Dual-Functional Superhydrophobic Textiles with Asymmetric Roll

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Dual-Functional Superhydrophobic Textile with Asymmetric Roll-Down/ Pinned States for Water Droplet Transportation and Oil-Water Separation Xiaojing Su, Hongqiang Li, Xuejun Lai, Lin Zhang, Xiaofeng Liao, Jing Wang, Zhonghua Chen, Jie He, and Xingrong Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15909 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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ACS Applied Materials & Interfaces

Dual-Functional Superhydrophobic Textile with Asymmetric Roll-Down/Pinned States for Water Droplet Transportation and Oil-Water Separation Xiaojing Su,a Hongqiang Li,*a,b Xuejun Lai,a,b Lin Zhang,a Xiaofeng Liao,a Jing Wang,a Zhonghua Chen,a Jie He*c and Xingrong Zeng*a,b a

College of Materials Science and Engineering, South China University of Technology,

Guangzhou 510640, China b

Key Lab of Guangdong Province for High Property and Functional Polymer Materials,

Guangzhou 510640, China c

Department of Chemistry, University of Connecticut, Storrs, CT 06269, USA

KEYWORDS: magnetic field, superhydrophobic textiles, pinned state, roll-down state, microdroplet transportation, oil-water separation

ABSTRACT: Superhydrophobic surfaces with tunable adhesion from lotus-leaf to rose-petal states have aroused much attention for their potential applications in self-cleaning, anti-icing, oilwater separation, microdroplet transportation and microfluidic devices. Herein, we report a facile magnetic-field-manipulation strategy to fabricating dual-functional superhydrophobic textile with asymmetric roll-down/pinned states on the two surfaces of textile simultaneously. Upon

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exposure to a static magnetic field, fluoroalkylsilane modified iron oxide (F-Fe3O4) nanoparticles in polydimethylsiloxane (PDMS) moved along the magnetic field to construct discrepant hierarchical structures and roughnesses on the two sides of textile. The positive surface (closer to the magnet, or P-surface) showed a water contact angle up to 165o and the opposite surface (or O-surface) had a water contact angle of 152.5o. The P-surface where water droplets easily slid off with a sliding angle of 7.5o appeared in the “roll-down” state as Cassie mode; while, the Osurface was in the “pinned” state as Wenzel mode, where water droplets firmly adhered even at vertical (90o) and inverted (180o) angles. The surface morphology and wetting mode were adjustable by varying the ratios of F-Fe3O4 nanoparticles and PDMS. By taking advantage of the asymmetric adhesion behaviors, the as-fabricated superhydrophobic textile was successfully applied in no-loss microdroplet transportation and oil-water separation. Our method is simple and cost-effective. The fabricated textile has the characteristics of superhydrophobicity, magnetic responsiveness, excellent chemical stability, adjustable surface morphology and controllable adhesion. Our findings conceivably stand out as a new tool to fabricate functional superhydrophobic materials with asymmetric surface properties for various potential applications.

INTRODUCTION Superhydrophobic materials with excellent water repellency have aroused enormous interests in recent years, for their fundamental research and potential industrial applications in selfcleaning,1,2 oil-water separation,3,4 microfluidic devices,5,6 anti-icing,7,8 anti-pollution,9 and so forth. Meanwhile, with wetting theory comprehensively developing, superhydrophobic surfaces


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have been further classified into five different states according to the contact angle (CA) hysteresis.10 As one of the most common states, the Wenzel state is described as a surface on which water droplets are pinned in a wet-contact mode and unable to slide off,11,12 as given in the following equation,

cos(θ ) = r cos(θ0 )

