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Ng, Chen Xian, [Naito, Masanobu](https://orcid.org/0000-0001-7198-819X), [Doi, Kotaro](https://orcid.org/0000-0002-5204-1088), [Tenjimbayashi, Mizuki](https://orcid.org/0000-0002-8107-8285), Kawase, Yudai

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[Coalescence delay of microbubbles on superhydrophobic/superhydrophilic surfaces underwater](https://mdr.nims.go.jp/datasets/c497b13b-4d2f-4f00-9e8b-9f0487dde1ad)

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Coalescence delay of microbubbles on superhydrophobic/superhydrophilic surfaces underwaterAppl. Phys. Lett. 113, 033705 (2018); https://doi.org/10.1063/1.5038910 113, 033705© 2018 Author(s).Coalescence delay of microbubbles onsuperhydrophobic/superhydrophilic surfacesunderwaterCite as: Appl. Phys. Lett. 113, 033705 (2018); https://doi.org/10.1063/1.5038910Submitted: 07 May 2018 . Accepted: 02 July 2018 . Published Online: 19 July 2018 Mizuki Tenjimbayashi, Yudai Kawase, Kotaro Doi, Chen Xian Ng, and  Masanobu NaitoARTICLES YOU MAY BE INTERESTED INSpreading of impinging droplets on nanostructured superhydrophobic surfacesApplied Physics Letters 113, 071602 (2018); https://doi.org/10.1063/1.5034046Robust laser-structured asymmetrical PTFE mesh for underwater directional transportationand continuous collection of gas bubblesApplied Physics Letters 112, 243701 (2018); https://doi.org/10.1063/1.5039789Droplet impact on cross-scale cylindrical superhydrophobic surfacesApplied Physics Letters 112, 263702 (2018); https://doi.org/10.1063/1.5034020https://images.scitation.org/redirect.spark?MID=176720&plid=1086294&setID=378288&channelID=0&CID=358612&banID=519992915&PID=0&textadID=0&tc=1&type=tclick&mt=1&hc=5bd1ca46492024a1db9bca71e55aadf07d3a139e&location=https://doi.org/10.1063/1.5038910https://doi.org/10.1063/1.5038910http://orcid.org/0000-0002-8107-8285https://aip.scitation.org/author/Tenjimbayashi%2C+Mizukihttps://aip.scitation.org/author/Kawase%2C+Yudaihttps://aip.scitation.org/author/Doi%2C+Kotarohttps://aip.scitation.org/author/Ng%2C+Chen+Xianhttp://orcid.org/0000-0001-7198-819Xhttps://aip.scitation.org/author/Naito%2C+Masanobuhttps://doi.org/10.1063/1.5038910https://aip.scitation.org/action/showCitFormats?type=show&doi=10.1063/1.5038910http://crossmark.crossref.org/dialog/?doi=10.1063%2F1.5038910&domain=aip.scitation.org&date_stamp=2018-07-19https://aip.scitation.org/doi/10.1063/1.5034046https://doi.org/10.1063/1.5034046https://aip.scitation.org/doi/10.1063/1.5039789https://aip.scitation.org/doi/10.1063/1.5039789https://doi.org/10.1063/1.5039789https://aip.scitation.org/doi/10.1063/1.5034020https://doi.org/10.1063/1.5034020Coalescence delay of microbubbles on superhydrophobic/superhydrophilicsurfaces underwaterMizuki Tenjimbayashi,1,a) Yudai Kawase,1 Kotaro Doi,2 Chen Xian Ng,1and Masanobu Naito1,a)1Research and Services Division of Materials Data and Integrated System (MaDIS), National Institutefor Materials Science (NIMS), 1-2-1 Sengen, Tsukuba Ibaraki 305-0047, Japan2International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS),1-2-1 Sengen, Tsukuba Ibaraki 305-0047, Japan(Received 7 May 2018; accepted 2 July 2018; published online 19 July 2018)Inspired by penguins, the formation of an air film on surfaces underwater has been well-researched forthe potential reduction of drag. However, the features that contribute to drag reduction of penguins arenot only the formation of an air layer but also the flow of bubbles along the air layer; basic investiga-tion of the wetting dynamics of a bubble scattered in an underwater environment has been overlooked.The focus of our research was microbubble contact on superhydrophobic/superhydrophilic surfacesunderwater. Unlike the adhesion of mist in air, a “coalescence delay” is observed when bubbles makecontact, which influences the deposition dynamics of an air film. The “coalescence delay” is propor-tional to the size of the bubbles. This study is helpful to understand air/solid/water systems as well asthe drag reduction. Published by AIP Publishing. https://doi.org/10.1063/1.5038910Nature has always