Biomimetic Bubble-Repellent Tubes: Microdimple Arrays Enhance

Dec 28, 2016 - WPI-Advanced Institute for Materials Research (AIMR), Tohoku University, 2-1-1, Katahira, Aoba-Ku, Sendai 980-8577, Japan. Langmuir , 2...
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Biomimetic Bubble-Repellent Micro Dimple Arrays Prepared by Ultrasonic Peeling of Self-Organized Honeycomb Films on Interior Surfaces of Tubes Jun Kamei, Hiroya Abe, and Hiroshi Yabu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04155 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on January 1, 2017

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Biomimetic Bubble Repellent Tubes; Micro Dimple Arrays Enhance Repellency of Bubbles Inside of Tubes Jun Kamei†, Hiroya Abe‡ and Hiroshi Yabu§* †

Innovation Design Engineering, Royal College of Art, London, UK



Graduate School of Environmental Science, Tohoku University, 468-1, Aramaki, Aza-Aoba,

Aoba-Ku, Sendai 980-0845, Japan §

WPI-Advanced Institute for Materials Research (AIMR), Tohoku University, 2-1-1, Katahira,

Aoba-Ku, Sendai 980-8577, Japan

KEYWORDS Self-organization, Bubble repellent, Superhydrophilic, Honeycomb Films, Tubes

The adhesion of bubbles underwater remains the greatest cause of malfunctions in applications such as microfluidics, medical devices and heat exchangers. Recently, combination of oxidization and peeling the top layer of self-organized honeycomb film with an adhesive tape

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resulted in formation of an ultra-bubble-repellent and pillared polymer surface structure. However, fabrication of honeycomb films on inner surface of tubes and formation of structured hydrophilic textures by peeling the top layer of honeycomb film are still remaining problems. In this report, the simple fabrication technique of honeycomb patterned polymer films interior of tubes by dip coating of polymer solution and blowing humid air in a tube. Furthermore, an ultrabubble-repellent dimple arrayed structure was fabricated by applying ultrasonication to the honeycomb structure formed on the interior surface of tubes.

1. Introduction The adhesion of bubbles underwater remains the greatest cause of malfunctions in applications such as microfluidics1, medical devices2 and heat exchangers3. Since microfluidic devices usually have narrower channels than capillary length of transported liquids, formation and adhesion of bubbles into the narrow channels causes inefficient transportation of liquids4. Contamination of bubbles into a medical tubing also induces fatal symptoms, thus, bubbles in a medical tube should be immediately removed. Furthermore, bubble formation and adhesion in piping of heat exchanger in aerospace crafts or a space station reduce the efficiency of heat exchange since formed bubbles can hardly be removed from the surface due to lack of buoyancy under microgravity environment5. There is therefore an emerging need for ultra-bubble-repellent surfaces. In efforts to create repellent surfaces, superhydrophobic and superhydrophilic surfaces inspired by nature have been an extensive ground for research6-10. Previous pursuit for repellency focused a lot on water and liquid repellency of superhydrophobic surfaces, which have been highly

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investigated by mimicking the surface chemistry and topography of lotus leaves. On the other hand, fish scales were reported to exhibit another type of repellency: when placed underwater, such surfaces showed high oil and air bubble repellency due to their hydrophilic nature and surface microstructures11-13. Previous research proved the efficiency of artificial hydrophilic micro-scale surfaces inspired from fish scale in creating a bubble repellent surface, exhibiting a higher contact angle and a lower adhesion force than a flat surface, thus opening a new research field on bubble repellency using superhydrophilic surfaces. Self-assembly and self-organization are attractive concepts for creating both nanoscale and mironscale features without highly sophisticated top-down lithography techniques14-16. Honeycomb patterned films can be prepared by casting a hydrophobic polymer solution under highly humid condition with templating condensed water droplets17-24. We have reported that combination of oxidization and peeling the top layer of honeycomb film with an adhesive tape resulted in formation of a hydrophilic and pillared polymer surface structure25,26. The previous study reveals the pillared surface prepared on a solid flat substrate to show high bubble repellency in water. Since water filled interspace among hydrophilic pillars and projected adhesive surface area was limited only at the top surface of the pillars, bubble adhesion was eventually prevented. The water thin layer formed between bubbles and real surface of the pillared surface also works as a lubrication layer for bubble locomotion and detouchment with water flow. Such a surface coating is useful for bubble free medical tubes, transport pipes, and so on, however, two problems remained to apply hydrophilic and pillared surface to these practical applications; one is how to fabricate honeycomb patterned films, which is a precursor of the pillared structure, and the other is how to convert the honeycomb structure to the pillared

