Facile fabrication of biomimetic slippery lubricant-infused transparent and multifunctional omniphobic surfaces

Abstract

Omniphobic coatings which can efficiently diminish the interfacial reactions between the underlying substrates and foreign liquids present broad technological impacts and enormous potential applications, whereas the current prepared superamphiphobic surfaces are constrained to their weak robustness owing to the vulnerability of the sophisticated hierarchical structures. Herein, we employed oxidative polymerization method to graft polyaniline (PANI) nanofibers on arbitrarily shaped surfaces that further modified with perfluoroalkylthiol and infiltrated with perfluoropolyether lubricant, constructing a slippery lubricant-infused porous surfaces (SLIPS). With the enlargement of the polymerization time, the coverage degree of PANI coating on the glass surface gradually increased and their transmittance reduced simultaneously. Meanwhile, the influences of the structure geometry and surface chemistry on the slippery behavior of foreign liquids on the SLIPS were investigated, further verifying that the synergetic effect of the adequate texture roughness and matched surface chemistry is the prerequisite for preparing steady and defect-free lubricant layer. Moreover, the prepared SLIPS could be applied in various promising applications such as anti-fogging, anti-fingerprint, three-dimensional droplet manipulation and crude-oil lossless transportation. More importantly, the lubricant layer remained stable on the surfaces after long-term storage in high/low temperature, water immersion and ultraviolet irradiation, and displayed superior mechanical resistance to water impact, sandpaper abrasions and knife scratches. Therefore, this strategy for fabricating nepenthes-inspired lubricant-infused surfaces is expected to further promote the cognition and manufacture of multifunctional omniphobic materials.

Introduction

Omoniphobic coatings, which can repel various contaminating liquids without staining the substrate, have become the subject of intense research in order to meet the emerging requirements in broad-ranging areas, such as microfluidic devices, anti-smudge protective coating, anti-fouling marine vessels and drag reduction surfaces [1, 2]. During the past decades, superhydrophobic or superoleophobic coatings inspired by natural lotus leaves have been extensively studied and greatly influenced interface science [3, 4]. The underlying design principle of these superamphiphobic surfaces is based on the combined effect of the micro/nano-hierarchical structure and low-surface-energy chemical composition, which is conductive to form trapped air pockets to reduce the contact region between the liquid droplet and solid substrate [5,6,7]. Despite considerable efforts, their repellent properties are prone to inevitable failure when the prepared surfaces were exposed to harsh circumstances including external pressure, high humidity or low-surface-tension organic liquids [8]. Moreover, the superamphiphobic surfaces are highly relied on sophisticated reentrant textures, which will result in the reduction in transmittance or even opacity due to undesired light scattering [9, 10]. Thus, it remains difficult to prepare thermodynamically stable coatings with good transparency and superior repellency to low-surface-tension liquids.

Recently, an alternative strategy for fabricating omniphobic coatings known as slippery lubricant-infused porous surfaces (SLIPS) has been exploited, inspired by the surface of the insect-catching nepenthes pitcher plant [11]. In contrast to the superamphiphobic surfaces, the SLIPS relied on their lubricant film rather than the trapped air to repel external liquids. For SLIPS, the modified rough porous nano/microstructure can be used to lock-in the lubricating liquid via capillary forces, forming an ultrasmooth, continuous and homogeneous lubricant layer, which can effectively prevent the foreign liquids from being in contact with the underlying substrates [12, 13]. In addition, the liquid nature of the lubricant endows the SLIPS with inherent self-healing property by repairing the destruction region through the redistribution of the mobility lubricant layer [14]. More importantly, the transmission of the porous film infiltrated with lubricant will increase owing to the decrease in the reflectance and scattering of light [15, 16]. Thus, SLIPS materials have been demonstrated to possess numerous potential applications such as self-cleaning, anti-icing, marine anti-fouling and biomedical protective materials because of their omniphobicity, good optical transparency and pressure tolerance [17, 18].

