Robust organic-inorganic composite films with multifunctional

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Materials and Interfaces

Robust organic-inorganic composite films with multifunctional properties of superhydrophobicity, self-healing and drag reduction Yibin Liu, Jin Liu, Yi Tian, Hao Zhang, Rumin Wang, Baoliang Zhang, Hepeng Zhang, and Qiuyu Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06302 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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Industrial & Engineering Chemistry Research

Robust organic-inorganic composite films with multifunctional properties of superhydrophobicity, self-healing and drag reduction Yibin Liu a, b, Jin Liu b, Yi Tian b, Hao Zhang b, Rumin Wang b, Baoliang Zhang a ,b, Hepeng Zhang a, b

a

and Qiuyu Zhang*, a, b

Research & Development Institute of Northwestern Polytechnical University in Shenzhen, 518057,

PR China b

MOE Key laboratory of Materials Physics and Chemistry under Extraordinary Conditions,

Northwestern Polytechnical University, Xi'an 710072, PR China.

Corresponding authors. Email address: [email protected] (Q. Zhang)

Abstract: Multifunctional films have attracted wide attention in scientific research and engineering applications. Based on thermodynamically driven metathesis reactions of disulfide bonds, a self-healing film with disulfide bonds was designed and prepared, which was hot pressed by fluorinated silica particles (F-SiO2) to prepare an organic-inorganic composite film with superhydrophobicity and self-healing properties of 70.29% self-healing efficiency measured by tensile experiments. The increasing F-SiO2 showed various hierarchical structures on the composite films, resulting in elevated contact angles with the maximum up to 168° at 2.0 mg/cm2 of F-SiO2 surface density. Have been hot pressed 4 layers F-SiO2, the composite films had excellent wear-resistance with keeping superhydrophobiciy after 10 m sandpaper abrasion scan. Subjected to acidic solution, bend, finger-wiping and knife-scratch damage, the composite films still had excellent superhydrophobicity. In addition, the organic-inorganic composite films exhibited excellent drag

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reduction property with the drag reduction rate up to 27.7%. Keywords：Organic-inorganic; Superhydrophobicity; Self-healing; Drag reduction

1. Introduction Multifunctional films are very promising materials in various areas, especially self-healing films and superhydrophobic films have attracted wide attention in scientific research and practical application. Self-healing films are normally made from self-healing polymers, which are promising smart materials1-2 because of their ability of autonomously repairing physical damages so as to extend their service life and reduce maintenance costs. Intrinsic self-healing is caused by non-covalent bonds such as hydrogen bonds3-4, metal ligands5-6, host-guest interactions7-8 or reversible (dynamic) covalent bonds such as Diels-Alder9, radical recombination10, olefin metathesis11, polysiloxanes12, boronic esters13 and other reactions14. Especially, disulfide bonds attracted great concerns because it can be activated at moderate temperature (60-90°C) without external intervention15-16. Odriozola17 et al. prepared self-healing poly(urea-urethane) elastomers by bis(4-aminophenyl) disulfide with quantitative self-healing efficiency at room temperature without external stimulus or catalysts. Hwang18 et al. made use of aromatic disulfides to prepare a transparent and easily processable thermoplastic polyurethane (TPU) with more than 75% self-healing efficiency at room temperature for 2 h. The self-healing films with superhydrophobic surface can broaden their applications because of excellent water-repellent property19-20, such as self-cleaning21-22, anti-icing23, anti-biofouling24, drag reduction25. Among them, the superhydrophobic surface with reduction of the flow resistance is desirable in maritime transport.


