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. 2019 Dec 24;5(1):718–725. doi: 10.1021/acsomega.9b03349

Laser-Induced Wettability Gradient Surface of the Aluminum Matrix Used for Directional Transportation and Collection of Underwater Bubbles

ZhiXia Zheng †,*, Huan Yang ‡, Yiqing Cao †, ZiYi Dai §
PMCID: PMC6964265  PMID: 31956822

Abstract

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The control of underwater bubble behavior on a solid surface has great research significance. However, the control of the spontaneous directional transport and collection of numerous underwater bubbles remains a challenge. A new technique of a metal mesh with superhydrophobic/hydrophobic properties is demonstrated here, which creates a wettability gradient coupled with a microporous array by means of pulsed fiber laser ablation and chemical modification of the aluminum sheet. The resultant wettability surface effectively achieved the spontaneous movement of bubbles along the directional wettability gradient (superaerophobicity to aerophilicity) and through the metal mesh (aerophilicity to superaerophilicity) in the direction of decreasing free energy. Theoretical analysis accounted first for the spontaneous sliding of bubbles on the wettability gradient surface as a result of the action of an unbalanced surface tension force and second for the spontaneous transition of bubbles from the aerophilic to superaerophilic side as a result of the combined action of Laplace pressure and buoyancy. A device with the capability of directional transportation and collection of underwater bubbles was designed based on the samples with a wettability gradient and a superhydrophobic/hydrophobic microporous array as the core components. The potential application is laser ablation of wettability gradient surfaces and metal mesh with superhydrophobic/hydrophobic properties for directional transportation and collection of underwater bubbles.

1. Introduction

Bubbles in aqueous media are a common encounter in our daily lives, both in nature and in engineered processes. Bubbles have great significance for life science engineering and industrial production and application to realize the manipulation or collection of underwater bubbles. They are advantageous for the industrial wastewater treatment by manipulating bubbles to reach the effect of pollution treatment;1 the use of solid–liquid interface nanobubbles caused by fluid boundary slippage and to reduce the flow of viscous drag;2 a sample with in situ tunable bubble wettability can be used as a light switch;3 humpback whales living in the depths of the ocean produce bubbles to hunt for food;4 and at the bottom of the ocean bed, a large amount of natural gas is present, which is important for human energy. However, bubbles also result in many adverse effects, for example, bubbles in microfluidic control chips cause obstruction;5 bubbles mixed with intravenous fluids cause increased pressure in pulmonary arteries;6 bubbles attached to the surface of the electrochemical reaction catalyst reduce the reaction efficiency;7 and bubbles attached to diving goggles or cameras will cause blurred vision.8,9

In nature, many living organisms have the characteristics of directional capture and transport of water droplets on their surfaces. The main reason is that these living organisms have unique multilevel micro–nanostructures on their surface. Such multilevel micro–nanostructures can generate Laplace pressure gradient and surface free energy gradient. Inspired by nature, researchers control the directional movement of droplets by constructing asymmetric microstructures or wettability gradients.10,11

Superhydrophobic/superhydrophilic solid surfaces in air are generally superaerophilic/superaerophobic in aqueous medium.12,13 This particular wetting phenomenon opens up the possibility of manipulating solid surface air bubbles in aqueous media. Generally, a solid surface has a certain degree of hydrophilicity, and water molecules can be easily captured underwater to form a water film. The repulsion of the water film to the bubble makes the friction between the bubble and the hydrophilic surface very small, and the bubble is difficult to be fixed on the hydrophilic surface or moved along the solid surface in a directional way, which is extremely unfavorable to the application of the bubble in drag reduction. Researchers used the characteristics of repulsive/adsorptive bubbles on superaerophobic/superaerophilic surfaces to collect, self-assemble, and reduce drag of underwater bubbles.14−17 The behavior of bubbles in waterborne media is dominated by upward buoyancy, which greatly impedes the transportation of bubbles in any other direction. Jiang et al. successfully created a superhydrophobic copper cone by the chemical coating of low surface tension on a conical shape to transport microbubbles spontaneously and directionally from the tip of the cone to the base underwater.18 Chen et al. manufactured microhole-arrayed polydimethylsiloxane with controllable wettability gradient for ultrafast underwater bubble unidirectional self-transport.19 Hu et al. successfully fabricated a channel-controlled Janus membrane by simultaneous laser ablation and nanoparticle deposition for underwater bubble manipulation.20 According to the different characteristics of hydrophobic and hydrophilic surface wettability, Hu et al. prepared thin rings with hydrophobic and hydrophilic alternation on the solid surface, which were used to nail the gas to reduce drag.21 Although the superhydrophobic copper cone can realize the directional delivery of microbubbles, the limitation in the delivery distance of 1–2 cm for single bubbles makes the continuous long-distance directional transport of bubbles remain a challenge at large. Previous studies have also shown that spontaneous and directional transport may occur between two bubbles of different volumes on the surface of superhydrophobic porous materials.22,23 However, the direction and efficiency of bubble transport depend on the difference in the volume of two bubbles. Therefore, the realization of single large bubble spontaneous and directional transport is still an urgent challenge. This work focuses on bioinspired wettability gradient solid surfaces through the construction of roughness gradient and free energy gradient to achieve the directional transport and collection of bubbles in water-based media.