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structure in an interior surface of tubes or pipes, whose size is usually less than the size to be accessed with adhesive tapes. In this study, we report the simple fabrication technique of honeycomb patterned polymer films interior of tubes by dip coating of polymer solution and blowing humid air in a tube. Furthermore, a ultra-bubble-repellent dimple arrayed structure was fabricated by applying ultrasonication to the honeycomb structure formed on the interior surface of tubes.

2. Experimental Section 2.1. Preparation of PS honeycomb and dimple arrayed films on flat substrates Polystyrene (PS; Mw =80 kg mol-1) and chloroform were purchased from Sigma Aldrich (St. Louis, MO) and WAKO Chemicals Inc. (Osaka, Japan), respectively. An amphiphilic copolymer (Polymer 1; Mw = 40 kg mol-1) was synthesized according to literatures (see supporting information, S1)7,27. 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 c.a. 90% and velocity 130 L min-1) and ambient temperature. An ultraviolet and ozone exposure method (OC-250615-D+A; IWASAKI DENKI, Co. Ltd., Tokyo, Japan) was used for hydrophilic treatment of the films. HC films were placed in the exposure chamber, and treated with UV-O3 for 3 min at ambient temperature and pressure.Ultrasonication was applied to the UV-O3 treated HC film immersed in water for 10 min and then, the dimple arrayed (DA) film was dried at room temperature. Typically, a 20 mm

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square sized sample of UV-O3 treated honeycomb film was immersed in a 50 mL beaker filled with water, and then ultrasonication was applied in a commercially available ultorasonic cleaner (Blanson 3510). Surface structures of films were observed by using a field-emission scanning electron microscope (S-5200; Hitachi, Tokyo, Japan). Static and dynamic contact angles were recorded with a drop shape analysis system equipped with a video camera. Hydrophilized DA films were placed on an inclinable arm with the surfaces facing downward and dipped in a square transparent glass vessel filled with deionized water. 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, which are contact angle of the most front and the most back of a bubble just removed from the substrate, and tilting angle of substrate when the bubble removed from the surface, respectively, were determined. Surface fraction of polymer was measured from the SEM image of the surface with using an imaging software (Image J, NIH).

2.2. Preparation of PS honeycomb and dimple arrayed films inside of tubes Chloroform solution of PS and Polymer 1 (10 g L-1, weight ratio PS : Polymer 1 = 10 : 1) was filled in a glass capillary tube whose interior diameter was ranging from 0.7 mm to 1.0 mm. Air flow (2 L min-1, r.h. 30 %) was blown into the glass capillary tube to form HC structures on the inner surface of the glass capillary tube. After UV-O3 treatment for 10 min, ultrasonication was applied to the tube immersed in water. The same treatment was performed for silicone tube, with an interior diameter of 3 mm.

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A bubble repellent property of DA structured tubes was examined by using handmaid air bubble supplier (see supporting information, S2). Water was filed in a bottle vial and then air flow was applied though a porous cartridge to make small bubbles. The water flow with air bubbles was injected into the non-treated or DA structured silicone tubes. The bubble movement was observed by video camera.

