Bioinspired Pressure-Tolerant Asymmetric Slippery Surface for

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Bioinspired Pressure-Tolerant Asymmetric Slippery Surface for Continuous Self-Transport of Gas Bubbles in Aqueous Environment Chunhui Zhang, Bo Zhang, Hongyu Ma, Zhe Li, Xiao Xiao, Yuheng Zhang, Xinyu Cui, Cunming Yu, Moyuan Cao, and Lei Jiang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00192 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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Bioinspired Pressure-Tolerant Asymmetric Slippery Surface for Continuous Self-Transport of Gas Bubbles in Aqueous Environment Chunhui Zhang,† Bo Zhang,† Hongyu Ma,† Zhe Li,‡ Xiao Xiao,† Yuheng Zhang,† Xinyu Cui,† Cunming Yu,†* Moyuan Cao,‡* Lei Jiang†§ †

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of

Education, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing 100191, China. ‡

School of Chemical Engineering and Technology, Key Laboratory for Green Chemical

Technology of Ministry of Education, Tianjin University, Tianjin 300072, China. §

Laboratory of Bio-Inspired Materials and Interface Sciences, Technical Institute of Physics and

Chemistry, Chinese Academy of Sciences, Beijing 100190, China.

ACS Paragon Plus Environment

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ABSTRACT

Bio-surfaces with geometry-gradient structures or special wettabilities demonstrate intriguing performance in manipulating the behaviors of versatile fluids. By mimicking natural species, i.e., the cactus spine with a shape-gradient morphology and the Picher plant with a lubricated inner surface, we have successfully prepared an asymmetric slippery surface by following the processes of CO2-laser cutting, superhydrophobic modification, and the fluorinert infusion. The asymmetric morphology will cause the deformation of gas bubbles and subsequently engender an asymmetric driven force on them. Due to the infusion of fluorinert, which has a low surface energy (~ 16 mN/m, 25 °C) and an easy fluidic property (~ 0.75 centipoise, 25 °C), the slippery surface demonstrates high adhesive force (~ 300 µN) but low friction force on the gas bubbles. Under the cooperation of the asymmetric morphology and fluorinert infused surface, the fabricated asymmetric slippery surface is applicable to the directional and continuous bubble delivery in an aqueous environment. More importantly, due to the hard-compressed property of fluorinert, the asymmetric slippery surface is facilitated with distinguished bubble transport capability even in a pressurized environment (~ 0.65 MPa), showing its feasibility in practical industrial production. In addition, asymmetric slippery surfaces with a snowflake-like structure and a star-shaped structure were successfully fabricated for the real-world applications, both of which illustrated reliable performances in the continuous generation, directional transportation, and efficient collection of CO2 and H2 micro-bubbles. KEYWORDS: asymmetric slippery surface, gas bubble, pressure-tolerant, continuous and directional transport, water electrolysis


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Generation, delivery, and collection of gas bubbles in aqueous solution are ubiquitous in modern science and technology, such as the electrochemical H2 production, photo-catalysis, CO2 capture/reduction, and so on.1-8 Regulating the bubble’s behavior in a controlled manner is of interest in the optimization of gas-involved processes. Functional interfacial materials with superhydrophobic wettability exhibiting strong air affinity underwater are considered as potential platforms for bubble manipulation.9-18 For instances, the diving bell spider can utilize its superhydrophobic abdomen to tightly adhere gas bubbles to survive in water for several days.10, 11

