Bioinspired Nanostructured Surfaces for On ... - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Bioinspired Nanostructured Surfaces for On-Demand Bubble Transportation Xin Tang,†,‡ Hairui Xiong,§ Tiantian Kong,*,‡,∥ Ye Tian,†,‡ Wen-Di Li,†,‡,⊥ and Liqiu Wang*,†,‡ †

Department of Mechanical Engineering, The University of Hong Kong, Pokfulam, Hong Kong HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), Hangzhou 311300, Zhejiang, China § Department of Radiology, Shenzhen Children’s Hospital, Shenzhen 518026, China ∥ Guangdong Key Laboratory for Biomedical Measurements and Ultrasound Imaging, Department of Biomedical Engineering, Shenzhen University, 3688 Nanhai Avenue, Shenzhen 518060, China ⊥ HKU-Shenzhen Institute of Research and Innovation (HKU-SIRI), Shenzhen 518000, China ‡

S Supporting Information *

ABSTRACT: The maneuver of small bubbles in a programmed way will advance numerous processes, including gas evolution reaction and aeration. Unlike in-air droplets, rapidly rising bubbles in liquid medium can hardly be steered through interaction with solid substrates, causing difficulties in maneuvering bubbles. We pattern and lubricate nanoporous substrates with regions of contrasting wettability that is similar to the back of Namib desert beetles and subsequently immerse the lubricated surface underwater to spontaneously form spatially patterned Nepenthes-inspired slippery surfaces after the dewetting of lubricants. As a result, bubbles are confined on lubricant-infused surfaces, with their high mobility well preserved. The interfacial states of attached bubbles are analyzed, and their dynamic sliding velocities are quantified. Using the lubricated patterned surfaces, we further demonstrate the predefined motion of bubbles driven by buoyancy at a small tiling angle, as well as a self-propulsion of bubbles driven by surface tension force at a tilting angle of 0°, respectively. The spatially lubricated surfaces simplify gas handling in liquid medium and have potential applications in fields where bubble handling is crucial. KEYWORDS: bioinspired, pitcher plants, pinning-free transport, on-demand bubble transportation, lubricated surface

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INTRODUCTION The maneuver of small droplets and bubbles in immiscible fluids impacts a variety of fields, including fluidic handling, therapeutic delivery, and energy conversion.1−3 Because of unique interactions among solid/liquid/gas phases, functional surfaces with special wettability have been developed as a useful platform for manipulating small droplets.4,5 The inspiration from biological organisms in nature is an effective tool to design and fabricate such functional surfaces. For instance, the legendary lotus leaf has inspired the microtextured surfaces that enable water droplets to roll off easily;6 the bumpy back of a desert beetle leads to the design of surfaces with intertwined hydrophilic/hydrophobic patterns for tuning surface adhesion toward water droplets.7,8 The pitcher plant becomes the inspiration for slippery surfaces that repel both high- and lowsurface-tension liquids, such as water and oil, respectively.9 Despite the success in manipulation of in-air droplets, as an important component of multiphase systems, the control over the movement of gas bubble in liquid environment is insufficiently explored. © XXXX American Chemical Society

The control of bubble behaviors is important for the biomedical and industrial processes, where bubbles are used as contrast agents or delivery vehicles.2,10,11 It also impacts chemical reaction and energy conversion systems in which bubbles are generated, collected, and stored.12,13 However, the bubbles are intrinsically difficult to handle because they rise rapidly in liquid medium due to buoyancy.14 Owing to composite solid−air interfaces, the superhydrophobic surfaces with air trapped among microstructures possess underwater affinity to air bubbles.15−17 However, the air trapped in the microtextured surfaces is susceptible to pressure and oil contamination and it merges with bubbles upon contacting, leading to undesired mass transfer.18 Very recently, oil-infused paper-based tracks have shown efficient guided transportation of bubbles underwater.19 However, the underlying physics of interfacial states and sliding dynamics are not sufficiently Received: September 23, 2017 Accepted: December 29, 2017

