The peristome of the pitcher plant is composed of hydrophilic components with regular microscopic structures, which secrete mucus and absorb rainwater, storing it within the microscopic structures to form a stable and uniform lubricating film. When prey such as ants land on the edge of the pitcher plant, the presence of the lubricating film prevents them from adhering and causes them to slide into the digestive fluid at the bottom^[25], as shown in Figure 6. Additionally, when the surface loses smoothness due to damage, the pitcher plant can self-repair by drawing internal fluids into the damaged area. Inspired by this, researchers^[26] first proposed and fabricated liquid-infused porous surfaces (LIS)/slippery liquid-infused porous surfaces (SLIPS), which rely on the capillary action and van der Waals forces of micro-nano porous structures (micro-structures, nano-structures, or micro-nano composite structures) to lock non-toxic chemically inert oil-phase substances, forming a stable surface. When physical scratches or wear create gaps, the lubricant can flow in and self-repair. Buddingh et al.^[27] pointed out that the term LIS is broader and used to describe different types of liquid surfaces, while the term SLIPS is more directional; hence, this review adopts the term SLIPS.

The construction of SLIPS is generally divided into three parts: a porous rough substrate, a substrate modified with low surface energy chemical substances, and infused low surface energy lubricating oil. The preparation principles^[26] are: (1) the lubricating fluid must be drawn in, wet, and stably adhere to the porous substrate; (2) the lubricating fluid and the external test liquid must be immiscible; (3) the substrate must be preferentially wetted by the lubricating fluid rather than the test liquid. Additionally, when preparing SLIPS, an excessively low surface energy may instead cause a cloak effect, where the lubricating fluid layer becomes unstable and is lost (Figure 7).

The roughness will not affect the fluidity of the lubricating oil, as the lubricant is retained in the rough structure due to its affinity for the pore walls and the capillary attraction of the pores, allowing the liquid with lower surface energy to slide on the liquid layer without penetrating into the rough substrate. However, no single lubricating oil is suitable for all external liquids^[28], so an appropriate lubricant must be selected based on specific applications. Bittner et al.^[29] infused polar ionic liquid 1-ethyl-3-methylimidazolium methyl sulfate into etched natural silicon and porous nanostructured silicon functionalized with ionic liquids. Different from nonpolar SLIPS formed by perfluorinated materials, the resulting fluorine-free SLIPS exhibited excellent repellency toward toluene and cyclohexane, with α <3°. However, due to the high miscibility of hydrophilic ionic liquids with other solvents, liquids such as hexadecane and tetrahydrofuran (THF) would wet the surface. He et al.^[30] used a thermally induced phase separation process to blend and extrude a mixture of ultra-high molecular weight polyethylene and SiO₂ nanoparticles, and prepared flat films through thermal compression. During the preparation process, polyethylene and SiO₂ acted as non-polar and polar components respectively, inducing the formation of microporous structures, onto which polar inks with various properties were printed to achieve functionalization. After injecting mineral oil/silicone oil as lubricants, multiple organic liquids could easily slide on the surface. Jing et al.^[31] grafted PDMS onto zinc oxide (ZnO) nanorods, infused silicone oil as a lubricant, and reacted under ultraviolet light (UV). Due to the strong intermolecular forces between the unbound silicone oil and grafted PDMS, the silicone oil was firmly locked onto the ZnO surface, showing strong repellence to both high-temperature and room-temperature liquids. Hu et al.^[32] cured Jeffamine, bisphenol A diglycidyl ether (DGEBA), and poly(glycidyl methacrylate)-graft-polydimethylsiloxane (PGMA-g-PDMS) on glass, then infused a large amount of silicone oil. Alkane-based liquids showed excellent sliding effects on the surface. The essence of SLIPS lies in using low-surface-energy lubricating oils to replace the air layer within rough structures, forming a liquid layer that prevents direct contact between external liquids and the substrate surface. By transforming the solid-liquid contact interface into a liquid-liquid contact interface, the roughness of the contact surface is reduced, allowing the test liquids to slip easily on the surface without leaving any sliding traces.

According to the environmental response characteristics, fluorine-free anti-oil SLIPS can be classified into temperature, electric, and light responsive surfaces, etc.