Various artificial superhydrophobic substrates, such as the superhydrophobic cones and the

superhydrophobic sponge, have been proved as promising materials for the delivery and collection of various types of gas bubbles.14, 17 Based on the strong gas-solid interaction, the textured superhydrophobic surface can spontaneously stabilize a Leidenfrost vapor layer and reduce its heat flux in the boiling water.18 However, with respect to the real-world underwater application, the superhydrophobic substrate is confronted with the drawback of invalidity in a pressurized environment or after a long-term soaking in aqueous medium due to the suppression or deterioration of the air films surrounding the superhydrophobic substrate.19-22 Therefore, another aerophilic material with high durability and pressure tolerance should be developed for application to bubble manipulation. In previous reports, Nepenthes-inspired “slippery” substrates infused by hydrophobic lubricants, have been considered as a pressure-stable pedestal for controlling droplet motion.23-26 Wong et al. proved that a decane droplet on a slippery surface maintained movable behaviors even under a high pressure of 67.6 MPa.23 Recently, our group demonstrated that an aerophilic slippery surface, under the assistant of buoyancy force, was a promising tool for manipulating air bubbles in aqueous environment.26 Originating from the pressure tolerance and bubble


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controllability, the lubricant-infused slippery surface should be an ideal optimization for improving the current bubble-manipulating systems. In addition, to achieve the arbitrary manipulation of gas bubbles underwater, another indispensable procedure is the introduction of directionality into the aerophilic surfaces.27-35 As a marvelous natural design, the cactus spine utilizes its shape-gradient morphology to generate the Laplace pressure to directionally move the harvested fog droplets from tip to root for continuous fog water collection.32 Similar to the cactus spine in governing the behavior of fog droplet, the geometry-gradient structure, like the trapeziashaped surface, should also engender an asymmetric driving force to guide the bubble’s behavior in aqueous environment. Herein, by combining the characteristics of Picher plant and cactus spine, i.e., the slippery surface and the shape-gradient structure, we have prepared a geometrygradient slippery surface (GSS) using the techniques of CO2-laser cutting, superhydrophobic modification, and fluorinert infusion. Employing the asymmetric driving force and low friction force, the GSS can spontaneously and directionally deliver the gas bubbles in aqueous environment. Notably, the transport distance is responsive to the apex angles of GSSs and the volumes of gas bubbles. Minor apex angles and large bubble volume lead to long bubble transport distances. In addition, completely different from the superaerophilic substrates with fragile air films, slippery surface with hard-compressed fluorinert shows a significant stability in bubble delivery under high pressure (~ 0.65 MPa), demonstrating a promising application in bubble’s transport and collection in deep water. Furthermore, based on the distinguished capability of bubble transportation, the integrated CO2 collecting apparatus (GSS with snowflake-like structure) and pressurized-electrochemical-H2-producing device (GSS with starshape structure) are designed to verify the reliability of multi-functions of micro-bubbles continuous generation, directional transportation, and efficient collection. The current finding


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should offer great opportunities to optimize the gas-participating chemical/physical processes, and can stimulate thinking in the promotion of interface science and technology. RESULTS AND DISCUSSION Figure 1A schematically illustrates the design concept of GSS with inspiration from the shapegradient morphology of cactus spine (Opuntia microdasys) and the slippery surface of Nepenthes pitcher plants. In aqueous environment, the geometry-gradient structures can engender the asymmetric Laplace pressure to directionally move the gas bubbles. Meanwhile, the fluorinertinfused slippery surface illustrates high affinity but low friction force to gas bubble. Consequently, the prepared GSS is capable of spontaneously and directionally transporting the gas bubbles in aqueous environment. In detail, the GSS originated from a rough PMMA sheet, can be facilely fabricated through the successive processes of CO2-laser cutting, superhydrophobic modification, and fluorinert infusion.36 Figure S1 is the characterized images of the corresponding surfaces (rough PMMA surface, superhydrophobic PMMA surface, and slippery surface) obtained by the scanning electron microscope (SEM), the confocal laser scanning microscope, and the contact angle measuring device. As shown in Figure S1A, there are numerous micro-sized gullies distributed on the PMMA sheet, endowing the PMMA surface with certain roughness. This rough surface is hydrophobic in air, with a water contact angle (WCA) of 103° ± 2°. In aqueous environment, the rough PMMA surface is aerophobic, on where the bubble contact angle (BCA) is 149° ± 2° and the bubble adhesive force is approximately 100 µN. This phenomenon is attributed to the wetting of the micro-structure of the rough PMMA surface, which impedes the fluorinert infusion. The superhydrophobic surface, which is covered by the air film under water, can be facilitated with the rapid infusion and strong interaction of fluorinert. Figure S1B illustrates the superhydrophobic surface (WCA, 152° ± 3°) was covered


