Appl. Phys. Lett. 116, 093706 (2020); https://doi.org/10.1063/1.5144593 116, 093706

© 2020 Author(s).

Spontaneous and unidirectionaltransportation of underwater bubbles onsuperhydrophobic dual railsCite as: Appl. Phys. Lett. 116, 093706 (2020); https://doi.org/10.1063/1.5144593Submitted: 08 January 2020 . Accepted: 15 February 2020 . Published Online: 06 March 2020

Suwan Zhu , Yucheng Bian, Tao Wu , Erqiang Li , Jiawen Li , Yanlei Hu , Dong Wu , and JiaruChu

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Spontaneous and unidirectional transportationof underwater bubbles on superhydrophobicdual rails

Cite as: Appl. Phys. Lett. 116, 093706 (2020); doi: 10.1063/1.5144593Submitted: 8 January 2020 . Accepted: 15 February 2020 .Published Online: 6 March 2020

Suwan Zhu,1 Yucheng Bian,2 Tao Wu,3 Erqiang Li,3 Jiawen Li,1,a) Yanlei Hu,1 Dong Wu,1,a)

and Jiaru Chu1

AFFILIATIONS1CAS Key Laboratory of Mechanical Behavior and Design of Materials, Department of Precision Machinery and PrecisionInstrumentation, University of Science and Technology of China, Hefei 230026, China

2School of Microelectronics, University of Science and Technology of China, Hefei 230026, China3Department of Modern Mechanics, University of Science and Technology of China, Hefei 230026, China

a)Electronic addresses: jwl@ustc.edu.cn and dongwu@ustc.edu.cn

ABSTRACT

Superhydrophobic/superhydrophilic surfaces (SBS/SLS) with excellent water repellency/adhesion are important in both academic researchand industrial settings owing to their intriguing functions in tiny droplet and gas bubble manipulation. However, most manipulationstrategies involving SBS/SLS are limited to their large-area fabrication or sophisticated morphology designs, which distinctly hinders theirpractical uses. In this paper, we design and fabricate superhydrophobic polydimethylsiloxane narrowing dual rails (SNDRs) beneath a super-hydrophilic stainless steel sheet by one-step femtosecond laser ablation. Our SNDR tracks are capable of transporting gas bubbles in variousvolumes from wide end to narrow end spontaneously and unidirectionally underwater, even when they are bent. The mechanical analysis fordiverse geometrical dual-rail configurations in bubble transportation performance is further discussed. Finally, we experimentally demon-strate the intriguing capability of lossless mixing of gas bubbles at a designed volume ratio on a multiple SNDR combination. This approachis facile and flexible, and will find broad potential applications such as intelligent bubble transport, mixing, and controllable chemicalreactions in interfacial science and microfluidics.

Published under license by AIP Publishing. https://doi.org/10.1063/1.5144593

The flexible manipulation of unidirectional motion, distribution,and interaction of gas bubbles plays a crucial role in both academicresearch and industrial settings, owing to its significant potentialsincluding sewage treatment, pressure sensors, micro-reaction, and min-eral floatation in aqueous ambient.1–4 Present approaches to gas bubblemanipulation mainly rely on the assistance of buoyancy force5–8 or thecooperation of wetting driving force and Laplace pressure differencearising from substrate’s morphology.9–17 Generally, most of these sub-strates can be divided into superhydrophobic/superhydrophilic surfaces(SBS/SLS)5,9,12,13 and lubricant-infused surface(s) (LIS).6–8,10,11

Nevertheless, current bubble manipulation strategies suffer from short-ages of additional operations or apparatuses and sophisticated substratedesigns. Specifically, for buoyancy-assisted strategies (no matter forSBS/SLS or LIS), the dominant buoyant force imposes great restrictionon motion control except floating up, and thus, the bubbles could onlybe driven via dynamically rotating and tilting the substrate.5–7 Besides,

recently reported smart LIS surfaces need additional sources (light,heat, magnetic field, etc.) for regulating their surface properties.8,18 Forthose strategies based on wettability gradient force and Laplace force,the bubble manipulation is subject to substrate’s complex morphology(e.g., tapered and helical shapes), which greatly hinders its practicalapplications.