Surface roughness gradient and free energy gradient can be created by chemical methods such as vapor-phase diffusion,24 photodegradation,25 and gradual immersion method.26 However, because of the poor controllability of chemical methods, it is difficult to obtain uniform gradient change. In recent years, the development and application of ultra-short-pulse technology, the nonlinear absorption effect of materials on ultrafast laser can be utilized to obtain the machining accuracy far less than the size of a laser-focused spot, and the construction of abundant micro–nanostructures on the surfaces of semiconductors, brittle materials, metals, polymers, ceramics, and biological materials.27−29 Laser manufacturing is a fast, precise, and flexible process to greatly improve the preparation efficiency and precision of functional micro–nanostructure surfaces. Lasers make it possible to prepare a micropore array with a wettability gradient. During laser texturing, the sample surface is generally perpendicular to the laser beam, which is usually focused on the sample surface to minimize the spot diameter and maximize the energy density absorbed by the sample surface, so as to optimize the interaction between light and substance. When the processing surface deviates from the focal plane, the laser spot becomes larger and the energy density decreases exponentially with distance from the focal plane. The shape of the laser-textured surface is closely related to the energy density injected by the laser. By adjusting the distance between the machining surface and focal plane, the surface with different roughnesses can be machined. It has been proved that by tilting the sample, the surface of the sample can be textured by laser to produce a roughness gradient.30

This work fabricated a device for the directional transportation and collection of underwater bubbles by laser ablation and chemical modification of an aluminum sheet. A uniform gradient of micro–nanostructures and micropore array was created on the sheet, which was then coated with stearic acid and laser-ablated to partially remove some stearic acid. The resultant surface was a wettability gradient and superhydrophobic/hydrophobic microporous array. This work provides reference for laser processing of wettability gradient surfaces and superhydrophobic microporous arrays, as well as a new method for the directional transportation and collection of underwater bubbles.

2. Experimental Methods

2.1. Preparation of Sample

Aluminum sheets (purity 99.6%, size 40 mm × 30 mm × 0.2 mm) were cleaned with ethanol and deionized water for 15 min to remove contaminants from the sample surface. A pulsed fiber laser scanning system (wavelength 1064 nm, pulse duration 5 ns, and repetition rate 90 kHz) was used to fabricate gradient roughness surfaces and microporous arrays. The matrix of microchannels and microholes was designed via the graphic interface of a control software, where the laser beam was focused normally to the substrate surface. The laser processing system is shown in Figure 1a. The laser passes through the attenuator and beam expander. Subsequently, it is coupled with a galvanometer scanner and focused by a F-θ lens to the aluminum substrate.

Figure 1.

Figure 1

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Schematic diagram of (a) laser processing system and (b) sample preparation process.

