Maskless Hydrophilic Patterning of the Superhydrophobic Aluminum

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Maskless hydrophilic patterning of superhydrophobic aluminum surface by atmospheric pressure micro-plasma jet for water adhesion controlling Jiyu Liu, Jinlong Song, Guansong Wang, Faze Chen, Shuo Liu, Xiaolong Yang, Jing Sun, Huanxi Zheng, Liu Huang, Zhuji Jin, and Xin Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19431 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018

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ACS Applied Materials & Interfaces

Maskless hydrophilic patterning of superhydrophobic aluminum surface by atmospheric pressure micro-plasma jet for water adhesion controlling Jiyu Liu1, Jinlong Song1, Guansong Wang1, Faze Chen1, Shuo Liu1, Xiaolong Yang1, Jing Sun1, Huanxi Zheng2, Liu Huang1, Zhuji Jin1, Xin Liu1

1 Key Laboratory for Precision and Non-traditional Machining Technology of the Ministry of Education, Dalian University of Technology, Dalian 116024, People’s Republic of China. 2 Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Hong Kong 999077, People’s Republic of China. KEYWORDS: micro-plasma jet, hydrophilic pattern, superhydrophobic aluminum surface, water adhesion, droplet transportation. ABSTRACT: Superhydrophobic surfaces with hydrophilic patterns have great application potential in various fields, such as microfluidic systems and water harvesting. However, many reported preparation methods involve complicated devices and/or masks, making fabrication of these patterned surfaces time-consuming and inefficient. Here, we propose a high-efficient, simple and maskless micro-plasma jet (MPJ) treatment method to prepare hydrophilic patterns like dots, lines and curves on superhydrophobic Al substrates. Contact angles, sliding angles, adhesive forces 1

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and droplet impact behavior of the created patterns are investigated and analyzed. The prepared “dot” patterns exhibit great water adhesion, while the “line” patterns show anisotropic adhesion. Additionally, MPJ treatment does not obviously change the surface structures, which makes it possible to achieve repeatable patterning on one substrate. Adhesion behavior of these patterns could be adjusted using MPJs with different diameters. MPJs with larger diameters are efficient for creation of patterns with high water adhesion, which can be potentially used for open channel lab-on-chip systems (e.g., continuous water transportation); while MPJs with smaller diameters are preferable in preparing patterns with low water adhesion for diverse applications in biomedical fields (e.g., loss-less liquid droplets mixing and cell screening). 1. INTRODUCTION Superhydrophobic (SH) surfaces show excellent water-repellency with contact angle larger than 150°, and has great application values in different fields, such as self-cleaning1, oil/water separation2, water harvesting3, lab-on-chip systems4 and antifogging5. According to different solid-liquid adhesions and sliding angles (SA), SH surfaces can be classified as low-adhesive (SA10°). Lotus-leaf6 inspired low-adhesive SH surfaces have been widely studied and prepared by various methods, including chemical etching7, coating8, 9, plasma nanotexturing10, 11 and photopolymerization12. The surfaces have preferable properties like self-cleaning thanks to their roll-off behavior. High-adhesive SH surfaces with sticky behavior can be obtained via two strategies. One is to prepare superhydrophobic surfaces with special microstructures by mimicking “petal effect”13-15. The second strategy is to fabricate (super)hydrophilic patterns on 2


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low-adhesive SH substrates by various techniques like ultraviolet irradiation16, 17, laser processing18,

19

, mechanical machining20,

21

and ink patterning22-24. Ultraviolet

irradiation is capable of changing surface compositions of exposed substrates, thus making it possible to be used in surface patterning with the combination of masks16, 17. However, ultraviolet irradiation can only change surface wettability of photocatalytic materials, which are subjected to degradation in some cases. Laser beam has high energy, and can be employed to prepare patterned surfaces with different wettability18, 19

. Yong et al.18 constructed line-patterned PDMS surfaces by a femtosecond laser.