(1)

where θ, θ0, and r represent the CA of rough surfaces, the CA of flat surfaces, and the ratio of actual surface area with respect to the projected surface area, respectively. Superhydrophobic surfaces in Wenzel state, such as rose petals, are suitable for no-loss microdroplet transportation due to its special character that water droplets are closely sticky to the surface even at an inverted angle.13,14 In contrast, the Cassie state is referred to a surface on which water droplets are unable to penetrate into rough cavities and form a non-wet-contact mode with a sliding angle (SA) below 10o,15,16 as described in equation 2,

cos(θ ) = rf SL cos(θ0 ) + f SL − 1　

(2)

where fSL is the fraction of solid surface area in contact with liquid. Superhydrophobic surfaces in Cassie state, such as lotus leaves, have exhibited the prominent superiority in self-cleaning application by means of its non-adhesive characteristics.17,18 Superhydrophobic materials in either pinned (Wenzel) or roll-down (Cassie) states are useful in a broad range of applications. Control of superhydrophobic surface with those two states is mainly achieved by tuning surface morphologies or compositions of surface coating under external stimuli such as magnetic field,19,20 electrical current,21,22 UV irradiation,23,24 temperature25,26 and pH value.27-29 For example, Lee et al. showed a number of magnetorheological films with variable CAs from 100o to 160o and SAs from 180o to 10o, by utilizing the alignment of modified carbonyl iron particles to control the surface topology under magnetic field.30 However, at the pinned state (high


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adhesion to water), such surfaces were unable to reach superhydrophobic. By the transformation between cis and trans states of the polymer containing fluorinated azobenzene moiety under a short UV irradiation, Jonas et al. designed a surface on rough silicon wafer to reversibly switch its wettability.31 Li et al. also achieved a superhydrophobic surface with transition mobility of water droplets from rollable state to pinned state by changing the molecular chain adaptability corresponding to the phase transition of side-chain liquid crystal polymers under different temperatures.32 However, these tunable adhesions are only responsive to the polymers with specific chemical structures. The above-mentioned transformations in adhesion states of superhydrophobic materials are achieved by the temporary introduction of external stimuli, which are inconvenient for practical applications. To the best of our knowledge, there are no reports on superhydrophobic materials simultaneously having both roll-down and pinned states in literature. We herein report a new strategy to fabricating dual-functional superhydrophobic textile with asymmetric roll-down and pinned states simultaneously on its two surfaces, by combining the magnetic response of fluoroalkylsilane modified iron oxide (F-Fe3O4) nanoparticles (NPs) to construct and regulate roughness with the low-surface-energy PDMS to endow hydrophobicity. After a simple dipping step with a solution containing F-Fe3O4 NPs and Sylgard 184 (see Figure 1), the textile was cured at 80 oC in the presence of a SmCo magnet. Driven by the magnetic field, superparamagnetic F-Fe3O4 NPs moved toward the top coating layer in close contact with the magnet, to construct distinct micro-nanostructures on the two surfaces of the textile. On the top surface of the textile (denoted as the positive surface or “P-surface” of the textile), F-Fe3O4 NPs showed more accumulation that resulted in hierarchical structures with high roughness. The P-surface where water droplets easily slid off with a sliding angle of 7.5o appeared in “roll-down”


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state as Cassie mode, due to the presence of many air pockets on the surface. On the opposite surface (or the O-surface), less F-Fe3O4 NPs appeared and this yielded a smoother surface accompanying with the curing of the PDMS layer. The O-surface was in Wenzel mode with the pinned state of water droplets. Given the asymmetric wettability in roll-down/pinned states, the as-fabricated textile was successfully applied in no-loss microdroplet transportation, and the release of water droplets was easily driven by a magnet. Additionally, the textile was also demonstrated to have excellent separation efficiency and reusability in oil-water separation process. Different from traditional methods for fabricating superhydrophobic materials, the novel magnetic-field-manipulation approach is facile, cost-effective and easy to control adhesion, and the fabricated superhydrophobic textiles with asymmetric roll-down/pinned states have great potential in a wider range of application fields such as microfluidics, biomedical devices and oilpollution treatment. Our findings conceivably stand out as a new tool to fabricate multifunctional superhydrophobic surfaces and coatings by using the combination of magnetic particles and magnetic-field manipulation. EXPERIMENTAL SECTION Materials. Fe3O4 NPs (particle size: 20 nm), oil red O, and 1H, 1H, 2H, 2Hperfluorooctyltriethoxysilane (PFOTES) were purchased from Aladdin reagent Co., Ltd (China). Polydimethylsiloxane (PDMS), a two-component crosslinkable resin (Sylgard 184), was supplied by Dow Corning Co., Ltd (USA). Hexane (A.R.), ethanol (A.R.), tetrahydrofuran (THF, A.R.), trichloromethane (A.R.), toluene (A.R.), hydrochloric acid (HCl, 37%, A.R.) and sodium chloride (NaCl, A.R.) were obtained from Guangzhou chemical reagent factory (China). Cylindrical SmCo magnet with a diameter of 50 mm and a height of 10 mm was provided by Yongsheng magnetic materials corporation (China). Polyester textile (plain weave textile, 155


