrometers and decreases from the scales’ central area to the edge. Abundant finer micro-pimples are present on every micro-papilla (Figure 1c). Fish scales are hydrophilic in air, and a small water droplet on the fish scale has a low CA of 25 ± 5° (Figure 1d). When the fish scale was dipped into water and an air bubble was released below the fish scale, the bubble could stand on the fish scale in the form of a spherical shape (Figure 1e). The CA to this bubble reaches up to 155 ± 2.5°. The bubble could easily roll away as the fish scale was tilted at an angle of 9° (Figure 1f and Movie S1 in Supporting Information). Such a high CA and low sliding angle (SA) value reveal that the fish scales exhibit underwater superaerophobicity and ultralow adhesion to a bubble. In fact, it is the underwater superaerophobicity that endows the fish scales with the anti-bubble ability, preventing air bubbles from adhering to the skin of the fish.

Figure 1. Underwater superaerophobicity of fish scales and underwater superaerophilicity of lotus leaves. (a) Photo of a carp. (b,c) SEM images of fish scale surface. (d) Water droplet on a fish scale in air. (e) Bubble on a fish scale in water. (f) Process of underwater bubble rolling on the fish scale. (g) Photo of lotus leaves. (h,i) SEM


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images of the lotus leaf surface. (j) Water droplet on a lotus leaf in air. (k) Process of a bubble being absorbed by the lotus leaf in water. Scale bars in (e,f,j,k) are 0.5 mm.

Water spiders and locusts are able to breathe in water.35,36 An air layer will form surrounding the body of those animals after they actively or accidentally fall into the water. They can then continue to breathe oxygen from this air layer even under water. Such an air layer is usually called the “physical gill” or “plastron”.35,36 Similarly, the floating water fern Salvinia also can capture air in water and has long-term air-retention ability.37 The gas in the water can be absorbed by these organisms, revealing strong superaerophilicity underwater. It is found that all of these organisms have a common characteristic: their surfaces show superhydrophobicity in air. An interesting conjecture emerges as a certain internal link exists between underwater superaerophilicity and in-air superhydrophobicity. It is well known that the most famous superhydrophobic surface is that of the lotus leaf (Figure 1g) .38,39 A high number of papillae with a diameter of about 10 µm are randomly distributed on the surface of a lotus leaf (Figure 1h). The microscale papillae are further covered with abundant nanoscale branch-like protrusions as well as a layer of hydrophobic wax crystals (Figure 1i). The combination of the micro/nanoscale binary structure and the low-surface-energy chemical composition results in a remarkable 38 superhydrophobicity of the lotus leaf. The lotus leaf shows a CA of 153 ± 2° (Figure 1j) and an SA of 4.5 ± 1° (Figure S1a and Movie S2, Supporting Information) to a water droplet in air. After immersion of the lotus leaf in water, a silver mirror-like reflectance can be directly seen by the eye, demonstrating that an air layer is trapped between the water and the lotus leaf surface (Figure S1b, Supporting Information).40,41 We released a bubble below the underwater lotus leaf and let it approach the lotus leaf. The bubble would spread out quickly once it contacted with the surface, and could be completely absorbed by the lotus leaf, as shown in Figure 1k and Movie S2 (Supporting Information). Such a small bubble CA of near 0° verifies the underwater superaerophilicity of the lotus leaf. To mimic the chemistry and surface characteristics of fish scales, a silicon wafer was used as the substrate due to is inherent hydrophilicity and was further ablated by a femtosecond laser to generate hierarchical microstructures on its surface. Femtosecond laser ablation has proven to be a powerful tool for the control of surface wettability.5,8,19,42-44 Figures 2a,b show the SEM images of the silicon surface after femtosecond laser ablation. A periodic hierarchical micro-mountains array was formed on the silicon surface. The period is approximately 10 µm, and the size of the micro-mountains is 7-8 µm. In addition, every micro-mountain is further covered by abundant nanoscale particles. The flat silicon surface is intrinsically hydrophilic with a water CA of 60 ± 2.5° (Figure S2a in Supporting Information). After laser ablation, the water wettability was amplified by the hierarchical rough microstructures.1 A water droplet that was dripped onto the laser structured surface could spread out rapidly, resulting in a very small water CA of 5 ± 1° (Figure 2d and Movie S3 in Supporting Information). Therefore, the femtosecond laser ablated silicon surface shows superhydrophilicity. The underwater bubble’s behavior on the silicon surface