(1) Temperature-responsive surfaces: The properties of temperature-responsive SLIPS change accordingly with temperature variations. As shown in Fig. 8(1), Wang et al. [33] successfully fabricated an anisotropic smooth surface with temperature-responsive characteristics by infusing a thermoresponsive phase-change wax lubricant into a directionally porous polystyrene (PS) film. When the surface temperature exceeds the melting point of the wax, droplets can slide easily on the material surface; when the surface temperature drops below the melting temperature of the wax, the material surface is covered with solid wax, preventing droplet movement. By adjusting the temperature, the reversible motion of dimethyl sulfoxide (DMSO) and ionic liquids was achieved. Meng et al. [34] also used porous PS films and employed wax/liquid paraffin as lubricants to prepare liquid-sliding surfaces with thermally assisted self-healing capabilities.

(2) Electric-responsive surfaces: Che et al^[35] used silicon oil/ion liquid (including non-conductive and conductive types) lubricants to fill oriented, porous, conductive reduced graphene oxide (rGO) films. By applying conductive lubricants for SLIPS preparation and reducing their response voltage, they successfully achieved reversible control of liquid sliding motion through electric actuation. Guo et al^[36] utilized oriented, porous, conductive poly(3-hexylthiophene) (P3HT) fibers and silicon oil to construct a semiconductor network structure on an ITO substrate surface, reporting electrically driven SLIPS with reversible control of droplet sliding. As the voltage was applied and removed, the droplets transitioned between the Wenzel-Cassie intermediate state and the Cassie state, respectively, as shown in Figure 8(2). Additionally, organic liquids such as propylene glycol could slide on the surface with a lower α.

(3) Light-responsive surface: Wooh et al.^[37] grafted PDMS brushes on the surface of photocatalytic mesoporous titanium dioxide (TiO₂) substrate, with the remaining ungrafted PDMS serving as a lubricant. Since the chemical properties of the lubricant and PDMS brushes are identical, the grafted PDMS brush layer is stably swollen by the lubricant PDMS, thereby preventing direct contact between the droplet and the solid substrate, enabling methanol and low surface tension fluorocarbon droplets to slide freely with extremely low α. As shown in Figure 8(3), the designed surface can also utilize UV-catalyzed degradation of organic dyes in the surface lubricating oil.

Table 2 shows a variety of fluorine-free oil-resistant SLIPS.

Long-term use of SLIPS can lead to problems such as lubricant depletion and reduced coating durability. Therefore, in the preparation strategy of SLIPS, reducing costs and minimizing lubricant loss become critical considerations to enhance their stability and resistance to wear over long-term use, thereby extending their service life.

Colloidal particles or macromolecules are interconnected through chemical crosslinking or physical entanglement, forming a spatial network structure. The dispersion system with liquid or gas as the dispersion medium filling the structural voids is called a gel, which exhibits basic properties similar to solid rheological characteristics. The research on fluorine-free oil-resistant gels includes types such as organic gels and hydrogels.

(1) Organic gels: As shown in Fig. 9, Urata et al^[38] used PDMS as the gel matrix and various organic liquids as the mobile phase to successfully develop a new type of transparent self-lubricating organic gel with multiple functions through a one-pot cross-linking reaction. When the surrounding environment (such as temperature) changes, the liquid inside the gel matrix can spontaneously release to the outer surface, and various liquids can freely slide on its surface.

(2) Hydrogels: Xu et al^[39] used water as a lubricant to design a hydrophilic and oleophobic binary polymer salt hydrogel coating (Figure 10), and due to the strong hydrophilicity of the hydrogel, water can fully wet the hydrogel and form a hydration layer on the surface, which can act as a lubricant to repel oily liquids, making them exhibit slipperiness on the hydrogel surface. The research results of fluorine-free anti-oil gel are shown in Table 3.

Compared with SLIPS, the properties of gels can be flexibly adjusted by simply altering the composition of the precursor mixture to accommodate different substrates, and they can form large-area free-standing thin films. However, as a liquid surface, gels also face the issue of lubricant depletion, which poses a challenge to their long-term performance.

