Ice-Inspired Polymeric Slippery Surface with Excellent Smoothness, Stability and Antifouling Properties

The ice is omnipresent in our daily life and possesses intrinsic slipperiness due to the formation of a quasi-liquid layer. Thus, the functional surfaces inspired by the ice show great prospects in widespread �elds from surface lubrication to antifouling coatings. Herein, we report an ice-inspired polymeric slippery surface (II-PSS) constructed by a self-lubricating liquid layer and a densely surface-grafted polymer matrix. The smooth polymer matrix could capture lubricant molecules via strong and dynamic dipole-dipole interactions to form a stable quasi-liquid layer that resembles the ice surface. The resultant II-PSS can be easily fabricated on various solid substrates (e.g., silicon, glass, aluminium oxide, plastics, etc.) with excellent smoothness (roughness ~ 0.4 nm), optical transmittance (~ 94.5%), as well as repellency towards diverse liquids with different surface tensions (22.3 ~ 72.8 mN m -1 ), pH values (1-14), salinity and organic pollutants. Further investigation shows that the II-PSS exhibits extremely low attachment for proteins and marine organisms (e.g., algae and mussel) for over one month. These results demonstrate a robust and promising strategy for high-performance antifouling coatings.


Introduction
Hundreds of years ago, the arti cial ice paths had been used for the transportation of heavy construction materials for building the Forbidden city in ancient China due to the super-lubrication properties of ice surface 1,2 (Fig. 1a).Nowadays, the ultra-low sliding friction of ice also accounts for a wide variety of natural phenomena and matters, including glacier movement 3,4 , frost heave 3 , automotive safety 5 , and winter sports 6,7 .The unique slippery property of ice can be attributed to the nanoscale quasi-liquid layers (Q-LL) 8 (approximately 1-100 nm) with ultralow coe cient of friction on the ice surface, which works even below the melting point of ice (i.e., 0 °C) 3,[9][10][11] .Although the precise nature of the Q-LL remains under controversial for over 160 years, the unique slippery property of ice surface has attracted considerable interests towards surface lubrication 1,12 and antibiofouling coatings 2 .
In the past decades, enormous efforts have been devoted to mimic the unique slippery features of ice surface.The main research trend is to introduce hydrophilic components such as hyaluronic acid 13,14 or zwitterionic polymer 2,15 on solid surface, which can adsorb water from the environment to generate a water-based lubricating layer for anti-icing and anti-cell adhesion.However, such slippery surface tends to fail in dry or high-temperature conditions 16 .In another scenario, an oil-based lubricating layer locked in the micro/nano-porous substrates has been proposed [17][18][19][20] , which could achieve a surface with ultralow friction coe cient [21][22][23] .However, the sophisticated preparation process of micro/nano-porous structures and the weakly bond of lubricating layer to the micro-structured surface (in contrast to the molecular-level interaction at ice surface) have been the main obstacles for mimicking the excellent lubricating and physical properties of ice surface [24][25][26] .
Herein, we report an ice-inspired polymeric slippery surface (II-PSS) on the basis of a densely surfacegrafted polymer chains (i.e., brush-like polymer matrix) [27][28][29][30] which could serve as a supporting layer to stabilize the lubricating molecules via dynamic dipole-dipole interactions 31 .The strong and dynamic interactions between polymer chains and lubricating molecules have been demonstrated by molecular dynamics simulations which generate a super-lubricated quasi-liquid layer (Q-LL) on solid substrates, and thus nicely mimic the functions of ice surface.As a result, the II-PSS surface shows excellent repellency towards diverse liquids (e.g., water, ethanol, toluene, dimethyl formamide, glycerol, etc.) and organic pollutants (e.g., milk, oil, coffee, soy sauce, etc.).Meanwhile, the slippery surface exhibits superior optical transmittance as well as mechanical durability and chemical stability against physical or chemical damages.Moreover, the II-PSS could near-completely inhibit the adhesion of protein, algae, and mussel in a long-term contact over one month, demonstrating its excellent practicability for antibiofouling coatings.

