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

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[Microbubble flows in superwettable fluidic channels](https://mdr.nims.go.jp/datasets/e3417c21-61bf-4f08-8d4d-c8bf53051bd4)

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Microbubble flows in superwettable fluidic channelsRSC AdvancesPAPEROpen Access Article. Published on 09 July 2019. Downloaded on 1/2/2021 1:49:22 PM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View IssueMicrobubble flowaResearch and Services Division of MateriaNational Institute for Materials Science (N305-0047, Japan. E-mail: TENJIMBAYASHI.nims.go.jpbResearch Center for Structural Materials,(NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki, 30† Electronic supplementary informationgeneration via electrolysis; the dimensivideos of the uid dynamics on superaeunder laminar and turbulent ow conditiCite this: RSC Adv., 2019, 9, 21220Received 4th June 2019Accepted 27th June 2019DOI: 10.1039/c9ra04212arsc.li/rsc-advances21220 | RSC Adv., 2019, 9, 21220–2122s in superwettable fluidicchannels†Mizuki Tenjimbayashi, *a Kotaro Doib and Masanobu Naito *aThe control of bubble adhesion underwater is important for various applications, yet the dynamics under flowconditions are still to be unraveled. Herein, we observed the wetting dynamics of an underwater microbubblestream in superwettable channels. The flow ofmicrobubbles was generated by integrating amicrofluidic devicewith an electrochemical system. The microbubble motions were visualized via tracing the flow using a high-speed camera. We show that a vortex is generated in the air layer of the superaerophilic surface underlaminar conditions and that the microbubbles are transported on the superaerophilic surface under turbulentconditions driven by the dynamic motion of the air film. Furthermore, microbubbles oscillated backward andforward on the superaerophobic surface under turbulent conditions. This investigation contributes to ourunderstanding of the principles of drag reduction through wettability control and bubble flow.Nature offers us ideas for the design of materials with super-wettability.1 In superwettable systems, the wetting of airunderwater has generated interest recently.2–6 For example,penguin feathers are superaerophilic, with an air layer formingon the surface underwater, which allows penguins to swim inthe sea with small amounts of drag.2,3 Inspired by this,researchers have theoretically and/or experimentally studiedthe inuence of wettability on drag reduction underwater.4–6 Inaddition, sh scales are superaerophobic, which offers the ideaof designing no-bubble adhesion electrodes that demonstratehigh and stable oxygen evolution reaction performance.7,8However, despite the development of superwettable materialsfor the controllable adhesion of air and/or bubbles underwater,9the wetting dynamics of bubbles under ow conditions, whichwe must consider in real environments, have not beeninvestigated.Herein, we generated microbubble ows parallel to super-wettable substrates inside a microuidic device10,11 and studiedthe wetting dynamics through integrating an electrochemicalsetup12 with a microuidic device, as shown in Fig. 1. Thebubbles were formed through the electrolysis of water (see theESI†). Two platinum plates were used: one as the workingelectrode and the other as the counter electrode. To increase thels Data and Integrated System (MaDIS),IMS), 1-2-1 Sengen, Tsukuba, Ibaraki,Mizuki@nims.go.jp; NAITO.Masanobu@National Institute for Materials Science5-0047, Japan(ESI) available: The mechanism of gasons of the microuidic devices; androphobic and superaerophilic surfacesons. See DOI: 10.1039/c9ra04212a4electrical conductivity, 2.0 mM K2SO4 was added to the water.The bath water–vapor interfacial tension, gLV, was 71.9 � 3.6mN m�1 (n ¼ 15) and the pH of the water was 7.8. We applieda current of �0.25 mA cm�2 to generate microbubbles witha diameter of 463.9 � 245.1 mm (n ¼ 120). The microuidicdevice was generated using a 3D printer and connected toa water-ow generator (see the ESI† for the dimensions of thedevice). The microbubbles generated around the