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However, the superhydrophobic surface is destroyed easily by mechanical abrasion or solvent corrosion, which exist widely in application.26-27 In order to overcome this disadvantage, a lot of researchers have made various attempts to obtain robust superhydrophobic surfaces, including creating multi-scale structure28-32, preparing cross-linking coating33-35, introducing self-healing function36-38, forming a chemical bond between the coating and the substrate39-41. Among these strategies, “paint + adhesive” method is an easily processable and cost-effective way, which can strengthen the robustness of superhydrophobic surface42-44. The adhesive is applied directly to enhance the physical and chemical bond between the substrate and the hydrophobic materials. Strong polar materials are generally selected as the binder such as epoxy resin, polyurethane, polyacrylonitrile45. For example, Chen39 et al. prepared a durable superhydrophobic surface by applying polyurethane adhesive and calcium fluoride carbonate in a two-step spray process. Pan46 et al. fabricated flexible, durable, and unconditioned superoleophobic/superhydrophilic surfaces, which applied reinforced superimposed structure to improve the wear-resistance. Lu41 et al. developed a powerful superhydrophobic coating by spraying modified multi-scale TiO2 nanoparticles on a binder coating

surface.

Wang47

et

al.

made

hydrophobic

silica

nanoparticles

modified

by

dichlorodimethylsilane spray on the adhesive coating to prepare superhydrophobic surfaces with wear-resistance, anti-corrosion and drag-reduction properties. Song's40 group also used tape pretreated hard/soft substrate to adhere to modified Cu powder to prepare a superhydrophobic surface. After the physical and chemical bonds between substrate and particles were strengthened by the adhesive, all coating surfaces reported had strong superhydrophobicity. But the ability of only one layer hydrophobic materials on the surface is limited to enhance mechanical strength of the superhydrophobic surfaces. The preparation of robust coating or films with superhydrophobic



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property remains challenge. Herein, in order to achieve robust superhydrophobic surface and excellent self-healing property, a composite film with organic-inorganic superimposed structure was designed and prepared. Firstly, based on self-healing ability of disulfide bonds and operational process of “paint + adhesive” method, a soft monomer trimethylolpropane tris(3-mercaptopropionate) (TMPMP) was choice and reacted with glycidyl methacrylate (GMA) via a thiol-ene click action to prepare a processable epoxy prepolymer, which was followed to mix 4-aminophenyl disulfide (APDS) curing agents with disulfide bonds to form self-healing resin matrix and harden under 130 ℃ to gain self-healing films. In order to calculate the self-healing efficiency of the self-healing films, tensile strength was measured by universal testing machine. Then an organic-inorganic composite film was designed and fabricated by hot-pressing fluorinated silica particles (F-SiO2) as the hydrophobic particles and self-healing resin matrix as adhesive agents repeatedly on the self-healing film. The organic-inorganic composite films surface structures were researched by scanning electron microscopy (SEM), and chemical elements composition and wetting behaviors were characterized by X-ray photoelectron spectroscopy (XPS) and contact angle analyzer, respectively. Sandpaper abrasion test and acid/base solution immersion were applied to evaluate the wear-resistance and acid/base stability of the composite films. Besides, the drag reduction property of the composite film was researched by measuring friction torques on rheometer at 500~1000 rpm rotation speeds. 2. Experimental 2.1 Materials Trimethylolpropane tris(3-mercaptopropionate) (TMPMP) was provided by Evans Chemetics. 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (FOTS), glycidyl methacrylate (GMA) and dimethyl


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phenyl phosphine (DMPPH) were purchased from Aldrich. 4-Aminophenyl disulfide (APDS) was gained from Adamas Reagent Co., Ltd. Ammonium hydroxide (25%), tetraethyl orthosilicate (TEOS) and ethanol were obtained from J&K Scientific Co.. 2.2 The preparation of self-healing films. The novel epoxy prepolymer was prepared by thiol-ene click reaction. In brief, the TMPMP and GMA at the equal mole ratio of S-H and C=C followed a phosphine-catalyzed nucleophilic Michael addition reaction with 0.1% wt DMPPh as catalyst at room temperature for 4 h. 5 g epoxy prepolymer above was mixed with 1.17 g APDS under stirring for 30 min at 80 ℃ to gain self-healing resin matrix, which was poured into the PTFE plate and cured at 130 ℃ for 2 h to form the self-healing films (1 mm of film thickness). 2.3 The preparation of organic-inorganic composite films. Firstly, hydrophobic silica particulate sol was prepared according to previously reported method48. Briefly, ethanol (200 mL) and ammonia (16 mL) were mixed uniformly and then TEOS (18 mL) was added. After magnetic stirring for 2 h, 1 mL FOTS was added to the reaction solution. The reaction was stirred for another 1 h at 25℃ to form the hydrophobic silica particulate sol (F-SiO2 concentration, 0.8 wt%). Secondly, the self-healing film in 2.2 was sprayed coating by hydrophobic silica particulate sol to from a layer F-SiO2. Then the self-healing film with F-SiO2 was applied 10 KPa pressure at 100 ℃ for 10 min, and finally gaining the organic-inorganic composite films. Besides, in order to improve the wear-resistance, the composite films can be covered and cured by more layers of the self-healing resin matrix in 2.2, and then hot-pressed by F-SiO2 to prepare more layers F-SiO2 composite films. 2.4 Characterization.