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g/m2) and stainless steel mesh (30 meshes) were bought from local stores. All chemicals were used as received without further purification, and deionized water was used for all the experiments and tests. Surface Modification of Fe3O4 NPs. Briefly, 1g of Fe3O4 NPs was dispersed in 100 mL of ethanol by an ultrasonic unit (Fisherbrand, USA) with frequency of 37 kHz and power of 100% for 5 min. 500 µL of PFOTES and 1 mL of deionized water were then added into the above solution. The mixture was further agitated for 12 h under N2 at 30 oC. The final product was purified by centrifuging at a speed of 10000 rpm for 20 min and washing with ethanol. PFOTEScoated Fe3O4 NPs was denoted as F-Fe3O4 NPs. Fabrication of Dual-Functional Superhydrophobic Textile. The fabrication process of dual-functional superhydrophobic textile is presented in Figure 1. Firstly, PDMS and its curing agent of Sylgard 184 at a weight ratio of 10:1 were dissolved in THF to form a uniform solution with a total mass concentration of 8 wt%. A predetermined amount of F-Fe3O4 NPs (e.g. the mass ratio of F-Fe3O4 to PDMS was varied at 0.1, 0.25, 0.4 and 0.55) was added to the PDMS solution. The mixture was sonicated for 30 min. Next, a piece of textile was cleaned with sonicating in ethanol for 10 min and then taken out to dry at 60 oC for 0.5 h. The dried textile was immersed into the above solution and stirred with a speed of 300 rpm for 30 min at ambient condition. The textile was subsequently removed from the solution and dried at room temperature for 10 min under a cylindrical SmCo magnet at a distance of 2 mm. The sample was then cured at 80 oC for 20 min in the presence of the magnet. The intensity of the magnetic field was measured by a Tesla Meter (F. M. Bell, USA) and the remanence of the magnet was 180 ± 10 mT. Here, the surface facing to the magnet was denoted as the positive surface (P-surface) of


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textile; and the other side of textile away from the magnet was denoted as the opposite surface (O-surface) accordingly. Water Droplet Transportation. The textile prepared with mass ratio of F-Fe3O4 to PDMS at 0.25 was selected to achieve no-loss water droplet transportation. A 6-µL of water droplet was firstly dropped onto the P-surface of a piece of as-fabricated textile, and then was adhered by the O-surface of another textile. With the O-surface moving to a position where there was a container just below, a magnet was directly placed above O-surface to promote the sliding off of water droplet into container. Oil-Water Separation. The textile prepared with mass ratio of F-Fe3O4 to PDMS at 0.4 was selected for oil-water separation. Stainless steel mesh was folded to be a small lidless box and covered with superhydrophobic textile outside. The box was put into a container containing 100 mL of hexane colored with red oil and 300 mL of water. By manipulating magnet and pump, oil was separated from water. Additionally, stainless steel mesh was bent into a miniature flat groove, and wrapped with superhydrophobic textile inside. A mixture containing 100 mL of oil and 100 mL of water colored with methylene blue was poured on the top of the groove, oil easily permeated the textile and fell into the under container, while water flowed to the end of the groove and was collected in water container, thus oil-water mixture was separated. Characterization. The morphology of F-Fe3O4 NPs was observed by JEM-2100 transmission electron microscopy (TEM, JEOL, Japan) at an accelerating voltage of 200 kV. The surface architecture of superhydrophobic textiles was examined using a EVO 18 scanning electron microscope (SEM, Carl Zeiss Jena, Germany) at an accelerating voltage of 10.0 kV. Elemental composition was characterized by energy dispersion spectrum (EDS) attached to SEM and X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD, UK) with Mg Kα