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DOI: 10.1021/acsami.7b14453 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Spatially lubricated Surfaces. (a) Image of the fog-harvesting Namib desert beetle and schematic illustration of desert-beetle-inspired surfaces with distinct water affinity. Insets in (a) are optical micrographs showing water contact angles of two contrastive regions on fabricated surfaces. (b) Image of the Nepenthes Pitcher plant and schematic illustration of the liquid-infused surface for in-air liquid repellency. (c) Schematic illustration of the patterned surface with locally confined oil. Insets in (c) are optical micrographs showing bubble contact angles of two distinct regions on the patterned surface. (d) Scanning electron microscopy image showing the cross-section of the fabricated porous substrate; (e) threedimensional representation of an atomic-force micrograph of 5 μm2 region of the porous substrate. (f) Time-sequence images showing the formation of patterned Nepenthes-inspired surfaces from oil-coated desert-beetle-inspired surfaces; blue lines represent the receding three-phase contact lines and the yellow lines represent the pinned three-phase contact lines at the peripheries of hydrophobic regions of nanoporous substrates. Images are adapted with permission from Wikimedia Commons (Namib desert beetle photograph by Moongateclimber, pitcher plant photography by Stewart McPherson).

studied. Moreover, the paper-based oil tracks are not durable in liquid medium, especially in extreme environments, which could be important for transferring bubbles for the organic reaction and electrochemistry applications; these paper tracks with rough edges may also have difficulties to be smoothly integrated into sophisticated systems.

In our study, we prepare desert-beetle-inspired porous silica substrates with precisely defined patterns of distinct wettability. As the substrates coated with lubricant-oil film are immersed underwater, lubricant oil in the superhydrophilic regions is spontaneously replaced by water, leading to patterned Nepenthes-inspired slippery surfaces in one step. Such spatially B


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Figure 2. Static bubble states. (a) A chronophotograph showing a 10 μL air bubble sliding on the lubricant-infused surface at a tilting angle of 2°, the time between each frame is 12 s; (b) schematic illustration of static bubbles’ states; (c) scanning electron microscopy image of the cross-sectional poly(dimethylsiloxane) (PDMS) ridge after removing the 1 μL bubble. The purple dashed line represents the contour of the removed bubble. Ridges are further enlarged in two images below; (d) scanning electron microscopy image of dissected PDMS ridge. Green dashed line circles the scratched thin PDMS film region which is formed due to thin oil layer beneath the bubble; (e) image of energy-dispersive X-ray spectroscopy mapping of oxygen in (d), indicating the existence of scratched PDMS thin film; (f) the increase of the lateral sizes and heights of the bubbles with increasing volume. The insets showing the deposited bubbles with minimum and maximum volumes.

conversion, where bubble collection and transportation are crucial.

lubricated surfaces have high-resolution oil patterns as smooth channels to facilitate directional bubble transportation. Similar to the slipping of preys on Nepenthes pitcher plants’ slippery peristomes, bubbles can readily slide on the underwater lubricated surfaces. By analyzing interfacial thermodynamic states, we find that the attached air bubbles are “sandwiched” between the oil/water and oil/solid interfaces; thereby, no contact lines nor adhesion exist. We further quantify the sliding velocities of microliter bubbles on tilted lubricant-infused surfaces through scaling analysis and attributed the sliding resistance to the Landau−Levich−Derjaguin-type viscous drag. Apart from bubble actuation by buoyancy, we also demonstrate and analyze a bubble movement driven by a surface tension force at a tilting angle of 0°. To demonstrate the potential in applications, the continuous and programmed bubble movement are used for gas mixing and for separating and collecting gas bubbles in reactions, respectively. The robust surface with spatially patterned lubricant can serve as a pinning-free platform for maneuver of gas bubbles in aqueous medium without energy input; thus, it bears potential for applications such as open-channel microfluidics, chemical reactions, and energy

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RESULTS Bioinspired Spatially Lubricated Surfaces. The pinningfree and programmed maneuver of bubbles are derived by combining the strategy of Namib desert beetles and Nepenthes pitcher plants. The desert-beetle-inspired surfaces comprise intertwined hydrophobic/hydrophilic regions to spatially confine water on hydrophilic parts (Figure 1a). The Nepenthes-inspired lubricant-infused surfaces immobilize lubricant within nanoporous textures, thereby providing frictionless contact (Figure 1b). Low-surface-tension lubricant can indistinguishably spread over both hydrophilic and hydrophobic regions due to the high surface energy and high oil affinity, respectively. Then, by immersing the lubricated surfaces into aqueous environment, energetically favored Nepenthes-inspired lubricant patterns defined by hydrophobic areas spontaneously formed through dewetting of lubricant on hydrophilic regions (Figure 1c). The hydrophilic regions repel both air bubbles and lubricant in aqueous medium, thus they C