Superhydrophobic surfaces (SBS), with the water contact anglegreater than 150� and the sliding angle less than 10�, are proven asexcellent water-repellent and bubble-adhesive functional surfaces andhave aroused enormous interest as they have useful applications inmicroscale fluid (e.g., droplets and gas bubbles) manipulations. It hasbeen demonstrated that SBS can be achieved by introducing low sur-face energy to roughened surfaces.19–21 On the other hand, superhy-drophilic surfaces (SLS) show high water adhesion and an ultralowwater contact angle of less than 10�, possessing the capability of waterharvest and bubble rejection. Recent studies show that the simple

Appl. Phys. Lett. 116, 093706 (2020); doi: 10.1063/1.5144593 116, 093706-1

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combination of SBS and SLS tracks exhibits great anisotropic wettingphenomena, demonstrating distinct contact angles and sliding anglesalong the parallel and perpendicular directions of the tracks fordroplets.22–25 These findings indicate that the fabrication of large-areaand complicated SBS/SLS patterns is not compulsory for droplet/bubble manipulation and hence significantly elevates the fabricationefficiency. It is worth mentioning that although similar works withrespect to liquid droplet anisotropic sliding on SBS/SLS tracks are spo-radically reported, the relevant configuration regarding underwatergas bubble manipulation is rarely explored. Consequently, it is inter-esting to develop a more facile and flexible approach for manipulatinggas bubbles without a sophisticated process.

Herein, we proposed a facile and effective platform which istermed superhydrophobic narrowing dual rail (SNDR) for manipulat-ing underwater gas bubbles. Our SNDR is constructed via the simplephysical combination of stainless steel SLS cover and the polydime-thylsiloxane (PDMS) SBS substrate through one-step femtosecondlaser ablation. By taking the advantage of Laplace pressure differencearising from geometric asymmetry, our SNDR track manifests thecapability of transporting gas bubbles spontaneously and unidirection-ally in various volumes underwater, even when it is bent. Furthermore,we systematically explored the transporting performance of variousbubble volumes with regard to the impact of diverse geometricalparameters including monorail width w, branch angle a, and narrow-end spacing D, followed by simplified mechanical model analysis.Finally, we experimentally deployed the proof-of-concept demonstra-tion in lossless mixing of gas bubbles at an arbitrary volume-ratio on amultiple SNDR combination. We envision that such a facile andflexible strategy could find its uses in the field of microfluidics andinterfacial science such as intelligent bubble transport, harvest, mixingand controllable chemical reactions in aqueous conditions.

We design the SNDR by satisfying four basic criteria: (1) the sur-face of the dual rail must be superhydrophobic for underwater bubbleadhesion; (2) the rest portion of the substrate must be superhydro-philic to repel the bubble adhesion on the surface; (3) there must be asufficient gas layer beneath the rail surface to guide the bubble achiev-ing smoothly sliding; (4) the configuration of SNDR should begeometrically asymmetric to meet the generation of Laplace pressuredifference.26–30 Based on these principles, mechanically polished stain-less steel sheets were tailored via femtosecond laser direct scanning at2mm s�1 for 25 repeat cycles, to form the hollowed dual-rail pattern.After that, we constructed the SNDR system by physically sticking theas-prepared hollowed steel sheet with a commercially available PDMSsubstrate (Hangzhou Bald Advanced materials Co., Ltd., thickness�250lm), as shown in Fig. 1(a). To enhance the surface roughness,the combination was then scanned through vertically crossed line-by-line femtosecond laser irradiation on the steel surface at an elevatedscanning speed of 20mm s�1 under 100 mW laser power. The scan-ning spacing between two adjacent lines was 20lm in both x and ycoordinate directions. The laser fabrication system and optical pathcan be found in our previous studies.31 Figure 1(b) represents thedigital photograph of 20ll gas bubbles sticking on the narrow-endof the SNDRs (monorail width w � 400lm, narrow-end spacing D� 70lm) with various branch angles a from 1 to 6� in aqueousambient. Due to the anisotropic superhydrophobic dual rail, the gas bub-ble would be easily adhered, guided, and driven along the SNDR system.As a paradigm, in situ observation of a 20ll gas bubble transportation

process on the SNDR with 6� branch angle is shown in Fig. 1(c), fromwhich one can see that the gas bubble slides spontaneously and unidirec-tionally from the wide-end to the narrow-end within 0.60 s.