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was investigated by immersing the sample in water and releasing an air bubble onto the surface. The CA of the bubble on the flat silicon surface is 125 ± 2° (Figure S2c in Supporting Information), yielding an underwater aerophobicity. Surprisingly, the bubble could maintain a sphere shape on the laser structured rough silicon surface at all times (Figure 2c). The bubble CA is as large as 162 ± 2°, revealing that the femtosecond laser ablated rough silicon surface shows superaerophobicity in water. If the sample was slightly tilted by 2°, the bubble would roll away freely (Figure 2e and Movie S3 in Supporting Information). Such a low SA value of 2° indicates an ultralow adhesion between the rough silicon surface and the bubble in water. The underwater superaerophobicity and very low adhesion to bubbles endow the rough silicon surface with great anti-bubble ability, very similar to a fish scale.

Figure 2. Superhydrophilicity and underwater superaerophobicity of the femtosecond laser structured rough silicon surface. (a,b) SEM images of the silicon surface after laser ablation. (c) Shape of a bubble on the rough silicon surface in water. (d) Process of dripping a water droplet onto the rough silicon surface in air. (e) Process of underwater bubble rolling on the femtosecond laser ablated silicon surface.

The PDMS surface also shows micro/nanoscale hierarchical rough structures after femtosecond laser ablation, as shown in Figures 3a,b. A high number of lumps with a size of several micrometers are observed to be distributed on the PDMS surface (Figure 3a). The surface of the lumps is not smooth but rather is decorated with many nanoscale protrusions (Figure 3b). Compared to the laser structured silicon surface, such a rough PDMS surface exhibits a completely opposite water wettability and underwater bubble’s behavior. The rough PDMS surface shows superhydrophobicity in air. A water droplet on the laser ablated PDMS surface can curl into a sphere. The CA of the droplet is 156.5 ± 1.5° (Figure 3c). For the dynamics, such a surface exhibits ultralow adhesion to a water droplet in air because the droplet will roll off as long as the surface tilt was 1.2° (Figure 3d and Movie S4 in Supporting Information). In addition, a water droplet falling from some height could bounce several times on the sample surface (Figure S3 and Movie S4, Supporting Information). When the superhydrophobic PDMS sample was placed into water, a silver mirror appears on the


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laser ablated area (Figure S4, Supporting Information). In water, an air bubble released below the sample surface is observed to rise toward the rough PDMS surface. The bubble then spreads out upon contact with the laser structured surface, as shown in Figure 3e and Movie S4 (Supporting Information). Within 35 ms, the bubble was completely absorbed by the rough PDMS surface. The bubble/water interface almost coincided with the sample surface, so that the CA value of the bubble can be considered to be ~0° (Figure 3e). Therefore, the femtosecond laser ablated PDMS surface shows superaerophilicity in water, which is the same behavior as that of the lotus leaf.

Figure 3. Superhydrophobicity and underwater superaerophilicity of the femtosecond laser structured rough PDMS surface. (a,b) SEM images of the PDMS surface after femtosecond laser ablation. (c) Shape of a water droplet on the rough PDMS surface in air. (d) Process of water droplet rolling on the femtosecond laser ablated PDMS surface in air. (e) Process of an underwater bubble being absorbed by the rough PDMS surface.

When a small water droplet is placed onto an ideal flat solid surface in an air environment, a spherical crown shape is usually formed, as shown in Figures 4a,b. The water CA (θw) of this droplet can be estimated using Young’s equation as:1 cos θ w =

γ SV − γ SL γ LV

(1)

where γ SV , γ SL , and γ LV are the interfacial energy/tension between the solid-vapor, solid-liquid, and liquid-vapor interfaces, respectively. When such a flat surface is immersed in water, the water molecules are in close proximity to the solid surface. If an air bubble is released onto this surface, the bubble must compete with the water film closely covering the surface to adhere to the substrate. In the case of an underwater bubble on the flat solid surface (Figure 4c), once a small perturbation occurs, the work change of this system can be estimated using the energy expression given by:45

δ w = γ SV dASV + γ SL dASL + γ LV dALV + ∆PdV


(2)


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where dASV , dASL , and dALV are the solid-vapor, solid-liquid, and liquid-vapor area changes, respectively. ∆P is the Laplace pressure change, and dV is the volume change. For the limiting case of a small perturbation, dASL = − dASV , dV = 0 , and dALV / dASV = cos θb for a semi-spherical underwater bubble. θb is the CA value of the underwater bubble. For δ w = 0 , Equation (2) can be simplified to: cos θ w =

γ SL − γ SV γ LV

(3)

By combining Equations (1) and (3), we simply obtain:

θ b = 180° − θ w

(4)

According to Equation (4), we can approximately infer that the hydrophilic surface will become aerophobic in water (Figures 4a,c) while the hydrophobic surface will become aerophilic underwater (Figures 4b,d). This corollary is in good agreement with the experimental results (Figure S2 in Supporting Information).