On smooth or structured organic or inorganic substrates, self-assembled monolayers with highly oriented and tightly packed functional groups can be prepared by grafting organic molecules such as silanes, amines, alkenes, alkynes, phosphates, carboxylates, isocyanates, etc., through gas-phase or liquid-phase reactions, which can achieve liquid slip effects on the substrate surface. Hozumi et al^[40] used cyclic 1,3,5,7-tetramethylcyclotetrasiloxane to perform CVD treatment on alumina and titania surfaces. The cyclic siloxane is highly temperature-sensitive. At 80 ℃, CVD produced a monolayer on the metal oxide surface with Δθ_n-hexadecane <3°, while at 180 ℃, hydrolysis and condensation reactions promoted particle deposition, significantly enhancing the influence of nano-microstructure morphology on hydrophobic properties, and also made the metal oxide surface completely oleophilic, indicating that the smoothness of the substrate has an important impact on liquid slip behavior. Fadeev et al^[41] performed chemical grafting using alkyl dimethyl chlorosilane and silicon wafers under three conditions: high-temperature gas phase, toluene/ethyl diisopropylamine (EDIPA), and high-temperature gas phase with toluene/EDIPA. They found that the bonding density was determined by the reaction conditions, and the gas-phase reaction could produce the densest monolayer. They also prepared a series of alkyl dimethylsilane surfaces with branched alkyl chain lengths of 1~22 carbons (Table 4) and found that the θ of water was independent of chain length, while the θ of n-hexadecane and diiodomethane decreased with increasing chain length. This indicates that these surfaces project disordered methyl groups towards water, and the monolayer topology achieves a barrier effect against water, whereas oily liquids can penetrate the monolayer and interact with methylene groups.

Since the average thickness of a single-layer film is generally about 2 nm, it is necessary to precisely control the generation conditions to achieve a tight and orderly arrangement of functional groups such as alkylsilanes on the surface. To achieve the ideal arrangement effect, the substrate surface must be as smooth as possible, otherwise, it will affect the sliding performance of the liquid. Therefore, smooth surface materials such as glass slides, silicon wafers, and aluminum sheets are often used when selecting substrates (Table 5).

Compared with the surface of single-layer membrane structures, the arrangement of surface functional groups on polymer brush-type surfaces is relatively non-directional and irregular, and PDMS is the most commonly used polymer in this type of surface. Compared to carbon-based polymers, the Si—O and Si—C bond lengths (1.63×10^-10 m and 1.90×10^-10 m) of PDMS exceed the C—C bond length (1.53×10^-10 m), and the Si—O—Si bond angle (143°) of PDMS is also larger than the C—C—C bond angle (109°). These structural differences give PDMS rotational and vibrational degrees of freedom^[42]. In addition, PDMS has excellent optical, mechanical, and thermal properties, biocompatibility, and can be flexibly grafted with various end groups and branched structures (such as hydroxyl —OH, amino —NH₂, and vinyl —HC=CH₂, etc.). Uncrosslinked PDMS is liquid at low n and semi-solid at high n (n refers to the degree of polymerization), typically having a longer carbon chain than perfluoroalkyl and containing a large number of methyl groups, with a surface energy of approximately 22~24 mN/m. Due to its extremely low glass transition temperature (T_g = -127 ℃), uncrosslinked PDMS can maintain highly mobile molecular segments at room temperature, used to prepare oleophobic surfaces that resist various polar or non-polar liquids^{[43⇓⇓-46]}. Liu et al.^[47] first aminated the substrate with APTES, then alternately treated the substrate with isophorone diisocyanate (IPDI) and bis(aminopropyl)polydimethylsiloxane (H₂N-PDMS-NH₂) to obtain a polymer brush surface. Additionally, they used comb-like PDMS oligomers with multiple side-chain amino groups and difunctional alkyl diamines as monomers to prepare copolymer coatings by sequential grafting with IPDI. Comparative studies found that the mobile PDMS chains in the copolymer are more conducive to the sliding of oily droplets on the surface (Figure 11).

PDMS can interact with organic materials and react with inorganic materials. It is mainly grafted onto the substrate surface through three methods, namely direct method, indirect method, and in situ growth method.