To investigate the transportation behavior of bubbles sliding onthe SNDRs, we first visualized the bubble profiles from differentperspectives via a high-definition video camera (Blackmagic URSAMini Pro 4.6K). A SNDR-array with branch angle a ¼ 3�, narrow-endspacing D ¼ 400 lm and diverse monorail width w from 300 to700lm was employed for demonstration. The static 20-ll-bubbleprofiles in wide-end, narrow-end, and top views are illustrated inFig. 2(a), from which we can see two pairs of distinct “gaseous feet” on

FIG. 1. Schematic fabrication of the SNDR and its spontaneous transportationbehavior of gas bubbles underwater. (a) The SNDR is constructed through the sim-ple physical combination of the femtosecond-laser-tailored stainless steel sheet andthe PDMS substrate using double-sided adhesive. (b) Top view of 20 ll gas bub-bles adhering on SNDRs with different branch angles underwater. (c) Spontaneousand unidirectional transportation of a 20 ll gas bubble from the wide-end of aSNDR with 6� branch angle.

FIG. 2. SNDR-attached bubble profiles from different perspectives in geometry. (a)Static profiles of a 20ll bubble on SNDRs with diverse monorail width w (from 300to 700 lm). (b) Side and top views of a 20 ll bubble on a SNDR (w ¼ 300 lm).The internal configuration between two rails illustrates the trapezoid-like solid-gasinterface along the axis direction. (c) Dynamic height H, length L, and width W of a20ll bubble transporting on the SNDR.

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the bottom of each bubble due to the existence of SBS (dual-rail) andSLS (substrate). The magnified side and top view of a single bubble onthe SNDR (w ¼ 300lm) are clearly shown in Fig. 2(b) (left and right).The internal configuration between two independent rails reveals thetrapezoid-like solid-gas interface (i.e., contact area) along the trackdirection. Moreover, the dynamic height H, length L, and width W ofa 20ll bubble transporting on the SNDR were calculated throughhigh-definition videos. It can be seen that the dynamic height of a bub-ble during self-transportation changed little (�3.1mm) from Fig. 2(c),and the dynamic length increased slightly with time from �3.2 to�3.6mm. Meanwhile, the time-dependent width of the bubble dropsfrom�3.3 to�3.0mm after�12mm displacement.

To elucidate the underlying mechanism responsible for sponta-neous and unidirectional bubble transportation, we further focusedonto the detailed configurations of solid–gas and gas–liquid interfacesbetween gas bubbles and surrounding ambient. When the bubbles areattached to the underwater SNDR filled with air, a unidirectionalmovement can be observed due to the disequilibrium of capillary forcearising from the asymmetric track configuration. The direction of theaxial capillary force is related to the interface wetting properties andthe configuration of the contact line. The solid-gas contact line config-uration is shown in Fig. 3(a) on the dual-rails, where a is the branchangle, w is the width of the monorail, d is the half-waist width of thesolid-gas contact area, and l is the length of the contact area. As drivenby the capillary force, the bubble will spontaneously transport to thenarrow-end of SNDRs to meet the lowest interfacial energy.Specifically, capillary force here can be expressed as32

Fx ¼ �ddx

cLSALS þ cLGALG þ cSGASG½ �; (1)

where c denotes the interfacial tension between the solid (S), liquid(L), and gas (G), and A is the corresponding contact area, respectively.

It is worth noting that our bubble configuration on SNDRs isanalogous to the hydromechanical paradigm: when a gas bubble is

injected into a hollow cone filled with water, if the internal surface ofthe cone tube is hydrophobic, the bubble configuration (cross sectionalong the cone axis) can be simplified as a concave trapezoid-likeshape, i.e., a trapezoid area (dl) without two semicircles (in diametersof d þ l tana and d � l tana, respectively) at either end [Fig. 3(a)]. Asmentioned earlier, during the movement of the bubble, the variationmagnitude of the length L, the widthW, and the height H of the bub-ble is negligible. Consequently, we assume that the area of thegas–liquid interface and the length l of the solid–gas interface areessentially unchanged. The two ends of the solid-gas contact line canbe approximated as semicircles; therefore, the area of the solid-gasinterface (i.e., bubble contact area Aca) is estimated to be ASG ¼ dl� p

