Letter pubs.acs.org/Langmuir
Biomimetic Ultra-Bubble-Repellent Surfaces Based on a SelfOrganized Honeycomb Film Jun Kamei,† Yuta Saito,† and Hiroshi Yabu*,‡ †
Graduate School of Engineering and ‡Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan S Supporting Information *
ABSTRACT: The adhesion of bubbles underwater remains the greatest cause of malfunctions in applications such as microfluidics, medical devices and heat exchangers. There is therefore an emerging need for ultra-bubble-repellent surfaces. Inspired by fish scales, which show high bubble repellency due to their hydrophilic nature and surface microstructures, we propose a novel method for preparing ultra-bubble-repellent surfaces by the hydrophilic treatment of self-organized microstructures. When in contact with air bubbles underwater, the artificial hydrophilic microstructured surfaces had a higher contact angle and a lower adhesion force than a flat surface. The mechanism leading to these properties is also investigated. Our method for the fabrication of ultra-bubble-repellent, hydrophilic, microstructured surfaces is simple and cost-effective, opening the way for its application in artificial devices, such as the inner surfaces of tubes, medical devices, and heat exchangers.
1. INTRODUCTION Water-repellent surfaces, often called superhydrophobic surfaces, have been studied extensively since the discovery of the lotus effect, opening the way to self-cleaning materials and antiwetting and antifouling coatings.1−5 In contrast, surfaces that repel air bubbles underwater have received much less attention,6,7 despite the fact that studies of air-bubble behavior underwater have considerable importance. For example, the existence of air bubbles in a heat-transfer system causes a drop in heat-transfer efficiency. In microfluidics, the adhesion of air bubbles to the inner surfaces of microtubes causes a deterioration of liquid flow, and when the microfluid is used in a lab-on-a-chip, this causes considerable damage to the cultured cells.8,9 In electric batteries, the adhesion of gas bubbles on the electrode causes a rise in the energy consumption of the system. The accumulation of air bubbles in medical tubing and medical devices used to send liquid directly into blood vessels must be avoided for health reasons. In macroscale systems, such bubbles are most often captured and removed from the system using the buoyancy force. However, for microfluidic systems and medical devices with millimeter-wide tubes, the surface tension becomes dominant, causing air bubbles to stick easily to surfaces and become difficult to remove. A similar difficultly is encountered when using heat exchangers and electric batteries in space, where the microgravity environment causes a drop in the buoyancy force. Several strategies have been developed to solve the problem of permanent adhesion of air bubbles on such surfaces. The first strategy, preventing the formation of bubbles inside the tubes or on the surfaces by controlling cavitation, electrolysis, or © XXXX American Chemical Society
boiling of the liquid, remains a challenge. The second strategy is to create a bubble-repellent surface, which is a surface on which bubbles adhere but are easily removed, by controlling the surface tension. However, to our knowledge studies of such surfaces have not been conducted. Finally, current technology is based on the removal of bubbles from the surface using an external force. Although techniques such as mechanical vibration and ultrasonic waves have been widely used, it remains a challenge in terms of the large amount of energy consumed during the process. It has recently been reported that fish can maintain a clean body surface, free from hydrophobic air bubbles and oil droplets.10−12 Fish scales are composed of calcium phosphate and protein. When they come in contact with water, water molecules can be trapped because of the hydrophilic nature of the scales, and an air/water/solid interface is formed when a bubble adheres to the surface. The introduced layer of water results in a decrease in the contact area between the bubble and the surface of the scale and therefore a decrease in the adhesion force between them, giving the scale its bubble-repellent properties. This leads us to the concept of biomimetic bubblerepellent surfaces that use microstructure and surface chemistry to control the interaction between solid surfaces and bubbles. However, for a further application of air-bubble-repellent surfaces on tubes, microfluidics, medical devices, and so on, the fabrication of microstructure has been a major issue. Received: September 4, 2014 Revised: November 16, 2014
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Figure 1. Schematic description of the utilized polystyrene and synthesized polymer 1 (a) and the experimental process, including the fabrication of honeycomb and pincushion films, hydrophilic treatment, and measurement of contact angles by the captive bubble method for underwater conditions (b).