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monochromatic X-ray source. The surface microstructure and roughness of fibers were tested with atomic force microscopy (AFM, Bruker Multimode 8, USA) in tapping mode with a scanning rate of 0.977 Hz at a 3 µm × 3 µm scale. Fourier transform infrared (FT-IR) spectroscopy was carried out on a Bruker Tensor 27 spectrometer (Bruker Optics, Germany) from 4000 to 400 cm-1 with a resolution of 4 cm-1. X-ray diffraction (XRD) spectra were recorded on a powder diffractometer (PANalytical, Holland) using Cu Kα radiation source of 1.54060 Å with a scanning rate of 1 o/min at room temperature. Magnetic studies of samples were performed on a PPMS integrated physical property measurement system (Quantum, USA) at ambient condition. Contact angle (CA) was measured with a contact angle meter (DSA100, Germany) equipped with a video capture using 6 µL of water as probe liquid at room temperature. The CAs of all samples were obtained from at least three different locations to calculate the mean value. Water shedding angle (WSA),33,34 instead of sliding angle, was used to evaluate the rolling property of textiles. Typically, the sample was fixed onto a glass substrate placed on a tilting table of contact angle meter, and the needle of a syringe was mounted above the sample with a distance of 10 mm. To determine WSA, the tilting table was firstly set at 30o, and water droplet was then released onto the sample at three different positions. If all droplets completely bounced or rolled down, the tilting angle was decreased by 2o (WSA>10o) or 0.5o (WSA≤10o). The measurement procedure was repeated until at least one droplet was unable to slide off the surface. The lowest tilting angle with all water droplets completely rolling down or bouncing off the surface was recorded as WSA. RESULTS AND DISCUSSION Preparation and Characterization of F-Fe3O4 NPs. Fe3O4 NPs is used to build up the roughness of the surface coating. Generally, pristine Fe3O4 NPs is hydrophilic and immiscible


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with hydrophobic PDMS, which will largely affect the formation of micro-nano structures during curing. Therefore, Fe3O4 NPs is first modified by fluoroalkylsilane through the hydrolysis and condensation reaction of alkoxy groups. The TEM images of Fe3O4 NPs before and after modification are shown in Figure 2. The average size of pristine Fe3O4 and F-Fe3O4 NPs was about 15-30 nm. No obvious change in size and size distribution was observed after surface modification. As can be seen from FT-IR spectra (Figure 2c), the absorption band of F-Fe3O4 NPs at 3435 cm-1 assigning to the stretching vibration of hydroxyl groups was obviously weaker than that of pristine Fe3O4 NPs. Several new absorption bands appeared on the spectrum of FFe3O4 NPs at 3000-2800 cm-1, 1250-1100 cm-1 and 1003 cm-1, corresponding to the stretching vibrations of C-H, C-F and Si-O bonds, respectively. It was clear that Fe3O4 NPs had been successfully grafted with fluorinated chains, which led to the formation of surface hydrophobicity of F-Fe3O4 NPs. F-Fe3O4 NPs could be easily dispersed in PDMS stock solution; and no obvious aggregation was observed after ultrasonic treatment. The XRD patterns of pristine Fe3O4 and F-Fe3O4 NPs are shown in Figure 2d. It was notable that the two curves were similar and appeared five diffraction peaks at 30.2o, 35.6o, 43.2o, 57.2o and 62.8o, corresponding to (220), (311), (400), (511) and (440) planes of inverse spinel Fe3O4 particles,35 respectively, implying that surface modification had little effect on the crystalline characteristic of Fe3O4 NPs. The magnetic properties of pristine Fe3O4 and F-Fe3O4 NPs are also examined and exhibited in Figure 2e. Both NPs were apparently superparamagnetic. The responses of the two NPs to magnetic field were linear but kept almost stable at high field, and appeared to be standard paramagnetic characteristics without hysteresis phenomenon after the removal of magnetic field. The magnetization value of F-Fe3O4 NPs at 34.2 emu/g was slightly


