Silicone Oil Swelling Slippery Surfaces Based on Mussel-Inspired

Sep 11, 2017 - Meanwhile, profited from the nanoscale topography of substrate, dust with ultralight weight and microscale size would not be trapped in...
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Silicone Oil Swelling Slippery Surfaces Based on Mussel-Inspired Magnetic Nanoparticles with Multiple Self-Healing Mechanisms Biyu Jin, Mingzhu Liu, Qinghua Zhang,* Xiaoli Zhan, and Fengqiu Chen Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China

Langmuir 2017.33:10340-10350. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/17/19. For personal use only.

S Supporting Information *

ABSTRACT: In this work, a novel substrate building block, magnetic Fe3O4 nanoparticles armed with dopamine molecules were developed via musselinspired metal-coordination bonds. Combined with glycidyl methacrylate, polydimethylsiloxane propyl ether methacrylate, and diethylenetriamine, the original silicone oil swelling slippery liquid-infused porous surfaces (SLIPS) were first prepared by reversible coordinate bonds and strong covalent bonds cross-linking process. The matrix mechanical characteristics and surface physicochemical properties were systematically investigated. Results showed that the mechanical property of copolymer matrix and surface wettability of SLIPS can be remarkably recovered, which were due to the synergistic interactions of magnetic nanoparticles’ intrinsic photothermal effect, reversible Fe-catechol coordination, and diffused lubricating liquid. After irradiating with sunlamp for 2 h and sequentially healing for 10 h under ambient conditions, the crack almost disappeared under optical microscopy with 78.25% healing efficiency (HEf) of toughness, and surface slippery was completely retrieved to water droplets. The efficient self-heal of copolymer matrix (66.5% HEf after eighth cutting−healing cycle) and recovering of slipperiness (SA < 5° and 5° < SA < 17° after fourth and eighth cutting−centrifuging−healing cycles, respectively) would extend longevity of SLIPS when subjected to multiple damages. Moreover, the prepared SLIPS displayed superb self-cleaning and liquid-repellent properties to a wide range of particulate contaminants and fluids.

1. INTRODUCTION Slippery liquid-infused porous surfaces (SLIPS) inspired by Nepenthes pitcher plants have attracted widespread attention due to its exceptional surface properties.1−4 Featuring the molecular-scale smooth liquid lubricants and the immiscibility of them with ambient fluid, SLIPS showed extremely weak interaction with ice,5,6 marine organism,7 bacteria,8 eukaryotic cells,9 and so forth. Typical strategy for fabricating SLIPS involves the infusion of lubricant into rough-structured substrates with low surface energy, which was similar to the fabrication process of superhydrophobic materials.10−14 However, it forces these SLIPS to inherit the disadvantage of superhydrophobic materials such as vulnerable roughness structure and sophisticated preparation technics.15,16 Meanwhile, on account of the migrating, evaporating, or leaking of the lubricant layer,17 roughness structure would be exposed in air and damaged. Thus, the previous defect-free surface would create sites for adhesion and lose its’ durability. Fortunately, a new strategy emerged which involves the use of selfreplenishing materials.4,18−20 These material can encapsulate or swell lubricant in matrix because of the comprised crosslinked networks and large free-volume structures. Due to the surface energy driven capillary action, the lubricant would autonomously secrete at the topmost surface.21,22 Although unremitting efforts have been made to enhance the durability, © 2017 American Chemical Society

problems still remain unsolved that once the substrates destroyed by external force, it would not be good reservoir for lubricant storage. Albeit surface wettability recovering of damaged areas can be achieved temporarily, lifetime-long functionalities will be snatched away eventually with rapid lubricant losing. Living organisms in nature increased longevity and the ability to adapt to environmental changes through self-healing process. Inspired by this fitness-enhancing functionality, scientists and engineers have been incorporating self-healing capabilities into synthetic materials by encapsulating healing agents or incorporating dynamic bonds.23 Introducing self-healing property to matrix of SLIPS may hint a promising pathway for efficiently extending those materials’ longevity when subjected to damage. For instance, self-regulated secretion SLIPS consisting of liquid-storage compartments in a supramolecular uPDMS matrix (copolymer of urea and polydimethylsiloxane) was reported to show completely self-healing property of matrix after 72 h.24 Through thermal stimuli selfrepair process, the textured surface can survive physical/ chemical damage because of vaporization and thermal Received: August 1, 2017 Revised: September 11, 2017 Published: September 11, 2017 10340