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with lots of hydrophobic silica nano-particles (NPs). Owing to their aggregation, the roughness of superhydrophobic surface is greater than the rough PMMA surface. Underwater, the fabricated superhydrophobic surface is superaerophilic, of which BCA approximately equal to 0° and the bubble adhesive force is greater than 200 µN. After introducing the fluorinert liquid, the roughness of the slippery surface is greatly reduced because of the coverage of the hydrophobic fluorinert liquid on the surface structure (Figure S1C). Therefore, the WCA on the slippery surface decreases to 118° ± 3°, indicating that the slippery surface is hydrophobic in air. When immersed in aqueous environment, the hydrophobic slippery surface turned to be aerophilic, on where the BCA is 63° ± 2° and the bubble adhesive force is up to ~ 300 µN. The large bubble adhesion guarantees the slippery surface with an ability to capture gas bubbles against buoyancy. To verify the ability for directional bubble transport, the GSS with an apex angle of ~ 6° was horizontally placed under water (Figure 1B and Movie S1). Gas bubble with a volume of ~ 20 µL was placed underneath the superaerophobic copper ring, which illustrates low affinity to gas bubble, contributing to the introduction of gas bubble to the GSS.14, 37, 38 From the side view, it could be observed that the gas bubble was immediately captured once contacted with the tip side of GSS. Subsequently, the captured gas bubble was spontaneously and directionally transported towards the root of GSS. In the opposite transporting direction, i.e., from the root side to the tip side, the captured bubble tended to stay at the original position instead of moving along the GSS, which demonstrated such transportation process was unfavorable (Figure S2). To further prove the essential role of the geometry-gradient morphology in bubble transporting process, the bubble’s behavior on the rectangle slippery surface without the shape-gradient morphology was also investigated, and the results showed there was no movement of gas bubbles on such a


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slippery surface (Figure S3). Therefore, the geometry-gradient morphology is of vital importance in realizing the directional transportation of bubbles. The apex angles of GSSs and the volume of gas bubbles are both crucial factors in determining the transport distances and the transport velocities (Figure S4). Figure 1C is the variation of the transport distances of gas bubbles with respect to the apex angles of GSSs. Generally, the minor apex angle is beneficial for long transport distances. For example, the transport distances of gas bubble (~ 20 µL) on the GSS with apex angles of 2°, 4°, 6°, 8°, and 10°, are 141.9 ± 12.9 mm, 96.2 ± 14.8 mm, 50.9 ± 2.5 mm, 34.5 ± 2.6 mm, and 31.5 ± 3.2 mm, respectively. In addition to the minor apex angle, large bubble volume also contributes to the long transport distances. On the GSS with an apex angle of 6°, the transport distances of gas bubbles with volumes of 5 µL, 10 µL, 15 µL, and 20 µL, are 26.6 ± 2.1 mm, 35.4 ± 2.1 mm, 39.0 ± 2.2 mm, and 50.9 ± 2.5 mm, respectively. Therefore, minor apex angle and large bubble volume result in long transport distances. Compared to the unambiguous variation tendency of transport distance, the transport velocities of gas bubbles on the GSS are relatively complicated but obtainable (Figure 1D). Typically, when the transport time is less than 0.1 s, the transport velocities are increased with the enlargement of apex angles. For instance, when the transport time is 0.05 s, the transport velocities of gas bubble (~ 20 µL) on various GSSs with apex angles of 10°, 8°, 6°, 4°, and 2°, are 136.7 ± 9.4 mm/s, 119.4 ± 10.6 mm/s, 115.1 ± 1.5 mm/s, 107.2 ± 8.5 mm/s, and 87.3 ± 8.7 mm/s, respectively. As bubble delivery continued, the transport velocities on large apex angles are dramatically decreased. As shown in Figure 1D, at the transport time of 0.3 s, the transport velocities of gas bubbles on various GSSs with apex angles of 10°, 8°, 6°, 4°, and 2°, are correspondingly reduced to 12.1 ± 1.4 mm/s, 16.6 ± 2.1 mm/s, 21.5 ± 3.3 mm/s, 30.6 ± 3.9 mm/s, and 18.0 ± 1.3 mm/s. When the transport time reaches at 0.6 s, the