(1) The direct method involves treating the substrate to expose functional groups such as -OH, thereby enabling the direct reaction between PDMS and -OH. Wang et al.^[48]used isopropanol and deionized water to rinse the substrate, effectively removing surface contaminants and exposing -OH groups. A solution containing dimethyldimethoxysilane was sprayed onto the surface where molecules reacted with -OH, forming a covalently grafted PDMS layer on the surface. Subsequently, silicone oil lubricant was added, resulting in a surface that repels liquids, bacteria, and viscoelastic solids with dynamic viscosity spanning over nine orders of magnitude, referred to as Liquid-entrenched Smooth Surface (LESS). Krumpfer et al.^[42]achieved the grafting of PDMS with different molecular weights onto silica surfaces after heat treatment by hydrolyzing PDMS with three methylsilane groups, undergoing condensation reactions with silanol (Si-OH) on the silica surface, and utilizing Si-OH for the acid-catalyzed direct silane decomposition of PDMS. As shown in Figure 12(1), PDMS can also be grafted onto metal oxide surfaces (such as TiO₂, aluminum oxide Al₂O₃, and nickel oxide NiO), but only PDMS grafted onto Si-based substrates can make nonpolar liquids exhibit a Δθ of less than 3°.

(2) The indirect method refers to chemically modifying the substrate to graft specific components onto the surface, followed by covalently bonding with PDMS. Cheng et al.^[17] applied a sol-gel coating of TEOS on glass slides, which provided numerous active silanol groups. Subsequently, they utilized CVD to hydrolyze and condense bifunctional chlorosilane monomers, grafting linear polysiloxane brushes and preparing g-PDMS coatings, on which various low-surface-energy liquids exhibited ultra-low Δθ. Huang et al.^[49] coated substrates with Sylgard 184 PDMS, where Sylgard 184A is polymethylhydrosiloxane and Sylgard 184B is polymethylvinylsiloxane. After vacuum degassing and curing at 125°C, a large number of Si-H groups remained due to the much higher proportion of A than B, forming hydrosilane (SiH-PDMS). This was followed by a hydrosilylation reaction between SiH-PDMS and vinyl ethylene glycol ether-PDMS (MV-PDMS), producing a transparent anti-oil coating. Cheng et al.^[50] performed vapor-phase treatment on silica substrates, forming a monolayer of 1,3,5,7-tetramethylcyclotetrasiloxane on the surface, which then underwent Pt-catalyzed hydrosilylation with PDMS terminated by -HC=CH₂. By controlling the molecular weight of linear PDMS grafted onto smooth silicon wafers and solvent effects, they successfully demonstrated that the dynamic dewetting behavior of alkane liquids can be finely tuned by physically controlling polymer chain mobility in PDMS brush films. Zheng et al.^[51] reported a room-temperature photocurable anti-oil coating. PDMS-OH and hydroxyethyl methacrylate (HEMA) were reacted with hexamethylene diisocyanate trimer (HDIT) to obtain PDMS (DM-PDMS) with two double bonds and a liquid trifunctional monomer (TM) mixture. Polyols (PO) were respectively reacted with 2-cyanoethyl methacrylate or with PDMS terminated by acyl chloride (PDMS-COCl) and then with 2-cyanoethyl methacrylate to prepare PO with introduced double bonds (POII) or POII-g-PDMS. Then, DM-PDMS and TM (F1), POII-g-PDMS and POII (F2), or POII-g-PDMS and TM (F3) were cast into films on glass together with 2-hydroxy-2-methylpropiophenone. Hexadecane could easily slide on F2 and F3 surfaces. After 30 writing and erasing cycles, the ink shrinkage capability on the F2 coating decreased, while that on the F3 coating remained unchanged. Through sol-gel chemistry, Zhang et al.^[52] used (2-(3,4-epoxycyclohexyl)ethyl)trimethoxysilane and ammonia water as raw materials to prepare cycloaliphatic epoxy-functionalized oligosiloxane (CEOS) via the sol-gel method and prepared PEMA-g-PDMS using commercially available poly(ethylene-alt-(maleic anhydride)) (PEMA) and monoamine-terminated PDMS as raw materials. CEOS and PEMA-g-PDMS were mixed in acetone and n-methyl-2-pyrrolidone mixed solvents. This mixed solvent could dissolve CEOS and PEMA but not the PDMS side chains. The incorporation of insoluble PDMS side chains led to intrachain or interchain micellization. Casting the micelle mixture onto the substrate surface, during solvent evaporation and dried film heating, CEOS and PEMA reacted with each other, and the anhydride groups of PEMA could open the epoxy rings of CEOS, ultimately fixing PDMS in the crosslinked CEOS/PEMA. The lower surface tension of PDMS promoted the migration of some PDMS clusters to the surface, resulting in a much higher concentration of PDMS on the surface than inside. Test liquids immiscible with PDMS (diiodomethane, methanol) had a larger α than test liquids miscible with PDMS (hexadecane, dodecane, and decane). Shen et al.^[53] reported the homogeneous grafting reaction of PDMS with polyarylethersulfone (PAES). Adding PDMS to PAES obtained the graft copolymer PAES-g-PDMS, whose coating showed targeted antifouling performance, with hexadecane droplets sliding at α<3° and oil-based ink easily retracting on the surface. Cheng et al.^[54] achieved oil repellency by grafting vinyl-terminated PDMS to Si-H groups of a 1,3,5,7-tetramethylcyclotetrasiloxane-derived monolayer on silica substrates through Pt-catalyzed hydrosilylation. Liu et al.^[44] amino-functionalized silicon wafers using APTES solution, then sequentially immersed them in IPDI and H₂N-PDMS-NH₂ solutions. Various low-surface-tension liquids could slide on the surface with extremely low Δθ, and fluorescein isothiocyanate (FITC) ink could be printed on the oil-repellent surface for surface patterning. Yu et al.^[55] treated glass with alkaline hydrothermal treatment, generating a large number of -OH groups as reaction sites for dehydration condensation with 3-mercaptopropyltrimethoxysilane (MPTS), followed by covalently grafting vinyl-terminated PDMS onto mercapto-modified glass through thiol click chemistry, as shown in Fig. 12(2). After high and low temperature, UV irradiation, water impact, and mechanical wear tests, it still exhibited durable anti-oil properties.