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The surface morphology of the organic-inorganic composite films were characterized by scanning electron microscope (SEM, VEGA 3 LMH, TESCAN). The stress-strain curves were measured by universal testing machine (CMT 6303). Contact angles (CA) and roll-off angles (ROA) were characterized on contact angle analyzer (Powereach, JC2000D1) with 8 μL water droplets at 25℃. The composite films surface chemical composition were measured by X-ray photoelectron spectroscopy (XPS, Kratos, AXIS Ultra DLD) with a monochromatic Al Kα source. The wear-resistance of the organic-inorganic composite films was evaluated by sandpaper abrasion test. The films thickness was measured by spiral-micrometer. Friction torques were measured by rheometer (Brookfield R/S plus) at 500~1000 rpm speeds at 25℃ to characterize the drag reduction effect of the composite films. 3. Results and discussion 3.1 The preparation and characterization of Self-healing films. The self-healing films were prepared as shown in Figure 1a. Firstly, an epoxy prepolymer was designed and prepared by reacting between TMPMP and GMA through thiol-ene click reaction, which can be easily carried out under air atmosphere with only 0.1 wt % catalyst of DMPPH, exhibiting rapid, efficient and no side products advantages. Secondly, the epoxy prepolymer mixed APDS and was scraped on a PTFE plate and cured under 130℃ for 2 h to gain the self-healing films. Figure 1b shows the typical FTIR spectra of the reactants (mixture of TMPMP and GMA) and products epoxy prepolymer. After thiol-ene click reaction, the characteristic absorption peak of thiol group of TMPMP at 2570 cm-1 and ene group of GMA at 1637 cm-1 disappeared on the FTIR spectra of epoxy prepolymer. In addition, the relative absorption peak intensity at 908 cm-1, 2980 cm-1 and 2920 cm-1, respectively, assigned to epoxy group, -CH3 and -CH2 stretching vibrations still existed.


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In order to characterize thermal dynamic property, the DMA curves of the self-healing films were measured and 17.6℃ of glass transition temperature (Tg) was gained through tg δ-temperature curve (Figure 1c). Comparing traditional epoxy resin, the self-healing films cured by epoxy prepolymer with more flexible molecule segments has low Tg, which is beneficial to self-healing property and hot-pressing process below. In addition, the thermal degradation temperature of the self-healing films was 238.5℃, which was measured by TGA (Figure S1).

Figure 1. a. The schematic of preparation of self-healing films; b. FTIR of epoxy prepolymer before and after click action; c. The DMA curves of self-healing films. There are disulfide bonds in APDS molecules, which undergo efficient metathesis at moderate temperature, so self-healing films have better self-healing ability, which can repair larger size cracks