electrodesmoved in the direction of the water ow and the coatedsubstrates were placed parallel to the ow.We used the microbubbles as tracers and analyzed their owas well as that of the water (i.e.microbubble image velocimetry),as shown in Fig. 2. We controlled the Reynolds number, Re ¼4Q(pDn)�1 (Q is the ow rate of the water, D is the tube diam-eter, and n is the kinetic viscosity of the water). Laminar owwas obtained at Re ¼ 79.21 and turbulent ow was obtained atRe ¼ 396.06 (Fig. 2A). Under laminar ow conditions, the owspeed was nearly constant and the ow direction was close toperpendicular to the substrate (4 z 0, where 4 is the anglebetween the microbubble direction of movement and the widthdirection of the substrate) in all areas; this behavior was time-independent (Fig. 2B and C). Under turbulent ow conditions,the ow speed was not constant, and the ow direction wasunstable (4 uctuated between �180 and 180�) in all areas. Weconrmed that the separation of ow did not occur, at leastduring the observation period, since the ow direction wasparallel to the superwetting microuidic device.We then prepared substrate coatings with superaerophilicityand superaerophobicity. Superaerophilic substrates werefabricated according to our previous study.12 Concisely, a glassplate was dip-coated with amixture of zinc oxide micro-tetrapodpowder for surface roughening and polydimethylsiloxane foraerophilization. Superaerophobic surfaces were preparedThis journal is © The Royal Society of Chemistry 2019http://crossmark.crossref.org/dialog/?doi=10.1039/c9ra04212a&domain=pdf&date_stamp=2019-07-08http://orcid.org/0000-0002-8107-8285http://orcid.org/0000-0001-7198-819Xhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/c9ra04212ahttps://pubs.rsc.org/en/journals/journal/RAhttps://pubs.rsc.org/en/journals/journal/RA?issueid=RA009037Fig. 2 The flow conditions of the microbubbles. We created laminar andturbulent flows through altering the Reynolds number. (A) Flow veloc-imetry of the microbubbles under laminar (left) and turbulent (right) flowconditions; scale bar: 10 mm. (B) Velocity and flow direction profiles ofmicrobubbles over the flow area under laminar (left) and turbulent (right)flow conditions. (C) Average velocity and flow direction fluctuations withtime under laminar (left) and turbulent (right) flow conditions.Fig. 1 A schematic illustration of the microfluidic device with an electrochemical setup. We generated a flow of microbubbles and investigatedthe influences of coating wettability and flow type on the microbubbles dynamics via high speed camera observations. Scale bar: 10 mm.This journal is © The Royal Society of Chemistry 2019Paper RSC AdvancesOpen Access Article. Published on 09 July 2019. Downloaded on 1/2/2021 1:49:22 PM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinethrough modifying a glass substrate with hydroxy groups usingan aqueous potassium hydroxide solution.13 The wettability ofthe superaerophobic surfaces in relation to bubbles wasconrmed via measuring the underwater bubble contact angle(q); the results are shown in Fig. 3. We calculated the adhesionforces of bubbles, Fadh ¼ pl2gLV(1 + cos q)/4,14 where l is thebubble–solid adhesion length. On the superaerophilic surface,the adhesion force was 3.2 � 103 mN, and on the super-aerophobic surface the force was 4.37 mN for 6 mL bubbles.In Fig. 4, we observed air lm formation on superaerophilicsurfaces under laminar and turbulent ow conditions. As wehave previously shown, when microbubbles are verticallydeposited on superaerophilic surfaces, a uniform air layer isformed.10 In the present study, under both laminar and turbu-lent ow conditions, a uniform air layer formed on the super-aerophilic surfaces, but the air layers grew non-uniformly withthe deposition of microbubbles owing to Rayleigh–Taylorinstability14 (Fig. 4A and B). In all ve independent observa-tions, the shape of the air layer was non-uniform; thus, the owof microbubbles inuenced the shape of the air layer. However,bubbles with l ¼ 4–7 mm formed on the surfaces under bothlaminar and turbulent ow conditions.Aer aging for 1000 s, a continuous air lm formed on thesuperaerophilic surfaces under turbulent conditions. However,the shape was unstable and changed with time (Fig. 4D). InFig. 