(3) The in-situ growth method refers to generating PDMS on the substrate surface through catalytic polycondensation and other methods. As shown in Fig. 12(3), Wang et al.^[56] proposed an acid-catalyzed condensation grafting method of dimethoxysilane monomers, placing oxygen plasma-treated silicon wafers exposing -OH into a mixed isopropanol solution of dimethoxysilane and sulfuric acid, where sulfuric acid catalyzes the hydrolysis and polycondensation of dimethoxysilane, enabling PDMS to be rapidly grafted onto the surface, allowing various droplets to slide on the coating surface with Δθ ≤1°. Zhao et al.^[57] subjected oxygen plasma-treated paper rich in -OH to vapor deposition with 1,3-dichlorotetramethyldisiloxane; free chlorosilane molecules undergo hydrolysis in the presence of water molecules, producing silanol end groups and releasing hydrogen chloride (HCl), and silanol-terminated siloxane molecules undergo polycondensation reactions to form highly mobile PDMS chains on the paper surface. Compared with polymer brushes grafted from solution, this method avoids physical deformation of the paper.

Table 6 displays a variety of fluorine-free oil-resistant polymer brush "liquid-like" surfaces. Compared with monolayer films, polymer brush coatings have greater thickness. Even after severe surface wear, newly released liquid polymer chains can replenish the worn areas, thus maintaining their anti-fouling performance over the long term.

By introducing other components into liquid-like oligomer/polymer coatings, composite coatings can be formed, keeping the surface with "liquid-like" properties while providing excellent physicochemical performance. The composite coatings can be mainly divided into three types: PDMS composite coatings, sol-gel hybrid coatings, and nanoscale rough "liquid-like" coatings.

(1) PDMS composite coating.

The PDMS component is added to cage-type silsesquioxane (GPOSS)/cage-type polysiloxane (POSS), polyurethane (PU), polyethyleneimine (PEI), epoxy resin (EP), and other resin coatings. By designing or synthesizing specific additives/precursors, they can spontaneously assemble or graft PDMS during the coating formation process, effectively reducing surface energy, enhancing coating adhesion, retaining a certain degree of liquid flow, and thus improving the overall performance of the coating.