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Figure 3. Dynamic bubble sliding. Schematic illustrations of (a) an air bubble slides on surfaces covered with lubricant underwater and (b) enlarged bubble front in (a); (c) dependence of sliding velocities on sine of tilting angles. Purple, orange, and red represent surfaces covered with 5, 20, and 100 cSt lubricants, respectively. Triangle, square, and circle represent 1.1, 10, and 34 μL bubbles, respectively. Purple and red lines represent predicted velocities of 34 μL bubble on the surface of 5 cSt lubricant and 1.1 μL bubble on the surface of 100 cSt lubricant using eq 4; (d) dependence of sliding velocities on bubbles’ spherical sizes. The color code of the data points is the same as in (c). Inverted triangle, diamond, and hexagon represent tilting angles of 1.4, 5.6, and 22°, respectively. Purple and red lines represent bubbles’ sliding velocities on surface of 5 cSt lubricant with 22° tilting angle and surface of 100 cSt lubricant with 1.4° tilting angle using eq 4; (e) collapse of all data in (c) and (d). Blue line is the linear fitting line with a slope of 1.49; (f) bubbles’ sliding velocities appear independent of initial oil thickness.

flat porous layer, which minimizes the structural hurdle from substrates below for sliding bubbles. Then, the porous substrates are lithographically patterned and treated with octadecyltrichlorosilane (OTS). After removal of photoresist, the nanostructures once protected by the sacrificial photoresist maintain their intrinsic hydrophilicity, leading to a water contact angle of 8°. Contrarily, the regions exposed to the OTS solution are made hydrophobic, giving rise to a water contact angle of 134° (Figure 1a). Subsequently, we infuse such beetleinspired nanoporous substrates with lubricant by spin-coating silicone oil of density ρo, viscosity μo, and surface tension σoa (Table S1). To examine the physical performance of the surface, silicone oils of various viscosities are subsequently used as lubricants. As such oil-infused substrates are submerged in water, oil is automatically replaced by water in the hydrophilic regions within 4 s, leading to spatially immobilized lubricant patterns (Figure 1f, Movie S1). Similar to water droplet

serve as energy barrier to prevent lubricant spreading over the entire surface, leading to the directional transport of bubbles along predefined lubricant trajectories (Figure 1c). According to above-mentioned mechanisms, to fabricate such spatially lubricated surfaces, we first pattern nanostructured surfaces with predesigned contrastive wettability (Figure 1a) and then infuse the patterned surfaces with lubricant (Figure 1b). By changing the environment to be underwater, lubricant patterns can spontaneously form (Figure 1c). To create a robust and durable surface with designed wettability, silicabased nanoporous substrates are readily prepared by spincoating a suspension of silica nanoparticles with a size of roughly 120 nm onto silicon wafers. The solvent is burned out after calcination at 600 °C, leaving a layer of porous sintered nanospheres (Figure 1d). The average roughness Ra of the resultant surface is measured to be roughly 15.24 nm using an atomic-force microscope (Figure 1e), indicating a uniform and D


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σoa)/(σwo + σoa)) (Figure 1c).20 Inside the oil ridge ring, a layer of thin PDMS film is barely visible (Figure 2d). As an indication of the oil layer situated between the bubble and the solid substrate, the existence of such a thin PDMS film is further verified through energy-dispersive X-ray spectroscopy mapping of oxygen (Figure 2e). Bubbles of volumes Ω ranging from 0.5 to 60 μL were deposited on the immobilized lubricant through micropipettes (Figure S2 and Supporting Note 2). When the bubbles’ size R 0 = 3 3Ω/4π is less than the capillary length Lc = σoa /ρo g , which is approximately 1.47 mm in our case,23 the shapes of bubbles are dominated by interfacial tensions. Within such range, the bubble length L and height H linearly increase with the increasing R0 with fitting slopes of 1.46 and 1.20, respectively (Figure 2f). Further increasing the bubble volume, the buoyancy forces start to deform the bubbles, leading to break of the linear relationship. The maximum bubble volume that can be attached to the immobilized lubricant is 60 μL. Bubbles of volume larger than that will break up and detach from the substrates. Dynamic Sliding Velocity. As the surface is tilted at an angle α, driven by buoyancy forces, the attached air bubbles slide up the slope with steady velocities (Figures 3a, S3 and Supporting Note 3). The photogrammetrically measured sliding velocities of air bubbles by high-speed imaging is strongly affected by the oil viscosities. To determine the dependence of the sliding velocities on physical parameters of the system, we use OTS uniformly modified smooth silicon wafers with silicone oil of different viscosities as lubricants (5, 20, and 100 cSt). The thickness of lubricant layer h0 on the surface is controlled to be approximately 10 μm. As substrates are immersed underwater, their oil layer is depleted through spontaneous spreading. Compared with their initial thickness h0, the depleted oil thickness can be neglected (Figure S4 and Supporting Note 4). For a given tilting angle α, coated lubricant oil of viscosity μo inclines to flow upwards at a typical speed of (ρw − ρo)g sin αh02/μo,23,27,28 which is at least 1 order of magnitude lower than the slowest bubble sliding velocity in our experiments, thereby, the effect of such flow can be neglected. To quantitatively determine the dependence of such steady velocities on the oil viscosities, the tilting angles α and bubbles’ sizes R0, we consider a balance between the viscous drag and buoyancy force. The viscous drag here associates with Landau− Levich−Derjaguin flow and is very similar to the cases in slides of pancake bubbles on an overhang plate,29 the pulling of bubbles from a liquid bath,30 dip-coating process, and movement of confined bubbles in tubes.31−33 By analyzing in the reference frame of bubbles, the surface is moving toward the bubble at the sliding velocity V (Figure 3b). The bubble front can be divided into three regions, including the static meniscus, the entrained lubricant film with constant thickness h, and the dynamic meniscus connecting the two (Figure S3b). The static meniscus is dominated by surface tension, hence, without disjoining pressure (ho,static ≫ 100 nm), its mean curvature is simply assumed to be ∇·nstatic ∼ 2/R0, where nstatic is the surface unit normal of the static meniscus; ∇·nstatic is the surface curvature of the static meniscus. The viscous-stressprompted dynamic meniscus is resisted by the surface tension. By matching the pressure at dynamic and static meniscus, we can have the length of the dynamic meniscus l ∼ R 0h/2 . In dynamic meniscus, balancing viscous stress gradient and