In order to prepare stable lubricant film, the underlying substrate should have a top layer whose roughness and surface chemistry are properly matched with the lubricate liquid; the textured rough structure is conductive to increase the capillary forces and wicking effect to the lubricant, and chemical affinity simultaneously supplies strong van der Waals forces to anchor the lubricant into the rough surface. Based on this strategy, many different approaches for constructing functionalized surfaces on various substrates have been introduced [19, 20]. Lubricant can be simply impregnated into the porous hydrophobic polymeric membranes such as poly(tetrafluoroethylene) [21], poly(tetrafluoroethylene)-poly(4-vinylpyridine) [22] and poly(vinylidene fluoride-co-hexafluoropropylene) [23], to prepare thermodynamically stable lubricant films. Also, the organogel monoliths with disulfide bond were prepared through addition reaction of various acrylic monomers, followed by infiltration of hydrophilic lubricant or fluorinated ionic liquids, obtaining the stable SLIPS with a certain self-healing and anti-icing properties [24, 25]. Whereas, both the hydrophobic membranes and polymeric blocks with poor formability could not be applied to other substrates. Likewise, regular rough structure can be prepared via photolithography [26] or nanosecond laser microfabrication [27] on various flat and smooth substrates which is inapplicable for the curved and coarse surfaces. Electrochemical etching [28, 29] or electrodeposition [30, 31] techniques can be efficiently applied for constructing oil container on various metal substrates using the conductive nanomaterial as building units, yet the substrate-dependent requirement limits their wild applications. Regular structures fabricated via multistep template method perform comparatively high mechanical strength. Nevertheless, this technique is inapplicable for scale-up production and the complex tubular substrates [32,33,34]. Although previous fabrication methods have significantly advanced the investigations of the basic theories and practical applications of SLIPS, great efforts are still highly required to explore a facile, versatile and scalable approach for preparing coatings with good transparency on materials regardless of their size, shape or composition.

Herein, we employed a versatile method to prepare omniphobic coatings by infusing perfluoropolyether (PFPE) lubricant into perfluoro alkanethiol-modified polyaniline (PANI) nanofiber arrays. Notably, PANI nanofibers could be uniformly and tightly grafted on different substrates including circular polymeric tubes and inorganic glasses via a chemical oxidation polymerization method. By adjusting the polymerization time, the coverage degree of PANI coating on the surfaces was controlled, and thus the surface structure and transparency simultaneously achieved regulation. Further modified with perfluoro alkanethiol, the obtained fluorinated PANI nanocoating could firmly capture the PFPE lubricant to form a stable and slippery liquid layer, which exhibited superior repellency to polar and non-polar liquids. The influences of the structure morphology and surface chemistry of the SLIPS on the liquids motion behavior were studied. By virtue of their omniphobicity, the prepared SLIPS exhibited outstanding performances in anti-fouling, anti-fogging, anti-fingerprint, three-dimensional droplet manipulation and lossless transportation of crude oil. Importantly, the obtained SLIPS also displayed long-term lyophobic durability and mechanical stability against various external destructions.

Experimental section

Materials

Glass plates (25 mm × 25 mm) and circular polypropylene (PP) tubes were commercially available. All glass and PP samples were sequentially cleaned with anhydrous ethanol and deionized water several times under sonication to remove any contaminants. Aniline, ammonium persulfate (APS), perchloric acid (HClO4), 1 H, 1 H, 2H, 2H-perfluorodecanethiol (97%) and other reagents were supplied by Sinopharm Chemical Reagent Co., Ltd. Crude oil was provided by SINOPEC Lanzhou Petrochemical Co., Ltd, China. The DuPont Krytox PFPE GPL 103 lubricant was purchased from the Chemours Company. All other chemical reagents were analytical grade and used without further purification. Deionized water was used throughout the experiment.

Preparation of various SLIPS

Firstly, 0.028 g aniline was dissolved into 10 mL of HClO4 solution (1 mol/L) in the circular polypropylene tube under sonication at 0–5 °C for 3 min, and then 0.046 g APS was dissolved into 20 mL of HClO4 solution (1 mol/L) under sonication at 0–5 °C for 3 min. Then the solution of APS was added into the circular polypropylene tube, and the reaction was allowed to proceed under stirring at 0–5 °C for different time including 2 h, 4 h, 5.5 h, 7 h, 8.5 h, 10 h and 11.5 h. After the reaction, the circular polypropylene tube coated with PANI coating called PP@PANI was obtained. Similarly, the polyaniline-coated glass named as Glass@PANI was prepared via the similar route, except that the glass plates used as the substrates were vertically placed into the reaction solutions. The obtained PANI-coated surfaces are defined as Glass@PANI-Xh and PP@PANI-Xh, in which X represents the reaction time.