Figure 2. Honeycomb film was obtained by depositing a chloroform solution of PS and polymer 1 (weight ratio PS/polymer 1 = 10:1) on a glass substrate under humid conditions and at ambient temperature. Images (a) and (b) show, respectively, the surface and cross-sectional scanning electron microscopy (SEM) images of honeycomb film after hydrophilic treatment. Images (c) and (d) show, respectively, the surface SEM image and cross-sectional image of the pincushion film obtained by peeling the top layer off a honeycomb film.
Conventional methods for microstructure fabrication are irrelevant for fabrication on intricate and nonflat surfaces such as the inner surfaces of tubes and also have the disadvantages of being energy-consuming and expensive. The breath figure method has been an area of extensive research because it enables the fabrication of highly ordered honeycomb structures by self-organization on the submicroscale and microscale.13−17 We have previously reported the fabrication of a self-organized, honeycomb-patterned, porous film using the breath figure technique to deposit a hydrophobic polymer containing an amphiphilic polymer solution under humid conditions.18,19 It was found that water droplets start condensing on the solution surface because of evaporation of the volatile polymer solution. The water droplets eventually grow in size while packing closely together into hexagonal arrays as a result of lateral capillary force and convectional flow.
A honeycomb-like porous structure is obtained after complete evaporation of the solvent and subsequent evaporation of template water droplets. Pincushion-like structures can also be obtained by simply peeling off the top layer of the honeycomb film using adhesive tape.20 This method has several advantages compared to traditional microfabrication methods using topdown processes: easy fabrication of large areas,21 high controllability of pore size from the submicroscale to microscale, and the possibility of fabrication on nonflat surfaces,22 which opens the way to the fabrication of highly ordered microstructures on the inner surfaces of devices. In addition, the surface can be easily treated to provide a hydrophilic effect.23 In this study, we report the simple fabrication of ultrabubble-repellent surfaces using a simple process based on selforganization. Microstructured surfaces were obtained by the B
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Figure 3. Static contact angle of an air bubble on underwater flat, honeycomb, and pincushion films. The plotted line shows the values calculated from the Cassie−Baxter equation (a). Dynamic contact angle of air bubbles on each of the surfaces. Contact angle hysteresis is shown by orange points, and the sliding angles of the air bubbles are shown by blue points (b). Schematic of air bubbles on flat, honeycomb, and pincushion films (c).
breath figure method and hydrophilized to give bubblerepellent properties.
2.3. Measurement of Air Bubble Contact Angle Underwater. Static and dynamic contact angles were recorded with a drop shape analysis system equipped with a video camera. Hydrophilized flat, HC, and PC films were placed on an inclinable arm with the surfaces facing downward and dipped into a square transparent glass vessel filled with deionized water (movie in Supporting Information). An air bubble (3 μL) was released from beneath the substrate using a microsyringe. The static contact angles were measured at a neutral tilting angle (0°). The inclinable arm was then tilted until the air bubble began to slide up the sample surface. Subsequently, the advancing (θA), receding (θR), and sliding (θS) angles were determined.
2. EXPERIMENTAL SECTION 2.1. Preparation of the Honeycomb-Patterned and Pincushion Films. Polystyrene (PS, Mw = 80 000 g mol−1, Figure 1a) and chloroform were purchased from Sigma-Aldrich (St. Louis, MO) and WAKO Chemicals Inc. (Osaka, Japan), respectively. An amphiphilic copolymer (polymer 1, Mw = 40 000 g mol−1, Figure 1a) was synthesized.15,16 Honeycomb-patterned (HC) films were prepared by depositing a 5 g L−1 chloroform solution of PS and polymer 1 (weight ratio PS/polymer 1 = 10:1) on a glass substrate of size 10 × 30 cm2 under humid conditions (relative humidity ca. 90% and velocity 130 L min−1) and ambient temperature.15,16 Pincushion-patterned (PC) films were prepared by peeling off the top layer of the honeycomb film using a sheet of adhesive tape.15 The glass substrates with the microstructured polymer films were then cut into 1 × 1 cm2 size pieces. As a control experiment, a flat film was prepared by spincoating a 20 g L−1 PS toluene solution on a glass substrate at 1000 rpm. 2.2. Hydrophilic Treatment of the Honeycomb-Patterned and Pincushion Films. An ultraviolet and ozone exposure method (OC-250615-D+A; IWASAKI DENKI, Co. Ltd., Tokyo, Japan) was used for the hydrophilic treatment of the films. As-prepared flat, HC, and PC films were placed in the exposure chamber and treated with UV−O3 for 3 min at ambient temperature and pressure. The contact angle of water droplets on the flat, nonhydrophilized PS was 91 ± 2°, but this decreased to 55 ± 3° after hydrophilic treatment, which is sufficient to provide a wetting state.24 No drop in the hydrophilicity of the flat hydrophilized PS was seen 6 month after the initial measurement (contact angle 56 ± 1°). Both the treated HC and PC films were examined using a field-emission scanning electron microscope (S-5200; Hitachi, Tokyo, Japan).