The laser beam emitted by the laser amplifier is coupled by the scanner and focused on the aluminum surface through the F-θ lens. The designed grid patterns are directly textured on the aluminum substrate. The laser scanning is performed line-by-line in the horizontal and vertical directions with an equivalent distance between the adjacent scanning lines. All processing is performed in an atmospheric environment. The sample preparation process is completed in five steps, as shown in Figure 1b. The first step begins by the design of laser ablation grids within region A (15 mm × 20 mm). There is a roughness gradient created throughout region A as the aluminum sheet was fixed on a stainless steel plate and end B was elevated by a glass slide at an angle of 2°. The focal length of the laser was focused at end A; thus, the ablation area resulted in end A having a stronger ablation and gradually lesser ablation toward the A–B interface. In the second step, microhole arrays were created in region B (area 10 mm × 15 mm, array 12 × 8 matrix, microhole diameter 600–800 μm, and microhole center-to-center distance 800–1000 μm). The sample was placed flat, at no elevation (the glass slide was removed). In the third step, laser grids were ablated in region B (ablation repetition times = 1) and also on the reverse side of region B (ablation repetition times = 3). This creates a metal mesh with hydrophobic/aerophilic (front of region B) and superhydrophobic/superaerophilic (back of region B) properties. Thereafter, the fourth step was a chemical modification on the entire sample, whereby the sample was immersed in stearic acid ethanol solution with a concentration of 0.01 M for 60 min. The purpose of this process was to reduce the surface free energy. Thereafter, the sample was removed, dried, and mounted in the same sample placement position as in step 1. Last, in the fifth step, laser grids were ablated to remove the stearic acid film partly within region A. This regained the surface free energy gradient. The laser processing parameters are shown in Table 1.

Table 1. Processing Parameters of Laser Ablation.

processing parameters values
processes 1, 3: microchannels pass 3
  laser fluence (J/cm2) 7.5
  speed (mm/s) 40
  scan spacing (μm) 70
process 2: micro-holes pass 2
  laser fluence (J/cm2) 10
  speed (mm/s) 3
  scan spacing (μm) 20
process 5: remove stearic acid pass 1
  laser fluence (J/cm2) 2.5
  speed (mm/s) 30
  scan spacing (μm) 20
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2.2. Characterization

The surface morphology of the laser-textured samples was studied by means of a scanning electron microscope (Gemini SEM 500, ZEISS, German). The roughness of the sample surface was analyzed using a laser scanning confocal microscope (LSCM, Smartproof 5, ZEISS). The water contact angle (WCA) and the underwater bubble CA of the rough aluminum sheet surface induced by the pulse fiber laser were measured using an optical CA meter (KSV CAM 200). To measure the WCA, a 4.0 μL drop of deionized water droplet was generated and deposited on the filter surface (in air) using a micrometric syringe by the sessile drop technique. To measure the CA of underwater bubbles, the sample was carefully immersed inside water and a 3.0 μL volume of air bubble was deposited on the sample surface. The final CA value was averaged by measuring three different points on the same surface at ambient temperature. The dynamic process of water droplets and bubbles on the sample surface was captured by IDS Imaging Systems (IDS, Germany) and Macro zoom lens (Nikon 60 mmf/2.8 d).

2.3. Directional Transport and Collection of Bubbles

An in-house device was set up as shown in Figure 2a to demonstrate the directional transportation of underwater bubbles. The sample was affixed beneath the fixture installed on the test tank to hold the sample horizontally. The system includes a horizontal regulator and a syringe connected to an automatic syringe pump. The horizontal regulator maintains the sample at a suitable level and the syringe was connected to an automatic syringe pump. Air is injected into the sample surface through a microinjection syringe that is affixed perpendicular to the bottom of the sample. The bubble released through the needle rises vertically to the superaerophobic end (end A) of the wettability gradient surface. The image acquisition system comprised a CCD camera, macrozoom lens, and a light-emitting diode light source, at a shooting speed of 50 fps. The camera system is connected to the computer, and the captured images are input into the computer through the video transmission acquisition card, and the computer is displayed and collected in real time through the control software provided by IDS.

Figure 2.

Figure 2

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Experimental setup for (a) directional transportation and (b) collection of underwater bubbles.

Another in-house device was set up to demonstrate the collection of bubbles underwater (Figure 2b). The sample was affixed to the bottom of a transparent plexiglass tube with a diameter of 20 mm and a base diameter of 50 mm, and the microporous array was located at the bottom of the tube diameter. The plexiglass tube was clamped and immersed into the water tank. The top of the plexiglass tube was sealed and attached to a soft tube to enclose the system. To verify the generation of bubbles, the other end of the soft tube is immersed into a water-filled beaker to collect the bubbles. A microair pump was connected to the microsyringe needle through a thin flexible tube and the tip of the needle points perpendicular to the base of the glass tube. The bubbles rose up toward the superaerophobic end (end A) and were directionally transported toward the aerophilic end (end B) of the sample surface through the microneedle, so as to observe the bubble emission of the thin flexible tube buried in the beaker.