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Langmuir Scheme 1. Schematic Representation of PG-MNP Nanocomposites Synthesisa

a

(a) Synthesis of DA modified Fe3O4 MNP; (b) synthesis of P(GMA-r-PDMSPGMA) copolymer; (c) preparation of PG-MNP.

migration of excess low energy silane molecules.25 Building an appropriate polymer network that synchronously keep balance among high healing efficiency, considerable mechanical strength along with slippery surface wettability is the key issue. Compared with the extrinsic self-healing, intrinsic self-healing is more attractive as it enables multiple rehabilitations of the same place.26 Thus far, various covalent and noncovalent interactions have been explored for the construction of intrinsic self-healing materials, including Dies-Alder reactions,27,28 disulfide exchange,29,30 host−guest recognition,31,32 hydrogen bonds,33−35 electrostatic interactions,36,37 metal−ligand interactions,38,39 and so forth. However, some of these approaches require the input of external stimuli40,41 or the assistance of catalyst,42 solvents,29,43,44 and plasticizers.45 Of the other examples of autonomous self-healing materials, most are based on moisture-sensitive hydrogen bonding and therefore lack stability against moisture.46 Meanwhile, the most serious dilemma of noncovalent self-healing materials is the poor mechanical performance,38 needless to say, for practical application with abrasion-bearing properties. As such, smart methodology including dual-cross-linking structure47,48 has been explored following the principle of bringing reversible weak bonds into polymers with strong covalent bonds thus achieving strength and toughness.49 The strong bonds, mostly covalent, form fundamental structure and maintain the integrity of the material. The dynamic weak bonds, acting as sacrificial bonds, rupture upon breaking to dissipate energy and reform

afterward to impart internal self-recovery and surface selfhealing of the damaged materials.50 On the other hand, incorporation of inorganic nanoparticles (NPs) into material matrix has also been verified to reinforce the mechanical strength.51 Surface functionalization with reactive functional molecule ligands via reversible interfacial cross-links can not only give rise to materials’ intrinsic self-healing capability but also increase the mechanical properties of nanocomposites through specific reactions with the polymer chains.52 In addition, nanoparticles will import a variety of stimuliresponsiveness, such as electricity,53 light,54 magnetic responsive,55 and so forth. To this end, we engineered magnetic Fe3O4 nanoparticles (MNPs) anchored with dopamine molecules via musselinspired metal-coordination chemistry at catechol−particle interface and glycidyl methacrylate-co-polydimethylsiloxane propyl ether methacrylate as silicone oil swelling SLIPS’ building blocks. Conferred by extremely high affinity of PDMS and silicone oil, it was a real snap for the nanocomposite to be infused by silicon oil and formed a slippery surface. Furthermore, nanosize Fe3O4 particles offered the surface a nanosized roughness that was conducive to long-term stability under extreme operating condition.56 In addition, the introduction of a Fe3O4-particle-reinforced coating layer obtained an improved mechanical property, which was further promoted by covalent cross-linking of the epoxy and amino groups in DETA. The synergistic interactions of magnetic 10341

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Figure 1. (a) FT-IR spectra of PG-MNP3, PG-MNP5, PG-MNP10, and PG-MNP15 from the top to bottom; (b) laser confocal Raman spectra of MNPs.

Moreover, visible increasing adsorption intensity of them was observed. The peaks at 908 cm−1 corresponding to epoxy groups disappeared in the spectra, indicating the complete curing. Considering the compromise of lubricant owing to evaporation or high flow shearing, we choose a lubricant with a minimal evaporation rate and high stability proved by our previous research57 to fabricate SLIPS, therefore enable prolonged operation. 2.2. Surface Topography. Apart from chemical structures, the surface topographies of the PG-MNP were further investigated by the SEM and AFM, respectively. Figure 2a,b

nanoparticles’ intrinsic photothermal effect, reversible Fecatechol ligand coordination together with diffused lubricating liquid resulting in the self-healing of matrix and recovery of the surface wettability. The effects of MNPs content on matrix mechanical properties and photothermal responsiveness as well as surface characteristics were systematically investigated. Moreover, liquid repellency and self-cleaning performance were also discussed to evaluate its potential application in antifouling.