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under certain conditions. To investigate the self-healing efficiency, self-healing films were cut in two pieces and then realigned and healed under 100 ℃ and 20 KPa for 2 h. Then tensile experiments and samples of original and cut-healed films were performed in Figure 2a and Figure 2b. From Figure 2c, it can be seen that the tensile strength of original films is 2.40 MPa, after cut off and healed this value became 1.76 MPa, and decreased to 0.80 MPa followed broke and repaired again, which showed that self-healing efficiency of the films were 73.33% in first healed and 33% in second healed. Furthermore, the healing efficiencies after 2 h at 25, 40, 60, 80 and 100 °C under 0, 10 and 20 KPa were also measured and shown in Figure 2e. It can be seen that the films hardly had healing ability without pressure or at room temperature. Only reaching to higher pressure and temperature, the self-healing property started to become obvious. From the previous reports18, self-healing mechanism of disulfide metathesis was shown in Figure 2d. The disulfide bonds in crack parts can undergo metathesis to make section healed. But because of the cross-linked structure of self-healing films and less content of APDS, the disulfide metathesis is easily hindered, thereby resulting in harsh repair conditions and less self-healing efficiency than some thermoplastic polymers18 with disulfide bonds.


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Figure 2. a. Tensile experiments on original and cut-healed films; b. The photographs of the self-healing films cut and healed; c. The tensile stress–strain curves; d. The self-healing process and mechanism of self-healing films; e. Healing efficiencies of the self-healing films after 2 h at 25, 40, 60, 80 and 100 °C under 0, 10 and 20 KPa. 3.2 The preparation and characterization of organic-inorganic composite films. Because of disulfide metathesis, the thermosetting films with disulfide bonds still have thermal processing ability.49 The self-healing films not only possessed disulfide bonds, but also had low Tg, so F-SiO2 can be easily pressed in them, which result in high adhesion between F-SiO2 and films. Therefore, an organic-inorganic composite film with superhydrophobicity and self-healing property was designed and prepared by “paint + adhesive” method. As is shown in Figure 3a, the self-healing film was covered by a layer of F-SiO2 and hot-pressed with 10KPa pressure under 100℃ for 10 min to gain the organic-inorganic composite films. Besides, the composite films can be alternately covered and cured by more layers of the self-healing resin matrix, and then hot-pressed by F-SiO2 to



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prepare more layers F-SiO2 composite films in order to improve the wear-resistance. In addition, the composite films still kept the self-healing property after hot-pressing a layer of F-SiO2 as is shown in Figure 3b. It can be seen that the tensile strength of original films is 2.39 MPa, after cut off and healed this value became 1.68 MPa, which showed that self-healing efficiency of the films was 70.29% after hot-pressed F-SiO2. Figure 3c shows the healing efficiencies of organic-inorganic composite films after 2 h at 25, 40, 60, 80 and 100 °C under 0, 10 and 20 KPa. It can be seen that only reaching to 100 °C and 20 KPa repair conditions, organic-inorganic composite films had better self-healing property, which was the same as the self-healing films.

Figure 3. a. The schematic of preparation of organic-inorganic composite films; b. The tensile stress–strain curves of organic-inorganic composite films original and healed (inserts: The photographs of organic-inorganic composite films cut and healed); c. Healing efficiencies of organic-inorganic composite films after 2 h at 25, 40, 60, 80 and 100 °C under 0, 10 and 20 KPa. The organic-inorganic composite films with different F-SiO2 surface density were prepared by controlling spraying-coating time of F-SiO2 and the surface SEM images were shown in Figure 4. It


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is found from Figure 4a that the composite film surface with 1.0 mg/cm2 appeared a lot of submicron F-SiO2, which was lack of continuity and uniformity disappointedly. With the increase of F-SiO2 surface density, the dispersion of F-SiO2 became more uniform and regular, and the surface of composite film with 1.2 and 1.4 mg/cm2 F-SiO2 surface density had obvious submicron structure from Figure 4b and Figure 4c. When the F-SiO2 surface density continued increasing, the surface became rougher, and appeared micron-scale aggregates accidentally, which mainly resulted from the F-SiO2 reunion. With the surface density increasing to 1.8 and 2.0 mg/cm2 (Figure 4e and Figure 4f), the reunion phenomenon became more serious, which appeared more block F-SiO2 aggregates, probably resulting in a lot of F-SiO2 not pressed in self-healing films. The chemical composition of self-healing films and composite films were measured by XPS. As is shown in Figure S2, the surface of self-healing films consisted of C, O, S and N elements. After hot-pressing of F-SiO2, the surface increased Si and F elements, which indicated that F-SiO2 existed on the composite films.