3. Results and Discussion

3.1. Effect of Laser Irradiation on Surface Morphology

Scanning electron microscopy (SEM) images confirmed the surface morphology on the laser-ablated aluminum sheet (Figure 3). The front and back of the samples are shown in Figure 3a,b, respectively. As shown in Figure 3a, there is a uniform color variation from dark to light (zone 1 to zone 3) because of the degree of surface oxidation and formation depth of microgroove area of laser ablation. Zone 1 is located at the starting end of region A, where the laser is in focus; hence, the degree of oxidation is greatest (Figure 3c,d). Zone 3 is located close to the interface of region A–B, where the laser is out of focus; hence, the degree of oxidation is lesser (Figure 3g,h). This is due to the laser ablation of the sample surface to form oxide and different surface roughnesses of the nonreflective properties. From the low-magnification SEM images in Figure 3c,e,g corresponding to the three regions, the microsquare grid formed by vertical and horizontal laser scanning can be seen, which is due to the microgroove grid formed by laser ablation. Magnified SEM images in Figure 3d,f,h showed that the edge of the microgroove and the unablated area of the laser have significant deposition of micro- and nanoparticles. Micro- and nanoparticles are thought to have formed from rapid cooling of the fluid ejected from the molten zone.31,32 Comparing the further magnified SEM images of zones 1 to 3, the ablated grooves within zone 1 is the deepest, with the side wall showing the most distinct V-slope and the micro- and nanoparticles being the most abundant. On the other hand, zone 3 has the shallowest groove and the least amount of micro- and nanoparticles.

Figure 3.

Figure 3

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Photos of the (a) front and (b) back of the sample. SEM images of the roughness gradient surface at zone 1 (c,d), zone 2 (e,f), and zone 3 (g,h) correspond to the low- and high-magnification SEM images of zone 1, zone 2, and zone 3, respectively.

The roughness of the sample surface at zone 1 was measured to be twice that of zone 3, with the average roughness of zone 1 around 2.6 μm, while that of zone 3 was around 1.3 μm. The roughness decreases uniformly along the X-axis (Figure S1). The results showed that laser-induced roughness can be controlled by adjusting the sample position perpendicular to the laser beam direction. The surface of the microhole array on the back of the sample was composed of micropores and microsquare grids. The boundary of the microsquare grid unit was formed by laser ablation to form a V-shaped groove, which is similar to that observed within zone 1. The edge of the trench forms a projection for nanostructure delamination, and there are more micro- and nanostructures than those in the central unablated area of the grid. There are also abundant micro- and nanoparticles distributed in the side walls of micropores (Figure S2). The abundant micro- and nanostructures on the surface of the microhole array enable it to have superhydrophobic properties.

3.2. Surface Wettability

The oxidized aluminum at different positions was investigated for its gradient wettability properties. The start of region A was taken as the origin and the gradient directionality from zone 1 to 3 as the X-axis. Process 1 in Figure 1b created microstructures and microgrooves on the surface with gradient roughness showing superhydrophilicity. It is observed that the water droplets of volume 1.0 μL in zone 1 spread out rapidly, and the diffusion speed along the X-axis was much faster than that along the Y-axis. The droplets were instantly absorbed by the surface, and the water traces were distributed in thin strips along the X-axis (Video S1). The distribution shape of droplets in zone 3 is the same as that in zone 1, but the diffusion direction is opposite, and the velocity is slow. Similarly, the surface infiltration area of a 1.0 μL water drop in zone 1 is larger than that of zone 3.

This indicated that the surface roughness structure caused the aluminum sheet to change from hydrophilic to superhydrophilic and the water droplets to have the characteristic of preferential diffusion along the gradient direction of roughness. After processes 2 and 3, the water CA was 0°, characterized by superhydrophilicity. After chemical modification, the relation between the water CA of the surface roughness gradient and the position of the X-axis is shown in Figure 4a. The water CAs of zone 1 and zone 3 were 161.5 ± 2 and 98.3 ± 1.5°, respectively. It indicated that the greater the surface roughness, the greater the abundance of micro- and nanostructure present on the surface and thus the larger the water CA. Therefore, the construction of a roughness gradient surface is an effective way to achieve a wettability gradient. The water CA on the surface of the aluminum sheet microhole array (region B) was measured to be 134 ± 1° (Figure 4b), while the reverse side of region B had a water CA measurement of 161.0 ± 2° (Figure 4c), which was similar to the water CA in zone 1.