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GSS with minor apex angles are endowed with large transport velocities, which are 5.7 ± 1.2 mm/s, 7.6 ± 2.1 mm/s, 11.5 ± 1.8 mm/s, 15.6 ± 0.8 mm/s, and 17.3 ± 1.8 mm/s on the GSS with apex angles of 10°, 8°, 6°, 4°, and 2°, respectively. Following this trend, all of the transport velocities are slowly reduced after one second of transport. Notably, the major proportion of the transport process can be accomplished in two seconds. Therefore, Figure 1D selected transport velocity in two seconds for measurement and analysis. Top-view images in Figure 2A and Movie S2 provided additional important information in understanding the behavior of gas bubble on the GSS. The gas bubble, due to its overlarge volume, was confined as it was primarily introduced on the GSS, which resulted in an asymmetric bubble shape. Subsequently, the gas bubble was elongated, resulting in the curvature radius behind the bubble is smaller than that of the front one. Accompanying with the bubble motion from the tip to the root of the track, the gas bubble was gradually transformed from an asymmetrical ellipse to a spherical crown. Inert images in Figure 2A are the Micro-CT observations of the confined and the unfolded states of gas bubbles placing on the GSS, demonstrating the confinement effect from the margins of GSS (Movie S3 and Movie S4). Based on the above images and observations, we carefully analyzed the forces determining the bubble’s behavior, which were presented in Figure 2B. In the initial transport stage, the gas bubble is pressed by the margins of GSS, resulting in a minor rear curvature radius (R1) and a large front curvature radius (R2). The Laplace pressures at the two respective sides of the gas bubble can be expressed as the following equation: 30-35 ~

, ~ (1)


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where R1 and R2 are the local radii of gas bubbles at the two opposite sides, γwater is the surface tension of water. According to Equation 1, a Laplace Pressure difference (∆ ) can be generated between the two opposite sides: 1 1 ∆ ~ − (2) Driven by the ∆ , the gas bubble exhibited the spontaneous and directional transport. When the bubble was unfolded, R1 is approximately equal to R2, resulting in a negligible ∆ . Subsequently, the gas bubble ceased its movement. Minor apex angle of GSS and large volume of gas bubble will result in a relatively long-lasting confinement of gas bubble, facilitating gas bubble with long transport distances (Figure 1C). The mechanisms of bubble transport on the GSS can also be explained from the viewpoint of energy (Figure S5).39 Before the gas bubble was introduced on the GSS surface, the total initial surface energy of gas bubble and GSS can be expressed as follows: "# = %# & + (# )*+#, + -# )*+#,/& (3) where A is the surface area of the bubble, B is the contact area of the bubble with the fluorinertinfused surface, C is the residue surface area of the GSS, γfluorinert is the surface tension of fluorinert, and γfluorinert/water is the interfacial tension between fluorinert and water. After the gas bubble stops on the GSS surface, the final total surface energy of gas bubble and GSS can be deduced as follows: ") = %) & + () )*+#, + -) )*+#,/& (4) Based on Equation 3 and 4, the difference in surface energy (∆") can be presented as follows:


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∆" = "1 − "2 = 3%) − %# 4 + 3() − (# 4)*+#, + 3-) − -# 4)*+#,/& (5) Based on Equation 5, ∆E ≈ -1.1×10-6 J (Details in Figure S5), which can be transformed into the kinetic energy of gas bubble, realizing the directional transport of gas bubble from the tip to the root. The variation trend of bubble’s transport velocities (~ 20 µL) on various GSSs were also schematically analyzed in our manuscript. For a better comparison of the experimental results, all of the tip sides of GSSs are kept at a width of 2 mm. Consequently, when the gas bubbles were initially introduced on the varied GSSs, their rear curvature radiuses were approximately equal to each other, i.e., 6 ≈ 66 ≈ 666 . Due to the relatively large track width, the GSSs with large apex angles are endowed with large curvature radiuses, i.e., 6 < 66 < 666 . According to Equation 2, a large ∆ will act on the gas bubble on the GSS with a large apex angle, which consequently results in high initial velocities. When transporting in aqueous media, the resistant forces generated by the liquid impeding and lubricant layer must be considered, and the resultant resistance force (FR) can be illustrated as follows:40, 41 9: =

1 - + ?= @ (6) 2 ;

where CD is the drag coefficient of water, ρ is the density of water, v is the transport velocity of gas bubble, S is the cross-sectional area of gas bubbles, a and k are the undefined parameters for the resistance force originating from lubricant layer. According to Equation 6, the increase in bubble velocity will lead to the enhancement of FR. Except for the variation of FR, the bubble motion on the GSS with a large apex angle has a large initial velocity and then decelerated dramatically owing to its shorter acceleration length compared to that on the GSS with a small


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apex angle. In accordance with the relatively large initial FR and quickly decreased ∆ , the velocity of gas bubble on the GSS with large apex angle decelerated more dramatically, which well explained the variation trend in Figure 1D. Based on the distinguished bubble transport capability, the prepared GSS could realize the anti-buoyancy bubble delivery. As shown in Figure S6, the gas bubble (~ 20 µL) could be transported as far as ~ 30 mm, when it was placed at a tilt angle of ~ 2°. We also fabricated the S-shaped GSS and the spiral-shaped GSS (Figure S7 and S8), both of which could achieve the directional transport of gas bubbles without extra energy input. In addition to the directional transport of single bubble, the fabricated GSS can also affect the continuous and directional transport of multi-bubbles as well. As shown in Figure 3A, the gas bubbles were continuously introduced on the tip side of the GSS via an air syringe. The first introduced bubble (1) could spontaneously move towards the root side. With the transport proceeding on, the first bubble gradually slowed down and finally ceased its movement once the confinement was moved. The later introduced gas bubble (3) could firstly caught up with the bubble (2) ahead and merged into a larger bubble (2+3). Then, the merged gas bubble returned to the confined state and continued to transport towards the root side, leading to the subsequent coalescence with the bubble (1). The volume of merged gas bubble (1+2+3) was enlarged, accommodating its state from unfoldment to confinement again (Figures 3A and 3B). Therefore, through the repeated “merging and motion” processes, the introduced gas bubbles were self-driven towards the root side of GSS, which facilitated the GSS with the capability of continuous capture and directional transport of multibubbles. Due to the enlarged volume of merging gas bubbles, the transport length of multibubbles is obviously longer than that of single bubble.


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Owning to the minute buoyancy forces, micro-bubbles will have a long residence time in aqueous environment and lead to serious corrosion of underwater metal facilities.42, 43 Timely removing the micro-bubbles is benefit to prolongate the service life of underwater equipments. CO2 supersaturated solution, as an efficient solution to generate acidic CO2 micro-bubbles, was selected in our experiment. The GSS was horizontally placed in the CO2 supersaturated solution. As shown in Figure S9 and Movie S5, there were numerous tiny CO2 micro-bubbles engendered and captured on the GSS’s margin. With the contiuous capture and coalescence of micro-bubbles, the merging bubbles were directionally transported to the root side of GSS. Based on the distinguished competence of GSS in the in situ collection and transport of CO2 micro-bubbles, a GSS with fractal structure, i.e., the snowflake-like slippery surface, has been fabricated in order to better demonstrate the application in collection of micro-bubbles in aqueous environment (Figure 3C). From the top view, it could be clearly observed that there were numerous tiny CO2 bubbles generated on the tips of snow-like slippery surface and captured by the aerophilic slippery track. With the fractal structure, lots of tiny CO2 bubbles could be successively transported to the center of the snowflake-like slippery surface. The collected CO2 bubble in the center of the GSS gradually grew up, and finally detached due to the overloaded buoyancy. The superaerophilic sponge with 3D interconnected porous structures can selectively absorb gas bubbles under water, which is applicable for the transfer and preservation of CO2 micro-bubbles. Integrating the snow-like slippery surface with the superaerophilic sponge could efficiently absorb enlarged gas bubbles once the bubble is large enough to contact the superaerophilic sponge (Figure 3D). The outstanding capability of the device demonstrates potential applications in removing and collecting micro-bubbles.