Afterward, the PANI-grafted surfaces were modified with perfluorodecanethiol ethanol solution (50  µL/30 mL, v/v) for 12 h, obtaining the fluorinated surfaces of PP@PANI-Xh@Thiol and Glass@PANI-Xh@Thiol, respectively. Finally, the surfaces were infiltrated with 1 mL of DuPont perfluoropolyether (PFPE, Krytox 103) and then vertically tilted for 10 min to achieve gravity-assisted drainage, preparing the slippery surfaces including PP@PANI-Xh@Thiol@PFPE and Glass@PANI-Xh@Thiol@PFPE.

Characterization

The surface morphology was observed on a field emission scanning electron microscope with Au-sputtered specimens (FE-SEM, JEOL JSM-6701F). The element distribution and percentage were analyzed by energy-dispersive X-ray spectroscopy (EDS, JSM-5600LV). X-ray photoelectron spectroscopy (XPS) measurement was carried out on an AXIS Supra. Contact angles (CAs) and sliding angles (SAs) were measured on a contact angle system equipped with a vertical rotating sample platform (JC2000DM, Zhongchen digital equipment Co., Ltd. Shanghai, China). Each value reported here was obtained by measuring five different sites of the same sample, and the error value was characterized by the standard deviation. The volume of the tested liquid was 10 μL. The transmittance of the samples in 300–800 nm was obtained using a UV–visible spectrophotometer (UV-2600). The optical images were taken by a digital camera and Olympus optical microscope (XY-MRT).

Results and discussion

Construction and characterization of SLIPS

The schematic diagram of the preparation process of SLIPS and the corresponding chemical reactions are demonstrated in Fig. 1a. The original substrates were firstly grafted with PANI nanofibers via a chemical oxidized polymerization method. Obviously, the original glass is smooth and clean (Fig. 1b). After polymerized for 4 h and 5.5 h, there were some sporadic polyaniline papillae appeared on the surface of Glass@PANI-4 h and Glass@PANI-5.5 h (Fig. 1c, Fig. S1a). Although massive PANI nanofiber arrays grown on the surface of Glass@PANI-7 h, the PANI layer was incomplete and there were many defects as shown in Fig. 1d. For the Glass@PANI-8.5 h, defects were significantly reduced and the PANI coating became more intense (Fig. S1b). Extending the reaction time to 10 h, the aligned PANI nanofibers were compactly and densely covered on the surface, forming a uniform PANI coating (Fig. 1e). Further increasing the growth time to 11.5 h, the structure of Glass@PANI-11.5 h showed no obvious changes but many agglomerates formed on the surface caused by the accumulation of the flexible PANI nanofibers (Fig. S1c). From the photographs of the PANI-coated glass and PP tubes (Fig. S2a, b and Fig. S3b), it can be clearly seen that the color of the samples became deeper green with increasing the reaction time. Meanwhile, the variation of transmittance of Glass@PANI with the growth time was quantitatively characterized as demonstrated in Fig. S3. Obviously, with the prolongation of the polymerization time, the polyaniline content on the glass surface constantly increased, while their transmittance at 490 nm decreased linearly from 90 to 62% due to the multidiffusion of visible light on the PANI nanostructure, leading to the deterioration of the glass transparency. Thus, the transmittance of the samples can be further employed to reflect the coverage degree of PANI coatings on the surface.

Figure 1
figure1

(a) Schematic diagram of the preparation process of the SLIPS. SEM images with different magnifications of original glass (b), Glass@PANI-4 h (c), Glass@PANI-7 h (d), Glass@PANI-10 h (e) and Glass@PANI-10 h@Thiol (f). (g) XPS spectra of different samples

Taking advantages of acid/base doping/dedoping property of PANI, the low-surface-energy modifier of perfluorodecanethiol with weak acidity could be doped into the long chain of PANI. Clearly, the thiol doping modification induced the formation of plenty of large particle aggregates on the surface (Fig. 1f). The alteration of the surface chemistry of samples was detected by the XPS spectra (Fig. 1g) and EDS (Fig. S4), respectively. As demonstrated in Fig. 1g, there were three peaks of the original glass at 103, 285 and 532 eV that corresponded to Si 2p, C 1s and O 1s, respectively. For the Glass@PANI-10 h, a significant N 1s peak at 399.8 eV was appeared. Moreover, the presence of new peaks at 834.2, 688.3 and 166.8 eV ascribed to F auger electron transition, F 1s and S 2s, indicating the successful modification of perfluorodecanethiol on the PANI nanofibers. The element contents and distribution of different samples were further investigated by EDS mapping as shown in Fig. S4. The original glass was mainly composed of C, O, Na and Si (Fig. S4a), and the characterized elements of N, F and S were uniformly distributed on the observed region of Glass@PANI-10 h (Fig. S4b) and Glass@PANI-10 h@Thiol (Fig. S4c), indicating that the PANI nanofibers and low-surface-energy modifier were homogenously coated on the surfaces.