Motif and fabrication process of II-PSS
To mimic the Q-LL of the ice surface (Fig. 1a), a densely surface-grafted poly(methyl methacrylate) (PGMA) layer was initially fabricated on initiator-functionalized SiO 2 /Si substrate by surface-initiated Cu(0)-mediated atom transfer radical polymerization (SI-Cu 0 ATRP, Fig. 1b and Supplementary Fig. 1) 28, 32- 35 .The surfaced-grafted PGMA layer on SiO 2 /Si substrate presented an ultra-smooth polymer matrix surface with a root-mean-square (RMS) roughness of ~ 0.4 nm and thickness of ~ 186.7 nm (Fig. 2a and Supplementary Figs. 2, 3).Then, a solution of per uoropolyether (PFPE) was spread on the PGMA matrix forming a dynamic liquid layer due to the molecular-level interactions (i.e., dipole-dipole) between PFPE and PGMA 31 (Fig. 1b inset).The cross-sectional scanning electron microscope (SEM) image of II-PSS showed a stable coating with thickness of ~ 925 nm on the surface of PGMA matrix (Supplementary Fig. 4).The X-ray photoelectron spectroscopy (XPS) analysis of II-PSS demonstrated the exitance of C (284.8 eV), O (532.7 eV) and F (688.7 eV) elements, while only C and O elements were detected for the PGMA lm layer (Supplementary Fig. 5), which further proved the formation of PFPE layer on the PGMA surface.
To verify the interactions between PFPE and PGMA, the interfacial adhesion energy and adhesion force were calculated via molecular dynamics (MD) simulation 36,37 .Meanwhile, the poly(methyl acrylate) (PMA) with only ester group and poly(methyl methacrylate) (PMMA) with ester and methyl groups (Fig. 2b inset) were selected and modeled with the PFPE for comparison.The results showed that the interfacial adhesion energy between PGMA and PFPE (PGMA-PFPE) was 76.9 kcal•mol −1 which was signi cantly higher than the two other models with 56.0 kcal•mol −1 (PMA-PFPE) and 58.6 kcal•mol −1 (PMMA-PFPE), respectively (Fig. 2b).In addition, the interfacial adhesion force of the PGMA-PFPE was also the largest and up to 6.6 nN (Fig. 2c and Supplementary Fig. 6).Furthermore, the contribution of the methyl group (-CH 3 ), ether group (C-O-C) and ester group (O-C=O) 31 to the interfacial adhesive energy were calculated.According to the results, the dipole-dipole interactions between ether group and -CF 2 or -CF 3 groups played the major role in the interacted process (~ 4.2 kcal/mol and ~ 4.9 kcal/mol), while the contribution of methyl with -CF 2 or -CF 3 groups was weaker (~ 1.5 kcal/mol and ~ 1.0 kcal/mol, Fig. 2d and Supplementary Fig. 7), which further theoretically con rmed the robust intermolecular forces of II-PSS The liquid repellency of the II-PSS was assessed by measuring the contact angles and sliding angles of a wide range of tested liquids, including water and organic liquids.As demonstrated in Fig. 2e, The II-PSS showed the highest water contact angle of 118.2° than blank substrate (36.4°),initiator-functionalized substrate (59.2°) or PGMA (78.2°), meanwhile the II-PSS also presented the lowest sliding angle of 3.8°.In addition, the SiO 2 /Si substrate coated with the II-PSS showed low sliding angles for liquids with a broad range of surface tensions.For example, the time-lapse images of water (γ = 72.8mN m −1 ) and ethanol (γ = 22.1 mN m −1 ) droplets slid easily off the surface with a tilting angle (TA) of 3° (Fig. 2e and Supplementary Fig. 8).Other organic solvents such as toluene, dimethyl formamide (DMF), diethylene glycol, dimethyl sulfoxide (DMSO), ethylene glycol, and glycerol, underwent slipping motion on the II-PSS by simply tilting the surface, and there were no any residual traces leaving behind (Fig. 2f, Supplementary Fig. 9 and Supplementary Movie 1).Although the contact angles increased with increasing surface tension of the liquids (Fig. 2g), the sliding angles of all the tested liquids were still as low as ~ 5° (Fig. 2h), indicating the excellent liquid repellency of the resultant slippery surface.
To demonstrate the generality of the coating, we prepared the II-PSS on various substrates such as glass, polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polyvinyl chloride (PVC) and aluminum oxide (Al 2 O 3 ).The water contact angles of II-PSS on different substrates were all increased compared to the blank substrates (Fig. 3a and Supplementary Fig. 10).At the same time, all the II-PSS on different substrates exhibited small water contact angle hysteresis (~ 6.5°) [CAH, difference between the advancing (q adv ) and receding (q rec )], which represented less lateral adhesion of the droplets to the surface and thus the low sliding angles (~ 5°) (Fig. 3b).As shown in Fig. 3c, the water droplets could quickly slide off from those II-PSS coated substrates at a tilt angle of 20°, even as short as only 1s on the coated polyethylene surface.Interestingly, a large area of II-PSS coated glass (200 cm 2 ) exhibited high transparency with negligible visibility fading (Fig. 3d insets and Supplementary Fig. 11).Compared to the bare glass (90.1%), the glass coated with II-PSS showed higher optical transparency (94.1%) at the visible light wavelength, which was due to the reduced light scattering through the II-PSS enabled by the replacement of the unwanted solid/air interface with a smoother liquid/air interface (Fig. 3d) 38 .