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lower than that of Fe3O4 at 43.4 emu/g. Clearly, F-Fe3O4 NPs still maintained the capability to respond and move to construct roughness under an external magnetic field. Fabrication and Formation Mechanism of Dual-Functional Superhydrophobic Textile with Roll-Down/Pinned States. To achieve the asymmetric surface coating on the textile, magnetically responsive F-Fe3O4 NPs was utilized to construct different roughness on the two surfaces of the textile driven by a magnetic field. A piece of cleaned textile was firstly immersed into the THF solution containing F-Fe3O4 NPs and Sylgard 184 at a predetermined mass ratio, then dried at room temperature for 10 min under a cylindrical SmCo magnet at a distance of 2 mm, and subsequently cured at 80 oC for 20 min in the presence of the magnet. Figure 3 shows the SEM images of P-surface and O-surface of the superhydrophobic textile with mass ratio of F-Fe3O4 to PDMS at 0.25. Different from the smooth surface of textile only coated with PDMS (Figure S1), it was obvious that the P-surface of the textile appeared many protuberant embossments with microscale sizes from 1 to 2 µm and nanoscale widths from 20 to 400 nm (Figure 3a and 3b). Moreover, the CA of P-surface obviously increased to 165o from 139o and the SA reached 7.5o. This is known as the “roll-down” state in which water droplets easily slide off. In comparison, the O-surface was only covered with nano grains smaller than 200 nm, and the CA of the O-surface attained 152.5o (Figure 3c and 3d). However, we found that water droplets were unable to roll off even O-surface was tilted vertically or turned upside down, which was considered to be in “pinned” state. Under the same fabrication conditions in the absence of the magnetic field, the two sides of the textile showed very similar morphology and hydrophobicity. The fiber surface was relatively smooth with many nanoscale grains (Figure S2a), similar to what presented on the O-surfaces. The symmetric textile had a CA of 154o, while water droplet was firmly pinned on both surfaces even with the tilt angle at 90o or 180o.


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Therefore, the manipulation of magnetic field is crucial to regulate the morphology and wettability of the textile surfaces. To testify whether F-Fe3O4 NPs was located at the top PDMS coating layer on P-surface of the textile, a piece of the fabricated textile was immersed in HCl solution with a pH=0 for 20 h. It was found that the protuberant embossments in the coating of P-surface constructed by F-Fe3O4 NPs disappeared, due to the etching of F-Fe3O4 NPs by acid. A large number of holes with micro and nano sizes appeared on the P-surface were seen (Figure S3a). Etched P-surface also showed a lower CA of 145o. However, no obvious holes were observed on the O-surface after HCl etching (Figure S3b). Instead, some nano-grains appeared, and the coating layer became thinner for the degradation of the outmost PDMS layer by strong acid solution. It further demonstrated that the construction of discrepant hierarchical structures by F-Fe3O4 NPs on the two sides of textile with the manipulation of magnetic field. Furthermore, no Fe 2p peaks were detected on both P- and O-surface by XPS, suggesting that F-Fe3O4 NPs was located underneath the PDMS coating layer (> 10 nm, Figure S4). To further investigate the differences in micro-nanostructures of P-surface and O-surface, AFM was used to examine the three-dimensional and two-dimensional topography of textile surfaces. Again, the pristine fibers appeared to be very smooth (Figure S5); and the roughness factor (r), defined as the ratio of actual surface area to projected surface area, was only 1.009. It was notable that the P-surface of the superhydrophobic textile appeared to be a hierarchical and rough structure constructed by F-Fe3O4 NPs and its aggregations. It had a roughness factor r of 1.133, as shown in Figure 4a and 4c. Furthermore, the root mean square roughness (RMS) as another important parameter to evaluate roughness,36 reached 32.5 nm. On contrary, the r and