Liquid repellency and transmittance of SLIPS

From the perspective of the physical chemistry of interfaces, the wettability is governed by the coordination effect of surface structure and chemical composition. Figure 2a, b presents the contact angles of water and chloroform droplets on different surfaces. Apparently, the glass before and after grafted with PANI coatings showed hydrophilicity with water contact angles (WCAs) of nearly 52.4° (Fig. 2a). Meanwhile, oil droplets fully spread on the glass and Glass@PANI-Xh with oil contact angles (OCAs) of 0° (Fig. 2b). After doping modification with the perfluorodecanethiol, WCAs of the Glass@PANI-4 h@Thiol, Glass@PANI-5.5 h@Thiol Glass@PANI-7 h@Thiol and Glass@PANI-8.5 h@Thiol were 98.3°, 113.8°, 117.5° and 135.0°, and their corresponding OCAs were 11.4°, 16.2°, 27.2° and 34.4°, respectively. Moreover, Glass@PANI-10 h@Thiol and Glass@PANI-11.5 h@Thiol exhibited superhydrophobicity with WCAs of 153.1° and 152.4° and OCAs of 43.4° and 43.5°, respectively. According to the Wenzel model, the wettability of surface can be further enhanced by the enlarged structure roughness constructed by the PANI nanofibers with increasing the polymerization time. Moreover, the adhesion effect of Glass@PANI-10 h@Thiol was also characterized by dynamic contact test of water droplet on the surface, on which droplet distorted under external pressure and entirely detached after lifting up (Fig. S5a). Besides, water droplet could easily roll off under the inclined surfaces with sliding angle of 5°, demonstrating their low water adhesive force (Fig. S5c). However, the chloroform droplet tightly adhered and could not slide along this surface (Fig. S5b and Fig. S5d). These results demonstrate that the prepared Glass@PANI-10@Thiol shows superhydrophobicity with low adhesion force and a certain degree of oleophobicity.

Figure 2
figure2

CAs of water (a) and chloroform (b) droplets on different samples. Schematic diagram of water droplets on different surface states (ce). f Time-sequence images of the mobility of water and chloroform droplets on Glass@PANI@Thiol@PFPE. g Variation of CAs and SAs of liquid droplets (10 μL) with surface tensions. h Optical transmittance spectra and photographs (inset) of Glass, Glass@PANI, Glass@PANI@Thiol and Glass@PANI@Thiol@PFPE

Further infiltrated with PFPE lubricant oil, CAs of the water droplets on the Glass@PANI-Xh@Thiol@PFPE that polymerized for 4-11.5 h remained about 106.2° and changed little. Meanwhile, OCAs of the Glass@PANI-Xh@Thiol@PFPE gradually increased from 20.9° to 44.8° with prolongation of the polymerization time from 4 to 11.5 h. Moreover, the sliding status of water and oil droplets on the tilted surface were tested as demonstrated in Fig. S6. When the growth time of PANI is less than 7 h, both water and oil droplets would adhere to the surface (Fig.  S6a–d). With respect to the Glass@PANI-7 h@Thiol@PFPE and Glass@PANI-8.5 h@Thiol@PFPE, water droplets could slide along the surface while the oil droplets were stuck on the lubricant film (Fig. S6e–h). On the contrary, water and oil droplets could effortlessly roll away along the Glass@PANI-10 h@Thiol@PFPE (Fig. S6i–l and Movie S1). Further extending the polymerization time to 11.5 h, surface structure and their wettability showed no obvious changes compared to Glass@PANI-10 h@Thiol@PFPE. Thus, in order to construct a stable SLIPS, the substrates should be coated with PANI nanofibers that polymerized for more than 10 h. For convenience, the Glass@PANI-10 h@Thiol@PFPE is abbreviated as Glass@PANI@Thiol@PFPE in the following discussion. In addition to the flat glass, the curved PP circular pipe used as the underlying substrates was functionalized modification, and the wettability of oil and water on the PP@PANI, PP@PANI@Thiol and PP@PANI@Thiol@PFPE is demonstrated in Fig. S2c, d.