adhering on desert-beetle-like surfaces, bubbles can be reliably attached on the lubricant patterns with an apparent contact angle of 68.5°, whereas be repelled on the water-infiltrated area (Figure 1c). Static Interfacial States. At the lubricant layer underwater, the attached discrete bubbles still retain their high mobility over the two-dimensional (2D) plane because they spontaneously move as the surface inclines at an angle as small as 2° (Figure 2a). For a free bubble, no observable sliding angle can be probed, implying no pinning force from underlying substrates. As an air bubble gets in contact with the lubricant layer, the lubricant/silicone oil instantly spreads and cloaks the bubble,20−22 owing to its positive spreading parameter on the water/air interface (Figure 2b).23 The enveloping oil/water interface presses the engulfed bubble against the solid substrate, causing the bubble’s attachment to substrates. Because the underwater receding contact angles of silicone oil on the OTSmodified silicon wafer as well as the OTS-modified silica are both 0° (Table S2), a thin layer of oil is maintained between the bubble and the solid substrate (Figure S1 and Supporting Note 1).22 As such, the thin oil film eliminates the pinning effect of substrates, thereby allowing the bubble to slide at small tilting angles. The contour of oil/air interface is described by augmented Young−Laplace equation as follows24,25 σoa∇·n + Π(e) = Pa − Po

(1)

where σ is interfacial tensions; n is the surface unit normal; ∇·n is the surface curvature; Π(e) is the disjoining pressure; e is the thickness of oil layer; P is the pressure. Subscripts w, o, and a represent water, oil, and air, respectively (unless otherwise specified). Disjoining pressure is determined as follows21,26 Π = AH /6πe 3

(2)

where AH is the Hamaker constant. On the basis of the augmented Young−Laplace equation, we can estimate the thickness of the oil layer separating the bubble and the solid substrate e1 and the one spreading over the top of bubble e2 (Figure 2b). By assuming that the Hamaker constant AH of air/ silicone oil/water interfaces is 10−18 J and the bubble is not compressed (Pa − Po = 2σoa/R0, where R0 is the radius of a spherical bubble of the same volume, such a pressure difference is 35.9 Pa for a 10 μL air bubble),20 for a 10 μL deposited air bubble, e1 and e2 are 121 and 167 nm, respectively. To examine the shape of oil/air interface, we coated a curable lubricant layer (freshly prepared poly(dimethylsiloxane) (PDMS)/crosslinker mixture) on a smooth OTS-modified silicon wafer. Then, the PDMS-coated silicon wafer was put in a vacuum chamber for 30 min to remove excessive gas bubbles. The coated wafer was subsequently immersed underwater, and then a 1 μL air bubble was immediately deposited on it. The PDMS-coated surface was kept still to allow slow cross-linking for 4 days. We use poly(dimethylsiloxane) (PDMS) as a lubricant only to examine the static oil/air interface. After complete curing of the PDMS and removing of the bubble, we obtain a solidified ridge ring that indicates the shapes of oil/water and oil/air interfaces (Figure 2c). As the ring of the oil ridge surrounds the perimeter of the deposited bubble, no three-phase contact line exists.21,22 The apparent contact angle θapp is measured by intersecting the substrate plane with the bubble outline and then fitting with the Young−Laplace equation.21 For a 5 μL bubble, the measured apparent contact angle is 68.5°, in consistence with the theoretical value, 69.2°, which is predicted by arc cos((σwo − E


