Switchable Underwater Bubble Wettability on Laser-InducedTitanium Multiscale Micro-/Nanostructures by Vertically CrossedScanningYunlong Jiao,† Chuanzong Li,‡ Sizhu Wu,‡ Yanlei Hu,† Jiawen Li,† Liang Yang,*,† Dong Wu,*,†

and Jiaru Chu†

†CAS Key Laboratory of Mechanical Behavior and Design of Materials, Department of Precision Machinery and PrecisionInstrumentation, University of Science and Technology of China, Hefei 230026, P.R. China‡School of Instrument Science and Optoelectronics Engineering, Hefei University of Technology, Hefei 230009, P.R. China

ABSTRACT: We present here a kind of novel multiscaleTiO2 square micropillar arrays on titanium sheets throughvertically crossed scanning of femtosecond laser. Thismultiscale micro-/nanostructure is ascribed to the combinationof laser ablation/shock compression/debris self-deposition,which shows superaerophobicity in water with a very smallsliding angle. The laser-induced sample displays switchablebubble wettability in water via heating in a dark environmentand ultraviolet (UV) irradiation in alcohol. After heating in adark environment (0.5 h), the ablated titanium surface shows superaerophilicity in water with a bubble contact angle (BCA) of∼4°, which has a great ability of capturing bubbles in water. After UV irradiation in alcohol (1 h), the sample recovered itssuperaerophobicity in water and the BCA turns into 156°. The mechanism of reversible switching is believed as the chemicalconversion between Ti−OH and Ti−O. It is worth noting that our proposed switching strategy is time-saving and the switchwetting cycle costs only 1.5 h. Then we repeat five switching cycles on the reversibility and the method shows excellentreproducibility and stability. Moreover, laser-induced samples with different scanning spacing (50−120 μm) are fabricated and allof them show switchable underwater bubble wettability via the above tunable methods. Finally, we fabricate hybrid-patternedmicrostructures to show different patterned bubbles in water on the heated samples. We believe the original works will providesome new insights to researchers in bubble manipulation and gas collection fields.

KEYWORDS: underwater bubble wettability, multiscale micropillar arrays, switchable superaerophobicity−superaerophilicity,femtosecond laser, bubble manipulation

1. INTRODUCTION

Underwater bubble wettability has attracted considerableattention because of its significant involvements in industrialmanufacture and daily life, such as water treatment,1 enhancingthe recovery of minerals particles from ores2 and improvingheat transfer in the ocean.3 Understanding the bubble’sbehaviors and realizing the controllable bubbles manipulationin a liquid medium are fascinating in various phenomena andapplications.4−16 As a kind of smart materials, titanium dioxidewith reversible wettability has become a hot issue because oftheir great potential in the fields of microfluidic devices,17

biosensors,18 and smart membranes.19 Inspired by its photo-sensitivity, a large number of experimental and theoreticalattempts have been made by researchers with the purpose ofachieving tunable in-air wettability and underwater−oilwettability.20−31 For example, Sakai et al.20 studied the kineticson the photoresponsive hydrophilic conversion processes oftitanium surface. It was indicated that the photoresponsivehydrophilic conversion competed with photocatalytic oxidationprocess under UV irradiation. Sun and co-workers21 fabricateda kind of nanocomposite intelligent films with TiO2 nano-

particles, which exhibited excellent reversibility betweensuperhydrophilicity and superhydrophobicity under UV lightand heat. Yong et al.22 presented a simple way to achievereversible switching between underwater superoleophobicityand underwater superoleophilicity on laser-ablated titaniummaterials, but the superhydrophilic sample needs to be placedin the dark for 48 h to recover superhydrophobicity. Recently,they achieved six different superwettabilities on the PDMSsurfaces by utilizing laser direct-writing technology, which mayexpand their potential values in tuning underwater bubblewettability.23 Yu and co-workers24 presented for the first timethe use of ethanol to switch underwater wettability of the TiO2surface. It is worth noting that abundant studies have beenconducted in the past 2 decades to reveal the mechanism ofreversible wettability on the micro-/nanostructured surfaces.However, all current studies mainly focus on in-air andunderwater oil tunable wettability. There are no reports on