2. RESULTS AND DISCUSSION 2.1. Fabrication of SLIPS. The copolymer PG was synthesized by free radical solution polymerization of PDMSPGMA and GMA (Scheme 1). As FT-IR spectra described in Figure S3, the peaks at 1057 and 1251 cm−1 correspond to the asymmetrical stretching vibrations of Si−O− Si and Si−CH3 bending deformation in PDMSPGMA. The existence of the epoxy groups in GMA were confirmed by the band at 908 cm−1. To further confirm the structure, 1HNMR spectra of copolymer had been provided in Figure S4. The chemical shift at 2.50−3.10 ppm were assigned to CHOCH2 and CHOCH2 from GMA. The sharp peak observed at 0.00 ppm was ascribed to SiCH3 from PDMSPGMA. Signals of CH2CH2 both disappeared in FT-IR (1637 cm−1) and 1 HNMR (two peaks among 5.40 to 6.20 ppm) results, signifying complete polymerization. For MNPs, the absorption bands at 3405, 1617, 875 cm−1 were exclusively ascribed to the N−H, NH2, and C−N stretching vibrations, whereas the peak appearing at 875 cm−1 was for the Fe−O group of MNPs, which indicate the successful synthesis of MNPs end-functionalized with amino groups. (Figure S3) Laser confocal Raman spectroscopy (Figure 1b) confirmed the presence of phenyl ring vibration peaks from ∼1250 to ∼1650 cm−1 in MNPs, derived from dopamine. Meanwhile, vibrations indicative of catechol−Fe coordinate interactions (from ∼400 to ∼800 cm−1) were present in DA-modified MNPs spectra. A simple cross-linking process was conducted to fabricate PG-MNP nanocomposite by mixing PG, MNPs and DETA (Scheme 1). The mixture of these reactants in butyl acetate and ethanol solution was sprayed on glass, followed by thermal curing to obtain PG-MNP. From FT-IR spectra in Figure 1a, broad bands around 3436 cm−1 attributed to N−H and −OH groups and peaks at 593 cm−1 for the Fe−O group appeared.

Figure 2. SEM images of PG-MNP3 (a) and PG-MNP15 (b) and the typical three-dimensional AFM height images of PG-MNP3 (c) and PG-MNP15 (d).

shows SEM images of PG-MNP3 and PG-MNP15. The white spots in the images are MNP nanoparticles. Under the same magnification, PG-MNP3 had a relatively smoother surface with sporadic apophyses (Figure 2a), while with the sequential addition of MNPs nanoscaled protuberances were denser and could be clearly seen from the surface of PG-MNP15 (Figure 2b). The dispersion of the nanoparticles is uniform in two cases, indicating that MNPs and copolymer are highly 10342

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Figure 3. Stress−strain plots (a) and calculated toughness (b) for PG-MNP3, PG-MNP5, PG-MNP10, and PG-MNP15. Error bars are standard deviation for three measurements.

Figure 4. (a) Self-healing process of PG-MNP15; the original undamaged material (1) was cut into two pieces (2), then could stand on two lids after adjoining the pieces at interface for 10 min (3), and sample after 2 h sunlamp irradiation (S.I.) followed by 10 h room temperature (R.T.) healing process (4). (3′) and (4′) are optical microscope images of A3 and A4’s incisions. (b) Stress−strain plots and (c) calculated toughness (integral area under the stress−strain curve) for PG-MNP15 healed under different conditions.