Figure 4. The SEM images of organic-inorganic composite films with different F-SiO2 surface density: a. 1.0 mg/cm2; b. 1.2 mg/cm2; c. 1.4 mg/cm2; d. 1.6 mg/cm2; e. 1.8 mg/cm2; f. 2.0 mg/cm2. Figure 5a shows the CA and ROA on the composite films with different F-SiO2 surface density.



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For the least F-SiO2 surface density (1.0 mg/cm2), the CA and ROA was 142° and 43° respectively, which didn’t show the superhydrophobic property. Starting at the 1.2 mg/cm2 sample, all the CA on the composite film surface exceeded 150°, and with the increase of F-SiO2 surface density, the CA also increased. Among the composite films the highest CA was 164° for the 2.0 mg/cm2 surface density samples (Figure 5b). Besides, ROA were also observed to decrease as F-SiO2 surface density increased with the minimum of 2° for the 1.8 and 2.0 mg/cm2 sample. In addition, a continuous water jet can completely bounce off shown in Figure 5c, without any impalement of water. Because of the superhydrophobic property, the composite films have excellent self-clean effect. As is shown in Figure 5d, the sand covered on the composite films surface (1.4 mg/cm2 F-SiO2 surface density) were washed off easily by water. Because of excellent superhydrophobicity and better F-SiO2 dispersibility, the composite films surface with 1.4 mg/cm2 F-SiO2 surface density were used to carry out various tests below.

Figure 5. a. The contact angles and roll-off angles on the composite films with different F-SiO2 surface density; b. The contact angles image of composite films with 2.0 mg/cm2 F-SiO2 surface


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density; c. Photographs of the composite films surface with 1.4 mg/cm2 F-SiO2 surface density with water bouncing off; d. The self-clean property of composite films. 3.3 Wear-resistance of organic-inorganic composite films. It is mentioned above that because of disulfide metathesis, F-SiO2 can be pressed into self-healing films with excellent adhesion. But only one layer of F-SiO2 was also destroyed easily under repeated friction. To solve this problem, inspired by thermal processing ability of self-healing films, an organic-inorganic superimposed structure that composed of self-healing resin matrix as adhesive agents and hydrophobic particles as the fillers was design and prepared as is shown in Figure 6a. Multilayer F-SiO2 was pressed into self-healing films and the section SEM images of organic-inorganic composite films with 1 layer and 4 layers F-SiO2 were shown in Figure 6b and 6c, respectively. It can be seen that there was a rough F-SiO2/resin composite layer with approximate 30 μm thickness on the self-healing film with 1 layer F-SiO2, which showed no obvious interface appeared between F-SiO2 layer and resin matrix. For the self-healing film with 4 layers F-SiO2, the thickness of the F-SiO2 /resin composite layer was 110 μm or so, which also didn’t show obvious interface between F-SiO2 layer and resin matrix.

Figure 6. a. The organic-inorganic superimposed structure; b. The section SEM images of organic-inorganic composite films with 1 layer of F-SiO2; c. The section SEM images of organic-inorganic composite films with 4 layers F-SiO2. The sandpaper abrasion test47, 50 was applied to characterize the wear-resistance of the composite films, as is shown in Figure 7a. A sample of the composite films (1 × 1 cm2) was posted on the