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the RMS of the O-surface were only 1.014 and 16.5 nm, respectively (Figure 4b and 4d). The AFM results were well-consistent with the SEM analysis. Similarly, when increasing the mass ratio of F-Fe3O4 NPs to PDMS to 0.40, the two surfaces of textile still presented the asymmetric roll-down/pinned states. P-surface became coarser with lots of micro-nano structure; and it showed a CA of 167o and a SA of 7o (Figure 5a and 5b). On the O-surface, more nano-grains with the larger size from 100 to 300 nm were seen, and water droplet was strongly pinned as well (Figure 5c and 5d). However, some bigger F-Fe3O4 aggregates appeared on the surface of the textile fabricated without magnetic field, and the CA and SA were 157o and 18o, respectively (Figure S2b). From AFM 3-D surface topography and the corresponding 2-D image, the P-surface was hierarchically rough with an r of 1.200 and a RMS of 48.9 nm (Figure S6a and S6c); while the O-surface only covered with nanoscale grains with an r of 1.050 and a RMS of 19.6 nm (Figure S6b and S6d). If further increasing the mass ratio to 0.55, both P- and O-surfaces exhibited “symmetric” roll-down state. The SAs of the Psurface and O-surface were 7.5o and 14o, respectively (Figure S7). Apparently, the concentration of F-Fe3O4 NPs was too high to construct large roughness on O-surface even under the magnetic field. However, when the mass ratio of F-Fe3O4 to PDMS decreased to 0.1, the topologies on the Psurface and O-surface showed no obvious differences as shown in Figure S8. Both surfaces were covered with a number of scattered nanoscale grains and they were in pinned states having a CA of >150o. Furthermore, the high mass concentration of PDMS also had an obvious influence on the morphology of P-surface and O-surface of textile (Figure S9). With a higher mass concentration of PDMS at 10 wt% (the mass ratio of F-Fe3O4 NPs to PDMS was 0.25), although the CAs were above 150o, both P-surface and O-surface were in the pinned state as Wenzel mode.


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It was mainly because that F-Fe3O4 NPs was difficult to move along magnetic field lines in highviscosity PDMS, only leading to the formation of nano grains on the two surfaces. Based on the above results, the formation mechanism of the asymmetric superhydrophobic textile with roll-down/pinned states can be deduced as follows. The textile was firstly dipped and coated by uniformly distributed F-Fe3O4 and PDMS. Under an external magnetic field, considerable amounts of F-Fe3O4 NPs gradually moved to the top coating layer of the P-surface along the magnetic field. After the curing of PDMS, F-Fe3O4 NPs and its aggregations were immobilized within the crosslinked PDMS layer to construct hierarchical structures with large roughness. Therefore, the P-surface is in Cassie mode and roll-down state. Water droplets can trap many air pockets on the P-surface as shown in Figure 6a. On the O-surface, F-Fe3O4 NPs moved away; and thus, the O-surface became much smoother (Figure 6b). Consequently, the Osurface has a lower hydrophobicity and it is in the pinned state as Wenzel mode where water droplets can easily penetrate the cavities of structures. Our hypothesis is also supported by SEMEDS analysis. The P-surface showed a higher Fe-to-Si weight ratio (1.85) than that on the Osurface (1.70) (Figure S10). According to the wetting theory, the wetting mode is mainly determined by wetting pressure (PW) and antiwetting pressure (PC).37 PC is closely related to the capillary pressure, and can be calculated according to the following equation38,39 PC = σ cos θ A

LC AC

(3)

where σ is surface tension of liquid, θA is advancing contact angle of flat surface, LC is capillary perimeter and AC is capillary area. It is clear that both the selection of materials and the specific geometry of surface roughness have the significant influence on anti-wetting pressure. On the superhydrophobic surface with hierarchical structures, the formation of capillary pressures is


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attributed to not only microstructure (PCM), but also nanostructure (PCN).37 When PCM > PW, PCN > PW, water droplets are unable to wet the surface, giving a roll-down state. If PCM < PW < PCN, water droplets can wet the microstructures on the surface but not nanostructures. Whereas, if PCM < PW, PCN < PW, water droplets fully penetrate into the cavities of the surface, and become very difficult to roll down, known as the pinned state. In our study, the discrepant hierarchical structures, constructed by the magnetic field induced migration of F-Fe3O4 NPs, likely led to the higher PCM and PCN than PW on P-surface and the lower PCN than PW on O-surface of the textile. Water Droplet Transportation and Oil-Water Separation. Taking advantage of the unique asymmetric wetting properties, the superhydrophobic textile shows very promising applications in no-loss water droplet transportation and collection. The scheme of the magnetic field controlled water droplet transportation is given in Figure 7a. When a water droplet was dropped on the P-surface in roll-down state, it could easily roll down. However, once contacting with the O-surface of another coated textile, the water droplet was quickly captured due to a higher adhesion force. Subsequently, the water droplet was transported to the location as needed. When placing a magnet over the textile, the water droplet adhered to the O-surface was quickly released and collected in the container without mass loss (Figure S11). This transportation process is based on the asymmetric roll-down/pinned states and magnetic responsiveness of the textile. Figure 7b displays the detailed process of water droplet transportation (also see Video S1). With

the

increasing

industrial

sewage

and

deteriorating

marine

environment,

superhydrophobic material has been considered to be an ideal candidate for oil-water separation.40 A simple oil-water separation device with lidless box shape can be designed using asymmetrically coated textile and a stainless steel mesh. It can freely float on water like a mini