ideally flat until the thickness of the lubricant became approximate to the height of the nanoscale features themselves, which would hardly happen even under harsh conditions. Whereas the length scale of hierarchical or microtextured surfaces were larger than capillary length of lubricant under high acceleration conditions. It lead to the easy drain of them from the valleys between the microscale ridges and the underlying solids ultimately exposed in air. The liquid interface would be no longer smooth which is required for best performing SLIPS. 2.3. Self-Healing of Copolymer Matrix and Recovering of Surface Wettability. As depicted in Figure 3a, tensile deformation of the solvent-free and flawless PG-MNPs were recorded as stress−strain plots to measure the mechanical properties. By enhancing the amount of MNPs, the modulus and tensile strength of PG-MNP were protuberantly improved without sacrificing the stretchability (Table S1). For example,

compatible in nanocomposite. TEM photographs (Figure S5) further verified loose distribution of the nanoparticles without apparent agglomeration even at the most MNP feeding rate. It is the anchored amino that provides potent advantage on uniform dispersion of MNPs. The homogeneous dispersion of MNPs in the polymer matrix is very important to the overall mechanical properties of PG-MNP which will be presented minutely later. According to AFM studies, both the PG-MNP3 and PGMNP15 surface presented with a typical nanoscale structure with a RMS roughness of 6.05 and 36.15 nm, respectively (Figure 3c,d). The uniformly nanotextured topography enlarged the surface area and was proved to show superior performance in long-term stability under extreme operating condition (e.g., high shear rate, elevated evaporation, and flowing aqueous immersion) compared to hierarchical or microtextured surfaces.56,57 Nanotextured surface was keeping 10343

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Figure 5. (a) Heating effect of copolymer matrix under sunlamp irradiation (100W); (b) DSC thermograms of PG-MNP3, PG-MNP5, PG-MNP10, and PG-MNP15. The heating rate was 10 °C/min.

Figure 6. Schematic illustration and images of surface wettability recovering.

compared with PG-MNP3, the Young’s modulus and tensile strength of PG-MNP15 were increased by 2.25 and 2.33 times, respectively, meanwhile the extensibility was 1.73 times longer. Moreover, the toughness (area under the stress−strain curves) of PG-MNP15 was improved by 4.24 times. All material compositions displayed good tensility in which 15 wt % MNPs showed the highest strains exceeding 1000% (Figure S6). In the resulting samples, the interparticles’ covalent cross-linking network among amino-modified MNPs, DETA, and GMA, associated with intraparticles’ Fe-catechol coordination interfacial interactions held together the substrate architecture. The coordination bonds acting in sacrificial manner would dissipate energy during stretching. The more MNPs on the one hand were propitious to improving modulus, on the other hand were positive for scattering more energy instead of tearing samples, consequently, achieving better malleability. The presence of the reversible coordinate bonds and magnetic Fe3O4 NPs endowed the copolymer matrix both spontaneous and stimuli-responsive self-healing properties. PGMNP15 with 15% MNPs were chosen as representative

samples for the self-healing test because of its better mechanical properties. As shown in Figure 4a, the fresh cut samples could support its self-weight without facture at the notch after rejoining for 10 min. Without any treatment, the damaged sample showed moderate healing capability with 47.13% healing efficiency (Toughnessrestored/Toughnessoriginal)36 after 12 h healing process. It is believed that reversible catechol-Fe coordination bonds in nanocomposites can dissociate and reconnect themselves to form a new network to heal themselves above glass-transition temperatures.52,58 Tg of PGMNPs are ranging from −4.26 to 7.34 °C (Figure 5b), indicating the intrinsic healing process can occur at room temperature. While if it was accelerated by exposing the sample under sunlamp during the initial 2 h, healing efficiency soared to 78.25% (Figure 4b). This conspicuous increase was contributed to the intrinsic ability of MNPs to absorb energy from the light.54 After 2 h sunlamp irradiation, surface temperature of PG-MNP15 have risen by 87.5% from room temperature (24 °C) to ∼45 °C. Figure 5a offered quantitative insights on how the surface temperature follows the light time. 10344

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Figure 7. (a) Stress−strain plots of PG-MNP15 samples that experienced 1 to 8 times healing cycles; (b) sliding angles of SPG-MNP15 versus healing cycles.