Figure 4.

Figure 4

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WCA on sample surface (a) at different positions on the roughness gradient surface, (b) at region B microhole array with microgrids textured (repetitive times = 1), and (c) at the back side of region B with microgrids textured (repetitive times = 3).

Stearic acid is a weak acid anionic surfactant with −COOH group and easily soluble in ethanol, acetone, and other organic solvents. This was used as a chemical modifier as it can react with the solid surface and graft long-chain alkyl groups with hydrophobicity and low surface energy on the solid surface to reduce the free energy of the solid surface.33 Stearic acid removal by laser ablation is a process in which the stearic acid film grafted on the sample surface is vaporized by the high energy of laser. By controlling the laser energy density and laser scan spacing, stearic acid can be removed in whole or in part without damaging the surface geometry of the sample. The removed parts of the stearic acid on the surface with gradient roughness from zones 1, 2, and 3 have water CAs measured to be about 13, 42, and 58° (Figure 5a and Video S2). On the contrary, the underwater bubble CAs were measured to be about 150, 123, and 96°, respectively (Figure 5b). According to Young’s equation, it can be deduced that the hydrophobic surface is aerophilic, while the hydrophilic surface is aerophobic in waterborne medium, and the water CA in air and the underwater bubble CA are approximately complementary to each other in principle.34 This conclusion is consistent with the experimental results. The stearic acid in the roughness gradient region was removed by laser ablation, and the water CAs on the front and back of the microporous array were not affected (Figure 5c,d). This indicates that a low-energy laser ablation can possibly remove some or all stearic acid in the ablation zone and form a free energy gradient surface. The removal of stearic acid is related to the energy of laser ablation and the elevation angle of the sample.

Figure 5.

Figure 5

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CA of sample surface: (a) water CA on a wettability-graded surface, (b) underwater bubble CA on a wettability-graded surface, (c) WCA on the positive surface of the microhole array, and (d) WCA on the back side of the microhole array.

3.3. Transportation and Collection of Underwater Bubbles

After the bubble is released from the microneedle, it rises vertically to the position of zone 1, and it is approximately spherical in shape on the sample surface. Immediately, it slides to zone 3. The shape of the bubble gradually transitions from spherical to near-spherical during the sliding process, and when it reaches zone 3, where the microporous array lines the region, the bubble is captured and it spreads out to form a gas film adhering to the lower surface of the microporous array. As bubbles accumulate on the lower surface of the microporous array, the gas film gradually expands and forms a larger bubble. When the gas accumulates to a certain extent, it will pass through the micropores and emerge from the upper part of the micropores to form the upper convex air layer (Figure 6a). With the continuous delivery of bubbles, the air layer above the micropores gradually forms bubbles, and the bubbles become larger with their shapes constantly changing.

Figure 6.

Figure 6

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Experimental process of micropore array surface transport of bubbles. (a) Bubbles passing through the microholes form a film on the upper surface, (b–h) bubbles gradually increase in volume permeating through the top surface of micropores, and the (i) gas film left after the bubble breaks away from the upper surface.

The shape of the bubble encountering the micropore was investigated as shown in Figure 6b–g. When a bubble is injected below the micropore, the shape of the bubble above changes and reaches a stable equilibrium quickly. When the bubble on the top surface of the microporous array is large enough, the buoyancy force is sufficient to overcome the adhesion force on the surface of the sample and instantaneously breaks away, floating on the top of the water tank (Figure 6h and Video S3). A layer of gas film was observed to remain at the upper surface of the micropores (Figure 6i). If the sample is oppositely oriented, the bubbles cannot be transferred from the superaerophilic side through the micropores to the aerophilic side and was observed to slide out of the sample edge and rise to the top of the water tank after a large amount of surface accumulation (Video S4). This indicated a dependence on wettability of the surface for the successful transfer of bubble from the lower to the upper surface of the micropore array. Previous studies have only shown that air bubbles can only be reached from the hydrophilic side, through the microholes to the superhydrophobic side.17 Our study has shown that the surface wettability gradient from zone 1 to 3 (superaerophobic to aerophilic), as well as from the lower to upper surface of the microporous array (aerophilic to superaerophilic), is a crucial factor in the single-directional transportation of air bubbles.