Figure 4. Schematics showing the relationship between the in-air water wettability and underwater bubble’s behavior on a flat solid surface. (a,b) Water droplets on a flat (a) hydrophilic surface and (b) hydrophobic surface. (c,d) Bubbles on the corresponding flat (c) hydrophilic and (d) hydrophobic substrates in water medium.

The experimental results reveal that the laser structured silicon surface shows superhydrophilicity in air and becomes superaerophobic after immersion in water. By contrast, the laser structured PDMS surface shows superhydrophobicity in air and becomes superaerophilic in water. It can be seen that the behavior of the bubble on a substrate in water is highly correlated with the substrate’s in-air water wettability. As shown in Figure 5a, when a water droplet is placed onto a superhydrophilic rough surface, the water will fully wet the surface and fill the space between the microstructures. This wetting state is in good agreement with the Wenzel model.1,46,47 Similarly, the rough microstructure of the superhydrophilic surface is also wetted by


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water as the sample is dipped into water and its superhydrophilic side faces down (Figure 5b). The water fills all the interspaces between the microstructures. It appears that the water is trapped by the rough microstructures. If a bubble is released onto the rough surface, it is difficult to effectively contact the rough microstructure because of the blocking effect arising from the trapped water layer. Since water inherently repels gas, the trapped water layer in the microstructure will prevent the contact between the air bubble and the solid substrate. This wetting state can be described by the underwater version of the Cassie model in an underwater water/gas/solid three-phase system (Figure 5c).1-3 The underwater bubble can only contact the tip of the rough microstructure. The bubble tends to maintain a spherical shape due to its being almost entirely surrounded by water. Therefore, the superhydrophilic surface has an anti-bubble function and shows superaerophobicity in water. The mechanism of the origin of underwater superaerophobicity is similar to that for the underwater superoleophobicity of in-air superhydrophilic materials.48-50 The spherical bubble shape will not change significantly over time unless it slowly dissolves into water (Figure 5d). An underwater bubble on a superaerophobic substrate is in a similar situation as a water droplet on an ultralow-water-adhesive superhydrophobic surface, only in this case, the heterogeneous surface underneath the bubble is composed of solid and water and the surrounding environment is water. Therefore, the bubble’s CA ( θ b ) on the underwater superaerophobic surface can be calculated using the Cassie equation.1 Generally, Cassie’s equation for the underwater bubble can be re-written as: (5)

cos θ b = f sb cos θ sb + f lb cos θ lb

Where f sb and flb are the area fractions of the solid part and the trapped liquid part, respectively, at the underwater bubble-substrate interface. θ sb is the intrinsic CA value of a bubble on a flat corresponding solid substrate, and θ lb is the CA value of a bubble on a water surface surrounded by water medium. Because water inherently repels gas and a bubble in water always remains spherical, the bubble’s CA on a water layer in a water environment can be defined as 180°. Substituting θ lb = 180° and f sb + flb = 1 into equation (5) results in cos θ b = f sb cos θ sb + f sb − 1

.

(6)

For the femtosecond laser structured rough silicon surface, it can be inferred from equation (6) that the f sb of the underwater bubble-solid interface is 11.5% because the measured θb and θsb are 162° and 125°, respectively, indicating that the bubble contacts with only a very small area of the rough silicon substrate.



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Figure 5. Formation mechanisms of the underwater superaerophobicity and underwater superaerophilicity. (a-d) In-air superhydrophilic substrate showing underwater superaerophobicity after immersion in water. (c) reveals the condition that a bubble is just placed onto the sample surface, while (d) reveals the state after a period of time since a bubble was released on the sample surface. (e-h) In-air superhydrophobic substrate showing underwater superaerophilicity after immersion in water.