Figure 8. (a) The evolution of CA of silicone oil (20 mPa·s) within 1.3 s; the inset image is the partial enlarged detail from 0.03 to 0.09 s. (b) CA of water on SPG-MNP surfaces. (CA, static water contact angle; AA, advancing water contact angle; RA, receding water contact angle.)

healing. With the exception of the assistance from autoheating by MNPs and the reversible bidentate coordination as explained above, the lubricant also played a crucial role in polymer-bond reorganization. Driven by low surface tension, silicon oil swarmed into the crack void and redistributed to form a uniform liquid film immediately, which provides a liquid medium for polymer chain diffusion, hence promotes spontaneous polymer reassemble. Actually, self-healing started from the deepest site of the crevice, where the polymer’s diffusion length is at its minimum value.24 For the sake of practical use, the healing property was qualitatively evidenced in the stress−strain and sliding angle measurements. As shown in Figure 7a, healing efficiency and max tensile strength were still maintained above 65% and 1.63 MPa, respectively, when the same area was subjected to successive cutting and healings. While a dramatic enhancement in the sliding angle, for example, about 14° increase from original slides, has been observed after eight cutting− centrifuging−healing cycles (Figure 7b). It was the centrifuging process that caused the sliding angle to be merely sustained below 5° at the previous four cycles. As a large amount of silicone oil was lost during high speed shearing (3500 rpm) process, nanotexture on the surface was exposed gradually which resulted in the increasing of surface roughness and degradation of the slipperiness.57

The maximum temperature on the magnetic nanocomposite surface increased with increasing magnetic nanoparticle content from 38 to 46 °C, making favorable effect on migration and entanglement of polymer chain. The healing capabilities were visualized using an optical microscope as displayed in Figure 4a. The ∼175 μm scar at the damaged interface almost disappeared after 2 h sunlamp irradiation and 10 h room temperature healing process although minor scars were still visible. As known to all, SLIPS own self-healing ability because the lubricant layer is dynamically reconfigurable. However, since the surface wettability self-healing is on account of the mobile infused liquid as demonstrated in most literatures,1,57 the destruction of liquid repellency is actually also in virtue of the lubricant draining. In other words, when the damaged SLIPS suffered from high shear rate up to 3500 rpm as demonstrated in Figure 6 and Video S1, the water droplet would be detained on the crack line ascribed to the no more molecular-scale smooth and detect-free surface. While, after irradiating with the sunlamp for 2 h and sequentially allowing it to heal for 10 h under ambient condition (room temperature in air) without any treatment, the surface retrieved slippery to water droplets. In addition, the cut on the polymer film was invisible to the naked eyes (Video S1). It is the intrinsic photothermal stimulus response of magnetic nanoparticles, reversible coordination cross-links of Fe-catechol ligand, coupled with diffused lubricating liquid that contributes the surface wettability self10345

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Figure 9. Photographs showing the dynamic mobility of a water droplet (7 μL) on SPG-MNP15 with a low tilting angle (∼5°).

Figure 10. Self-cleaning tests by (a) and (b) using copper(II) chloride dehydrate (CuCl2·2H2O) particles and (c) using silica gel (mSiO2·nH2O, 75 to 150 μm) particles. Images (a,c) were on the surface of SPG-MNP15 and (b) was performed contrastively on glass.

2.4. Surface Wettability. The affinity of silicone oil (20 mPa·s) and PG-MNP were assessed with oil contact angle measure as shown in Figure 8a. Although the original CAs ranged between 62.6° and 79.9° were relatively high compared to traditional superoleophilic surfaces, the CAs promptly decreased to an ultralow value below 17.3° within 1.3 s. As wetting time prolonged to 20 s (Figure S7a), CAs came down to ∼0° finally and the silicone oil permeated into the substrates completely. The extra lubricant formed a continuous fluid layer on the surface. Even the high-viscosity oils (ca. 100 mPa·s) only required ∼120 s to completely penetrate into the substrate (Figure S7b), declaring high compatibility between silicone oil and the designed substrates. In order to comprehensively represent surface oil affinity of the coatings than the observed silicone oil contact angle, adhesive work Wa was put forward to describe the ability of liquid wetting a solid surface, Wa = γl × (1 + cos θ), where γl is silicone oil surface tension and θ is oil contact angle. The larger value of Wa, the easier the solid surface is to be infiltrated. A few minutes later, the silicon oil CAs could be considered as 0°, hence the Wa values approached to about 2γl (maximum value), declaring high oleophilicity. Note that although the CA of silicone oil of four sample surfaces with different MNPs content were approximately similar, the original values were quite at odds with each other. The CA of PG-MNP3 was 62.6°, while the corresponding value of PG-MNP15 reached up to 79.9°, which was 17.3° more than PG-MNP3. It is the higher cross-linking that attenuates the permeability of lubricant. Amino-functionalized MNPs, the basic donator of the surface roughness, can meanwhile react with epoxy groups which were suspended on the polymer main chain and, as a consequence, stronger cross-linking and mechanical property were generated with the increasing MNPs. The static CA, AA, and RA as well as the CAH of water droplets on all samples after silicone oil (20 mPa·s) infusion