The core component of the underwater bubble collecting device is the sample fabricated by laser ablation. The base of the plexiglass tube was immersed in water. After the bubble was injected vertically below zone 1, it rose to zone 1 and slid to zone 3 through the wetting gradient surface. Under the action of inertia, it continued to slide along the surface of the microporous array and was captured by the surface of the microporous array to form an air layer. The individual bubbles float upward and were collected when they reach the opening of the plexiglass tube, which was connected to a hose immersed into a water beaker to visually verify the effect of the bubble collection. Bubbles were observed to emerge from the underwater hose in the beaker (Video S5), which demonstrated the successful and direct method to collect gas bubbles. The efficiency of gas collection needs to be further investigated, and it is postulated to be dependent on the rate of bubble release from the microneedle, the spacing and diameter of the micropores in the array, the thickness of the aluminum sheet, and the depth which it is submerged at, as well as the transport speed of the collected gas.

3.4. Mechanism of Spontaneous and Directional Transport of Gas Bubbles

The mechanism of directional transport of bubbles on the surface with underwater gradient wettability is shown in Figure 7a.

Figure 7.

Figure 7

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Diagram schematic of bubble movement mechanism in (a) force analysis of bubbles on the wettability gradient surface and (b,c) schematic diagram of force analysis of bubbles on the superhydrophobic surface and hydrophilic surface of the microporous array, respectively.

The behavior of the horizontal transportation of gas bubble on the sample is primarily determined by the wettability gradient force arising from the unbalanced surface tension force (FS), the drag force of water (FD), and the hysteresis resistance (FH). When bubbles are released from the microneedles and raised to the surface of the sample, the initial velocity in the horizontal direction (X-axis) is zero. At this juncture, the force in the X-axis direction is mainly the unbalanced surface tension. Let the small displacement along the X-axis direction in which the gradient surface energy decreases be with respect to x, and the change in the interface energy is

3.4. 1

where ΔG is the interface free energy, γSV is the solid–gas surface tension, γSL is the solid–liquid surface tension, and lSVL is the length of the three-phase contact line. The driving force (FS) of the bubble on the surface of the gradient surface energy aluminum sheet is the same as the rate of change of interface energy on the surface of the aluminum sheet (eq 2).

3.4. 2

and according to Young’s equation

3.4. 3

where θ is the three-phase CA. Then

3.4. 4

where θ1 and θ2 are the front and backside CAs of the bubble. Equation 4 is consistent with the previous report.30 On the aerophobic surface, the free energy decreases along the X-axis, so the CA on both sides of the bubble becomes θ1 > θ2, and since FS > 0, the unbalanced surface tension produces the driving force for the bubble to slide toward the direction where the free energy decreases. FH can be expressed as follows35

3.4. 5

where θR and θA are the receding and advancing CAs, respectively. In the case of a horizontally fixed sample, θR ≈ θA, therefore FH is extremely small. When FS > FH, the bubble acceleration in the direction of the surface energy decreases. FD can be demonstrated as follows

3.4. 6

where CD is the drag coefficient of water, ρ is the density of water, ν is the transportation velocity of gas bubbles, and A is the cross-sectional area of gas bubbles. As the bubble velocity increases, so does the resistance to water. At that moment, the force of the bubble reaches equilibrium, and the movement speed no longer increases. When the bubbles reach the surface of the micropore array, under the action of Laplace pressure difference, they will rapidly merge with the gas film captured on the surface, and then spread out to form a larger gas film.23 Continuous delivery of bubbles makes the gas film larger and larger, forming large bubbles that adhere to the lower surface of the micropore array (Figure 6a). Whether a large bubble can break through the water film above the micropores and form a convex shape above the microholes to realize the bubble transfer is dependent on the wettability of the upper and lower surfaces of the micropore array. The surfaces of the superhydrophobic/hydrophilic micropore arrays are easy to transport bubbles,17 but the moving bubbles can easily slip off the surface of the microporous array because of inertia. Therefore, the surfaces of the superhydrophobic/hydrophobic micropore arrays are able to easily capture bubbles and also provide favorable channels for bubble transfer. The wettability on both surfaces (lower and upper) of the micropore array is different, and the corresponding water CA is also different. For a superhydrophobic/hydrophilic micropore array, the upper surface WCA is greater than 150°. The convexity of the water–gas interface faces down (Figure 7b). For a hydrophilic/hydrophilic array, the upper WCA <90°, liquid–gas interface of convex is upward directional (Figure 7c). For a gas bubble in aqueous medium, the Laplace pressure PS acting on the bubble is