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Figure 4. Gas-mixing microfluidic device. (a) Schematic illustration and (b) fluorescence image of the gas-mixing microfluidic device. Red dashed lines in (a) illustrate the capture patch; (c) time-sequence images showing microfluidic mixing of two discrete bubbles from two separate inlets.

Figure 5. Self-propulsion of gas bubbles. (a) Image of a cactus; schematic illustrations showing (b) self-propulsion of liquid drops on an asymmetrical cactus spine; (c) 0° tilting-angle propulsion of bubbles on the triangular lubricant pattern; (d) parameters for self-propulsion of bubbles on leveled triangular lubricant pattern; (e) time-sequence images showing the self-propulsion of bubbles; (f) the halt position is inversely proportional to the tip angles of triangles. The blue line is the linear fitting line with a slope of −1.04; (g) time-sequence images of antibuoyancy transport of bubbles. The cactus image is adapted with permission from Wikimedia Commons (Cactus photograph by CactiLegacy).

By substituting h, l, and L (L = 1.46R0), we can have

curvature pressure gradient yields the thickness of entrained oil film h ∼ R0Ca2/3, where Ca is the capillary number μoV/σoa. When bubble slides, the steady velocity is attained through the balance between buoyancy force and viscous force ρo R 03g

sin α ∼ μo VlπL /h

V ∼ 0.17(ρo g sin α)3/2 R 03σoa−1/2μo−1

(4)

Ca ∼ 0.17(sin αBo)3/2

(5)

where Bo is the Bond number ρogR20/σoa.

(3) F


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Figure 6. Gas-collecting electrodes. (a) Image showing the experimental setup for water electrolysis; (b) schematic illustration showing shapes of the vascular lubricant pattern on the copper electrode; micrograph images showing surfaces of gas-collecting electrodes without (c) and with (d) applied voltage. Red dashed lines in (c) outline regions of the vascular lubricant pattern. In (d), copper surfaces with pinned bubbles are shaded in blue and lubricant surfaces with sliding bubbles are shaded in red; (e) enlarged schematic illustration in (b) showing the typical removing pathway of the bubbles on gas-collecting electrodes; (f) time-sequence enlarged micrographs in (d) showing the actual removing pathway on gas-collecting electrodes; (g) current density of water hydrolysis using a vascular-patterned (red) and nonpatterned (blue) copper electrodes, respectively.

Maneuver of Bubbles. To demonstrate the continuous guided bubble transportation on the lubricant patterns, we fabricate a serpentine lubricant pattern to deliver a string of bubbles (Figure S6, Supporting Note 6 and Movie S2). Moreover, we fabricate a Y-shaped lubricant pattern serving as a gas-mixing microfluidic device to coalesce and mix bubbles (Figure 4a). The fabricated continuous fluorescent-dyecontained lubricant is shown in Figure 4b. As shown in Figure 4c, a bubble deposited on one of the two inlets moves upward and is captured by a patch at the junction. Then, another deposited bubble moves along the other inlet line and coalesces with the immobilized one. The merged bubble has higher buoyancy force; thereby, it overcomes Laplace pressure difference and squeeze up along the outlet path. In contrast to conventional microfluidic channels, the gas-mixing microfluidic devices do not rely on any pumps or actuators and provide convenient and controlled bubble handling; thus, such patterned-lubricant-enabled device may have potential applications for gas analysis. Except for buoyancy-actuation, bubbles on lubricant patterns can also be driven by surface tension forces at a tilting angle of 0°. Similar to the strategy of directional transport of captured fog drops on conical cactus spines, we fabricate a skinnytriangle lubricant pattern with different tip angles β (Figure 5a− c).34 Bubbles on the oil triangle can automatically move from the tapered side toward the broad side to adopt minimum surface energy configurations, thereby, achieving 0° tilting-angle