Received: February 15, 2018Accepted: April 25, 2018Published: April 25, 2018

Research Article

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© 2018 American Chemical Society 16867 DOI: 10.1021/acsami.8b02812ACS Appl. Mater. Interfaces 2018, 10, 16867−16873

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switchable underwater bubble wettability and correspondingtheory is still not systematic enough. In addition, the currenttunable method of finishing a wetting switching circle is time-consuming (mostly exceeds 48 h), which may largely decreasetheir potential applications in underwater bubble manipulationand gas collection. It is highly desirable to find a more time-saving solution for rapidly realizing switchable bubblewettability on the smart materials.Here, we fabricate a kind of multiscale micropillar arrays on

titanium sheets with a one-step femtosecond laser-inducedmethod. Every multiscale square micropillar is mainlycomposed of four microcones and random smaller nano-particles. The formation of the multilevel structures is the resultof three effects: laser ablation, shock compression, and debrisself-deposition. The modified titanium surface shows super-aerophobicity in water and has a great antibubble ability with asmall sliding angle. Furthermore, we investigate the switchableunderwater bubble wettability on the laser-modified titaniumsurface by heating in the dark (0.5 h) and UV irradiation inalcohol (1 h). The reversible wetting method proposed in thepresent study is time-efficient and the wetting switching cycleonly costs 1.5 h. However, 48 h were needed to finish a wettingswitching cycle in the other reported studies.22,24 Additionally,we fabricate a hybrid-patterned titanium surface like five-pointed star, triangle, circle, and square for obtaining differentpatterned bubbles via switchable underwater bubble wettability.The present study provides new insights into switchable bubblewettability in water, which should provide significant guidanceto researchers and engineers to control well bubbles’ behavioron the smart materials in a water medium.

2. EXPERIMENTAL DETAILSMaterials. The Ti sheet (99.5% purity, 0.2 mm thick) was

purchased from New Metal Material Tech. Co., Ltd., Beijing, China,and was cut into small pieces (2 × 2 cm) for laser processing. Beforemanufacturing, the Ti piece was cleaned ultrasonically in acetone. Thedistilled water (H2O, 1 g/cm

3 density) and NPT (normal pressure andtemperature) air (20 °C, 1 atm, 1.205 × 10−3 g/cm3 density) wereserved as test material in the in-air and underwater contact anglemeasurement.

Micro-/Nanostructure Fabrication. The Ti piece was ablated byvertically crossed line-by-line femtosecond laser scanning. The laserbeam (104 fs pulses) with a repetition rate of 1 kHz at a centralwavelength of 800 nm from a regenerative amplified Ti:sapphirefemtosecond laser system (Legend Elite-1K-HE, Coherent, USA) wasemployed for ablation. The laser beam was guided onto the titaniumsurface through a galvanometric scanning system (SCANLAB,Germany), which was equipped with a 63 mm telecentric fθ lens tomake the laser beam focus and scan along the x/y coordinate direction.The schematic of the femtosecond laser system and scanning path isshown in Figure 1a. The scanning spacing between two adjacent linesranged from 50 to 120 μm in both x and y coordinates direction. Thelaser processed area is 8 × 8 mm and scanning speed is 1 mm/s.

Instrument and Characterization. Scanning electron micros-copy (SEM) photos were utilized to analyze the surface topography oflaser-modified Ti pieces via use of a field-emission scanning electronmicroscope (JSM-6700F, JEOL, Japan). The contact angles of waterin-air and bubble underwater were measured on the as-prepared Tisurfaces with a contact-angle system (CA100C, Innuo, China) by asessile drop method. The volume of water droplet and air bubble is setto be 4 μL. The average values were obtained by measuring five dropsat different locations on the same surface. Furthermore, to measure thesliding angle of the air bubble in water, the Ti pieces were slowly titledwith an increment of 1° until the air bubble started to slide.Continuous photos were taken every 200 ms by the contact-anglesystem with a computer-controlled charge-coupled device (CCD)camera to display the sliding behavior of the air bubble on the as-prepared Ti surfaces in water. All the contact angle measurements