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Bubble-participated industrial processes in a pressurized aqueous environment are very crucial in various fields, such as the exploration of natural gas hydrate. The GSS infused with a layer of fluorinert, which is uncompressible and stable in pressurized media, should maintain its bubble transport ability even in pressurized environment. To prove this hypothesis, the tested surfaces and a bubble (~ 10 µL) were placed under a high pressure of ~ 0.65 MPa, which approximately equal to a water depth of ~ 65 meters (Figure S10). Similar to the phenomena at normal pressure, the gas bubble was immediately captured and directionally transported towards the root side of GSS (Figure 4Ai), revealing the reliability of the bubble transport under pressure. The behavior of gas bubble on the superhydrophobic geometry-gradient superface in pressurized environment was also investigated in our experiment. As shown in Figure 4 Aii, once contacted with the superhydrophobic surface, the bubble was immediately squeezed and finally released into the aqueous environment, indicating that the superhydrophobic surface could not transport the gas bubbles under high-pressure system. This phonemenon is resulted from the compress or fragile of air film arround the superhydrophobic surface in a high-pressure environment. Therefore, the durable bubble transport ability of GSS should offer a great opportunity to optimize the current application system. Water electrolysis, as a promising approach to generate pure hydrogen, has attracted wide attentions. However, to our best knowledge, the collection of H2 bubbles generated by water electrolysis in high pressure is difficult and rarely reported. In this manuscript, we have fabricated the star-shaped slippery electrode (SSE), of which margins were covered by the conductive copper wires. The copper electrode was connected with the negative electrode of a battery through the copper wire. After a voltage (~ 9.0 V) was applied, hydrogen microbubbles were massively generated on the copper electrode. When the hydrogen bubbles grew up and


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contacted with the slippery surface, they were captured and transported towards the SSE center. The transported bubbles will be coalesced and finally released under the effect of enlarged buoyancy. Figure 4C is the representative optical images of the SSE to generate, transfer, and collect hydrogen. With the continuous capture and transport, the generated H2 bubble was vertically collected by a tube above the SSE center. Therefore, the fabricated SSE realizes continuous electrolysis and efficient collection of hydrogen, which demonstrates potential functions in high pressure medium. CONCLUSION In summary, we have accomplished the directional and continuous bubble transport in aqueous environment by complying with two biomimetic principles: 1) the cactus-inspired geometrygradient structure generates an asymmetrical driving force to directionally move the gas bubbles, and 2) the Picher plant-inspired slippery surface possessing a fluorinent layer demonstrates a high affinity and low motion resistance to gas bubbles. Due to the capability of directional bubble transport on GSS, a snowflake slippery surface with fractal structure was fabricated, on which the micro-bubbles in a saturated CO2 solution can be directionally transported to the center through the repeated “merging and motion” processes. In addition, originated from the hard-compressed property of fluorinert, the surface exhibits reliable H2 bubble transport and collection abilities in a high-pressurized environment. The current concept of bubble manipulation should find practical applications in both industrial production and daily life, and stimulates the development of manipulating bubbles in more complex situations. EXPERIMENTAL SECTION