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bottom of a 100 g weight. Then the weight was pulled and pushed onto a 600# sandpaper with each abrasion cycle moving 10 cm. After every 10 abrasion cycles, the films thickness of the self-healing films and composite films were measured and shown in Figure 7b. It was seen that with the increase of abrasion cycles from 0 to 100 cycles, the films thickness decreased from 1000 to 760 μm for the self-healing films and from 1130 to 1030 μm for composite films with 4 layers F-SiO2, which supposed that multilayer F-SiO2 can improve mechanical strength of composite films. In addition, after every 5 abrasion cycles, the CA and ROA on the sample surface were measured and shown in Figure 7c. It can be seen that after 10 abrasion cycles, the CA of 1 layer F-SiO2 decreased from 158° to 142°, and ROA increased from 9° to 31°. However, for 2 layers F-SiO2 and 3 layers F-SiO2 composite films, the obvious drop trend for CA and raise trend for ROA appeared at 30 and 70 abrasion cycles, respectively. Besides, when 4 layers F-SiO2 were hot-pressed into the self-healing films, after 120 abrasion cycles the CA still exceeded 150°, and water droplet still moved easily on the surface, showing excellent wear-resistance. In order to explain the reason, the SEM images of 4 layers F-SiO2 composite films after 0, 50, 100 and 135 abrasion cycles were shown in Figure 7d. It is found that the composite film surface without abrasion appeared a lot of F-SiO2 particles with uniform and regular dispersion, which showed obvious submicron structure. After 50 abrasion cycles the surface became rougher with obvious micro/submicron structure and submicron particles could also be discovered, which supposed that there existed still a large number of F-SiO2 particles on the composite films. After 100 abrasion cycles, obvious hierarchical structure still appeared on the films surface, which composed of some resin residues rubbed down and F-SiO2 particles and can supply rough structure and low surface energy for superhydrophobicity. But when abrasion cycles were 135, the submicron-scale aggregates and F-SiO2 disappeared obviously, and only a little micron-scale


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friction defects existed on the surface, resulting in the loss of superhydrophobicity of the composite films. These results above confirmed that the superimposed structure of F-SiO2 and resin matrix can improve the wear-resistance of the composite films through adding F-SiO2 layer. As is shown in Table S1, comparing with some previous reports, organic-inorganic composite films have better ability to keep their superhydrophobic property under severe conditions of higher pressure and longer abrasion distance.

Figure 7. a. The schematic diagram of sandpaper abrasion test; b. The films thickness of the samples; c. The contact angles and roll-off angle on the sample surface; d. The SEM images of 4 layers F-SiO2 composite films after 0, 50, 100 and 135 abrasion cycles. In addition, the healing efficiency after 2 h at 100 ℃ under 20 KPa of the composite films with different F-SiO2 layers was shown in Table S2. It is found that when the F-SiO2 layers increased, the healing efficiency of the composite films decreased conversely with the minimum efficiency of 62.89% for the composite films with 4 layers F-SiO2, which supposed that more F-SiO2 can hinder disulfide metathesis, thereby resulting in the reduced of self-healing efficiency. Therefore, in order to maintain better self-healing property of the composite films, excessive F-SiO2 layers were unnecessary. 3.4 Acid/base and mechanical durability of organic-inorganic composite films.



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In addition to mechanical abrasion treatment, the composite films encounter probably corrosive liquids in practical usages, such as strong acid or base solution. As shown in Figure 8a, the silvery sheen of the air layer above the films was still obvious even when immersed in a 1 M HCl solution for 24 h, and the CA and ROA were nearly the same as them before immersion from Figure 8c1. But immersed in 1 M NaOH solution for 2h, air layer of the composite film surface disappeared and CA decreased to 88° (Figure 8b and Figure 8c2). However, after sandpaper abrasion several times, the silver air layer on the composite film appeared again, and CA increased to 154°, which showed that the superhydrophobic property of the composites films was recovered (Figure 8b and Figure 8c3). As is shown in Figure 8d, the chemical compositions of the composite film surfaces before, after the immersion of basic solution and after sand abrasion measured by XPS. It is found that after basic solution immersion, F element contents decreased from 23.26% to 9.79% obviously, but increased to 14.51% after sandpaper abrasion, which supposed that the loss of superhydrophobicity of the composites films resulted from degradation of perfluorooctyl molecule chains. Besides, it can be found in XPS that Si element content increased to 16.01% after sandpaper abrasion, which indicated that abrasion made more F-SiO2 expose to the surface of the composite films, thereby resulting in the restore of the superhydrophobic property. In addition, as is shown in Figure 8e, the composite film was suffered from bending 200 times, finger-wiping 10 times and knife-scratch test, the water droplets still rapidly rolled off in tilted direction, which indicated that the self-healing films had strong adhesion with F-SiO2, resulting in excellent mechanical durability of organic-inorganic composite films.