4 ðd2 þ l2tan2aÞ, where ddx d ¼ �a. Note that the exact bubble con-

tact area Aca is difficult to obtain and varies during bubble motion.Finally, the axial capillary force on the bubble is

Fx ¼ cSG � cLGð Þ l � p2d

� �a : (2)

To evaluate the disparity of driving force among diverse parame-ters including branch angle a, narrow-end spacing D, monorail widthw, and bubble volume V, we measured the sliding angle h of the gasbubbles on the SNDR through gradually tilting the substrate with anincrement of 0.1� until the gas bubble started to move. We deployed ahomemade instrument equipped with a screw rod in the accuracy ofmicrometer order, as shown in Fig. 3(b). When the component ofbuoyancy force along the dual-rail axis exceeds the capillary force (i.e.,the tilted angle is large enough), the bubble will start to move

h ¼ arcsincSG � cLGð Þ l � p

2d

� �a

qL � qGð ÞgVeff

: (3)

According to Eq. (2), the magnitude of the capillary force is propor-tional to the branch angle a and negatively related to the half-waistwidth d (proportional to narrow-end spacing D), which is in good

FIG. 3. Mechanical analysis of spontaneous transportation of gas bubbles on the SNDR. (a) Detailed configurations of solid–gas and gas–liquid interfaces between gas bub-bles and surrounding ambient. (b) Sliding angle h of gas bubbles on the SNDR is measured through a homemade instrument equipped with a screw rod in the accuracy ofmicrometer order. (c) The relation between sliding angle h and various parameters including branch angle a, narrow-end spacing D, monorail width w, and bubble volume V.

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consistency with Fig. 3(c). In addition, due to the effect of capillaryforce from the dual rail, the injected gas will be trapped inside the rail,thus decreasing the effective volume Veff of the gas bubble [Fig. 2(a)],which indicates the proportional relation between sliding angle h andmonorail width w. On the other hand, an increased bubble volume Vwould lead to the elevation of buoyancy force, which could accountfor the decrease in sliding angle h [Fig. 3(c)].

The transport performance of gas bubbles on horizontal SNDRswith diverse parameters including the monorail width, bubble volume,narrow-end spacing, and branch angle was systematically quantifiedvia a high-speed video camera (Phantom VOE 710S) at a typical rateof 1000 fps. As a paradigm, Fig. 4(a) shows the dynamic transportationof a 20ll gas bubble on SNDRs with different monorail widths. It canbe obviously seen that the horizontal displacement x of gas bubbles oneach SNDR varies distinctly at time t ¼ 0.33 s. Going further, the rela-tion of bubble displacement x vs time t (i.e., moving velocity) underthe above-mentioned parameters is shown in Fig. 4(b). Our experi-mental results demonstrate that the average velocity of gas bubblestransporting on the SNDR increased with the decrease in bubble vol-ume V as well as narrow-end spacing D. For instance, a 5ll gas bubblecould quickly slip along the SNDR in �0.5 s, covering a displacementof �14mm. While for a 25ll gas bubble, this time would be close to0.8 s. In the meantime, the completion time for a gas bubble movingon a D ¼ 300lm SNDR was shorter than that on a D ¼ 700lmSNDR (0.75 s vs 1.05 s). In contrast, the elevated monorail widthw and branch angle a would lead to the decline of the average velocity[Fig. 4(b)]. In fact, during the transporting process, bubbles would beinfluenced not only by the capillary force arising from the asymmetricconfiguration, but also the viscous friction stemming from the relativemotion with the ambient matter. At low Reynolds numbers, the dragforce of an ideal non-deformable spherical bubble moving under waterfollows the Hardamard–Rubczynski formula33,34

FD ¼ 4plRU; (4)

where l, R, and U are the dynamic viscosity of the water, the radius ofbubble, and the velocity of bubbles, respectively. It should be notedthat the bubble radius R is not only proportional to the bubble volume,but the elevation of the width of the monorail w would also diminishthe sectional area of the bubble perpendicular to the movement direc-tion. By equating the viscous friction to the driving force above, we getthe bubble characteristic velocity

U �cSG � cLGð Þ l � p

2d

� �a

4plR: (5)

Our experimental results in Fig. 4(b) are consistent with the theoreticaldescription.