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Fabrication of Morphology-Gradient Slippery PMMA Sheets: Firstly, a poly (methyl methacrylate) (PMMA) sheet with thickness of 1 mm was carefully rubbed by 240-mesh sandpaper and cut into the anticipated geometry-gradient morphology under the assistant of the CO2-laser device with a cutting speed of 100 mm/s and the output power of 27 W (LSC 30, HGLASER, China). Then, the superhydrophobic solution consisting of 1.0 g hydrophobic SiO2 nanoparticles (Aerosil R202, average particle size 14 nm, Evonik Degussa Co.) in 100 ml nhexane (Beijing Chemical Works) and 5 ml CH3Cl (Beijing Chemical Works) was painted on the rubbed PMMA surface with a brush and placed at room temperature for several minutes to ensure the complete volatilization of solvent. After that, the fabricated superhydrophobic geometry-gradient PMMA sheet was covered with a thin layer of fluorinert (FC-3283, 3M, America) introduced through the micro-injector and then the geometry-gradient slippery PMMA sheets (GSS) were successfully prepared. The other geometry-gradient slippery PMMA sheets including the S-shaped surface, the spiral-shaped surface, the six-pointed star shape surface, and the fractal surface, could be fabricated with the similar process as well. Methods for Determining The Bubble Transportation Length (L) and The Bubble Transportation Instantaneous Velocity (v): The bubbles transport distance (L) was achieved via the high-speed camera at a rate of 50 fps (i-SPEED 3, Olympus, Japan). The starting point and ending point of gas bubbles on the GSS surface were defined as the moment of gas bubble introduced on the minor side of GSS surface and the moment of gas bubble ceasing its movement on the GSS surface, respectively. The transportation length of gas bubble (L) on the whole GSS surface was the distance between the front edges of gas bubble between the starting point and ending point.


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The bubble transportation instantaneous velocity (v) on the horizontally placed GSS could be obtained by utilizing the high-speed camera as well. Several representative frames were taken from the high-speed video (1500 fps), of which interval time is 0.05 s (75 frames). Thus, the corresponding transportation time of gas bubble on the representative frames can be defined as ti = 0.05 i. The transportation distance (Li) was the transport distance of gas bubble at the moment of ti. Therefore, the instantaneous velocity (v) at the moment of ti can be deduced as follows: ==

B#C − B#D E#C − E#D

Instruments and Characterization: SEM images of the samples were obtained with a desktop scanning electron microscope (Phenom Pro, Phenom World, Netherlands). The behaviors of the gas bubble on the GSS and other shape-gradient slippery PMMA sheet were recorded by using a high-speed camera (i-SPEED 3, Olympus, Japan). The water and bubble contact angles were measured using a video-based contact angle measuring device (OCA 40, Data-physics, Germany). The three-dimensional (3D) images of confined bubble and unconfined bubble are achieved by the Micro-CT (MicroXCT-200, Xradia Inc.

USA). The roughness of various

surfaces, i.e., the rough PMMA surface, the superhydrophobic surface, and the slippery PMMA surface is obtained by Laser confocal microscope (OLS-4500, Olympus, Japan). The interaction force between the gas bubbles and the slippery surface can be accessed by a high-sensitivity micro-electromechanical balance system (DCAT 11, Data-physics, Germany). ASSOCIATED CONTENT Supporting Information Available: The detailed characterized images of the rough PMMA surface, the superhydrophobic PMMA surface, the slippery PMMA surface by the SEM, the confocal laser scanning microscope, and the surface wettability measuring device. The optical