Nevertheless, laser devices are quite expensive and complicated. Mechanical machining can change wettability of SH substrates by the removal of low energy substance and microstructures20, 21, but the induced structure changes make it difficult to remove and regenerate different wettability patterns on the same substrate. Ink patterning technology uses ink with high surface energy to pattern substrates22-24. Lai et al.23, 24 employed ink patterning to construct extreme wettability patterns on SH surfaces by changing surface structures and wettability. As a high-efficient and green modification method, plasma treatment has been widely employed for wetting control. Gogolides et al.25 employed low pressure oxygen plasma and stencil masks to prepare multifunctional microfluidic devices on polymer surfaces. Tuteja et al.26 fabricated oil wettability patterns using low pressure oxygen plasma and stainless steel masks. Zhu et al.27 achieved reversible oil wettability transition on copper surfaces by masked plasma treatment and chemical modification. These works clearly demonstrate the preferable features of plasma 3



treatment in wettability control, and have promising application values in microfluidic and biomedical fields. However, the low pressure plasma treatment requires relatively expensive vacuum devices and specific masks, which also restrain the treatment efficiency and sample sizes. Atmospheric plasma jet is generated under 1 atm, and the absence of vacuum devices enables the atmospheric plasma jet to be low-cost, more efficient and easy-to-operate.28-30 Thanks to these advantages, there have been several research applying atmospheric plasma jets to adjust surface wettability. Kostov et al.31 successfully modified various engineering polymer surfaces using atmospheric plasma jets. Bónová et al.32 found that atmospheric plasma treatment could efficiently hydrophilize flat aluminum surfaces. We previously employed atmospheric plasma jets and specific masks to prepare wettability patterns capable of realizing directional droplet transportation33. The works also demonstrated that atmospheric plasma treatment had little effect on the micro-nano structures of the substrate, therefore, reversible patterning could be easily realized on one SH substrate. Although plasma exposure has been successfully used for functional patterns fabrication on SH surfaces, designed masks are required for specific patterns creation, which would be a time-consuming and undesired process. Therefore, it is of great significance to develop a maskless plasma treatment method to directly prepare wettability patterns and adjust adhesive forces of SH surfaces, which, to the best of our knowledge, has been rarely studied. In this paper, we propose a maskless plasma treatment method to prepare wettability patterns and adjust surface adhesion by atmospheric pressure 4


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micro-plasma jets. The movements of MPJs are controlled by a self-made numerically-controlled three axis motion platform, and the removal of mask preparing process makes the treatment method more efficient. The influences of diameters of MPJs on adhesion behaviors of the treated patterns are investigated, and the results indicate that adhesion forces and sliding angles can be well adjusted just by changing diameters of MPJs. Large-area patterns that can be effectively prepared by MPJs with larger diameters have great application potential in continuous and antigravity liquid transportations; while small-size patterns with relatively lower adhesion created by MPJs with smaller diameters could be used for loss-less droplet mixing. 2. EXPERIMENTAL SECTION 2.1 Materials. 6061 aluminum sheets (60×60×2mm) were bought from Dalian Al material manufacturer (China). Sodium chloride (NaCl) was purchased from Tianjin Kermel Chemical Reagent Manufacturer (China). Fluoroalkylsilane (FAS) was obtained from Deguassa Co. (Germany). All chemicals are analytically pure and were used as received. 2.2 Fabrication of SH surfaces. SH aluminum surfaces were fabricated by previously

reported

method34.

Briefly,

polished

aluminum

surfaces

were

electrochemically etched using 0.1 mol/L NaCl aqueous solution as electrolyte, and the etched surfaces were then modified by 1 wt% FAS ethanol solution to lower the surface energy. 2.3 Micro-plasma treatment. The SH surfaces were treated by atmospheric DBD MPJs with different diameters (as shown in Fig.1, Fig.S1, Supporting information). 5



Conductive copper tape was closely wrapped on quartz tubes and served as high-voltage electrode. Quartz tubes with outer diameter of 1mm and inner diameters of 60, 150, 300 or 500 µm were used as working gas channel and to control the diameters of MPJs (MPJ60, MPJ150, MPJ300, MPJ500). Helium was used as working gas. The helium flow rate was set as 16 mL/min, 100 mL/min, 400 mL/min and 1.1 L/min for MPJ60, MPJ150, MPJ300, MPJ500, respectively. The helium flow rate was proportional to the outlet area of the glass tube used for generating MPJs to ensure same gas velocity for each MPJ. Frequency of the AC power supply was fixed at 60 kHz, and the vertical distance between the MPJs outlet and the SH surfaces was about 8 mm. The MPJs were generated by applying 6.5 kV AC voltage on the high-voltage electrode. Preparation processes of various patterns are summarized as follows: Dot-patterned surfaces: the SH surfaces were treated for 5 s by no-moving MPJs (Video S1, Supporting Information). Dots with different sizes could be prepared by adjusting diameters of MPJs. Line-patterned surfaces and other patterned-surfaces (curve, wedge-shape, etc.): the SH surfaces were modified using different MPJs whose motions were controlled by the three axis motion platform with a scanning rate of 2 mm/s (Video S2 for “line” pattern, Video S3 for curve and wedge-shape patterns). 2.4 Characterization. Contact angles (CAs) were measured using an optical angle meter (SL200KS, KINO, USA), and sliding angles (SAs) were investigated by a precision whirler. CA and SA values were measured five times at different locations. 6