3.4. 7

where r is the surface tension of water, R is the radius of curvature of the liquid–gas contact surface, C is the perimeter of the micropore, A is the cross-sectional area of the microhole, and θ is the WCA. From eq 7, on the superhydrophobic surface, θ > 90°, the direction of PS is upward. When the diameter of the microhole is fixed, the larger the WCA, the greater the PS will be, and it is easier for the bubbles to push apart the water film on the surface of the micropores and pass through the microhole under the combined action of buoyancy and Laplace pressure. On the contrary, the hydrophilic surface has WCA θ < 90°, cos θ > 0. Hence, the direction of PS is downward, which is opposite to the buoyancy force, and the resistance prevents the bubble from passing through the micropores. Therefore, it was crucial for the upper surface to exhibit superhydrophobicity to facilitate the gas bubbles passing through the micropores, while hydrophilicity or superhydrophilicity hindered the gas from passing through the micropores. Similarly, the bigger the bubble on the lower surface of the microporous array, the bigger the radius of curvature of the liquid–gas interface, therefore the larger the PS is. This is indicative of larger buoyancy force on the gas bubble, which is more favorable for it to pass through the micropore. Consequently, the superhydrophobic upper surface and hydrophilic lower surface are conducive for the directional transportation of bubbles and the transfer of bubbles from the lower to the upper surface.

4. Conclusions

The surface exhibiting wettability gradient and a superhydrophobic/hydrophilic micropore array (to trap bubbles more easily, the lower surface is hydrophobic), was fabricated by nanosecond laser ablation of aluminum sheets, and the behavior of underwater bubbles on the prepared surface was investigated. The laser-induced gradient surface roughness was hydrophobic after chemical modification. The wettability gradient surface demonstrated the directional transportation of the bubbles along the direction of the decrease in free energy. The superhydrophobicity/hydrophilicity of the upper/lower surface of the laser-ablated micropore array can generate upward Laplace force, which is conducive to the transfer of bubbles from the lower to the upper surface through the micropores. The results and analysis demonstrated that the surface wettability gradient combined with the superhydrophobic/hydrophilic micropore array can realize the directional transportation and collection of underwater bubbles. The preparation method of the device and application to the directional transport and transfer of underwater bubbles have important significance for potential application for the control of underwater bubbles.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b03349.

  • LSCM images of zone 1, zone 2, and zone 3 and surface topography of the micropore array shows that there are abundant micro–nanostructures near the laser ablation grid and micropores (PDF)

  • Water droplets diffuse along a gradient surface of roughness (MP4)

  • Water drops on zone 3, zone 2, and zone 1 (MP4)

  • Process by which bubbles are transported across the sample surface and penetrate the micropores (MP4)

  • Bubbles are prevented from passing through the micropores (MP4)

  • Progress of bubbles collection (MP4)

Author Contributions

Z.Z. wrote the original manuscript, involved in the preparation, and performed the experiments; H.Y. and Z.D. performed the formal analysis and experiments; and Y.C reviewed and edited the manuscript.

The authors gratefully acknowledge the support of FuJian Provincial Department of Science and Technology (no. 2017H0032) and open fund supported by FuJian Laser Precision Machining Engineering Technology Research Center (no. 2017JZA001).

The authors declare no competing financial interest.

Supplementary Material

ao9b03349_si_001.pdf (385.2KB, pdf)
ao9b03349_si_002.mp4 (16.5MB, mp4)
ao9b03349_si_003.mp4 (14.7MB, mp4)
ao9b03349_si_004.mp4 (40.2MB, mp4)
ao9b03349_si_005.mp4 (6.8MB, mp4)
ao9b03349_si_006.mp4 (7MB, mp4)

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Supplementary Materials

ao9b03349_si_001.pdf (385.2KB, pdf)
ao9b03349_si_002.mp4 (16.5MB, mp4)
ao9b03349_si_003.mp4 (14.7MB, mp4)
ao9b03349_si_004.mp4 (40.2MB, mp4)
ao9b03349_si_005.mp4 (6.8MB, mp4)
ao9b03349_si_006.mp4 (7MB, mp4)

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