3. RESULTS AND DISCUSSION 3.1. Scanning Electron Microscope Images of Honeycomb and Pincushion Films. The obtained HC and PC structures were examined with a scanning electron microscope (SEM). An SEM image (Figure 2a) clearly shows the hexagonally arranged micropores of a highly ordered honeycomb-patterned film with 8 μm pores. From the cross-sectional SEM image in Figure 2b, we see that the film consists of two layers separated by pillars at each vertex of the hexagons. By peeling off the top layer using adhesive tape (Figure 2c), each pillar was broken at its center and a highly ordered pincushion-like structure with a pillar height of 2 to 3 μm was obtained. The minimum distance between two adjacent pillars was 7 to 8 μm (Figure 2d). Thus, each of the obtained films had a size on the order of several micrometers, which matches the size of the microstructures in fish scales. 3.2. Static Contact Angles of Air Bubbles on Underwater Flat, Honeycomb, and Pincushion Films. To evaluate the bubble repellency of the hydrophilized microstructured films, contact angles of a bubble with a volume of 3 C
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recorded, and the contact angle hysteresis (Δθ) is defined as θA − θB. Figure 3b shows the sliding angle and the contact angle hysteresis for flat, HC, and PC films. The sliding angle decreased with an increase in the fraction of water at the interface and was the lowest with the PC film, a result that matches the contact angle measurement in the static environment. However, the contact angle hysteresis was the highest for the HC film, suggesting that the pinning effect is higher than on the PC film. This phenomenon can be explained by the difference in the site on the honeycomb structure on which the air bubble is pinned. Normally, for bubbles on a flat surface, the advancing and receding sides of the bubble are both in contact with PS, resulting in a PS/bubble interface as shown in the schematic in Supporting Information Figure 2. In the case of the PC film, the advancing and receding sides of the bubble are both in contact with water, resulting in a water/bubble interface. We suggest that for the HC film the pinning site on the advancing side of the air bubble is situated on the outermost edge of the honeycomb structure and is surrounded by a water-filled area of the honeycomb. However, the receding part of the air bubble is pinned on the outermost edge of the honeycomb rim structure, causing the air bubble to be mostly in contact with the hydrophilized PS surface. Because the contact angle of the bubble increases with an increase in the fraction of water at the interface, the advancing side of the bubble has a high contact angle and the receding side has a low contact angle. Therefore, the difference between the advancing and receding contact angles is greater than for the other two surfaces, explaining the high contact angle hysteresis of the air bubble on the honeycomb structure. 3.4. Conclusions. We have demonstrated that an underwater ultra-bubble-repellent surface can be easily fabricated using the breath figure method followed by hydrophilic treatment of the obtained film. Upon submerging the microstructured hydrophilic film, water filled the pores of the fabricated surface. The contact angle of an air bubble on the microstructured hydrophilic surface increased with an increase in the fraction of water at the interface and was greatest on a hydrophilized PC film. A low adhesion force of the air bubble was also observed for the PC film because of the low contact area between the bubble and the hydrophilized surface. The simplicity of this method allows the fabrication of highly ordered microstructures on large areas18 and on curved surfaces.19 Therefore, this technology has broad application as a method for producing a bubble-repellent coating for the inner surfaces of tubes, medical devices, heat exchangers, and so on. The microstructured surfaces can also be patterned24 to achieve functional surfaces, such as bubble traps, using differences in bubble adhesion. Furthermore, the fabricated microstructure can also be metalized by electroless plating25 or used as a mask for the etching25 of metal substrates to provide the robustness needed for further application.