To verify the above predictions, we experimentally measure the sliding velocities in systems with varied inclined angles α, bubbles’ sizes R0, and oil viscosities μo. Representative data for experiments of gradually varying angles and gradually changing bubbles’ sizes are shown in Figure 3c,d, respectively. In both figures, purple and red solid lines are theoretical predictions for data of upper and lower boundaries. The predicted values are roughly on the same order of magnitude as that of the measured ones. Then, all data in Figure 3c,d are nondimensionalized and plotted in Figure 3e. The data collapse onto a single line with a linear fitting slope of 1.49, which is in excellent agreement with the prediction (slope of 1.5). The confirmation of eqs 4 and 5 also indicates that the sliding velocities are independent of initial lubricant thickness h0. For verification, we varied lubricant thickness h0 in a range as large as possible by controlling the spin-coating parameters. As shown in Figure 3f, the measured velocities indeed appear to be independent of the initial oil thickness. To examine the thinnest lubricant layer for bubble sliding, we investigate the performance of the lubricated surfaces with sub-micrometer thickness (Figure S5a and Supporting Note 5). We found that when the initial oil thickness is below a threshold value (2 μm), the bubble sliding velocity gradually decreases with decreasing oil thickness. Above this threshold value, the bubble sliding velocity plateaus when the oil layer becomes thicker (Figure S5a). G


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directionally transported from their back to mouth under the gravitational forces. Such timely bubble removal on the vascular-patterned electrode allows the effective electrode area to remain relatively constant, thus generating a stable current density (Figure 6g). On the contrary, bubbles nucleate and continuously grow on the nonpatterned copper electrode. Only when bubbles are sufficiently large, they detach from the electrode; thereby, the effective electrode area fluctuates, resulting in an oscillating current density. Our lubricant patterns simplify the gas handling process in a hydrogenevolving reaction, where the generated bubbles can be selfremoved in time to increase the electrode/electrolyte contacting area. The oil-covered bubble transportation can also decrease the dissolution of hydrogen in electrolyte and prevent mixing between hydrogen and oxygen.

directional transport (Figure 5d,e). We assume bubbles should move along the triangular lubricant as long as the actuation force exists. Such actuation force preserves when bubbles are deformed by the triangles, we can have the halt position s as follows (Figure S7 and Supporting Note 7)

s /R 0 ≈ 2.92/β

(6)

To verify the analysis, we fabricate triangular lubricant patterns with tip angles β varying from 4 to 12°. Bubbles with volume ranging from 1 to 10 μL are deposited on lubricant triangles’ tapered sides. Their temporal positions on the lubricant triangles are recorded using a high-speed camera (Figure S7 and Supporting Note 7). On lubricant triangles, the longest halt position we achieved is 4.5 cm and the fastest instantaneous velocity is 6.8 cm s−1. By plotting data in logarithmic axes (Figure 5f), the fitting power law exponent is −1.04 which is consistent with the predicted inversely proportional relationship between s/R0 and β. Such pump-free propulsion can even move bubbles in antibuoyancy direction. As shown in Figure 5g, when the tapered side is tilted up for 10°, bubbles on the triangular lubricant pattern move against the upward buoyancy force, owing to dominating surface tension forces. Such surface-force actuation enables the bubble directional transport on a 2D plane without any external energy supply. As bubbles move on the lubricant pattern, the lubricant oils are continuously sheared by bubbles thus being depleted. To study the long-term durability of the lubricated surfaces, we use the surface to continuously deliver bubbles (Figure S8, Supporting Note 8, and Movie S3). Hundreds of bubbles can be delivered before the first breakdown of the lubricant oil layer. After a short restoring time, the depleted lubricant recovers because of oil wicking and wetting through the porous nanostructures. After each cycle of depletion and recovery, the oil layer become thinner and the number of bubbles that can be delivered gradually reduces. Gas-Collecting Electrodes. The bubble maneuver on lubricant patterns can be utilized to remove and collect gas bubbles for electrodes in gas evolution reactions. To demonstrate its potential, we fabricate lubricant-patterned electrodes to collect and directionally transport hydrogen in water electrolysis (Figures 6a, S9 and Supporting Note 9). On the copper electrode, regions of high oil affinity are fabricated by coating 10 μm thick PDMS. Then, 20 μm thick silicone oil is coated on the surface. After immersing underwater, the silicone oil is retained on PDMS. Because both silicone oil and PDMS are dielectrics, the PDMS-covered regions are well insulated and only the oil-free regions are in contact with water (Table S3). Similar to the vascular structure of leaf veins, we fabricate lubricant networks on a cathodic copper sheet composing of a primary pattern and many branched secondary patterns (Figure 6b,c). Similar to fog seeding on hydrophilic bumps of Namib desert beetles, the bubble initially nucleate and grow on the lubricant-free copper surface. As the growing or coalesced hydrogen bubbles come into contact with water/oil interfaces, they can be immediately captured by secondary lubricant patterns (Figure 6d). Driven by buoyancy, these captured bubbles slide on lubricant. They frequently coalesce with each other, owing to oil-ridge-induced lateral capillary attractions (Figure 6e,f). All bubbles on secondary lubricant patterns eventually arrive at the primary pattern and are transported away (Movie S4). The process is analogous to the desert beetles’ fog harvest process, in which captured water drops are