2.3. PB honeycomb films and photo-crosslinking 1,2-poly(butadiene) (PB, RB820) was kindly provided from JSR Co. Ltd., Japan. A HC film of PB was prepared from 5 g L-1 chloroform solution of PB and Polymer 1 (weight ratio PS : Polymer 1 = 10 : 1) under the same condition of the PS HC film preparation. After placing a photomask on the surface of PB HC film, UV-O3 treatment was performed for 30 min and ultrasonication process as same as the PS HC film case was also performed. The surface structure of the film was observed by optical microscope (BX51, Olympus).

3. Results and Discussion A honeycomb-patterned polymer film was obtained according to the literature. An SEM image shows a hexagonally arranged porous structure (Figure 1 (b)). As shown in the scheme (Figure 1 (a)), the honeycomb-patterned polymer film was treated with UV-O3 treatment and then, the hydrophilic honeycomb-patterned films were immersed in water. Ultrasonication was applied to the film for 10 min, the top layer of the honeycomb-patterned film was completely removed and dimple arrayed structure was successfully formed. Figure 1 (c) shows a SEM image of dimple

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arrayed structure. Hexagonally arranged dimples were formed and the surface show high bubble contact angle (164˚). Based on the Cassie-Baxter equation, the underwater bubble contact angle on a solid substrate can be calculated by following equation;

cosθbubble=fpolymer(cosθpolymer+1)-1

(1)

, where θbubble, fpolymer and θpolymer are actual contact angle of a bubble, surface fraction of polymer and contact angle of a bubble on a flat polymer surface, respectively. Since fpolymer and θpolymer were measured as 0.017 and 50˚, respectively, θbubble was calculated as 166˚. This value was quite match with the experimental value. Furthermore, advancing (θA) and receding (θR) contact angles were 163˚~170˚ and 152˚~160˚, respectively. These values were high enough and the contact angle hysteresis was also less than 30˚, which value is low enough to repel air bubbles. The sliding angle of bubble (θS) removal from the surface was only 3˚. These results indicate that the dimple arrayed surface has high bubble repellent property. The adhesion force of a bubble in this experiment can be calculated from the volume of a bubble and adhesion area of a bubble on the surface. Since the removal force of a bubble was caused by buoyancy of a bubble and the effect of gravity can be negligible in the case of bubble, the buoyancy of a bubble should be calculated. In this case, c.a. 30 µL of air was introduced into the water to form an air bubble, the buoyancy of bubble can be calculated as 30 µN from the Archimedes’s law. The surface area of adhered bubble was not so much changed when the substrate was tilted since the hysteresis of the contact angle was negligible, the adhesion area of the bubble was calculated as 0.20 mm2 from the contact angle measurement result. From these parameters, the adhesion force

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of the bubble was c.a. 150 µN/m2. These results indicate that the dimple arrayed surface has high bubble repellent property. To fabricate the dimple arrayed structure onto the interior surface of tubes, dip coating technique followed by blowing humid air into the tube was examined. Figure 2 (a) shows a schematic illustration of coating honeycomb-patterned film inside of a glass capillary. The commercially available glass capillary (minimum diameter of capillary less than 1.0 mm) was dipped in a polymer solution containing PS and amphiphilic copolymer 1. After dipping and drawing the glass capillary, humid air was blown into the glass capillary. Interior surface of glass capillary was observed by SEM (Figure 2 (b) and 2 (c)). Figure 2 (b) shows a cross-sectional SEM image of a cut glass capillary. When the interior surface was close-upped, honeycombpatterned film was successfully prepared. Due to the viscosity and capillary force of polymer solution, the thin liquid layer of solution was kept on the interior or exterior surface of glass capillary, and breath figures were formed only on the interior surface of the glass capillary due to the rapid evaporation and water droplets condensation induced by applying humid air blow. The concentration of polymer solution, mixing ratio between polymer and amphiphilic copolymer 1 and flow rate of the humid air were optimized by changing these parameters (see supporting information). Dimple arrayed surface was successfully prepared onto interior surface of the glass capillary by UV-O3 treatment and ultrasonication in water. As shown in a SEM image of the interior surface of glass capillary (Figure 2 (d), dimple arrayed structure was successfully fabricated. This process can be widely applied to fabricate honeycomb and dimple arrayed surfaces onto inner surface of various tubes and pipes including silicone, polypropylene, and so on.