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Surface morphology was observed using a scanning electron microscopy (SEM, JSM-6360LV, Japan). Droplet impact behavior on original SH and plasma-treated patterns was recorded by a high-speed camera (Fastcam SA4, Photron) at a frame rate of 8,000 fps.

Fig.1 Treatment process of micro-plasma jet. 3. RESULTS AND DISCUSSION 3.1 Controllable adhesion by preparing dot-patterned surfaces. As shown in Fig.2, adhesion behavior of dot-patterned surfaces changed obviously compared with original SH surfaces. The CAs of droplets with smaller volumes (V 0 > cosθRSH, and the following relation is valid: Fdot − Foriginal Wdot γL cosθRdot -cosθRSH F > 0

(4)

According to Eq.4, the treated dot-patterned surfaces have an additional adhesion force F, and the SAs of droplets on the treated surfaces are therefore much larger. In addition, F is proportional to Wdot, indicating that water adhesion behavior of the surfaces could be adjusted by changing diameter of plasma jets: surface treated by MPJ with larger diameter has larger Wdot, F is larger, and SA on this patterned surface is therefore larger. Moreover, we also measured the perpendicular adhesive forces of water droplets depositing on SH surfaces and dot-patterned surfaces (Fig.S3, Supporting Information), the results also demonstrated that the surfaces treated by 10


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MPJs with larger diameters showed stronger adhesion towards water. In order to better illustrate sticky superhydrophobicity of the dot-patterned surfaces, we observed water droplet impact behavior on the original SH and the treated surfaces. Droplets with volume of 20 µL were allowed to free fall at a height of 2 cm to impact on surfaces. As shown in Fig.3, the droplets firstly spread rapidly on each surface dominated by inertial effects38. After reaching maximum spreading, the droplets started to retract and attempt to rebound. For the original SH surface with low adhesion, there is little resistance for droplet to move at the drop edge39, and the droplet retracted quickly with sufficient energy and rebounded from the surface with a short contact time (21.4 ms), as shown in Fig.3(a) and Video S4. Water droplet could also rebound from dot- patterned surface treated by MPJ60, but the contact time was longer (33.5 ms), which was attributed to more energy loss during retraction induced by strong adhesion of the hydrophilic dot. By contrast, for dot-patterned surfaces treated by MPJ150, MPJ300, MPJ500, although the droplets attempted to jump, stronger adhesion of the hydrophilic dot held them back (Fig.3, Video S4, Supporting Information). The different droplet impact behavior on SH and dot-patterned surfaces illustrates the change of surface adhesion induced by micro-plasma treatment.

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Fig.3 Droplet impact behavior of (a) original SH surfaces and (b-e) dot-patterned surfaces. 3.2 Anisotropic adhesion of line-patterned surfaces. The line-patterned surfaces showed different CAs and SAs in two directions (Fig.4). For instance, for 15 µL droplets on surface treated by MPJ150, the CA parallel to the line was about 92° (Fig.4 a1), while that perpendicular to the line was approximately 148° (Fig.4 a2). Droplets with volume of 15 µL could slide away from the line-patterned surfaces along the direction parallel to the line, but stuck on the surfaces along the direction perpendicular to the line (Fig.4 b, Video S2, Supporting Information). Additionally, the SAs of droplets slide on the line-patterned surfaces along the parallel direction increased with diameters of plasma jets used for pattern preparation: SAs of 30 µL droplets on line-patterned surfaces treated by MPJ60, MPJ150, MPJ300, MPJ500 were 12