For the superhydrophobic surface, an in-air water droplet on the surface is at the Cassie wetting state and is repelled by the superhydrophobicity of the rough substrate (Figure 5e).1,51,52 An air cushion is usually trapped between the water droplet and the rough surface microstructure.53 Such an air cushion only allows the droplet to contact the peaks of the rough microstructure. For the superhydrophobic surface immersed in water, the air cushion beneath the water droplet will turn into an air layer that is trapped between the substrate and water (Figure 5f). Here, when a bubble contacts the surface, the gas in the bubble will spread out rapidly along the trapped air layer under pressure; that is, the bubble will merge with the air in the trapped air layer, as shown in Figure 5g. The bubbles can be completely absorbed by the superhydrophobic microstructures as long as the area of the rough surface is large enough. Therefore, such a superhydrophobic surface exhibits superaerophilicity in water (Figure 5h). According to the experimental results and the above analysis, we find that the in-air superhydrophilic surface will become superaerophobic in water while the in-air superhydrophobic surface will become superaerophilic in water. The underwater superaerophobic surface generally has strong anti-bubble ability. On the contrary, the


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underwater superaerophilic surface can absorb and capture air bubbles in a water medium. The bubbles’ behavior on a through-holes underwater superaerophobic or superaerophilic surface was also investigated. An aluminum sheet with a thickness of 0.1 mm was first drilled by a mini drill to generate a through-microholes array (Figure 6a). The diameter of the holes was ~312 µm, and the period was set as 1 mm. Then, both sides of the sheet were scanned by the femtosecond laser. As a result, both the rim of the microholes and the rest area between the microholes are covered by a micro/nanoscale hierarchical rough structure (Figure 6a). Such a porous aluminum sheet shows underwater superaerophobicity with a bubble CA of 151 ± 3° (inset of Figure 6a). A large amount of bubbles was released below the sheet in the water medium, as shown in Figure 6b and Movie S5 (Supporting Information). As the first bubble arrived at the aluminum sheet, it stopped rising and was intercepted by the sheet. Similarly, the second bubble was also intercepted when it reached the sheet. These two bubbles merged into a bigger bubble as soon as they touched each other. As an increasing number of bubbles rose up to the sheet, all of them were unable to pass through the superaerophobic sheet. The bubbles merged with each other, generating increasingly larger merged bubbles. Therefore, the underwater porous superaerophobic sheet shows an interception function for the bubbles, preventing the bubbles from passing through the sheet. This sheet can also be applied for the removal of the bubbles from water. Figure 6c depicts the mechanism of the bubble interception of the underwater porous superaerophobic sheet. This functionality is a direct result of the superaerophobicity-induced gas repellency in water. The underwater superaerophobicity allows the sheet to repel all bubbles irrespective of the number of bubbles reaching the sheet. Therefore, the bubbles are forced to merge into larger bubbles and continue to grow.

Figure 6. Bubble-interception function of the femtosecond laser ablated porous aluminum sheet. (a) SEM images of the rough porous aluminum sheet. (b) Release of a large amount of bubbles below the as-prepared sheet. (c) Schematic illustration of the bubble-interception function of the underwater porous superaerophobic sheet. Scale



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bar in (b) is 2 mm.

An underwater porous superaerophilic sheet was prepared through the same processing procedure used for the preparation of the rough through-microholes aluminum sheet. A polytetrafluoroethylene (PTFE) sheet (thickness = 0.3 mm) was selected as the substrate material because it is intrinsically hydrophobic.54 The surface topography of the as-prepared porous PTFE sheet is shown in Figure 7a. The diameter of the holes was ~287 µm. The surface was also fully covered by a rough microstructure. The porous PTFE sheet shows underwater superaerophilicity with a bubble CA of near 0°. The experiment of releasing many bubbles below the sheet was also performed in the water medium, as shown in Figure 7b and Movie S6 (Supporting Information). The first bubble was absorbed by the sheet instantaneously upon contact with the sheet. Indeed, this is a direct result of the underwater superaerophilicity of the porous PTFE sheet. Similarly, the second bubble could also be completely absorbed by the sheet. With an increasing number of bubbles captured by this sheet, the trapped air layer in the through microholes and the surface microstructures started to bulge on the top side of the sheet. Then, the air bulge grew larger by capturing more bubbles. Its shape changed from “crescent” to “bell” and then to “wine goblet”. After enough accumulation, the air bulge was so big that the buoyancy force acting on the air bulge could drive the air bulge to leave the porous sheet and continue to rise up. In this way, the bubbles successfully pass through the underwater superaerophilic porous sheet, one after another. In this case, it is the underwater superaerophilicity that allows the bubbles to pass through the sheet. The mechanism of the bubbles passing through the porous PTFE sheet is schematically shown in Figure 7c. After the underwater superaerophilic PTFE sheet that is also superhydrophobic in air was immersed in water, a trapped air layer was formed in the microstructures.40,41 Once a bubble touches the trapped air layer, the gas in this bubble will enter into the trapped air layer. This will lead to the merging of the trapped air layer and the bubble. As the rough porous PTFE sheet absorbs an increasing number of bubbles, the volume of the trapped air layer increases. At some point, the high inner pressure allows the trapped gas layer to lift the water that presses on the top side of the sheet. The trapped air layer stretches its top out of the porous sheet, forming a big air bulge. Finally, driven by buoyancy force, the gas in the form of a big bubble will leave the sheet and rise up. It is found that the underwater superaerophilic porous sheet can absorb bubbles and further allow the bubbles to pass through in the water medium. Such a sheet can be potentially used in underwater gas collection and filtration.