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To examine bubble repellent properties of DA tubes, flat and DA surface were fabricated into silicone tubes and then water with bubbles was injected into the coated and flat silicone tubes. Silicone is known as one of the hydrophobic materials which repels water but adheres oils and bubbles. Since bubbles are frequently trapped in the hydrophobic silicon tube due to its high hydrophobicity, bubble removal from such surface is demanded. Figure 3 (a) and 3 (b) shows time laps photographs of flat and coated tubes with dimple arrays during water and bubble injection. In the flat tube, bubbles are stuck randomly on the interior surface of tubes. Only large size bubbles (white arrow) moves due to water flow. On the other hand, bubbles in the coated tube were located on the upper side of the tube, which implies that an adhesion force of bubble onto the interior surface of tube was weaker than the buoyancy of bubble at the bottom side, and then bubbles removed from the bottom of the tube, and accumulated at the top side of the tube. Furthermore, bubbles were removed from the interior surface of tube with water flow and only a small bubble (red arrow) was stuck on the surface. At the top of tube, the bubbles strongly pressed onto the surface due to the adhesion force between bubble and the inner surface of tube and the buoyancy of bubble itself. In such a case, bubble locomotion induced only by water flow. Since the water pressure was equally applied on the cross section of air bubble, larger cross section, i.e. larger size bubble can easily removed from the surface. This is the reason why the small bubble has still stuck on the DA surface. Furthermore, bubbles were removed from the interior surface of tube with water flow and only a small bubble (red arrow) was stuck on the surface. Since the buoyancy of bubble decrease with decreasing its size, there is a threshold size to remove the bubbles from the surface. Figure 3 (c) shows the size dependency of bubble states, which means bubbles removed by water flow or not, in the non-coated flat tube and dimple arrayed coated tube. From the plot, the threshold sizes in coated and flat tubes to remove bubbles

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from the interior surface of tube were c.a. 900 nm and c.a. 180 nm, respectively. This result indicates that 5 times smaller bubbles were removed from the dimple arrayed surface comparing with flat surface, in turn, adhesion force to bubbles of dimple arrayed surface is 125 times smaller than that of flat surface since buoyancy depends on the volume of bubble. Ultrasonic peeling process is also applicable to fabricated combined surface comprised of patterned honeycomb and dimple array structures. As reported previously, honeycomb-patterned films comprised of 1,2-polybutadiene (PB) can be patterned by UV light cross linking followed by melting of non-cross linked region28,29. This photo-patterning process can be applied to photopatterning of honeycomb and dimple arrayed structure. UV light generates free radicals at side chain double bonds of PB, the free radical reacts and cross links with other intra- and intermolecular double bonds immediately. After this photochemical cross linking process, PB has chemically stable and mechanically stiff as same as vulcanization of natural rubber30. Figure 4 (a) shows schematic illustration of patterning and formation of surface patterns. PB honeycombpatterned films were prepared by breath figure technique as same as PS case (Figure 4 (b)). A photo mask was placed on the honeycomb-patterned films and then, UV light was irradiated through the photo mask for 15 min. After irradiation of UV light, the film was immersed in water and applied ultrasonic for 10 min. Figure 4 (c) shows a optical micrograph of patterned surface. Inverted letter “2” was clearly imaged. Figure 4 (d) shows a close up image of honeycomb and UV-irradiated region. From the image, only UV irradiated region forms dimple arrayed structure and the other region kept the honeycomb structure. Since UV irradiation induce photo cross linking of PB, the irradiated region became stiff and fragile. Ultrasonic broke only the fragile UV irradiated region and patterned surface comprised of honeycomb and dimple arrays were finally fabricated. This technique is useful for site selective patterning of surface structures and control

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bubble repellent properties of surfaces. We intended to create such a patterned surface for bubble repellent surfaces to achieve efficient bubble repellency. It is well known that a superhydrophobic and superhydrophilic patterned surface is useful for efficient collection of water droplets, which phenomenon is known as mimicry of dessert beetles31.