Finally, based on the capability of bubble sliding anisotropy onSNDRs, demonstrative applications such as transporting bubbles incontrollable direction and intelligent mixing gas bubbles in desiredvolume ratios were explored on our specially designed SNDR system.In Fig. 5(a), we designed a kind of arc-shaped SNDR through the com-bination of two different monorails following the radius ratioR1 : R2 ¼ 35 : 40, which has potential applications for intelligent bub-ble transportation. It can be seen that when the gas bubble wasattached onto the wide-end of the arc-shaped SNDR, it slid along thecurved track driven by Laplace pressure difference and reached thenarrow-end. For another promising multiphase chemistry application,intelligent gaseous chemical reactions in the microscale should meetthe requirements for arbitrary mixing ratio and contamination-freemixing-triggering strategy. Our further exploration of a multipleSNDR combination should open a new avenue for the diversificationof current lab-on-a-chip studies. Figure 5(b) vividly illustrates thedetailed configuration of the multiple SNDRs, which comprise twonarrow-end-closed SNDRs. Two bubbles with various volume ratioV1 : V2 (1:1, 1:2, and 1:3) were initially placed onto the wide-end ofeach SNDR component, and then the two bubbles transported towardthe narrow-end. When the bubbles came in contact with each other,they would merge into a big bubble within several seconds. In general,the multiple-SNDR system in Fig. 5(b) can be flexibly designed viadiverse secondary configurations and morphologies, which can be eas-ily realized under various laser-ablation conditions.

To conclude, we proposed a facile and effective superhydropho-bic narrowing dual rail (SNDR) system for manipulating underwatergas bubbles, which was constructed through the simple physical com-bination of the femtosecond laser tailored stainless steel sheet and thePDMS substrate. Our SNDR enables the gas bubbles to transportspontaneously and unidirectionally underwater from the wide end tonarrow end, but prevents the bubbles from sliding along the oppositedirection. The basic physics of transportation behavior was briefly ana-lyzed and attributed to the Laplace pressure difference arising fromgeometric asymmetry of the narrowing dual rails. Moreover, trans-porting performance of gas bubbles on the SNDRs in diverse parame-ters including the monorail width, bubble volume, narrow-endspacing, and branch angle, was systematically explored. Finally, theproof-of-concept demonstration in lossless mixing of gas bubbles at anarbitrary volume ratio on a multiple SNDR combination was vividlyelucidated. We believe that such a facile and flexible strategy couldfind its potential applications in intelligent bubble manipulation andcontrollable chemical reactions in microfluidics.

FIG. 4. The transport performance of gas bubbles on horizontal SNDRs withdiverse parameters including monorail width w, bubble volume V, narrow-end spac-ing D, and branch angle a. (a) Dynamic transportation behavior of a 20 ll gas bub-ble on SNDRs with different monorail widths. (b) Relation between bubbledisplacement x and time t (i.e., moving velocity) under different parameters includ-ing branch angle a, narrow-end spacing D, monorail width w, and bubble volume V.

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Appl. Phys. Lett. 116, 093706 (2020); doi: 10.1063/1.5144593 116, 093706-4

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This work was supported by the National Natural ScienceFoundation of China (Nos. 51805508, 51875160, 11772327, 11621202,61505047, 51875544, and 61805230), Fundamental Research Funds forthe Central Universities (Nos. WK2090090025 and JZ2017YYPY0240),the National Key R&D Program of China (2017YFB1104303), and theChina Postdoctoral Science Foundation (No. 2018M642534). Weacknowledge the Experimental Center of Engineering and MaterialSciences at USTC for the fabrication and measurement of samples.

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FIG. 5. Potential applications of transporting gas bubbles on the specially-designed SNDR system. (a) An arc-shaped SNDR, which consists of two different monorails followingthe radius ratio R1 : R2 ¼ 35 : 40, shows the capability of intelligent bubble transportation in desired direction. (b) Intelligent mixing gas bubbles in desired volume ratios on amultiple SNDR combination that comprises two narrow-end-closed SNDRs.

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Appl. Phys. Lett. 116, 093706 (2020); doi: 10.1063/1.5144593 116, 093706-5

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