Chemical and mechanical stability of II-PSS
The chemical and mechanical stabilities of II-PSS are very important for practical application and thus further investigated.The water contact angles of the liquid droplets of different pH values (from 1 to 13) on II-PSS tended to be stable (~ 114.6°) and the sliding speed changed very slightly (Fig. 4a), as proven by the approximate spherical shape of those dyeing droplets (Fig. 4a insets).The same tendency was obtained to liquid droplets with various salinity (0-7wt.%,Fig. 4b), indicating the excellent chemical stability of the II-PSS.A continuous droplets impacting test was then carried out to assess the durability of the II-PSS towards external mechanical effects 39 .The water droplets were released continuously on the II-PSS coated glass at a tilt angle of 30° (Supplementary Movie 2), and it was gratifying to see the sliding speed of droplets were almost unchanged between 0-400 drops.Although the sliding speed tended to decrease gradually with the continuously increase of the number of droplets, the sliding property did not disappear (Fig. 4c).Furthermore, when the II-PSS was rotated at 0-7000 rpm/min for an indicated time interval 40 , the water contact angle changed marginally from 115.3° to 117.9°, and the sliding velocity remained relatively steady at 27.4 mm•s -1 , which demonstrated that the lubricating layer remained on the polymer matrix surface (Fig. 4d).

Self-cleaning and anti-fouling properties
It is well acknowledged that the slippery surfaces have great application prospects in the elds of selfcleaning and antifouling [41][42][43][44] .As such, the self-cleaning property of the II-PSS was investigated with four typical liquids that were commonly used in daily life including milk, soy sauce, cooking oil, and cola.The testing samples were tilted to 20°, and all of the tested pollution liquids could quickly slide off the II-PSS coated glass without leaving any residues, which was in clear contrast to the control glass substrate (Fig. 5a, Supplementary Fig. 12 and Supplementary Movie 3).
Subsequently, the anti-fouling properties of II-PSS were examined, and uorescently labeled bovine serum albumin (BSA) was used as model to test anti-protein adhesion 45 .The uorescence microscopy photos showed that there was almost no detectable uorescence signal from protein on the II-PSS but obvious green uorescence on the blank surface and PGMA surface (Fig. 5b).Similarly, mussels attach indiscriminately to virtually any surface via adhesive elastomeric foot protein 46,47 .As expected, the II-PSS could also e ciently prevent the adhesion of mussels (Fig. 5c and Supplementary Fig. 13).In addition, algae are also one of the main biological pollutants in the marine fouling process 48 .We immersed the samples in a suspension of chlorella and observed few green spots on the uorescent microscope picture of II-PSS, but massive signals on blank substrate, initiator-functionalized substrate and PGMA matrix, indicating that the II-PSS remained effectively for anti-chlorella adhesion (Fig. 5d).
We nally investigated the long-term marine antifouling behavior of the II-PSS lasting for one month by choosing phaeodactylum tricornutum as a model organism.The samples were immersed in suspension of phaeodactylum tricornutumin for 1, 3, 7 and 30 days, respectively, and then photographed by confocal laser scanning microscope.We found that only a few uorescent spots appeared on the II-PSS with the increase of time, on the contrary, the amount of algae adhesion on other three kinds of the sample surfaces increased signi cantly with time (Fig. 6a and Supplementary Fig. 14).In order to observe a more pronounced contrast of the experimental phenomena, one sample was processed into a half-blank and half-II-PSS surface and the same algal adhesion experiment was carried out.As expected, there was a clear dividing line for algal adhesion between the II-PSS coated surface and blank counterpart (Fig. 6b).In addition, the calculation result showed that the adhesion area of algae on the II-PSS was only 0.7% after 30 days (Fig. 6c).The aforementioned results demonstrated that the II-PSS has an excellent durability for practical marine antifouling.