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respectively 18°, 30°, 46° and 67° (Video S2, Supporting Information). When droplets contacting with the line-patterned surfaces, the contact interface is consisted of two parts, the droplet-SH surface interface, and the droplet-hydrophilic line interface. When the droplets sliding on the line-patterned surfaces along the direction parallel to it, they just need to conquer two forces: the viscous force of water, and the interfacial force between droplets and SH surfaces. However, for the droplets sliding along the direction perpendicular to the pattern, they are confined by the liquid tension, which is much larger than the two forces20. Therefore, it is much more difficult for droplets to slide along that direction, and the line-patterned surfaces with patterns of different sizes show anisotropic sliding behavior to droplets of different volumes. The anisotropic sliding behavior of line-patterned surfaces indicates their potential application values in droplet directional transportation. According to the measured velocities of droplets moving along different line-patterned surfaces with various tilted angles (Fig.S4, Supporting Information), the droplets moved faster across the narrow patterns treated by MPJs with smaller diameters (MPJ60, MPJ150). Additionally, when the tilted angle was relatively smaller (15° and 30°), the droplet spread along the wide patterns treated by MPJ500 and MPJ300, and could not roll off. The results demonstrate that the narrow patterns treated by MPJs with smaller diameters are relatively better patternings to realize rapid liquid transportation under various tilted angles.

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Fig.4 (a) CAs and (b) SAs of droplets with different volumes on line-patterned surfaces treated by different plasma jets. (a1-a2) CA images of 15 µL droplets on line-patterned surface treated by MPJ150 along the (a1) parallel direction and (a2) perpendicular direction. (b1-b2) SA images of 15 µL droplets on line-patterned surface along the (b1) perpendicular direction and (b2) parallel direction. 3.3 Surface morphology and repeatability. SEM images of the original SH surface and the patterned surface were observed to investigate the influence of plasma treatment on surface structures, as shown in Fig.5. We found that the surface morphology did not change obviously after being treated by MPJs [Fig.5(a-b)]. According to the XPS spectra of untreated SH surface and MPJ-treated pattern (Fig.S5, Supporting Information), the chemical content of SH surface had been obviously changed after MPJ treatment: O content increased largely while F content decreased apparently. The high resolution C1s peaks indicated that the contents of -CF2 and -CF3 decreased significantly after plasma treatment, while those of C-O and C=O increased obviously. Therefore, the wettability change of treated pattern is mainly attributed to chemical composition change induced by reactive particles in the plasma jet, which has been reported by previous research27, 40-44. 14


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Since the surface structures of the surface were well retained during plasma treatment, it is possible for the patterned surface to recover its original superhydrophobicity just by low surface energy modification. As shown in Fig.5 (c-d), wettability of the “line” pattern treated by MPJ500 could recover to be SH after being immersed in 1 wt% FAS-ethanol solution for just 2 min. The adhesive forces of the recovered SH surface were similar with the original SH surface even after 5 cycles [Fig.5 (e)], demonstrating excellent recoverability. Therefore, one SH substrate can be used for repeated maskless patterning (Fig.S6, Supporting Information), enabling rapid and low-cost reversible extreme wettability pattern fabrication. The reversibility of wettability prepared by MPJs has important application values in various fields, such as smart surfaces43 and bidirectional oil/water separation45.

Fig.5 Surface microstructures and repeatability of micro-plasma treatment. (a-b) Micro structures of (a) original SH surface and (b) plasma-treated surface; (c-d) pictures of 5 µL droplets on (c) SH surface and (d) plasma-treated patterned surface, the left droplet was photographed along the direction perpendicular to the line; (e) SAs of 30 µL droplets and adhesion forces. 3.4 Applications. The prepared patterns show relatively stronger adhesion towards 15



water droplets, which makes it possible to confine droplets in designated position and control droplets movements. As the adhesion behavior of treated surfaces can be adjusted by changing diameters of MPJs, the MPJs with different diameters have dissimilar advantages for preparing specific wettability patterns. The MPJs with larger diameters have much higher efficiency for patterning wider patterns and larger closed areas (Fig.S2, Supporting Information). Since the wider patterns show stronger adhesion towards water, they are more appropriate patterns for continuous transportation of water droplets. As shown in Fig.6 (a), water droplets could be continuously transported along the curve pattern that could be prepared by MPJ500 within 30 s (Video S3, Supporting Information). Antigravity water transportation [Fig.6 (b), Fig.S7, Supporting Information] could also be realized on the wedge-shape pattern prepared by MPJ500 within 2min (Video S3, Supporting Information). Furthermore, glycol droplets can also be transported along the curve pattern prepared by MPJ500 on superoleophobic substrate (Fig.S8, Video S3, Supporting Information). On the other hand, the smaller scanning areas of MPJ150 and MPJ60 enable them to be good candidates for preparing small-size patterns with relatively lower adhesion. The narrower patterns with lower adhesion are more preferable ones for rapid liquid transportation (Fig.S4, Supporting Information). The patterns with smaller sizes are also able to control droplets adhering and mixing, which is potentially valuable in cell screening. As shown in Fig.6 (c), 15 µL droplets were confined on the dot array patterns fabricated by MPJ150, and the droplets could be controlled to mix together 16


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using a syringe needle (Video S3, Supporting Information).