inspired ideas for the design of super-wetting materials (e.g., lotus, nepenthes and water strider).1–3Some experimental and theoretical investigations haveshown that the penguin-inspired approach of introducing anair film on a superhydrophobic surface is an effective methodfor the drag reduction underwater.4–8 However, the featuresof a penguin’s skin are not only the formation of an air layerbut also the flow of bubbles along the air layer. Therefore, areal analogue can be considered to be the interfacial dynam-ics of superhydrophobic surfaces in bubbles scattered under-water.9 Despite the development of superwetting materialsagainst air in underwater conditions as well as their potentialapplication in sensors, transportation, and reactors,10,11 thewetting dynamics of bubbles scattered underwater have notbeen investigated.Herein, we prepared superhydrophobic and superhy-drophilic surfaces and studied the wetting dynamics ofcontacting microbubbles scattered underwater, as shown inFig. 1(a). The bubbles were formed through the electrolysisof water. A carbon steel plate and a carbon plate were usedas a working electrode and a counter electrode, respec-tively. To increase the electrical conductivity, 2.0� 10�3 MK2SO4 was added to the water. The bath water-vapor inter-facial tension was 71.9 6 3.6 mN m�1 (n¼ 15), and the pHvalue was 7.8. An applied current of 0.2–0.3 mA generatedsubmicrometer-sized bubbles (diameter: 54.1 6 29.6 lm,n¼ 1000, see supplementary material). The formed bubblesmoved in a vertical direction to the substrates by buoyancy ata speed of �10 mm s�1 [Fig. 1(b)]. Figure 1(c) shows that theimpact speeds of the microbubble to the substrates weremostly uniform (within 5 mm s�1). A few bubbles moved at ahigher speed owing to the coalescence on the working elec-trode, although this was extremely rare.A superhydrophobic surface was prepared by dipping aglass plate into a mixture of zinc oxide micro-tetrapod pow-der and polydimethylsiloxane (PDMS). The zinc oxide pow-der was used for roughening the surface structure, andPDMS was used because of its low surface energy. A super-hydrophilic surface was prepared by treating a glass platewith plasma. Figure 2 shows the wettability of the differentsurfaces that were prepared. Theoretically, the sum of theYoung contact angle of water and air is approximately180�.12 Therefore, as the more hydrophobic the solid gets,FIG. 1. (a) A schematic diagram of the experiment. The wettability-controlled surface is faced down in a water bath. Bubbles are formed by theelectrolysis of water. The bubble-coating interface is observed using a high-speed camera. (b) Gas bubble velocity distribution mapping. (c) Time-lapsedistribution of the impact velocity of the bubbles.a)Authors to whom correspondence should be addressed: TENJIMBAYASHI.Mizuki@nims.go.jp and NAITO.Masanobu@nims.go.jp0003-6951/2018/113(3)/033705/4/$30.00 Published by AIP Publishing.113, 033705-1APPLIED PHYSICS LETTERS 113, 033705 (2018)https://doi.org/10.1063/1.5038910https://doi.org/10.1063/1.5038910https://doi.org/10.1063/1.5038910ftp://ftp.aip.org/epaps/appl_phys_lett/E-APPLAB-113-052829mailto:TENJIMBAYASHI.Mizuki@nims.go.jpmailto:TENJIMBAYASHI.Mizuki@nims.go.jpmailto:NAITO.Masanobu@nims.go.jphttp://crossmark.crossref.org/dialog/?doi=10.1063/1.5038910&domain=pdf&date_stamp=2018-07-19the more aerophilic the solid becomes.10,11,13–15 In the case ofour samples, the superhydrophobic surface was aerophilicunderwater and capable of forming an air film (i.e., unmeasur-able receding contact angle in a decreasing bubble volume) asshown in Figs. 2(a)–2(d), whereas the superhydrophilic sur-face was superaerophobic underwater and the adhesion of thebubble was relatively low, as shown in Figs. 2(e)–2(h) (i.e.,Wad¼ cLV(1þcos h), where Wad denotes the adhesion work,cLV the liquid vapor interfacial tension, and h the droplet con-tact angle).16 Thus, we anticipated that the wetting dynamicsof the scattered bubbles underwater could correspond to mistadhesion in air, in which microbubbles simply coalesce oncontact with other bubbles or the air film and grow.17Interestingly, unlike the