μL were measured on each of the different substrates. The contact angles on hydrophilized PS flat, HC, and PC films were 128.0, 144.8, and 163.2°, respectively (Figure 3a). Because each of these surfaces has the same surface chemistry, the differences between the contact angles are generated by the surface structure. The wettability of solid substrates with surface patterns has been widely researched and can be modeled using two different models: the Wenzel model2 and the Cassie model.3 The Wenzel model takes into account the increase in the surface area induced by the surface roughness, which geometrically enhances the wettability. The Cassie model suggests that the liquid is in contact with a surface composed of two different components with different wettabilities. In this model, the superficial contact angle of a surface composed of two components is given by a formula based on the fraction of each component3 cos φ = f1 cos θ1 + f2 cos θ2
(1)
where f1 and f 2 are the fractions of components 1 and 2 and θ1 and θ2 are the contact angles of a liquid on a flat surface composed of component 1 or 2. From our results, the contact angles of the bubble for HC and PC films were higher than those of the flat film, suggesting that the contact angle of bubbles on HC and PC films increased because the bubble is in contact with a composite surface of water and hydrophilized PS. Although Cassie’s equation was originally applied to a liquid droplet on a composite surface in air, it can also be applied to air bubbles on a solid surface in the presence of a liquid. In this case, we assume that an air bubble underwater is in contact with a composite surface of water and hydrophilized PS. The schematic in Figure 3c represents the contact area between a bubble and hydrophilized PS on flat, HC, and PC films. In this case, the Cassie equation can be expressed as cos φ = fPS cos θPS + fwater cos θwater
(2)
where f PS is the fraction of PS at the interface, θPS is the contact angle of an air bubble on a flat surface, f water is the fraction of PS at the interface, and θwater is the contact angle of an air bubble on a layer of water. Equation 2 can be simplified by noting that the sum of f PS and f water is 1. cos φ = (1 − fwater )cos θPS + fwater cos θwater
(3)
The contact angles calculated from eq 3 with various values of f water calculated from surface scanning electron microscopy are plotted in Figure 3a. The calculated values from the Cassie− Baxter equation matched the measured contact angles, which increase with decreasing PS fraction in the composite surface consisting of water and PS. This result suggests that water is held inside the microstructures and that by introducing a pillared structure the contact angle of the air bubble on the surface can be increased, thereby decreasing the contact area between the bubble and the surface. 3.3. Dynamic Contact Angles of Air Bubbles on Underwater Flat, Honeycomb, and Pincushion Films. When tilting the substrate with a bubble on its surface, it reaches an angle at which the adhesion force between the bubble and the surface equals the buoyancy. Above this angle, the air bubble slides up the surface of the film. We call this the sliding angle (θS) and use it to evaluate the adhesion force between the substrate and the air bubble. The advancing (θA) and receding (θB) angles of the sliding bubbles were also
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ASSOCIATED CONTENT
S Supporting Information *
Ultra-bubble-repellent surface demonstration. Experimental setup. Schematic image of air bubble on tilted flat, honeycomb, and pincushion films. Supporting Information is available. This material is available free of charge via the Internet at http:// pubs.acs.org. D
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Vulcanization of Honeycomb Structured Polymeric Films. Soft Matter 2011, 7, 546−552. (23) Yabu, H.; Inoue, K.; Shimomura, M. Multiple-Periodic Structures of Self-Organized Honeycomb-Patterned Films and Polymer Nanoparticles Hybrids. Colloids Surf., A 2006, 284, 301−304. (24) Nakamichi, Y.; Hirai, Y.; Yabu, H.; Shimomura, M. Fabrication of Patterned and Anisotropic Porous Films Based on Photo-CrossLinking of Poly(1,2-Butadiene) Honeycomb Films. J. Mater. Chem. 2011, 21, 3884−3889. (25) Nakanishi, T.; Hirai, Y.; Kojima, M.; Yabu, H.; Shimomura, M. Patterned Metallic Honeycomb Films Prepared by Photo- Patterning and Electroless Plating. J. Mater. Chem. 2010, 20, 6741−6745.
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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REFERENCES
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