Discussions
In conclusion, we report an ice-inspired polymeric slippery surface on the basis of a densely surfacegrafted polymer chains matrix.The polymer matrix could serve as an excellent supporting layer to stabilize the lubricating molecules via dynamic dipole-dipole bonds.The resultant II-PSS exhibits a high optical transmittance up to 94.1% which allows it to coat on a variety of solid substrates without changing their appearance.Meanwhile, the slippery surface displays superior repellency towards various liquids with surface tensions from 22.3 to 72.8 mN m -1 and pH values from strong acid to alkali as well as high salinity and organic pollutants.The II-PSS also shows outstanding mechanical durability that maintaining sliding properties after continuous droplet impact and shear rotation with 7000 rpm/min.Finally, we demonstrate that the II-PSS can e ciently inhibit the attachment of aquatic microorganisms such as algae, proteins and mussels, revealing the great possibility as a new type of antifouling coating.

Preparation of ice-inspired polymeric slippery surface (II-PSS)
Silicon wafer with a nanoscale oxide layer (or glass, PE, PP, PET, PVC, Al 2 O 3 , etc.) was washed alternately with methanol and deionized water for 3 min, and then ionized by oxygen plasma (PEC-6, 600 W, sykejing company) for 600 s.The substrates were functionalized with 5 μL 2-bromo-2-methyl-6trimethoxysilylhexanoate 49 by vapor deposition for 3 h at 60 ℃.Mixing 500 μL glycidyl methacrylate (methyl acrylate or methyl methacrylate), 2 mL water, 1 mL methanol, and 37 μL PMDETA to obtain the polymerization solution.The Cu sheet was washed with 3M HCl and methanol (v: v = 1: 1) under ultrasonication for 3 min and dried by nitrogen stream.Next, the Cu sheet and the initiator functionalized substrate were placed in parallel with a distance of 0.18 mm.The assembly was submerged into the polymerization solution and taken out after polymerizing for a period of time at room temperature (1 h unless otherwise speci ed) 33 .After that, the substrate was washed with water and ethanol, and nally dried under nitrogen ow to obtain polymer matrix.Finally, an excess amount of per uoropolyether was spread onto the Polymer matrix for 1h, and then placed vertically for 10 min to remove excess lubricant to obtain a dynamic slippery surface.

Protein adhesion assays
The protein adsorption assay was adapted from a previous report by Yang et al 45 .First, the prepared testing samples (10 mm × 10 mm) were sterilized under ultraviolet light for 1 h and equilibrated for 2 h in phosphate buffer solution (PBS, pH = 7.4).Subsequently, the samples were immersed in prepared BSA solutions (1 mg/mL) to incubate at 37 °C and 200 rpm for 4 h.Each sample was gently washed three times with PBS and dried by nitrogen stream.Finally, the confocal laser scanning microscope was used to image the amount of attached protein.

Algae settlement and adhesion assays
Phaeodactylum tricornutum and chlorella were selected for algal adhesion assays.The prepared testing samples were immersed in phaeodactylum tricornutum and chlorella suspensions (the cell concentrations were approximately 8 × 10 6 cells mL -1 ), respectively, and cultured at 25 °C for 1 day, 3 days, 7 days, and 30 days (12 h light and 12 h dark cycles).Then, the samples were rinsed with deionized water after taken out, dried with nitrogen stream.Optical microscope and confocal laser scanning microscope was used to image the amounts of attached algae, and ImageJ was used to determine the algae intensity from the obtained micrographs.The average value and standard deviation of the algal coverage were calculated for each collected image.

Molecular dynamics simulation
First, the Amorphous Cell modeling was used to build the model of polymer matrix and per uoropolyether (PFPE), and the density of PMA, PMMA and PGMA were 0.1 g/cm 3 .Then the MD simulation was performed using the ReaxFF package in LAMMPS software and the parameters of the ReaxFF force eld used were provided by Wood et.al 37 .The simulation system was the NVT ensemble with the time step of 0.25 fs and photo acquisition was created using VMD software.The system is entirely run in a periodic cubic box in all directions during the MD simulations with temperature of 300 K and time step of 0.25 fs.Finally, molecular dynamics simulations were performed when running NVT for 100 ps and then the stable structure of polymer matrix and PFPE was obtained.To simulate the van der Waals forces and interfacial adhesion energy between polymer matrix and PFPE, the loads are applied by using steer MD module in LAMMPS.The loading velocity is 0.01 Å/fs and the spring constant is 0.01 nN/Å.

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