Fig.6 Applications of micro-plasma jets. (a1-a4) Continuous water transportation along the curve pattern prepared by MPJ500; (b1-b4) antigravity water transportation along the wedge-shape pattern prepared by MPJ500, the tilted angle was 5°; (c1-c4) droplet mixing on the dot array pattern prepared by MPJ150. 4. CONCLUSION In summary, we proposed a maskless MPJ treatment method to prepare hydrophilic patterns on SH aluminum surfaces. Contact angles, sliding angles, adhesive forces, droplet impact behavior and SEM images of the patterned surfaces were investigated. The results indicated that micro-plasma treatment could effectively adjust adhesion behavior of SH surfaces while had little influence on their surface structures. On the basis of these preferable features, rapid and repeatable preparation of various patterns was realized by micro-plasma treatment and low energy modification. The “dot” and “line” patterned surfaces demonstrated great water adhesion, and adhesion behavior 17



of these surfaces could be adjusted by changing diameters of MPJs. The MPJs with larger diameters (MPJ500, MPJ300) are better candidates for patterning larger closed areas and wider patterns, which have promising application values in open channel lab-on-chip systems (e.g., continuous and antigravity water transportations). On the other hand, the MPJs with smaller diameters (MPJ150, MPJ60) have smaller scanning areas, and are therefore capable of preparing small-size patterns with low adhesion, having bright application prospects in biomedical fields (e.g., loss-less liquid droplets mixing and cell screening).

ASSOCIATED CONTENT

Supporting Information

Figure S1: Micro-plasma treatment process. Figure S2: Investigation of wettability change of plasma-treated area. Figure S3: Adhesive forces of untreated SH surfaces and dot-patterned surfaces. Figure S4: Transportation velocities of 30 µL droplets on different patterns. Figure S5: XPS spectra of untreated superhydrophobic surface and pattern treated by MPJ500. Figure S6: Repeatability of micro-plasma treatment. Figure S7: Antigravity transportation of water droplets on wedge-shape pattern under various tilted angles. Figure S8: Transportation of glycol droplets on curve pattern. Video S1: Video of droplets sliding on dot-patterned surfaces. 18


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Video S2: Video of droplets sliding on line-patterned surfaces. Video S3: Applications. Scene 1: Preparing curve pattern. Scene 2: Continuous water droplets transportation on curve pattern. Scene 3: Glycol droplets transportation on curve pattern. Scene 4: Preparing wedge-shape pattern. Scene 5: Antigravity water droplets transportation on the wedge-shape pattern. Scene 6: Preparing “pentagram” pattern. Scene 7: Droplet mixing. Video S4: Water droplets impact behavior on the original SH and dot-patterned surfaces.

AUTHOR INFORMATION

Corresponding Author

*Tel.: 86-411-84708422. E-mail: [email protected]

AUTHOR CONTRIBUTIONS

Xin Liu, Jinlong Song, Jiyu Liu and Faze Chen conceived the research. Xin Liu and Jinlong Song supervised the research. Jiyu Liu, Guansong Wang, Faze Chen and Shuo Liu designed and carried out the experiments. All authors discussed the results and commented on the manuscript.

Notes

The authors declare no competing finical interest.

ACKNOWLEDGEMENT

19



The authors acknowledge the financial support from National Basic Research Program of China (Grant No.2015CB057304), National Natural Science Foundation of China (NSFC, Grant No. 51305060 and 51605078) and the Fundamental Research Funds for the Central Universities.

ABBREVIATIONS

MPJ, micro-plasma treatment; SH, superhydrophobic; CA, contact angle; SA, sliding angle; SEM, Scanning Electron Microscopy.

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Graphical Abstract 35x21mm (600 x 600 DPI)


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Maskless Hydrophilic Patterning of the Superhydrophobic Aluminum

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