Figure 1. Experimental setup and the formation mechanism of the multiscale micro-/nanostructure on the titanium surface. (a) Schematic diagramof femtosecond laser fabrication. (b) Schematic of fs laser scanning path. (c−e) SEM images of laser-induced titanium surface. It can be seen thatsquare micropillar arrays are regularly distributed on the ablated surface (Figure 1c) and every micropillar is composed of four microcones (Figure1d) and nanoparticles (45° titled in Figure 1e). (f, g) SEM images of vertically crossed scanning to show the formation mechanism of the multiscalemicro-/nanostructure. (h) Schematic illustration of laser shock compression and debris deposition process.

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were conducted under 10% humidity and 20 °C temperature,respectively.Switchable Wetting Strategy. The laser-induced sample

equipped with opaque aluminum foil was heated for 30 min at 200°C on a heating plate (Lab Tech, EH35A plus). For UV lightirradiation, the samples were immersed in a transparent box withadequate alcohol for 60 min. A 36 W UV lamp with a wavelength of365 nm was placed above the heat-treated surface at a distance ∼8 cmas the light source. The reversible switching of underwater bubblewettability between superaerophobicity and superaerophilicity wasverified in cycle tests (five cycles) by using the proposed strategy.Different hybrid-patterned titanium surfaces were fabricated forobtaining different patterned bubbles via switchable underwaterbubble wettability.

3. RESULTS AND DISCUSSION

Titanium surface with regular square micropillar arrays werefabricated by vertically crossed femtosecond laser scanning(Figure 1b) with laser power of ∼450 mW and scanninginterspace ∼80 μm. From the low-resolution SEM image(Figure 1c), we can see that square micropillar arrays (∼80 μmin length) are distributed regularly on the modified titaniumsurface. Every square micropillar (Figure 1d) is composed offour microcones with 25−30 μm in height and 25−40 μm indiameter. In addition, nanoparticles with 20−300 nm size arerandomly located on the microcones (Figure 1e). To show theformation mechanism of the multiscale structure, SEM imagesof the scanning process were obtained on the modified titaniumsurface (Figure 1f,g). The initial groove arrays with the spacingof ∼80 μm formed on the surface along the x-axis directionafter the first horizontal scanning (Figure 1f). Then the squaremicropillar arrays were obtained by the second vertical laserscanning along the y-axis direction (Figure 1g). During thelaser-ablating process, microcones and nanoparticles wereinduced on both sides of the pits under the combined effectof laser shock compression and debris deposition (Figure 1h).Therefore, we concluded that the multiscale structures wereachieved by the combination of laser ablation, shockcompression, and debris self-deposition from the processedregion to the unprocessed region. The height of the

mircropillars and microcones increases with the laser powerand scanning times.To study the water wettability in air and the bubble

wettability in water, contact angle measurements wereemployed on the original and laser-modified surfaces,respectively. Before laser manufacturing, the unprocessed areashowed hydrophilicity with a water contact angle (WCA) of∼48° and aerophobicity with a bubble contact angle (BCA) of∼135° (Figure 2a). After laser manufacturing, the processedarea (Figure 2b) showed superhydrophilicity with a WCA of∼0° and superaerophobicity with a BCA of ∼156° (Figure 2c).Moreover, the original surface showed high adhesive force ofthe bubbles. Even when the surface was titled at an angle of90°, the air bubble could not roll (Figure 2a). While the bubbleon the laser-induced surface shows lower adhesive force with asliding angle (BSA) of ∼3°, the sliding behavior of the bubblewas recorded every 200 ms by the contact-angle system with aCCD camera (Figure 2c).The laser-fabricated surface displays switchable water