were investigated (Figure 8b). All samples retained a similar water CA (∼100°) and CAH (∼5°). The similarity might be explained by the same lubricant used, which was the main impact factor of wetting properties.57 These ultralow CAH values confirmed a super slippery surface and guaranteed outstanding self-cleaning property as described in the following part. Furthermore, the suspending and sliding motion of a water droplet over the slippery surface was also justified visually by the digital pictures as shown in Figure 9. A water droplet of 7 μL slipped from the SPG-MNP15 surface with a low tilting angle (∼5°). Thanks to the high affinity of the substrate and lubricant, there was no replacement of the oil by water in the prepared substrate. 2.5. Self-Cleaning Properties and Liquid Repellency. Self-cleaning properties of SLIPS were rendered with dirtremoval tests using hydrophilic copper(II) chloride dehydrate (CuCl2·2H2O) and hydrophobic silica gel (mSiO2·nH2O, 75 to 150 μm) particles as model pollutant. As shown in Figure 10 and Video S2, a handful of dust was sprinkled on the surface with a tilt angle (∼30°) and continuously added water dropwise onto it. For hydrophilic particles, benefitting from splendid solubility in water, it would be dissolved rapidly once in contact with a few water droplets, and then carried away as the droplets slide from the surface (Figure 10a). However, the blank glass surface remained contaminated by the CuCl2 powders at the same time scale (Figure 10b and Video S2). For hydrophobic powders, attributed to the tendency to gather at the water−air interface, white hydrophobic particles could be picked by the sliding droplets without impeding its motion, thus the slippery surface could be undefiled again under water flow (Figure 10c). Both surfaces of coatings became completely clean within no more than 14 s due to the low contact angle hysteresis. Meanwhile, profited from the nanoscale topography of substrate, dust with ultralight weight and microscale size would not be trapped into the surface, which would inversely 10346

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Figure 11. Snapshots of the sliding of milky tea (a) and honey (b) droplets on SPG-MNP15-coated glass plates. The tilt angle of plates was about 30°.

of MNPs and reversible Fe-catechol ligand, coupled with diffused lubricating liquid, both the mechanical property of copolymer matrix and surface wettability of SLIPS could be effectively healed with 78.25% healing efficiency of toughness and completely retrieved slippery to water droplets. After multiple destructive tests, healing efficiency of PG-MNP15 was still maintained above 65% (after eight cutting−healing cycles) and SAs of SPG-MNP15 were inferior to 5° and 17° after the fourth and eighth cutting−centrifuging−healing cycles, respectively. This study offers a promising strategy for constructing SLIPS with self-healing ability to efficaciously extend those materials’ longevity when subjected to damage.

happen to hierarchical or microtextured substrates in all likelihood. As a consequence, dust would float on the slippery surface and then be collected and removed away by water droplets. These results indicate outstanding fouling-release performances of the nanocomposite slippery coatings. Therefore, the prepared surfaces might possess potential application in self-cleaning field to various solid pollution in reality. Meanwhile, in actual use, taking advantage of the self-healing ability of prepared SLIPS, the longevity of it would be extended in significant measure when subjected to damage. In addition to repelling water droplet, the SPG-MNP15coated coatings effectively repel complex solutions, which is of great significance for practical application. In this section, a series of liquids, which generally appeared in our daily life such as milky tea and honey (Figure 11a,b), were used to test the special liquid repellency of slippery coatings. Because of their high viscosity and complicated component, honey and milky tea are difficult to be wiped away from most existing surfaces. Milky tea droplet slid away quickly in 1.96 s, while the honey droplet left the surface in 16.22 s because its high viscosity (Video S3). Remarkably, these droplets left no traces along their paths, indicating that they could not impale the substrate and displace the lubricant. These desirable properties came from the incompatibility of lubricating and impinging test liquids and the preferential wetting of substrate by lubricant rather than by the outcast liquid, correlating well with the criteria proposed by Aizenberg.1 As displayed in Video S3, all the drops of test liquids, including soy sauce and yellow wine (Figure S8) floated away quickly in less than 2 s with no traces left. In contrast, the untreated glass surface pinned those liquids under the same circumstance (Figure 11c and Video S3). The satisfying lyophobic nature of our nanocomposite slippery coating was also in favor of cleaning the surface with a wide range of particulate contaminants by washing with not only water but also a broad assortment of fluids.