4C and E, we observe the formation of a vortex on thehemispherical air lm under laminar ow conditions (seeMovie S1†). This phenomenon is interesting because underlaminar ow conditions a vortex should not be generatedRSC Adv., 2019, 9, 21220–21224 | 21221http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/c9ra04212aFig. 3 Wettability of the coatings. (A) schematic illustration of themeasurement of the underwater air contact angle. The contactbehavior of 6 mL microbubbles underwater on superaerophilic (B) andsuperaerophobic (C) surfaces.RSC Advances PaperOpen Access Article. Published on 09 July 2019. Downloaded on 1/2/2021 1:49:22 PM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online(Fig. 2A); this cannot be explained using Bernoulli's theorem15and the generation of a vortex suggests the separation of ows,which works to decrease ow resistance at the interface. VortexFig. 4 Microbubble deposition behavior on a superaerophilic surface unparts of the images are the initial and time-aged stages, respectively. (C) Tflow conditions. (D) Microbubbles transported on a superaerophilic surfafilm. (E) and (F) Schematic representations of the microbubble behavior21222 | RSC Adv., 2019, 9, 21220–21224generation may be due to the coalescence of microbubbles withthe air layer, causing a change in the curvature of the hemi-spherical air lm. This, in turn, would result in a change in theLaplace pressure of 2DkgLV, where Dk is the change in curva-ture. There is a uctuation in the vertical force torque togenerate the vortex, and the force should be balanced bya Kutta–Joukowski force in the form of 2gLV dk/dtz rGU, wherer is the density of ows, G is the vortex constant, and U is thevelocity of the constant laminar ow.16In Fig. 4D and F, we observe that microbubbles on the airlm were transported as the shape of the air lm dynamicallychanged to a wave-like nature; however, the microbubbles andair lm did not coalesce (see Movie S2†). This indicates thata thin water layer exists between the microbubbles and the airlm to prevent coalescence, whereas microbubbles are trappedon the air lm by the buoyancy force of themicrobubbles, whichz(Dr)Ug, where Dr is the difference in densities betweena bubble and water, U is the volume of a microbubble, and g isgravitational acceleration.We then observed the dynamics of the microbubbles on thesuperaerophobic surfaces (Fig. 5). As we have previously shown,der laminar (A) and turbulent (B) flow conditions. The top and bottomhe vortex motion of microbubbles on deposited air films under laminarce under turbulent conditions driven by the dynamic motion of the airfrom (C) and (D), respectively. All scale bars: 10 mm.This journal is © The Royal Society of Chemistry 2019http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/c9ra04212aFig. 5 Microbubble deposition behavior on a superaerophobic surface under laminar (A) and turbulent conditions (B). The top and bottom partsof the images are the initial and time-aged stages, respectively. (C) Linear motion of the microbubbles on the surface under laminar flow. (D) Theoscillating motion of microbubbles on the surface under turbulent flow. (E) Motion distance and (F) velocity analysis of microbubbles underlaminar (left) and turbulent (right) conditions for different bubble diameters (2R). All scale bars: 10 mm.Paper RSC AdvancesOpen Access Article. Published on 09 July 2019. Downloaded on 1/2/2021 1:49:22 PM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinewhen microbubbles are vertically deposited on super-aerophobic surfaces, they are uniformly deposited on thesurface and have a spherical shape.12 Under both laminar andturbulent ow conditions, microbubbles were deposited on thesuperaerophobic surfaces with spherical shapes but with non-uniform deposition (Fig. 5A and B). We then observed themotion of bubbles in contact with the superaerophobicsurfaces. Under laminar ow conditions, microbubblesadhering to the surface moved in the direction of the ow(Fig. 5C and Movie S3†). In contrast, turbulent ow conditionscaused the