Although chemical nature of the surfaces functionalized with perfluoroalkylthiol was well matched to the perfluoropolyether lubricant, different surface morphologies constructed by the PANI nanofiber arrays displayed an important impact on the stability of the formed lubricant layer. For the Glass@PANI-4 h@Thiol@PFPE, the low roughness of the PANI papillae could not wick enough lubricant to form an integral lubricant layer, resulting in the adherence of water and oil droplets (Fig. 2c). As for the Glass@PANI-7 h@Thiol@PFEP, a relatively complete lubricant film could repel water droplet with high surface tension. However, the foreign oil liquids would impregnate through the lubricant layer under gravity and were trapped into the large gaps and interstices among the PANI arrays. Thus, the produced noncontinuous solid–liquid–lubricant three-phase contact area increased the adhesive force of liquids on the Glass@PANI-7 h@Thiol@PFPE, which was in accordance with the Cassie impregnating wetting state (Fig. 2d). With further prolongation of reaction time to 10 h, PANI nanofibers arrays are densely and integrally coated on the surfaces, forming a continuous well-sized nanoscale structure with plenty of tiny gaps, which is conductive to effectively capture the lubricant on the surface and generate a steady and defect-free oil layer (Fig. 2e).

Except for the exploration of the influences of structure morphology on the slippery behavior of the SLIPS, the effects of the surface chemistry on motion of liquids and the stabilization of lubricant layer were also studied. Figure 2f shows the time-lapse images of water and chloroform droplets sliding along the Glass@PANI@Thiol@PFPE with a tilt angle of 4°. However, for the Glass@PFPE prepared by infusing the lubricant into the glass, water and oil droplets adhered and completely spread out on the surface, which is ascribed to the failure of forming lubricant layer in the absence of oil-storing structure (Fig. S7a, b). With regard to the Glass@PANI@PFFE that was fabricated by infiltrating the PFPE into the Glass@PANI-10 h, water droplets could readily roll off without leaving any residue whereas chloroform droplet slid along the surface accompanied by constantly penetration into the lubricant film (Fig. S7c, d). Despite the oil-storage space provided by the PANI coating, the surface without modification with fluorinated alkyl performed inferior affinity to the lubricant and massive oleophilic sites of PANI nanofibers were partially exposed, leading to strong attachment of oil on the surface. Overall, for the capillary-stabilized lubricant film of SLIPS, the sufficient texture roughness paired with matched surface chemistry to lubricant determines the effective attachment and storage of lubricant on the solid.

The omniphobic capability of the Glass@PANI@Thiol@PANI to various liquids with different surface tensions was tested as shown in Fig. 2g. The CAs of different droplets gradually increased from 40° to 110° with the improvement in surface tension from 18.43 mN/m (ethanol) to 72.8 mN/m (water). In contrast, all the droplets could easily slide along the surfaces with SAs of nearly 5° regardless of their surface tensions, indicating extreme low adhesion force of SLIPS to various liquids. Figure 2h presents the optical photographs and optical transmittance spectra of different glass. After grafted with the PANI nanocoating, the light transmission rate of Glass@PANI in the visible light region within range of wavelength number between 300 and 700 nm all exceeded 45% and reached the highest transmittance value of 71% at 490 nm; the uniform PANI nanocoating could generate multidiffusion of light, leading to the reduction of their transparency. The thiol modification showed little effect on the transmission rate of the Glass@PANI@Thiol. Moreover, the transmission rate of Glass@PANI@Thiol@PFPE achieved a higher value of 73.1% due to their anti-reflection properties afforded by the smaller refractive index of lubricant.