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Figure 8. a. The photograph of the organic-inorganic composite films immersed in acidic solution; b. The photographs of the organic-inorganic composite films immersed in base solution and immersed in water after sandpaper abrasion; c. The contact angles and roll-off angles on the organic-inorganic composite films immersed in acidic solution, base solution and water after sandpaper abrasion, respectively; d. XPS of the chemical compositions of the surfaces before, after the immersion of basic solution and after sandpaper abrasion; e. The photographs of dropping tests after the bending, finger-wiping and knife-scratch. 3.5 Drag reduction effect of organic-inorganic composite films. The drag reduction acted as an important application of superhydrophobicity at low flow speed attracted more and more attentions with lack of energy materials in modern life. The drag reduction property of the composite films was evaluated by testing friction torques on rheometer

47, 50

. The

schematic diagram of the rheometer is shown in Figure 9a. The space between upper and lower plates was 0.2 mm. When the upper plate rotated for 1min, the friction torque value can be read from the rheometer. The drag reduction rates of the samples are calculated as followed Eq. (1):

DR  1  M sa M sm

(1)

where, Msa and Msm represent the torques of the sample surface of the composite film and the smooth



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surface, respectively. Because lower viscosity of water, the rotation speeds were chosen between 500 and 1000 rpm to prevent water from being spilled.

Figure 9. a. The schematic diagram of the drag reduction rate measurement device; b. The friction torque of smooth surface and sample surface at different rotating speed; c. The schematic diagram of slip boundary. As is shown in Figure 9b, the friction torque was plotted as a function of rotation speed. It is found that with the increase of rotation speeds from 500 to 1000 rpm, the friction torque also increased from 2.1 to 36.3 μNm for smooth surface, and from 1.5 to 26.2 μNm for the superhydrophobic surface. According to Eq. (1), the drag reduction rate of the composite films reached to maximum of 27.7%, which indicated that the composite films have excellent drag reduction effect. The friction torques of the composite films with 4 layers F-SiO2 after sandpaper abrasion were also measured. As is shown in Figure S3, after 50 and 100 abrasion cycles, there weren’t significant changes in the friction torques of the superhydrophobic surface, which is supposed that the composite films still


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possessed excellent drag reduction after sandpaper abrasion at least 100 cycles. When the plate of the rheometer rotates at the speed from 500 to 1000 rpm, the flow between two plates presents a laminar flow with a boundary layer. Under laminar flow conditions, it is considered that the air layer on the superhydrophobic surface of the sample is the main factor in drag reduction behavior. As is shown in Figure 9c, the mechanism of drag reduction of superhydrophobic surface in laminar flow is currently considered to be mainly velocity slip at the interface proposed by Navier51. According to the expression of this model, slip velocity u0 is proportional to velocity gradient, which is expressed by Eq. (2).

u 0  b u y

(2)

where b is the slip length. Because b is a finite value, a slip speed should exist at the boundary. As is shown in Figure 9c, an air layer filled in the hierarchical structures on the superhydrophobic surface separates water from the solid-liquid interface by the gaps between the hydrophobic posts, and the water flowing over the air layer experiences little friction, which can make effective slip generate, resulting in reduction of the flow resistance47. Therefore, superhydrophobic surface of the organic-inorganic composite films have drag reduction effect. 4. Conclusions In summary, an organic-inorganic composite with multifunctional properties film was prepared by repeatedly hot-pressing F-SiO2 and self-healing resin matrix on the self-healing films, which had the 70.29% self-healing efficiency. With the increased F-SiO2 surface density on the organic-inorganic composite films, the CA also elevated with the maximum to 168° at 2.0 mg/cm2 of F-SiO2 surface density. The composite films with 4 layers F-SiO2 have excellent wear-resistance with keeping superhydrophobiciy after 10 m sandpaper abrasion scan. The composite films surface still kept better



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superhydrophobicity immersed in acidic solution for 24 h, but CA decreased to 88° in base solution. After sandpaper abrasion several times, CA of the composite film surface increased to 154°, and water droplet on it easily move again. Subjected to bend, finger-wiping and knife-scratch damage, the composite films still have excellent superhydrophobicity. Rheometer results showed the drag reduction rate of the organic-inorganic composite films can reach up to 27.7%. This research is expected to supply a multifunctional film with superhydrophobic, self-healing and drag reduction properties in various realms.