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“boat” when put into hexane-water mixture (Figure S12). Due to superhydrophobicity of the textile, water was impeded while hexane automatically penetrated into the boat through the textile. After enough hexane was collected in the mini boat, a pumping system was utilized to transfer hexane into a container. Conveniently, the floating box also could be magnetically manipulated to move to capture more hexane on water in whole area by a magnet (Video S2). After several pumping-magnetic manipulation circles, hexane was cleaned up, leaving a clear water surface. The collection process of oil by the superhydrophobic mini boat is displayed in Figure 8a. The mini boat driven by magnet is only suitable for separating light oil from water. Accordingly, a miniature flat groove for separating various oil-water mixtures was also designed and made with stainless steel mesh and superhydrophobic textile as illustrated in Figure 8b. When oil-water mixture was poured in the groove covered with superhydrophobic textile, oil rapidly permeated through the textile and dropped into the oil container, while water easily flowed to the end of the groove and was collected in water container. The separation processes of hexane-water and trichloromethane-water mixtures are displayed in Figure 8c and 8d, respectively (Video S3 and S4). Separation efficiency, defined as the ratio of the weight of oil collected in oil container to that added initially, was measured to be 93.5% for hexane-water mixture and 92.2% for trichloromethane-water mixture. In practical oil-water separation, reusability is an important factor for the superhydrophobic material, which is mainly determined by its chemical stability.41,42 Hence, the dual-functional superhydrophobic textile was separately immersed in hexane, toluene, trichloromethane, NaCl solution (3.5 wt%) and acidic/alkaline solutions to testify the stability in terms of superhydrophobicity. Notably, the CAs of P-surface of the textiles remained above 150o even after being immersed in different solvents for 15 days


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(Figure S13). Besides, the P-surface of dual-functional textile kept superhydrophobic after being immersed in acidic/alkaline solutions with pH value from 0 to 13 for 12 h (Figure S14), exhibiting excellent chemical stability. CONCLUSIONS In summary, we demonstrated a magnetic-field-manipulation approach to fabricating dualfunctional superhydrophobic textiles with asymmetric roll-down/pinned states. In the presence of the magnet, F-Fe3O4 NPs in PDMS migrated along the magnetic field, resulting in the formation of discrepant hierarchical structures and roughnesses on the two sides of textile. The P-surface close to the magnet in the “roll-down” state showed a CA of 165o and a SA of 7.5o, as Cassie mode; while, the O-surface away from the magnet was in the “pinned” state as Wenzel mode, where water droplets firmly adhered even at vertical (90o) and inverted (180o) angles. The accumulation of F-Fe3O4 NPs on the P-surface along the magnetic field within the crosslinked PDMS layer constructed hierarchical structures with large roughness. Although the surface morphologies on the two surfaces were dependent on the mass ratio of F-Fe3O4 NPs to PDMS and the concentration of PDMS, the magnetic field was found to be essential to create asymmetric hydrophobicity on the two surfaces. The dual-functional superhydrophobic textile with roll-down/pinned states was successfully applied in no-loss microdroplet transportation and oil-water separation. The fabrication procedure is facile, cost-effective, easy to control adhesion, and the novel smart dual-functional superhydrophobic textiles with asymmetric roll-down/pinned states have great potential in a wider range of application fields such as microfluidics, biomedical devices and the treatment of oil pollution.


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FIGURES

Figure 1. Schematic illustration for fabricating dual-functional superhydrophobic textile with roll-down/pinned states.