Self-cleaning and anti-fouling property of SLIPS

Thanks to their superior omniphobicity, the prepared slippery surfaces of Glass@PANI@Thiol@PFPE displayed excellent anti-fouling (Movie S2) and self-cleaning (Movie S3) property. As shown in Fig. 3a–c, the lubricant oil film can prevent various routine beverages including coke, milk and honey from direct contact with substrates. The coke and milk droplets were fallen away from the SLIPS within 1 s and 6 s. It took 12 s and 14 s for the crude oil and honey to slide off from the SLIPS, which was ascribed to their higher viscosity (Fig. S8a). On the contrary, the coke, milk, honey and crude oil all spread on the original glass. Similarly, the stain-resistant properties of PP circular tubes against crude oil were tested as demonstrated in Fig. S8b–d. Obviously, the crude oil fully spread out on the original PP, PP@PANI and PP@PANI@Thiol. While for the PP@PANI@Thiol@PFPE, the crude oil could readily pass through without leaving any residues, implying that the prepared SLIPS can be applied for the lossless pipeline transportation of viscous liquids (Fig. S8e). Moreover, the SLIPS also exhibited excellent self-cleaning property (Fig. 3d, e). Clearly, the ketchup and salad dressings adhered on the surfaces of Glass@PANI@Thiol@PFPE were removed by water scouring effect owing to the weak interaction between solid materials and lubricant layer. And yet, the ketchup and salad were tightly stained on the glass and could not be sorted out by water. Hence, the formed lubricant film can effectively decrease the adhesion forces of liquidus and solid contaminants on the substrates.

Figure 3
figure3

Anti-fouling property of Glass@PANI@Thiol@PFPE to coke (a), milk (b), honey (c) and self-cleaning property of Glass@PANI@Thiol@PFPE to ketchup (d), salad (e)

Anti-fingerprint and anti-fogging property of the SLIPS

By virtue of the excellent repellency to water and oil of the wicking lubricant layer, the prepared transparent SLIPS performed superior anti-fogging and anti-fingerprint properties. For the original samples, the underlying handwriting was clearly observed as shown in Fig. S9a. After exposure to boiling water for 20 s, moisture vapor quickly condensed and formed massive tiny water droplets over the entire surfaces of original glass, Glass@PANI and Glass@PANI@Thiol, respectively (Fig. 4b). When light passed through these glasses, refraction and reflection effects caused by the water layer greatly lower their surface transmittance, on which the font changed completely blurred and invisible (Fig. S9b). On the contrary, words could be clearly seen and there were no moisture drops accumulated on the Glass@PANI@Thiol@PFPE due to the excellent water repelling nature of slippery coatings, showing excellent anti-fogging property of SLIPS (Fig. S9b).

Figure 4
figure4

Optical micrographs of the a original sample, b after exposure to hot water vapor, c after finger pressing. All micrographs have the same magnification

Fingermarks mainly composed of water, free fatty acids, wax esters and glycerides threaten the cosmetic aspects, clarity and gloss of touch surfaces, which unavoidably bring about the decrease in transparency. Thus, elevating the resistance to fingerprinting of the surface is of great significances for practical applications. In this work, the anti-fingerprint property of various samples was measured by observing the human fingerprint region created by the same finger and compression strength. As shown in Fig. 4a, the optical microscope images of each samples were clean and smooth without any stains. After finger compression under same pressure, surface of the original glass, Glass@PANI and Glass@PANI@Thiol emerged fingerprints with different clarities. As noted, the trace of fingerprints on the PANI-modified surfaces was more apparent because of the stronger adsorption capacities of the grafted PANI nanofibers to the oily and waxy matters secreted by glands. While, there was no fingerprint marks on the Glass@PANI@Thiol@PFPE due to the excellent fluidity and oleophobicity of lubricant film (Fig. 4c).

Three-dimensional liquid manipulation on the SLIPS

The prepared omniphobic Glass@PANI@Thiol@PFPE also displayed tremendous applied potential in microfluidics area, on which the liquid can be readily moved directionally by external stimulus. As a conceptual simulation, the magnetic liquid droplets fabricated by dispersing the Fe3O4 nanoparticles into the water or glycerol were employed to test their controllable behaviors including migration, mixing and lossless transportation on the lubricant-infused surfaces under magnetic field. As shown in Fig. 5a, b, magnetic water and glycerol drops horizontally moved on the Glass@PANI@Thiol@PFPE under external magnetic field, respectively. Clearly, the movement speed of the corresponding drops depends on their own mass and viscosity. Furthermore, the magnetic particles dispersed into the glycerol were moved to the water droplet dyed with oil red pigment. Once in contact with each other, the glycerol and water drops were gradually agglomerated and formed a bigger droplet (Fig. 5c). In addition to the movement in the horizontal direction, the droplet could be reversibly manipulated perpendicular to the SLIPS. As demonstrated in Fig. 5d, a magnetic glycerol drop lying on the bottom substrate bounced up and was completely transported to the upper surface driven by a ferromagnet. When the magnet was removed, the droplet went back to the bottom surface under the gravitation. Notably, the returned magnetic droplet could still slide along the surfaces. Thus, the switchable transportation of droplets could be achieved on the prepared SLIPS in three dimensions, which is expected to apply for the microfluidic devices.

