1 Introduction
With the advancement of industry and the improvement of living standards, people are paying more attention to the protection against various types of oil pollution. Whether it is the environmental protection and resource utilization problems caused by oil spills or common phenomena such as oil stains on clothing affecting aesthetics, the design of oil-resistant surfaces has attracted widespread attention in the field of functional finishing. Materials with oil-resistant properties exhibit excellent anti-pollution, self-cleaning, antioxidant, and antibacterial characteristics, which can reduce maintenance costs and increase service life. Therefore, they play important roles in the textile industry, daily stain prevention, environmental protection, oil extraction industry, energy production, liquid transportation, self-cleaning, wastewater/oil purification, anti-fouling and anti-corrosion, chemical shielding, biomedicine, and liquid handling.
The ability or tendency of a liquid to spread on the surface of a solid material is called wettability, which is the basic inherent property of a solid surface and is mainly determined by surface energy/surface tension. Surface energy refers to the extra energy possessed by surface particles relative to internal particles, which originates from the energy difference caused by the uneven distribution of surface atoms or molecules. The surface energy per unit area is the surface tension. The fluorine (F) element has the highest electronegativity and possesses the smallest van der Waals atomic radius except for hydrogen, thus the repulsive force of lone pair electrons in fluorine molecules is quite large. Moreover, because fluorine does not have available d orbitals and cannot form d-pπ bonds, the F—F bond energy is very small, only 157 kJ/mol. However, fluorine's chemical properties are extremely active and it can chemically react with almost all elements at room temperature. The carbon-fluorine bond (C—F) has a large bond energy, reaching 485 kJ/mol, and exhibits excellent physical and chemical properties. Fluorination treatment can significantly reduce the surface energy of solids, so fluorocarbon compounds (fluorinated compounds containing a large number of —CF2 and —CF3 groups) as low surface energy finishing agents can effectively improve the super-liquid-repellent (super-hydrophobic/super-oleophobic) properties of substrates and have always been the preferred material for (super) dual-repellent finishing of material surfaces. However, with the rapid development of science and technology, people gradually recognize that perfluorocarbons (PFCs) bring adverse effects on human health and environmental safety: fluorides are bioaccumulative, disrupt metabolic balance, and persist in nature for a long time, being difficult to degrade, causing serious health and environmental problems. In 2008, the European Union began implementing a ban on the sale and use of perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA); in 2019, China banned the production, circulation, use, and import and export of PFOS and its salts and perfluorooctyl sulfonyl fluoride except for acceptable uses, therefore, finding and developing fluorine-free alternatives have become the focus of attention in the field of surface functional finishing.
At present, the alternatives to PFC are mainly divided into two categories: one is short-chain fluorides, which still have a certain degree of bioaccumulation and toxicity that is relatively lower compared to long-chain PFCs, and their usage and application scope are gradually being restricted by various countries; the other category consists of non-fluorine compounds such as aliphatic hydrocarbons, siloxanes, mercapto compounds, fatty acids, aromatic azides, and low surface energy polymers with 3D spatial structures (including bio-based polymers), etc. Because they do not contain fluorine elements, they are more compliant with environmental and health safety standards. According to Young's equation
[1], on smooth and uniform surfaces, when the solid-liquid interface is in an equilibrium state without external forces, the wetting behavior of the liquid is primarily influenced by the surface energy of the substrate. Studies have shown that the surface energy of the substrate must be at least one-fourth lower than the surface tension of the liquid to exhibit repellency to that liquid
[2]. Given the high surface tension of water (approximately 72.8 mN/m), it is relatively easier to achieve non-fluorinated (super)hydrophobic surface treatment technologies. In recent years, many non-fluorinated low surface energy compounds have replaced traditional fluorinated finishing agents, such as long-chain alkanes and organosiloxanes. These compounds can interact with organic materials and also react with inorganic materials and graft onto the substrate surface to form strong chemical bonds; simultaneously, they can form encapsulating chemical coatings through their own cross-linking condensation, successfully achieving (super)hydrophobic effects. By comparison, due to the surface tension of hydrocarbon oils typically ranging from 20 to 36 mN/m, achieving oleophobic properties requires even lower surface energy of the substrate. Due to the limitations of the surface energy of non-fluorinated materials themselves, meeting this requirement poses a certain challenge.