①PDMS-POSS composite coatings: Zhang et al^[58] obtained GPOSS-g-PDMS by reacting GPOSS with PDMS-NH₂, then incorporated GPOSS-g-PDMS into a GPOSS-based coating, and subsequently prepared a highly transparent, highly flexible, and highly wear-resistant anti-oil-fouling coating through solvent evaporation and photolysis processes; various organic solvents easily slide on the coating surface, and ink and paint tend to contract. Zheng et al^[45] adopted a one-step method to prepare a semi-interpenetrating entangled network design, rapidly fabricating the coating through a thiol-ene click reaction between POSS and styrene-butadiene copolymer (SBS) (Figure 13); POSS and —OH terminated PDMS (flexible macromolecular brushes) reduced the substrate's surface energy, and the novel structural design avoided complex grafting processes; the coating exhibited excellent liquid sliding performance, strong interfacial bonding, flexibility, and mechanical stability; various oils and organic liquids could slide freely without leaving traces.

②PDMS-PU composite coatings: Shiraki^[59] heated hydrogenated poly(farnesene) (HHPF) with 4,4'-diphenylmethane diisocyanate (MDI) under the catalysis of dioctyltin dilaurate (DOTDL) to generate PU chains. After the newly prepared PU was coated on a silicon wafer, the contact angle of hexadecane reached 68°, demonstrating a certain oil-repellent effect. To further enhance the oleophobic performance, the PU coating can be mixed with materials such as PDMS. When preparing PDMS-PU composite coatings, in order to eliminate the phase separation issue between PDMS and the PU matrix, additives can be introduced or reaction conditions optimized to reduce or eliminate phase separation, thereby obtaining a uniform and stable composite coating. Khan et al.^[60] treated NH₂-PDMS on a semi-crosslinked PU coating; NH₂-PDMS would penetrate from the top into the PU matrix, and its —NH₂ groups reacted with the isocyanate groups (—NCO) of partially crosslinked PU, forming covalent bonds, which solved the phase separation problem of PDMS in PU, thus preparing an oil/ink repellent coating. Ye et al.^[61] fabricated a composite coating on paper consisting of a PU bottom layer (first layer) and a PDMS top layer (second layer), which still maintained its oil resistance after 4 months of outdoor storage. Rabnawaz et al.^[62] grafted polyols (styrene/methyl methacrylate oligomers) onto PDMS terminated with acyl chloride (—COCl), then added HDIT; the reaction of polyols with HDIT could produce PU, developing a fluorine-free transparent anti-smudge PU coating by eliminating large phase separation between PDMS and the PU matrix, which retained its anti-oil stain properties even after extensive wear. Hu et al.^[63] mixed polyols, polyol-grafted PDMS copolymers, and HDIT, added free silicone oil or a silicone oil mixture, cast them on glass slides, successfully preparing a PU-based coating with excellent anti-oil stain properties. As shown in Figure 14, Huang et al.^[64] co-dispersed blocked polyisocyanates and graft copolymer polyol-g-PDMS (composed of water-dispersible polyol main chains and PDMS side chains) in water containing dipropylene glycol monomethyl ether, cast it on substrates allowing solvent evaporation, followed by thermal curing, to prepare solvent-borne PU coatings. During the formation and curing of the coatings, due to the covalent bonding of liquid PDMS with the polyol backbone, PDMS was incompatible with the coating matrix, leading to microphase separation instead of large phase separation, forming spherical phases (diameter <30 nm) dispersed throughout the matrix. Hexadecane droplets caused the grafted PDMS chains to swell, stretching the surface PDMS chains into solvent droplets, leaving nanoscale pools in the air and covering the coating surface to lower surface energy. Due to the surface enrichment effect of PDMS, the coating surface transitioned from solid to liquid, and test liquids with surface tension higher than 23 mN/m easily and cleanly slid off the surface. Additionally, marker ink and paint contracted on the coating and were easy to remove.

③PDMS-PEI composite coatings: Hu et al^[65] grafted PEI onto PDMS, then added bisphenol A diglycidyl ether (DGEBA) and a hardener (including Jeffamine curing agent, triethanolamine, and piperazine) into the coating system. PDMS is incompatible with the coating system, but PEI is soluble in it. Through the reaction between PEI and DGEBA, a polymer with multiple pendant epoxy rings was formed. The epoxy rings reacted with the hardener, allowing PEI to be evenly distributed while dragging the side PDMS chains. The incompatible PDMS formed nanodomains (crystals with nanostructures) uniformly distributed in the cross-linked epoxy resin matrix. Due to the low surface tension and high mobility of PDMS, more PDMS chains were enriched near the air interface. Grafting PEI onto PDMS solved the large phase separation problem between PDMS and other components, and the coating exhibited oil resistance and ink shrinkage capabilities.