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Figure 7. Bubbles passing through the femtosecond laser ablated porous PTFE sheet. (a) SEM images of the rough porous PTFE sheet. (b) Release of a large amount of bubbles below the as-prepared sheet. (c) Schematic illustration of the process of bubbles passing through the underwater porous superaerophilic sheet. Scale bar in (b) is 2 mm.

4. Conclusions In conclusion, we found that fish scales show superaerophobicity while lotus leaf shows superaerophilicity in the water medium. Inspired by these living organisms, a silicon sample was ablated by a femtosecond laser to generate micro/nanoscale hierarchical structures on the surface. The resultant silicon surface showed superhydrophilicity in air and underwater superaerophobicity. By contrast, the femtosecond laser ablated PDMS surface showed superhydrophobicity in air and superaerophilicity underwater. The experimental results revealed that an in-air superhydrophilic surface is generally superaerophobic in water and that an in-air superhydrophobic surface is generally superaerophilic in water. The underwater superaerophobic surface shows great anti-bubble ability, while the underwater superaerophilic surface can absorb and capture air bubbles in the water medium. By combining the underwater superaerophobic/superaerophilic property and the characteristics of through microholes, an underwater superaerophobic porous aluminum sheet and an underwater superaerophilic porous PTFE sheet were prepared, respectively. The underwater superaerophobic porous sheet was able to intercept and then remove the bubbles from water due to its superaerophobicity. The underwater superaerophilic porous sheet could allow the bubbles to pass through the sheet, due to the superhydrophobicity and underwater superaerophilicity of the sheet. The designed underwater superaerophobic/superaerophilic surfaces will have potential applications in excluding bubble-induced hazards and in the clever use of underwater bubbles in some cases.



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Acknowledgements This work is supported by the National Science Foundation of China under Grant nos. 51335008 and 61475124; the NSAF Grant no. U1630111; the National Key Research and Development Program of China under Grant no. 2017YFB1104700; the China Postdoctoral Science Foundation under Grant no. 2016M600786; the Collaborative Innovation Center of Suzhou Nano Science and Technology; and the International Joint Research Center for Micro/Nano Manufacturing and Measurement Technologies. The SEM work was done at the International Center for Dielectric Research (ICDR), Xi’an Jiaotong University.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . Water droplet rolling on a lotus leaf in air, photo of a lotus leaf being immersed in water (Figure S1); intrinsic wettabilities of the flat silicon surface and PDMS surface (Figure S2); water droplet bouncing on the femtosecond laser ablated PDMS surface (Figure S3); photo of a femtosecond laser ablated PDMS sample in water medium (Figure S4). Process of underwater bubble rolling on the fish scale (Movie S1). Water droplet rolling on a lotus leaf in air and the process of a bubble being absorbed by the lotus leaf in water (Movie S2) Process of dripping a water droplet onto the rough silicon surface in air, and the process of an underwater bubble rolling on the femtosecond laser ablated silicon surface (Movie S3) Process of water droplet rolling on the femtosecond laser ablated PDMS surface in air, water droplet bouncing on the femtosecond laser ablated PDMS surface, and the process of an underwater bubble being absorbed by the rough PDMS surface (Movie S4) Bubble-interception function of the femtosecond laser ablated porous aluminum sheet (Movie S5) Bubbles passing through the femtosecond laser ablated porous PTFE sheet (Movie S6)

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Laser Ablated Durable Superhydrophobic PTFE Films with Micro-Through-Holes for Oil/Water Separation: Separating Oil from Water and Corrosive Solutions. Appl. Surf. Sci. 2016, 389, 1148-1155.


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Bioinspired Design of Underwater Superaerophobic and

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