4. EXPERIMENTAL SECTION 4.1. Materials. Polydimethylsiloxane propyl ether methacrylate (PDMSPEMA, X-22-2475, 420 g/mol) was purchased from Shin-Etsu Chemical Co. Ltd. Ferric chloride (FeCl3, CP), sodium acetate (NaOAc, purity ≥99.0%), ethylene glycol (EG), dopamine hydrochloride (DA, 189.64 g/mol, 98% purity), glycidyl methacrylate (GMA), diethylenetriamine (DETA) and butyl acetate (CP, 98% purity) were purchased from Aldrich. 4.2. Sample Preparation. 4.2.1. Synthesis of Dopamine Modified Fe3O4 MNP. Fe3O4 NPs were synthesized following previous reported methods.52,59 Briefly, FeCl3 (0.973 g) was dissolved in ethylene glycol (30 mL) to form a tawny solution, followed by the addition of NaOAc (1.97 g) and DA (56.9 mg). The mixture was vigorously stirred for 30 min and then sealed in a Teflon-lined stainless-steel autoclave. The autoclave was heated to 200 °C for 12 h. After cooling to room temperature, the black products were washed three times with ethanol and then redispersed in ethanol (100 mL) added with dopamine (100 mg). The mixture was homogenized by pulsed sonication (pulse, 20 s on + 5 s off; power, 125 W) for 1 h, then centrifuged at 10 000 rpm for 10 min to remove aggregates. Finally the modified Fe3O4 NPs were collected by an external permanent magnet, dried at 60 °C for 12 h and noted as MNPs. From thermal gravimetric analysis (TGA) and photocorrelation spectroscopy (PCS) results, the weight percent of Fe3O4 in the modified MNP powder was determined to be 85−87% (Figure S1) and the hydraulic radius was 550 nm (Figure S2). 4.2.2. Synthesis of Copolymer P(GMA-r-PDMSPEMA). The copolymer P(GMA-r-PDMSPEMA) was prepared by solvent polymerization. A typical copolymerization process is described as follows. PDMSPEMA (7.5 g) and GMA (2.5 g) were dissolved in butyl acetate (25 mL) with nitrogen protection. Then AIBN (0.10 g) as an initiator was added into the reactor at 80 °C. After 12 h of polymerization, the crude reaction solution was purified by rotary evaporation to remove most of the solvent. Then, the copolymer was precipitated in methanol three times and finally dried in a vacuum oven at 50 °C. The synthesized functionalized copolymers were designated as PG. 4.2.3. Preparation of PG-MNP and SPG-MNP. As shown in Scheme 1, the PG copolymer was dissolved in butyl acetate. Then MNPs dispersed in ethanol mixed with 1 wt % diethylenetriamine (DETA) were added and ultrasonicated for 10 min. For surface wettability tests,

3. CONCLUSION In summary, original SLIPS, owning ability to heal the matrix itself, were efficiently fabricated through cross-link reaction of dopamine-functionalized magnetic Fe3O4 NPs, DETA, and glycidyl methacrylate-co-polydimethylsiloxane propyl ether methacrylate, followed by swelling silicone oil into matrix. The designed SLIPS demonstrated excellent self-clean and liquid-repellent properties to a wide range of particulate contaminants and fluids. Besides, the nanocomposite matrix of SLIPS showed high affinity to lubricant (silicone oil) with 20 s soaking time, considerable mechanical strength with 8.9 MJ/ m3 toughness, and conspicuous warming up to ∼45 °C after 2 h sunlamp irradiation. Moreover, because of photothermal effect 10347