wettability in air and bubble wettability in water via heat inthe dark and UV irradiation in alcohol. After being heated in adark environment (0.5 h), the laser-induced superhydrophilicsample became superhydrophobic in air with a WCA of ∼152°(Figure 3a). In contrast, the bubble wettability in waterchanged from superaerophobicity with a BCA of ∼156° tosuperaerophilicity with a BCA of ∼4°. After UV irradiation inalcohol (1 h), the sample recovered its superhydrophilicity inair and the WCA turned into only 5°. Therefore, the laser-modified sample finished a wetting switching circle betweensuperhydrophobicity and superhydrophilicity. The underlyingmechanism is mainly related to the conversion between Ti−OH and Ti−O (Figure 3b). On the basis of the previoustheoretical studies,26−28 it can be concluded that the photo-excitation would contribute to the formation of oxygenvacancies on the titanium surface where ambient H2O couldfavorably compete with O2 for dissociative adsorption. Whenthe original sample was modified by laser ablation, besides fromthe wettability amplification effect of rough surface micro-structure,22 there was a large amount of hydrophilic hydroxyl

Figure 2. Water wettability in air, bubble wettability in water, and rolling behavior of the bubble on the original and laser-ablated titanium samples.(a) WCA and BCA on the original surface. The bubble adheres to the surface even the sample rotates 90°. (b) Digital photograph of unprocessedarea and laser-processed area on the titanium surface. (c) WCA and BCA on the laser-ablated surface. The bubble slides with low adhesive force andsliding angle of ∼3°.

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groups (−OH) on the laser-induced sample.26 Hence, theobtained sample shows stable superhydrophilicity in air andultralow bubble adhesive in water. However, the chemical bondTi−OH would be easily replaced by Ti−O when the as-prepared sample was heated in a dark environment. Theambient O2 rapidly replaced the −OH groups and formed morestable oxygen absorption28 on the heat-treated surface whichendows the sample with stable superhydrophobicity. After thesample was irradiated by UV light, unstable oxygen vacanciesformed on the titanium surface. These oxygen vacancies

showed a stronger propensity for dissociative adsorption of−OH supplied by alcohol. The UV-treated sample recovered itssuperhydrophilicity because of the high surface energy of thechemical bond Ti−OH.Moreover, the switching trend of bubble wettability in water

reveals that the bubble wettability in water intensively dependson the water wettability in air. As shown in Figure 3c, thetitanium surface after laser scanning was superhydrophilic in air.Following the Wenzel model, when the laser-induced samplewas immersed in water, the water rapidly filled into the

Figure 3. Switchable water wettability in air and bubble wettability in water on the laser-induced sample by heating in a dark environment and UVirradiation in alcohol. (a) WCA and BCA on the laser-induced samples after heating and UV irradiation. (b) Mechanism of chemical compositionconversion between Ti−OH and Ti−O. (c) Schematic illustration of underwater superaerophobicity and underwater superphilicity. (d) and (e)show the evolution of WSA and BSA with heating and UV-light time, respectively. (f) Five reversible circles are repeated to demonstrate theexcellent reproducibility and stability of the proposed reversible method. (g) Time consumption of different reversible wetting methods.

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microstructures and formed a water layer so that the surfacehad a great antibubble ability and displayed superaerophobicityin water. We speculated that the air bubble stayed on the peaksof the microstructure, leaving the water trapped in the pits.After being heated in a dark environment, the sample exhibitedsuperhydrophobicity and ultralow water adhesive. Shown inFigure 3c was the water droplet staying at the bottom of therough microcones, leaving the air trapped in the pits which wassatisfied with the Cassie−Baxter model. When such a samplewas immersed in water, the surface exhibited superaerophilicity.Therefore, it is revealed that superhydrophilic surface in airgenerally shows superaerophobicity in water and has a greatantibubble ability.Parts (c) and (d) of Figure 3 respectively showed the

evolution of the WCA and BCA with time under heating in adark environment and UV irradiation in alcohol. While theheating time increases from 0 to 30 min, the WCA graduallyincreased from 3° to 152°, indicating that the sample switchedfrom superhyodrophilic to superhydrophobic states. In contrast,the bubble wettability in water changed from superaeropho-bicity to superaerophilicity. To demonstrate the excellentreproducibility and stability of the proposed reversible method,we have repeated five switching cycles on the reversibility(Figure 3f). In addition, time consumption of differentreversible wetting methods was compared (Figure 3g) andthe switching wetting cycle cost only 1.5 h by heating in thedark and UV in alcohol. It is worth noting that the tunable timefrom superhydrophilicity to superhydrophobicity needed only0.5 h by heating in a dark environment, which was far less thanthat shown in existing reports, needing 48 h longer to recoverits superhydrophobicity in air.22,24