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CONCLUDING REMARKS We design and fabricate Nepenthes-inspired lubricated surfaces on the basis of nanostructured desert-beetle-inspired surfaces with intertwined hydrophilic/hydrophobic regions. The resultant surfaces are robust and smooth and have precisely defined, high-resolution lubricant patterns as microchannels to transfer bubbles in a programmed manner. The interfacial states of the attached bubbles are analyzed, which indicates a pinningfree bubble transportation on such surfaces. We further quantify the steady velocities of sliding bubbles with respect to the bubble sizes, tilting angles, and the oil viscosities through scaling analysis. Apart from the buoyancy-driven bubble movement, we also achieve an energy-free bubble maneuver driven by the surface tension force at a tilting angle of 0° on levelled 2D planes. Our approach has promising potentials for applications including gas analysis, enhancement of boiling heat transfer, and bubble collection.

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METHODS

Fabrication of Surfaces with Patterned Lubricant. Silicon wafers were purchased from Lijingkeji (Zhejiang, China). Polished copper sheets were purchased from Gaosheng Instruments (Dongguan, China). All materials were used as received. The following chemicals were used as received without further purification: tetraethyl orthosilicate (Sigma-Aldrich, ≥99.0%), ammonium hydroxide (Acros, 28−30%), ethanol (Merck, absolute), acetone (Merck, ACS), isopropanol (RCI Labscan, 99.8%), octadecyltrichlorosilane (Acros, 95%), toluene (Sigma-Aldrich, 99.8%), silicone oil (Sigma-Aldrich, 5 cSt), silicone oil (Sigma-Aldrich, 20 cSt), silicone oil (Sigma-Aldrich, 100 cSt), silicone oil (Aladdin, 10 mPa s), SYLGARD 184 silicone elastomer kit (Dow Corning), photoresist (Merck, AZ P4620), photoresist developer (Merck, AZ 351B), Nile red (Acros, 99%), nhexadecane (Acros, 99%), and sulfuric acid (Chengda, 95−98%). To fabricate silica nanoparticles with diameter of 120 nm, 50 mL of tetraethyl orthosilicate-in-ethanol solution (0.44 M) was first prepared; then, another solution containing 2.76 mL of ammonium hydroxide, 28.44 mL of water, and 18.8 mL of ethanol was added into the tetraethyl orthosilicate ethanol solution and mixed at 300 rpm for 6 h at room temperature. After reaction, solvent of 50 mL silica nanoparticle colloidal was allowed to evaporate to reduce the total colloidal volume to be 15 mL. To prepare the spin-coating solution, 7.5 mL of glycerol was then added and mixed with the 15 mL concentrated silica colloid to increase the solution viscosity. Silicon wafers were successively rinsed in water, acetone, and isopropanol and treated with oxygen plasma to enhance their hydrophilicity. For nanoporous substrates, prepared silica nanospheres in water−glycerol mixture were spin-coated on hydrophilic silicon wafers at 1000 rpm for 10 s; then, the coated wafers were baked on a hot plate at 150 °C for 1 min to remove the solvent. The coating and baking processes were repeated five times, and the coated substrates H