mist adhesion in solids orliquids in air, we noticed that there was a delay between thecontact and coalescence of bubbles (termed “coalescencedelay”), which influenced the deposition mechanism of thebubbles on the surface. As shown in Fig. 3(a), microbubblesuniformly adhered onto the superhydrophilic surface (seesupplementary material). This phenomenon was owing to a“coalescence delay”. When a floating bubble (bubble I)driven by a buoyancy force (Fb) comes into contact with abubble that is adhered on the surface (bubble II), bubbles Iand II do not coalesce owing to the coalescence delay andbubble I rolls along the curvature of bubble II, as schemati-cally shown in Fig. 3(b) (left). We defined the time taken bybubble I to adhere onto the surface as tad. On the superhydro-philic surface, the highest tad was 20 ms, which was muchsmaller than the coalescence delay time (td) of most of thebubbles observed. In the case tad < td, the bubbles adheredon the surface rather than coalesce with other bubbles. Incontrast, on a superhydrophobic surface, an air film wasformed, first owing to its wettability, and bubbles that floatalong the air film were absorbed by the air film. Whenbubble III comes into contact with the air film, a coalescencedelay occurs, and bubble III starts to roll along the air film.However, the bubble adhesion time is larger than the coales-cence delay time (tad > td) owing to the large surface area ofthe air film. Thus, bubble III is absorbed by the air filmbefore adhering onto the solid surface.When a coalescence delay occurs, the bubble “hovers”on the air film surface (i.e., contact angle�180�). This phe-nomenon indicated that a thin water layer existed betweenthe bubble and the air film. Therefore, the delay is the life-time of the thin water film. Recently, some reports performedair film mediated hovering of the liquid droplet.18,19Comparing the bubble coalescence underwater with the cor-responding liquid droplet coalescence in air, the differencebetween the thin film conditions was the mass of the bubble/droplet and the viscosity of the surrounding space. For bub-bles to coalesce underwater, the bubble must first penetratethe thin water film; however, the coalescence time is delayedbecause of the small momentum of the bubbles and thehigher viscosity of the water compared with that of air. Thisphenomenon indicated that the delay time was in a positivecorrelation with the buffering time of the energy dissipationby interfacial change through coalescence.20 The dissipationenergy through coalescence (Edis) is expressed as Edis ¼ DEkinþ DEinterf þ Evis,21 in which DEkin is the change in the kineticenergy (�qu2r3, in which q is the bubble density, u is the bub-ble velocity, and r is the radius of the bubble), DEinterf is thechange in the interfacial energy (� �pcLVr2 as long as the sizeof the air film is much larger than the size of the bubble), andEvis is the viscous dissipation energy (�Wq�0.5DlcLV0.5r1.5, inwhich W is the scale factor of the thin water film, q is the gasdensity, and Dl is the difference in viscosity between the gasand water). The bubbles used in this study were obviouslysmaller than the capillary length (r < lcw ¼ cLV0.5qw�0.5g�0.5� 2.7 mm� lcg ¼ cLV0.5qg�0.5g�0.5� 286.2 mm, in which lc isthe capillary length and g is the gravitational acceleration);therefore, the gravitational influence and the change in themomentum energy could be ignored. In addition, the mass ofthe bubble is small; therefore, the kinetic energy can beneglected in this experiment (DEkin� 0). Considering thecoefficients in the approximation formula of the plot in Fig.3(c), the measurement was reasonable in terms of the coeffi-cient of r3 being almost 0, which means that there is no influ-ence of kinetic energy on the delay time. Thanks to the largevalue of DEinterf and Evis, a coalescence delay was apparentlyobserved under the studied conditions, and a positive correla-tion between the bubble size and the delay time was obtained.We confirmed that a relatively large bubble (2r� 2.1 mm)was not absorbed in the air layer td > tad even on a superhy-drophobic surface, as shown in Fig. 3(d). The lifetime of thetarget bubble highlighted in yellow in Fig. 3(d) was�1700 ms, whereas the adhesion time on the air layer was�140 ms, which fulfills the criterion of tad > td. The bubblerolls on the air film and settles next to the neighboring airfilm. After 1732 ms, the air film and the bubble coalesced.In conclusion, we studied the wetting dynamics of super-hydrophobic/superhydrophilic surfaces in the condition ofbubbles scattered underwater. Unlike in air, a “coalescencedelay” of the bubbles was observed under the studied condi-tions, and the delay time had a positive correlation with theFIG. 