Figure 5
figure5

Migration of Fe3O4 nanoparticles dispersed into water (a) and glycerol (b) droplet along the horizontal direction. c Coalescence of magnetic glycerol drop with water droplet. d Lossless transportation of magnetic glycerol droplet (8 μL) on lubricated surface. All the surfaces are placed horizontally

Stability evaluation of the SLIPS

Indeed, the prepared lubricant-infused textured surfaces are suffering from the lubricant losses caused by the volatilization and entrainment during continuous exposure to flow of air or external shear stresses. Here, the long-term durability of the lubricant film of the SLIPS against high/low temperature, UV irradiation and NaCl immersion for 7 days was tested as shown in Fig. 6a–d. Clearly, CAs and SAs of water droplets on the SLIPS were maintained stable during the tests. Meanwhile, the mass retention ratios of SLIPS after different treatments during 7 days were measured as demonstrated in Fig. S10, from which it can be clearly seen that all the samples existed a degree of mass loss due to the volatilization, degradation and attrition of the lubricant. Especially, the lubricant film showed the maximum quality loss (up to 50%) when immersed into the NaCl solution, which might be caused by the entrainment and partial miscibility into the solution of the exterior oil layer. However, the inner oil layer was still firmly attached to the surface via wicking effect and remained stable and integrity, which endowed the surface with stabilized repellency to the foreign liquids. In addition, after vertically placed for 20 days, SLIPS maintained hydrophobicity with WCAs larger than 114° and WSAs lower than 5°, displaying outstanding long-term omniphobic stability (Fig. 6e).

Figure 6
figure6

WCAs and WSAs of Glass@PANI@Thiol@PFPE after treatments in high temperature (a), low temperature (b), UV irradiation (c) and NaCl solution (d) for 7 days. Variation of WCAs and WSAs of the Glass@PANI@Thiol@PFPE against long-term vertically static storage (e) and several cycles of water impact tests (f)

Furthermore, the mechanical robustness of the SLIPS was characterized by applied different external damages. Water-resistant impact test was employed by flushing the SLIPS surface using water flow with striking velocity of 1.34 m/s for 30 s. After water impact for ten cycles, WCAs remained stable at nearly 108° and WSAs slightly increased to 5° (Fig. 6f). Besides, the SLIPS were subjected to mechanical destructions including knife scratches and sandpaper abrasions for ten cycles, on which water and crude oil could still easily slide off (Fig. S11 and Movie S4). The excellent repellent durability is attributed to the stable adhesion of PFPE to the fluorinated PANI nanofibers and the mobility nature of lubricant layer, which acted as the self-healing coating to rapidly restore its anti-wetting performances.

Conclusions

In summary, robust, transparent and omniphobic SLIPS was prepared by in situ polymerized PANI nanofibers on underlying substrates regardless of their shapes and components, followed by fluorinated modification with perfluoro alkanethiol and infiltration with PFPE lubricant. By altering the polymerization time, the coverage degree of PANI and the visible light transmittance of the glass realized controllable regulation. In addition, the influences of the structure geometry and surface chemistry on the stability of lubricant layer and motion behaviors of foreign liquids were studied in detail, further revealing their synergistic effect for building robust SLIPS. Moreover, the prepared SLIPSs showed great potential applications in various areas such as anti-fouling, anti-fogging, anti-fingerprint, three-dimensional droplet manipulation and lossless transportation of viscous liquids. The presented approach with simplicity and versatility is conductive to supply a new pathway to generate multifunctional superwetting interfacial materials.

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Acknowledgements

This work was financially supported by the Fundamental Research Funds for the Central Universities (JUSRP11916, JUSRP51907A and JUSRP21933). We also thank Jiangnan University for supporting in the course of research.

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Correspondence to Mingming Liu or Shaohai Fu.

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Yu, M., Liu, M., Hou, Y. et al. Facile fabrication of biomimetic slippery lubricant-infused transparent and multifunctional omniphobic surfaces. J Mater Sci 55, 4225–4237 (2020). https://doi.org/10.1007/s10853-019-04243-8

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