④PDMS-EP composite coatings: To suppress the phase separation between PDMS and the EP matrix, Khan et al^[66] did not directly add PDMS-NCO to the EP coating but first prepared the EP coating through Jeffammine-D230 and bisphenol A diglycidyl ether BADGE. On the basis of the partially crosslinked EP coating, PDMS-NCO was then added, and the -NCO reacted with the amino group of Jeffamine D-230. Curing was carried out at high temperatures to achieve complete curing of the remaining Jeffammine-D230 and BADGE, ultimately successfully preparing an oil-resistant epoxy resin coating with excellent adhesion. Hexadecane could easily slide on the surface with a lower α, and the PDMS-EP composite coating is shown in Figure 15.

⑤PDMS-other resin composite coatings: Wu et al^[67] used a mixture of methyl methacrylate (MMA), butyl acrylate (BA), and acrylic acid (AA) as the pre-emulsion, with hydroxypropyl methacrylate (HPMA) participating in the particle growth stage of polymerization to prepare the coating precursor via emulsion polymerization, namely particles with densely packed crosslinkable -OH on the surface (PPH). Highly etherified melamine-formaldehyde resin (HEMF) connects PPH and siloxane surfactant (SSH) with PDMS and -OH hydrophilic side chains through crosslinking reactions, ultimately forming a fully aqueous polymer crosslinking system composed of polymer particles, silicone surfactants, and melamine-formaldehyde resin, exhibiting excellent resistance to crude oil adhesion performance.

Research on composite coating systems composed of PDMS and resin/polymer is shown in Table 7.

(2) Sol-gel hybrid coatings.

A sol is a colloidal system with liquid characteristics, formed by dispersing small particles or molecules in a solvent, with a particle size ranging from 1 to 1000 nm, characterized by fluidity and no fixed shape. The sol-gel method utilizes compounds rich in highly chemically active components as precursors, which are uniformly mixed in a liquid environment and undergo hydrolysis and condensation reactions, thereby forming a stable and transparent sol system in the solution. Through the aging process, the colloidal particles within the sol slowly aggregate and form a three-dimensional network structure of gel, which can be used to prepare transparent and high-performance coating materials.

Masheder et al^[68] simply combined stearic acid (SA) with zirconium tetrapropoxide (ZTP) to prepare a zirconia-based inorganic-organic hybrid membrane. The molecular structure of stearic acid (including linear stearic acid LSA and branched-chain stearic acid BSA) would affect the physical and chemical properties of the surface of the hybrid film. Due to its better thermal stability, the Zr:BSA hybrid membrane exhibits a fully cross-linked Zr-O-Zr network structure under 200 ℃ high-temperature curing. Compared with the hybrid membrane prepared by LSA, it shows superior hardness, and n-hexadecane droplets slide easily. Urata et al^[69] successfully prepared polymethylsiloxane films by simply carrying out methyltriethoxysilane sol-gel reaction, which had excellent high-temperature resistance, durability, and temperature-dependent oil-repellent performance at about 350 ℃. Urata et al^[70] prepared hybrid coating surfaces using sol-gel solutions containing dodecyltriethoxysilane (DTES) and tetramethyl orthosilicate (TMOS). As TMOS acted as a molecular spacer and solvent effect, it enhanced the mobility of alkyl chains on the surface, making organic liquids easy to slide on the surface. On this basis, Urata et al^[71] developed a series of alkyl silane sol-gel hybrid coatings and pointed out the influence of alkyl chain length on liquid sliding performance, as shown in Figure 16, when the alkyl chain length n =3~10, low-surface-energy alkane liquids can slide at α <5º.