2. Wettability of a superhydrophobic surface (a)–(d) and superhydro-philic surface (e)–(h). The wettability analyses were the static water contactangle in air [static: (a) and e and dynamic: (b) and (f)] and underwater aircontact angle [static: (c) and (g) and dynamic: (d) and (h)]. (n¼ 10).033705-2 Tenjimbayashi et al. Appl. Phys. Lett. 113, 033705 (2018)ftp://ftp.aip.org/epaps/appl_phys_lett/E-APPLAB-113-052829bubble size. When penguins swim in the sea, flows of scat-tered bubbles are formed along the air layer.9,22 This bubblescattering phenomenon can function as a drag reduction toolfor penguins underwater. So far, the biomimetics for dragreduction relies on a simple air film formation,23 yet the coa-lescence delay of bubbles strongly influences the interfacialenergy change with time. Since this change can be convertedto kinetic energy,21 the influence of microbubbles on the dragreduction underwater cannot be negligible. We predict thatthe coalescence delay is a key to unravelling penguin-inspired wetting dynamics.See supplementary material for the mechanism of gasgeneration by electrolysis, the structure of superhydrophobiccoating, and the bubble adhesion behavior on superhydro-phobic and superhydrophilic surfaces.This work was supported by Acquisition, Technology &Logistics Agency. We are grateful to Ms. MegumiKawakami for her experimental help. We also acknowledgeDr. Yoshihiro Yamauchi and Dr. Sadaki Samitsu for theirhelpful comments.1X. Feng and L. Jiang, Adv. Mater. 18, 3063 (2006).2T. Darmanin and F. Guittard, J. Mater. Chem. A 2, 16319 (2014).3M. Tenjimbayashi and S. Shiratori, J. Appl. Phys. 116, 114310 (2014).4H. Dong, M. Cheng, Y. Zhang, H. Wei, and F. Shi, J. Mater. Chem. A 1,5886 (2013).5G. McHale, M. I. Newton, and N. J. Shirtcliffe, Soft Matter 6, 714 (2010).6E. Jenner and B. D’Urso, Appl. Phys. Lett. 103, 251606 (2013).7S. Wang, Z. Yang, G. Gong, J. Wang, J. Wu, S. Yang, and L. Jiang,J. Phys. Chem. C 120, 15923 (2016).8W. Y. L. Ling, G. Lu, and T. W. Ng, Langmuir 27, 3233 (2011).9R. J. Daniello, N. E. Waterhouse, and J. P. Rothstein, Phys. Fluids 21,085103 (2009).10C. Yu, P. Zhang, J. Wang, and L. Jiang, Adv. Mater. 29, 1703053 (2017).11R. Ma, J. Wang, Z. Yang, M. Liu, J. Zhang, and L. Jiang, Adv. Mater. 27,2384 (2015).12X. Tian, V. Jokinen, J. Li, J. Sainio, and R. H. A. Ras, Adv. Mater. 28,10652 (2016).13X. Chen, Y. Wu, B. Su, J. Wang, Y. Song, and L. Jiang, Adv. Mater. 24,5884 (2012).14R. Xu, X. Xu, M. He, and B. Su, Nanoscale 10, 231 (2018).15C. Yu, M. Cao, Z. Dong, J. Wang, K. Li, and L. Jiang, Adv. Funct. Mater.26, 3236 (2016).16M. E. Schrader, Langmuir 11, 3585 (1995).17M. Cao, J. Xiao, C. Yu, K. Li, and L. Jiang, Small 11, 4379 (2015).18J. De Ruiter, R. Lagraauw, D. Van Den Ende, and F. Mugele, Nat. Phys.11, 48 (2015).19M. Tenjimbayashi, T. Matsubayashi, T. Moriya, and S. Shiratori,Langmuir 33, 14445 (2017).FIG. 3. Bubble adhesion dynamics on superhydrophobic and superhydrophilic surfaces. (a) Time-lapse images and (b) schematic illustrations of underwaterscattered bubble deposition on the surfaces (left: superhydrophilic and right: superhydrophobic). (c) Bubble diameter versus coalescence delay time. The corre-lation coefficient was ca. 0.891. A plot is fitted by td ¼ 0.001r3 þ 14.0634r2 þ 563.954r1.5. (d) Time-lapse image of a large size bubble making impact with anair film formed on a superhydrophobic surface (controlled as td < tad).033705-3 Tenjimbayashi et al. Appl. Phys. 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