Acknowledgements This work was supported by the Science, Technology and Innovation Commission of Shenzhen Municipality (Grant JCYJ20160331142330969) and the Science, Technology and Innovation Commission of Shenzhen Municipality (No. JCYJ20170306154725569).

Appendix A. Supplementary data

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Figure 1. a. The schematic of preparation of self-healing films; b. FTIR of epoxy prepolymer before and after click action; c. The DMA curves of self-healing films. 72x80mm (300 x 300 DPI)


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Figure 2. a. Tensile experiments on original and cut-healed films; b. The photographs of the self-healing films cut and healed; c. The tensile stress–strain curves; d. The self-healing process and mechanism of selfhealing films; e. Healing efficiencies of the self-healing films after 2 h at 25, 40, 60, 80 and 100 °C under 0, 10 and 20 KPa. 89x76mm (300 x 300 DPI)



Figure 3. a. The schematic of preparation of organic-inorganic composite films; b. The tensile stress–strain curves of organic-inorganic composite films original and healed (inserts: The photographs of organicinorganic composite films cut and healed); c. Healing efficiencies of organic-inorganic composite films after 2 h at 25, 40, 60, 80 and 100 °C under 0, 10 and 20 KPa. 84x66mm (300 x 300 DPI)


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Figure 4. The SEM images of organic-inorganic composite films with different F-SiO2 surface density: a. 1.0 mg/cm2; b. 1.2 mg/cm2; c. 1.4 mg/cm2; d. 1.6 mg/cm2; e. 1.8 mg/cm2; f. 2.0 mg/cm2. 77x51mm (300 x 300 DPI)



Figure 5. a. The contact angles and roll-off angles on the composite films with different F-SiO2 surface density; b. The contact angles image of composite films with 2.0 mg/cm2 F-SiO2 surface density; c. Photographs of the composite films surface with 1.4 mg/cm2 F-SiO2 surface density with water bouncing off; d. The self-clean property of composite films. 82x59mm (300 x 300 DPI)


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Figure 6. a. The organic-inorganic superimposed structure; b. The section SEM images of organic-inorganic composite films with 1 layer of F-SiO2; c. The section SEM images of organic-inorganic composite films with 4 layers F-SiO2. 64x21mm (300 x 300 DPI)



Figure 7. a. The schematic diagram of sandpaper abrasion test; b. The films thickness of the samples; c. The contact angles and roll-off angle on the sample surface; d. The SEM images of 4 layers F-SiO2 composite films after 0, 50, 100 and 135 abrasion cycles. 96x45mm (300 x 300 DPI)


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Figure 8. a. The photograph of the organic-inorganic composite films immersed in acidic solution; b. The photographs of the organic-inorganic composite films immersed in base solution and immersed in water after sandpaper abrasion; c. The contact angles and roll-off angles on the organic-inorganic composite films immersed in acidic solution, base solution and water after sandpaper abrasion, respectively; d. XPS of the chemical compositions of the surfaces before, after the immersion of basic solution and after sandpaper abrasion; e. The photographs of dropping tests after the bending, finger-wiping and knife-scratch. 63x43mm (300 x 300 DPI)



Figure 9. a. The schematic diagram of the drag reduction rate measurement device; b. The friction torque of smooth surface and sample surface at different rotating speed; c. The schematic diagram of slip boundary. 57x54mm (300 x 300 DPI)


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19x11mm (300 x 300 DPI)


Robust organic-inorganic composite films with multifunctional

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