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Langmuir the obtained mixture was sprayed onto glass slides or silicon wafers by a spray gun (nozzle diameter, 0.5 mm; HD 180, NO 302, Taiwan) at a spraying pressure of 1 bar and distance of 10 cm. The amount of mixture sprayed was about 0.11 mL/cm2. For mechanical tests, the obtained mixture was poured into a silicone mold and then dried in vacuum oven under high temperature to give uniform rectangular specimens without remaining solvents or any bubbles.60 The above samples were allowed to cure for 2 h at room temperature, followed by an additional 12 h curing in an oven at 120 °C and named as PGMNP. After immersing in silicone oil (20 and 100 mPa·s) for 2 h and spinning at 1000 rpm for 1 min to remove superfluous lubricant, slippery films were obtained and named as SPG-MNP. Eight film samples with different MNPs charging rate were prepared in this work. The detailed compositions of nanocomposites were listed in Table 1.

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mass ratioa/% a

SPG-MNP3 PG-MNP3 3

SPG-MNP5 PG-MNP5 5

SPG-MNP10 PG-MNP10 10

*E-mail: [email protected]. Phone: +86-571-8795-3382. Fax: +86-571-8795-1227. ORCID

Qinghua Zhang: 0000-0003-1350-6388 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No.21476195, 21576236 and 21676248) and Zhejiang Provincial Major Project of Science & Technology for Award (No. 2014C13SAA10006).

SPG-MNP15 PG-MNP15 15

Mass ratio represents the m (MNPs); m (PG).

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4.3. Characterizations. 4.3.1. Chemical Structure and Composition. 1HNMR (Bruker Advance DMX500, 25 °C) and FT-IR (Nicolet 5700) characterizations were used to analyze the chemical composition of samples. Raman spectra (LabRAM HR Evolution, 532 nm) were used to confirm chemical structure of MNPs. Differential scanning calorimeter (DSC Q100) was used to examine the glass transition temperature (Tg) of PG-MNPs. 4.3.2. Surface Topography and Wettability. The surface morphologies was observed under FESEM (Hitachi TM-1000) at an accelerating voltage of 20 kV and AFM (Veeco, U.S.A.) operated in tapping mode using Muti Mode. The scanning range was 5 μm × 5 μm and the root-mean-square (rms) roughness values were calculated from the obtained AFM images. TEM images of PG-MNP15 were obtained by JEOL JEM-1230L microscope by applying an acceleration voltage of 120 kV. The ultrathin sections were prepared using LEICA Ultracut ultramicrotome cooled in liquid N2 and collected on a copper grid. Surface wettability was investigated by water contact angle (WCAs) measurements performed on CAM 200 optical contact-angle goniometer (KSV Co., Ltd., Helsinki, Finland) at room temperature. Advancing and receding contact angles were measured for macroscopic droplets (∼7 μL) using the goniometer at ambient condition by slowly increasing and decreasing the volume of the droplet to induce sliding, then analyzed the obtained images to find the best fitting contact angles. The water contact angle hysteresis reported here is the difference between the advancing water contact angle and receding water contact angle. Water sliding angles (SAs) were measured by adjusting the homemade mutable plane slowly from 0° to higher angles until a 10 μL droplet start to roll off; the inclined angle was recorded as the sliding angle. 4.3.3. Mechanical Property tests of Copolymer Matrix. Mechanical properties were conducted by a Zwick/Roell Z020 universal material tester. Typically, a synthesized rectangular shape of polymer sample was clamped at one end and pulled at a constant rate (100 mm/min) of elongation at the other clamped end. All tests were performed at room temperature.

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AUTHOR INFORMATION

Corresponding Author

Table 1. Compositions of MNP-PG Nanocomposites sample symbol

Video S3, showing liquid repellency of SPG-MNP15 (AVI) Figures S1, S2, S3, S4, S5, S6 and S7 and Table S1 (PDF)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b02691. Video S1, showing recovering of surface wettability (AVI) Video S2, showing self-cleaning properties of SPGMNP15 (AVI) 10348

DOI: 10.1021/acs.langmuir.7b02691 Langmuir 2017, 33, 10340−10350

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