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were subsequently sintered at 600 °C for 1 h to complete the process. Copper sheets were cleaned through successive rinse in water, acetone, and isopropanol and subsequently dried through nitrogen flow. For surface functionalization, nanoporous substrates were immersed in an OTS-in-toluene solution (10 mM) for 15 min. Then, treated substrates were subsequently flushed with toluene and baked in a furnace at 100 °C for 30 min. To fabricate substrates with functionalized patterns, substrates covered with an inverted photoresist pattern were fabricated before the surface functionalization process. Positive photoresist, AZ P4620, was spin-coated on substrates at 1500 rpm for 30 s, followed by hot-plate baking at 100 °C for 8 min. Serpentine, Y-shape, triangular, and vascular photolithography masks were fabricated through laser-cutting on 1 mm thick steel plates. After exposing the photoresist-covered substrate in masked UV for 133 s, the patterns were developed in 20 vol % water-diluted developer for 10 min. Then, above-mentioned functionalization processes were conducted and sacrificial photoresists were subsequently removed by rinsing in ethanol. To pattern vascular lubricant patterns on copper electrodes, 66.6 wt % n-hexadecanediluted PDMS was coated on photoresist-patterned substrates through a sharp blade. After curing of PDMS, the sacrificial photoresist was dissolved in ethanol, leaving PDMS networks on copper sheets. On the fabricated beetle-inspired surface of mixing water affinity, a uniform silicone oil layer was infused through spin-coating. To complete the fabrication process, the coated substrates are subsequently immersed underwater to trigger the dewetting of lubricant on hydrophilic regions, thereby giving rise to locally confined lubricant patterns. Instruments and Characterization. The contact angles were measured using a self-built measuring system and were analyzed using the DropSnake plugin in ImageJ (1.46). Advancing contact angles were measured by first dispensing 2 μL of liquid droplets on substrates; then, liquid was added to the droplets at a rate of 0.1 μL s−1 using a precision pump (Longer Pump, LSP01-2A). After the volume of droplets increased to be 10 μL, liquid pumping was stopped and advancing contact angles were measured after 30 s when the droplets were stabilized. Receding contact angles were measured by a reverse process of the measurement of advancing contact angles. In cases of 0° receding contact angles, deposited liquid droplets were withdrawn as much as possible by the pump without observation of any receding of three-phase contact lines. The nanoporous substrates were observed using a field emission scanning electron microscope (Carl Zeiss, LEO 1530). The surface profile and roughness were measured using an atomic force microscope (Bruker). Lubricant oil was deposited using a spin coater (IMECAS, KW-4B). To control the inclined angles of substrates, a tilting stage (SIGMAKOKI, GOH-40A35) was fixed on a leveled optical table (Newport, PG Series). A water tank of the size of 10 × 12 × 5 cm3 was fixed on the tilting stage. The tested surfaces were subsequently immersed in the water tank. The slides of bubbles were recorded using a high-speed camera (Phantom, Miro 110) coupled with a camera lens (Sigma, 30 mm/F1.4/DC/HSM). The bubble volumes were controlled through micropipettes (Dragon lab, 0.5−10 and 10−100 μL). To determine the spin-coated oil thickness, the substrates were weighed before and after oil deposition using an analytical electronic balance (Mettler Toledo, XSE105). Photolithography was performed using the exposure system (IOECAS, URE-2000/35). Water electrolysis was performed in a container of the size of 5 × 5 × 7 cm3. The electrolyte was 0.1 M H2SO4 aqueous solution. Platinum wire was used as anode, and the gas-collecting electrode served as cathode. A voltage of 3V was applied during water separation (GW Instek Power Supply, GPC-3030D). The current density of the water electrolysis was measured by using an electrochemical workstation (CHI Instruments). Data Availability. The authors declare that the data supporting the findings of this study are provided in the article and its Supporting Information.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b14453. Interfacial states of static bubbles; control of the bubble volume; steady sliding velocity; control of lubricant thickness; bubble slides on the surface of the thin lubricant; continuous bubble delivery; halt position of bubbles on triangular patterns; durability test of lubricated surfaces; fabrication of a vascular pattern on the copper electrode (PDF) Spontaneous formation of spatially lubricated surfaces underwater (AVI) Transportation of air bubbles on spatially lubricated surfaces underwater (AVI) Self-recovery of lubricated surfaces underwater (AVI) Collection and transportation of hydrogen bubbles on copper electrodes covered with vascular lubricant patterns (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.K.). *E-mail: [email protected] (L.W.). ORCID

Xin Tang: 0000-0002-7004-8494 Wen-Di Li: 0000-0002-7005-2784 Author Contributions

X.T. and L.W. conceived the project. X.T. and L.W. designed the project. X.T. performed the experiments. X.T., X.H., Y.T., and W.L. analyzed the data. X.T., T.K., and L.W. wrote the manuscript. L.W. supervised the study. All authors commented on the article. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank Dr. Shien-Ping Feng for equipment support. The financial support from the Research Grants Council of Hong Kong (GRF 17237316, 17211115, and 17207914) and the University of Hong Kong (URC 201511159108 and 201411159074) is gratefully acknowledged. This work was also supported in part by the Zhejiang Provincial, Hangzhou Municipal, and Lin’an County Governments.

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REFERENCES

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