microbubbles to oscillate backward and forward(Fig. 5D and Movie S4†). The velocimetry proles in Fig. 5E andF conrm that the bubble motion is linear in time underlaminar ow, but it varies under turbulent ow (with thevelocity periodically becoming negative). Despite the periodicnegative velocity under turbulent ow conditions, the bubblesgo forwards in the ow direction, which is not due to thelaminar boundary but because the turbulent ow has morepositive components than negative ones. This is because thelength of positive motion under turbulent ow conditionsincreases with the size of the bubbles, obeying Newton'sThis journal is © The Royal Society of Chemistry 2019viscosity law.17 Thus, we conrmed that the motion of bubbleson superaerophobic surfaces is inuenced by the ow condi-tions. The bubble motion distance on superaerophobic surfacesincreased with bubble diameter.ConclusionsWe investigated the wetting dynamics of microbubbles onsuperwettable surfaces under laminar and turbulent owconditions. The microbubbles were non-uniformly depositedon the surfaces and the motion of the bubbles was inuencedby the substrate wettability and the ow conditions. To discussair adhesion on superwettable surfaces, the inuence of owmust be considered, which strongly inuences the hystereticbehavior of bubbles.18 For instance, vortex ow formation maybe a crucial factor in deciding whether air can adhere to a solidor not; Taylor instability of the air layer dynamically changes thecurvature and/or air/water contact area, strongly affecting thebalance of solid–liquid–air interfacial energies. Moreover, thedirect in situ observation of bubble ow will help create anunderstanding of the interfacial phenomena demonstrated bypenguins, sh, and other swimmers in nature. In addition, thisRSC Adv., 2019, 9, 21220–21224 | 21223http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/c9ra04212aRSC Advances PaperOpen Access Article. Published on 09 July 2019. Downloaded on 1/2/2021 1:49:22 PM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinework may be helpful for understanding the inuence of bubbleleakage or cavitation on microuidic systems.19–21Conflicts of interestThe authors declare no conicts of interest associated with thiswork.AcknowledgementsThis work was supported by the Acquisition, Technology &Logistics Agency and partially supported by JSPS KAKENHI No.18K14006. We are grateful to Ms Megumi Kawakami for herexperimental help. We also acknowledge Dr Sadaki Samitsu, DrYasuyuki Nakamura, and Dr Kazuaki Kato for their helpfulcomments.References1 T. Darmanin and F. Guittard, J. Mater. Chem. A, 2014, 2,16319–16359.2 C. L. Williams, J. C. Hagelin and G. L. Kooyman, Proc. R. Soc.B, 2015, 282, 20152033.3 A. Partt and J. Vincent, J. Bionic. Eng., 2005, 2, 57–62.4 K. B. Golovin, J. W. Gose, M. Perlin, S. L. Ceccio andA. Tuteja, Philos. Trans. R. 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Cabot and A. W. Cook, Nat. Phys., 2006, 2, 562–568.16 D. Saranadhi, D. Chen, J. A. Kleingartner, S. Srinivasan,R. E. Cohen and G. H. McKinley, Sci. Adv., 2016, 2, e1600686.17 U. Pesavento and Z. J. Wang, Phys. Rev. Lett., 2004, 93,144501.18 F. Zami-Pierre, R. de Loubens, M. Quintard and Y. Davit,Phys. Rev. Lett., 2016, 117, 074502.19 I. Mirzaee, M. Song, M. Charmchi and H. Sun, Lab Chip,2016, 16, 2254–2264.20 P. Rademeyer, D. Carugo, J. Y. Lee and E. Stride, Lab Chip,2015, 15, 417–428.21 X. Hou, Y. Hu, M. Khan and J. Aizenberg, Nature, 2015, 519,70–73.This journal is © The Royal Society of Chemistry 2019http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/c9ra04212a Microbubble flows in superwettable fluidic channelsElectronic supplementary information (ESI) available: The mechanism of gas generation via... Microbubble flows in superwettable fluidic channelsElectronic supplementary information (ESI) available: The mechanism of gas generation via... Microbubble flows in superwettable fluidic channelsElectronic supplementary information (ESI) available: The mechanism of gas generation via... Microbubble flows in superwettable fluidic channelsElectronic supplementary information (ESI) available: The mechanism of gas generation via...