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images of the gas bubble transporting from the root side to the tip side. The optical images of the gas bubble transporting on the rectangle slippery surface. The optical images of the fabricated GSS with apex angles of ~ 2°, ~ 4°, ~ 6°, ~ 8° and ~ 10°. The schematically illustration of the bubble transport process on the GSS from the view-points of energy. The optical images of antibuoyancy bubble delivery on the GSS. The optical images of gas bubble transporting on the SShaped and the spiral GSS. Optical images of CO2 micro-bubbles transporting on the GSS. The pressure device provides the high pressure of ~ 0.65 MPa for our experiment. This material is available free of charge via the Internet at http://pubs.acs.org. Notes: The authors declare no competing financial interest. AUTHOR INFORMATION Corresponding Author Email: [email protected] Email: [email protected] ORCID: Cunming Yu: 0000-0002-6824-9035; Moyuan Cao: 0000-0002-1528-3096; Lei Jiang: 0000-0003-4579-728X. Author Contributions The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript. C. M. Yu, M. Y. Cao, and L. Jiang conceived and designed the experiments. C. H. Zhang, B. Zhang, H. Y. Ma, Z. Li, X. Xiao, Y. H. Zhang, and X. Y. Cui performed the experiments. C. M. Yu and M. Y. Cao wrote the manuscript. ACKNOWLEDGMENT


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Figure 1. Overview of fabricating the GSS and its performance in transporting gas bubbles. (A) The present strategy of utilizing the GSS to directionally transport gas bubbles in aqueous environment takes inspirations from the cactus spine and pitcher surface. The GSS can be facilely fabricated by utilizing the processes of laser cutting, superhydrophobic modification, and fluorinent coating. Of note, the fluorinert liquid completely covers the surface structure. (B) The prepared GSS could be capable of spontaneously and directionally transporting the gas bubbles from its tip side to root side. (C) Variation of the transportation distances of gas bubble (5 µL, 10 µL, 15 µL, 20 µL) on the GSSs with apex angles of 2°, 4°, 6°, 8°, 10°. (D) Variation of transport velocities with respect to the transport time of gas bubble (~ 20 µL) on various GSSs with apex angles of 2°, 4°, 6°, 8°, 10°. Scar bar: 1 cm.


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Figure 2. Optical images and underlying mechanism of gas bubble moving on the GSS. (A) The top-view of directional motion of gas bubble on the GSS. Inert images are the three dimensional images of the confined and the unfolded states of bubble on the GSS. (B) The underlying mechanism of gas bubble transporting on the horizontally placed GSS. Scale bar: 1.0 cm.

Figure 3. Continuous and directional delivery of gas bubbles on the prepared GSS and its application in the collection of tiny CO2 bubbles. (A) Through the merging of the introduced gas


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bubbles, the GSS could be capable of achieving the continuous and directional bubble delivery. (B) The mechanism of the continuous and directional bubble delivery on the GSS. (C) Top view of the continuous and directional delivery of CO2 bubbles on the GSS with fractal structures. The generated CO2 micro-bubbles on the edges could be directional transported to the center via the continuous coalescence of gas bubbles. (D) After integrating the fractal structures with the superaerophilic sponge, the continuous CO2 capture, transfer and collection in saturated CO2 solution can be successfully accomplished. Scale bar: 1.0 cm.

Figure 4. The directional bubble transport on the GSS in pressurized environment and its applications in water electrolysis. (A) In high pressure aqueous environment (~ 0.65 MPa), the GSS could still directionally transport the gas bubbles; however, the geometry-gradient superhydrophobic surface failed in transporting the gas bubbles. (B) Schematic illustration of the star-shaped slippery electrode for continuous H2 generation, directional transportation, and efficient collection in aqueous environment with high pressure. (C) Optical images of the distinguished performance of the star-shaped slippery electrode in continuous H2 bubble generation, directional transport, efficient collection in high pressure environment. Scale bar: 0.5 cm.


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Table of Contents

Inspired by cactus spine and pitcher surface, a geometry-gradient slippery surface (GSS) was fabricated, which could engender asymmetric Laplace pressure to directionally transport gas bubbles underwater. In addition, the GSS realized the continuous collection of micro-bubbles and also demonstrated stable capability of transporting bubbles in high pressure environment, which has been applied in micro-CO2 collection and water electrolysis in high pressure medium.


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Bioinspired Pressure-Tolerant Asymmetric Slippery Surface for

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