In recent years, certain research progress has been made in the preparation of fluorine-free oil-repellent surfaces. By introducing micro-nano multi-scale structures and synergizing with low surface energy compounds, solid surfaces with oleophobic properties can be obtained. Additionally, by constructing "liquid" and "liquid-like" surfaces, oil liquids can slide freely on the substrate without adhesion, demonstrating excellent oil resistance performance. However, there are issues such as the difficulty in constructing solid surfaces and their weak oleophobic properties; at the same time, the coatings of "liquid" and "liquid-like" surfaces face problems like easy depletion of liquid lubricants and susceptibility to contamination. Therefore, the development of fluorine-free oil-repellent technology remains a significant challenge.
This article first introduces the performance characteristics and research progress of fluorine-free anti-oil surfaces and analyzes their basic formation mechanism. Currently, the design core of fluorine-free anti-oil surfaces focuses on liquid coatings and "liquid-like" coating surfaces. Significant progress has also been made in the synergistic preparation method of combining low surface energy compounds with micro-nano multi-scale structures on solid surfaces. This article lists typical research achievements of fluorine-free anti-oil surfaces, detailing recent preparation strategies for fluorine-free anti-oil surfaces and their anti-oil effects. Finally, it discusses the application prospects, challenges faced, and future development trends of fluorine-free anti-oil surfaces in actual production and daily life.
2 Performance Characteristics of Fluorine-Free Oleophobic Surfaces
The lyophobicity of a solid substrate surface (i.e., the degree to which a liquid wets and spreads on the substrate surface) is typically evaluated by the contact angle (
θ). As shown in
Figure 1(1), the contact angle refers to the angle between the solid-liquid interface and the gas-liquid interface at the triple-phase boundary of solid, liquid, and gas. For any given surface,
θ < 90° indicates that the substrate has lyophilic properties; whereas
θ ≥ 90° suggests that the substrate is lyophobic; when
θ ≥ 150°, it signifies that the substrate is super-lyophobic and the liquid hardly wets the substrate surface. The tilt angle (
α) or contact angle hysteresis (Δ
θ) is used to characterize the slip effect of the liquid on the substrate surface. As illustrated in
Figure 1(2),
α represents the minimum tilt angle of the substrate at which gravity overcomes lateral adhesion force causing the droplet to slide or roll, and Δ
θ is the difference between the advancing contact angle
θadv (the maximum contact angle before the droplet advances) and the receding contact angle
θrec (the minimum contact angle before the droplet recedes). According to the Kawasaki and Furmidge equation
[3-4]: the force
F required for a droplet to slide off the substrate surface can be expressed as
F =
kwγL(cos
θrec - cos
θadv) =
ρVgsin
α, where
γL represents the liquid surface tension,
w is the width of the droplet's contact with the surface,
k is a dimensionless factor,
ρ represents the liquid density,
V denotes the droplet volume, and
g stands for gravitational acceleration, with
α or Δ
θ serving as indicators of the ease or difficulty of droplet movement. When the contact angle
θ for oily liquids on the substrate surface is ≥ 90° (showing oleophobicity) or
α/Δ
θ is small (easy to slide and non-adhesive), the surface can be considered an anti-oil-fouling surface.
3 Preparation Strategy of Fluorine-Free Oil-Resistant Surface
This section will focus on the preparation strategies for fluorine-free anti-oil contamination functional finishing on material surfaces, as shown in
Figure 2, separately elaborating on solid surfaces, liquid surfaces (including slippery liquid-infused porous surfaces and gel types), and "liquid-like" surfaces (including monolayers, polymer brushes, and composite coating forms).
3.1 Solid Fluorine-Free Oil-Resistant Surface
At the traditional solid-liquid interface, the wetting behavior of a liquid on a solid surface can usually be explained by the Young model, Wenzel model, Cassie model, and Wenzel-Cassie transition state model (
Fig. 3).