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Figure 2. TEM images of (a) Fe3O4 and (b) F-Fe3O4 NPs. (c) FT-IR spectra, (d) XRD patterns and (e) magnetization curves at 298 K of Fe3O4 (black) and F-Fe3O4 NPs (red).


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Figure 3. SEM images (inset was the CA optical image at different tilting angles) of (a) Psurface and (c) O-surface of textile when the mass ratio of F-Fe3O4 NPs to PDMS was 0.25. (b) and (d) were the corresponding higher magnification images.


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Figure 4. AFM 3-D surface structures and the corresponding 2-D images of (a), (c) P-surface and (b), (d) O-surface, respectively. The mass ratio of F-Fe3O4 NPs to PDMS was 0.25.


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Figure 5. SEM images (inset was the CA optical image at different tilting angles) of (a) Psurface and (c) O-surface of textile with mass ratio of F-Fe3O4 NPs to PDMS at 0.4. (b) and (d) were the corresponding higher magnification images.


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Figure 6. Different superhydrophobic states of (a) P-surface and (b) O-surface.

Figure 7. (a) Schematic process and (b) detailed process of water droplet transportation.


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Figure 8. (a) Process of the lidless box made by superhydrophobic textile and stainless steel mesh for oil spill accidents. (b) Schematic illustration of miniature flat groove made by superhydrophobic textile and stainless steel mesh for separating various oil-water mixtures. Process of (c) hexane-water and (d) trichloromethane-water separation with the miniature flat groove, respectively.


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ASSOCIATED CONTENT Supporting Information The following files are available free of charge. Additional SEM image of textile only coated with PDMS (Figure S1), SEM images of textile with the mass ratio of F-Fe3O4 NPs to PDMS at 0.25 and 0.4 without external magnetic field (Figure S2), SEM images of P-surface and O-surface after immersing in aqueous HCl solution with a pH=0 for 20 h (Figure S3), XPS spectra of P-surface and O-surface (Figure S4), AFM images of pristine textile (Figure S5), AFM images of P-surface and O-surface with mass ratio of F-Fe3O4 NPs to PDMS at 0.4 (Figure S6), SEM images of P-surface and O-surface with mass ratio of F-Fe3O4 NPs to PDMS at 0.55 (Figure S7) and 0.1 (Figure S8), SEM images of P-surface and O-surface with mass concentration of PDMS at 10 wt% (Figure S9), EDS spectra of Psurface and O-surface (Figure S10), magnetization curves of the fabricated textile at different mass ratios of F-Fe3O4 NPs to PDMS (Figure S11), scheme of lidless box for oil spill accidents (Figure S12), CA changes of the superhydrophobic textiles after immersing in different solvents for 15 days (Figure S13) and acidic/alkaline solutions with pH value from 0 to 13 for 12 h (Figure S14), process of water droplet transportation (Video S1), hexane-water separation process with a mini boat (Video S2), hexane-water and dichloromethane-water separation process with a miniature flat groove, respectively (Video S3 and S4). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected], [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The work was financially supported by the National Natural Science Foundation of China (51403067) and the Pear River S&T Nova Program of Guangzhou (201710010062). The authors are grateful for the generous help from Dr. Li Gong for AFM measurement at analytical and testing center, Sun Yat-Sen University. REFERENCES (1) Fu, Y.; Jiang, J.; Zhang, Q.; Zhan, X.; Chen, F. Robust Liquid-Repellent Coatings Based on Polymer Nanoparticles with Excellent Self-Cleaning and Antibacterial Performances. J. Mater. Chem. A 2017, 5, 275-284. (2) Suryaprabha, T.; Sethuraman, M. G. Fabrication of Copper-Based Superhydrophobic SelfCleaning Antibacterial Coating over Cotton Fabric. Cellulose 2016, 24, 395-407. (3) Gao, J.; Huang, X.; Xue, H.; Tang, L.; Li, R. K. Y. Facile Preparation of Hybrid Microspheres for Super-Hydrophobic Coating and Oil-Water Separation. Chem. Eng. J. 2017, 326, 443-453. (4) Su, X.; Li, H.; Lai, X.; Zhang, L.; Wang, J.; Liao, X.; Zeng, X. Vapor-Liquid Sol-Gel


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Dual-Functional Superhydrophobic Textiles with Asymmetric Roll

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