4. Conclusion We showed that preparation of ultra-bubble-repellent surface with simple irradiation of UV-O3 and ultrasonication treatment of self-organized HC films. Furthermore, HC and DA structures fabricated inside of the tubes and DA structured tube show high bubble repellent properties comparing with non-coated one. This process can be applicable to fabricate combined surface comprised of HC and DA structures. 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.

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Figure 1. Schematic illustration (a) of HC and ultra-bubble-repellent DA films preparation, SEM images of HC (b) and DA (c) structures and bubble contact angle on a DA structured film (d). An inset image of (d) shows advancing and receding contact angles of bubble when the substrate tilted 5 ˚. The scale bar indicates 5 µm.

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Figure 2. Schematic illustration of HC and DA structured surface prepared in tubes (a), crosssectional SEM image of glass capillary after formation of HC structure (b) and close-up SEM images of HC (c) and DA (d) structures, respectively.

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Figure 3. Photographs of bubble adhered non-coated (a) and DA coated (b) tubes, and plot of bubble diameters adhered on the interior surface of tubes (c), respectively.

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Figure 4. Schematic illustration of photo patterning process (a), optical micrograph of HC film, photo-patterned and ultrasonicated HC film (b) and close-up image of dimpled region (d), respectively. The inset image of (c) shows an optical micrograph at the border between HC and DA.

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AUTHOR INFORMATION Corresponding Author H. Yabu ([email protected]) *Give contact information for the author to whom correspondence should be addressed. Author Contributions J. K. prepared honeycomb, dimple structures, and patterned surfaces and measured bubble repellency. H. A. prepared honeycomb structures inner surface of tubes. H. Y. directed the whole project and wrote a manuscript. Funding Sources This work has partly been supported by Grant-in-Aid for Exploratory Research (16K14071). ACKNOWLEDGMENT HY thank Ms. Minori Suzuki, WPI-AIMR, Tohoku University for helping observation of SEM and fabrication of honeycomb structures. ASSOCIATED CONTENT Supporting information is available free of charge via the Internet at http://pubs.acs.org/.

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Yabu, H.; Jia, R.; Matsuo, Y.; Ijiro, K.; Yamamoto, S.-A.; Nishino, F.; Takaki, T.; Kuwahara, M.; Shimomura, M. Preparation of Highly Oriented Nano-Pit Arrays by Thermal Shrinking of Honeycomb-Patterned Polymer Films. Adv. Mater. 2008, 42004204.

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Nakamichi, Y.; Hirai, Y.; Yabu, H.; Shimomura, M. Fabrication of Patterned and Anisotropic Porous Films Based on Photo-Cross-Linking of Poly(1,2-Butadiene) Honeycomb Films. 2011, 21 (11), 3884-3889.

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Kamei, J.; Yabu, H. On-Demand Liquid Transportation Using Bioinspired Omniphobic Lubricated Surfaces Based on Self-Organized Honeycomb and Pincushion Films. Adv. Funct. Mater. 2015, 25 (27), 4195–4201.

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Kagiya, V. T.; Takemoto, K. Crosslinking and Oxidation of 1, 2-Polybutadiene by UV Irradiation. Journal of Macromolecular Science—Chemistry 2006, 10 (5), 795–810.

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Parker, A. R.; Lawrence, C. R. Water Capture by a Desert Beetle. Nature 2001, 414 (6859), 33–34.

Biomimetic Bubble-Repellent Micro Dimple Arrays Prepared by Ultrasonic Peeling of SelfOrganized Honeycomb Films on Interior Surfaces of Tubes

Jun Kamei, Hiroya Abe and Hiroshi Yabu*

Ultra-bubble-repellent surface having dimple array structures were prepared in a tube by simple irradiation of UV-O3 and ultrasonication treatment of self-organized honeycomb films.

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