Urata et al. ^[72] also used sol-gel-derived hybrid films containing decylsilyl groups as model surfaces to investigate the reasons for the movement of organic liquids on smooth alkyl-terminated sol-gel hybrid coating surfaces. When droplets are at a chemical interface, they can spontaneously move from regions of low surface energy to regions of high surface energy. Compared with droplets with higher dielectric constants, when test liquids with lower dielectric constants come into contact with the hybrid film surface, —CH₃ groups are preferentially exposed at the solid-liquid interface, and the surface energy at the solid-liquid interface is lower than that at the solid-gas interface, allowing the test droplet to flow smoothly on the surface with a smaller α. As shown in Figure 17, the polarity of the test liquid can induce alkyl chains to reorient at the liquid-solid interface to achieve the most stable conformational state.

Table 8 shows the surface of fluorine-free oil-resistant sol-gel hybrid coatings.

(3) Nanoscale rough "liquid-like" coating.

On traditional super-liquid-repellent surfaces, droplets can maintain mobility only in the Cassie state, and once they transition to the Wenzel state, their mobility is lost (Fig. 18(1)). Therefore, in most cases, "liquid-like" fluorine-free oleophobic surfaces are more suitable for functional finishing of relatively smooth substrates. However, by designing nano-scale rough structures, rough surfaces can also exhibit "liquid-like" oleophobic performance. Fan et al.^[73] reported that by combining specific surface textures with liquid surface chemistry, a "liquid-like" polymer polyfluorinated ether (PFPE) coating applied on micropillar arrays achieved dual mobility in both Cassie and Wenzel states for solid super-repellent surfaces (Fig. 18(1)). As shown in Fig. 18(2~3), by adjusting the pillar diameter D, pillar height H, and center-to-center distance L between pillars, the grafted PFPE chains covalently connect to the substrate at one end while the rest can freely rotate and move, forming a nanoscale thick "liquid-like" layer that significantly reduces interfacial adhesion. Although this study used fluorinated compounds, its theoretical model provides reference for designing fluorine-free "liquid-like" coatings. As shown in Fig. 18(4), the model divides the contact line of the droplet in the sliding direction into front and rear sides, separately calculating their adhesion forces to explain the movement differences on different surfaces. The total lateral adhesion force is the sum of the front and rear adhesion forces: F_La = F_La^Front + F_La^Rear. As shown in Fig. 18(5), the adhesion force mainly comes from the projected parts of the contact line (texture top and bottom) as well as the pillar side walls. The front-side contact line hinders droplet movement due to a larger contact angle, F_La^Front = D_c(-γcosθ_a + 2γH/L), where D_c is the droplet contact diameter, γ is the liquid surface tension, and θ_a is the inherent advancing contact angle. The rear-side contact line's adhesion force can be negative, especially on "liquid-like" coated surfaces, which further aids droplet sliding, F_La^Rear = D_c(γcosθ_r + 2γsinθ_rH/L), where 𝜃_r is the inherent receding contact angle. The "liquid-like" coating increases the droplet's receding contact angle, reducing or even reversing the rear-side adhesion force, thereby promoting fluidity. By decomposing the front and rear adhesion forces, the high mobility of Wenzel-state droplets on liquid-coated surfaces was analyzed, providing theoretical guidance for optimizing surface texture and chemical properties.

At present, researchers have developed fluorine-free oil-repellent nanoscale rough "liquid-like" coatings. Hegner et al^[74] used PDMS as the "liquid-like" material, combined with dual-scale micro-nano structure design, and prepared a series of SiO₂ particle-based surfaces through three methods: liquid flame spraying, spraying, and paraffin soot deposition, thereby achieving oleophobic properties and successfully preparing surfaces that can repel liquids with surface tension as low as 31 mN/m. Singh et al^[75] amino-functionalized glass slides using APTES, grew zirconium-based metal-organic frameworks (Zr-MOFs) layer by layer through covalent amide bonds, and then functionalized them by reacting —OH in the MOF linker with alkylsilane, as shown in Figure 19, to obtain fluorine-free oil-repellent MOF films with a nano-textured hierarchical structure. The nanoscale rough "liquid-like" coating (Table 9) combines the advantages of rough structures and "liquid-like" surfaces, enabling excellent oil-repellent performance.

On the "liquid-like" surface, various polar or non-polar liquids exhibit excellent sliding properties, which has become an important strategy in the preparation of fluorine-free anti-oil fouling surfaces. However, the service life limits its wide application. By introducing other components into the liquid oligomer/polymer coating, a composite coating can be formed, which maintains the "liquid-like" properties while improving its fastness, thereby extending the service life.