Finally, we studied the effect of scanning spacing on thewetting switching cycle of the femtosecond laser-modifiedrough titanium surface. As shown in Figure 4a, with theincrease of scanning spacing, the micropillars induced by

crossed laser ablation became more and more obvious.Multiscale micropillars (microcone, nanoripple) on the ablatedsample with spacing of ∼120 μm were observed in the high-resolution SEM image (Figure 4a). The reversible wettingresults indicated that the laser-induced samples with differentscanning spacing all exhibited switchable in-air and underwaterwettability via the above switchable methods (Figure 4b).Additionally, we designed and fabricated different hybrid-patterned titanium surface-like five-pointed star, triangle, circle,and square to display a patterned bubble in water via heating ina dark environment. As shown in Figure 4c, the laser-ablatedarea with different patterns showed superhydrophobicity in airand superhydrophilicity in water by heating in a darkenvironment. The water droplets looked like a ball and seatedon the heated surface in air. While the samples were immersedin water, different patterned bubbles formed on the super-aerophilic areas because of high adhesive force of the sample.To show the fabrication accuracy and controllability of thefemtosecond laser micro-/nanofabrication, complex patternswere achieved without masking, such as airplane and bicycle(Figure 4d). These microstructures may exhibit switchingwetting properties on the titanium materials via femtosecondlaser manufacturing.

4. CONCLUSIONIn summary, we report a novel multiscale micro-/nanostructureon the titanium surface induced by femtosecond laser verticallycrossed scanning for the first time, which efficiently and rapidlyachieves switchable water wettability in air and bubblewettability in water via heating in the dark (0.5 h) and UVirradiation in alcohol (1 h). The multiscale structure is realizedby the combination of three effects: laser ablation/shockcompression/debris self-deposition. Furthermore, the reversiblewetting strategy proposed in the present study is time-savingand the switching time of finishing a wetting recycle between

Figure 4. Effect of scanning spacing on the reversible wetting cycle and some patterned microstructures induced by fs laser. (a) SEM images of laser-induced surfaces with different scanning spacing ranged from 50 to 120 μm. (b) WCA on the laser-induced surfaces with different scanning spacingafter heating in the dark and UV in alcohol. (c) Different patterned bubbles on the laser-induced surfaces heating treatment. (d) Multiple patternsfabricated by fs laser, such as airplane and bicycle.

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superaerophobicity and superaerophilicity needs only 1.5 h. It isalso revealed that bubble wettability intensively depends on thewater wettability, and the switching trend of bubble wettabilityin water is opposite to that of water wettability in air. Theheated surface has a great ability of absorbing and capturingbubble in water medium. Finally, hybrid-patterned micro-structures such as five-pointed star, triangle, circle, and squareare fabricated to show different patterned gas bubbles on theheated sample. It is worth noting that the present work mayprovide new insights to researchers in capturing/manipulatingbubble fields and gas collection under aqueous environments.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: dongwu@ustc.edu.cn.*E-mail: ygliang@ustc.edu.cn.ORCIDYunlong Jiao: 0000-0001-7718-7342Liang Yang: 0000-0001-6103-6451Jiaru Chu: 0000-0001-6472-8103NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the National Natural ScienceFoundation of China (Nos. 61505047, 51275502, 61475149,51405464, 51605463, 61675190, and 51675503), the ChinaPostdoctoral Science Foundation (Nos. 2015M571922),Fundamental Research Funds for the Central Universities(No. JZ2017YYPY0240), and the “Chinese Thousand YoungTalents Program”.

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