As shown in
Figure 3(1), the Young's model typically refers to an ideally flat surface without roughness. According to the Young's equation
[1]: cos
θ = (
γsv -
γsl) /
γlv,
θ depends on the solid-gas interfacial tension
γsv, solid-liquid interfacial tension
γsl, and liquid-gas interfacial tension
γlv. Generally, a solid surface with lower surface energy will cause the contact angle of the droplet to increase, indicating a state that is harder to wet. Therefore, liquids with higher surface tension on the Young’s model surface with low surface energy tend to form larger equilibrium contact angles
θY. Nishino et al.
[5] found that achieving super-liquid repellency on the Young’s model surface by using only low-surface-energy substances is impossible; it is also necessary to design the micro-rough structures of the material. The Wenzel theory reveals the effect of roughness on wettability
[6]. When a droplet contacts a rough surface, its actual contact area is larger than the apparent contact area. Therefore, high roughness amplifies the inherent wettability of the droplet (
Figure 3(2)). The Wenzel theory suggests that when the liquid completely fills the space between surface protrusions, its contact angle
θw can be derived from the equilibrium contact angle
θY of the Young’s equation, i.e., cos
θw =
r cos
θY, where the roughness coefficient
r is the ratio of the actual area of the solid surface to the horizontal projected area. Cassie et al.
[7] further described the wetting state when a droplet is suspended over surface rough structures. When the pores on the solid surface caused by surface roughness are difficult to fill with liquid and air exists, the contact angle of the droplet increases, showing lower adhesion, making the droplet easier to roll or slide (
Figure 3(3)). In practical applications, pure Wenzel states and pure Cassie states rarely occur; more commonly, there are transitional states between them (
Figure 3(4)), where the droplet can partially penetrate into the pores of the rough structure.
Barthlott et al
[8] and Jiang Lei's research group
[9] revealed the mechanism of the "lotus leaf effect", where the micro-nano level "papillary" structures on the lotus leaf surface and the secreted low surface energy wax make water droplets roll disorderly on its surface, thus achieving superhydrophobic properties. Cai et al
[10] mixed tetraethyl orthosilicate (TEOS) sol catalyzed by alkali with methyltriethoxysilane sol catalyzed by acid to successfully prepare a monolayer silica (SiO
2) antireflective film with good antireflective performance and stable optical properties. While being hydrophobic, the
θ of oil is only 78.4°. This indicates that in traditional micro-nano hierarchical surfaces, droplets with high surface tension (such as water) are more likely to remain in the Cassie state, while liquids with low surface tension (such as oil) tend to penetrate into the surface texture, forming the Wenzel state, thus increasing the difficulty of constructing oleophobic surfaces.
To promote the stable Cassie state of droplets and inhibit their transition to the Wenzel state, various design strategies have been proposed, including chemical modification using low surface energy coatings and constructing multi-scale micro-nano rough surfaces that lock air inside microspaces, thereby reducing the solid-liquid contact area. Compared with traditional micro-nano structures, inverted structures, textured structures, reentrant corner structures, T-shaped structures, and dual-reentrant structures mimicking the cuticle of springtails
[11], micro-nano multi-scale rough structures with specific morphologies can effectively delay the transition of oil droplets and organic solvents to the Wenzel state, making droplets more inclined to exhibit the Cassie suspended state, which helps improve the stability of the surface's hydrophobic and oleophobic performance
[12-13]. Liu et al.
[14] designed a set of SiO
2 surfaces composed of nanoscale vertically overhanging micropillars close to the ideal dual-reentrant structure. Compared with vertical and reentrant structures, the surface super-repels various liquids (oil, organic solvents, and even fluorinated solvents), as shown in
Figure 4.
θY is the intrinsic contact angle of the material, and the apparent contact angle
θC of the suspended droplet in the Cassie state is described by the Cassie-Baxter model
[7] as cos
θC=
fscos
θY-
fg, where
fs is the liquid-solid contact fraction, i.e., the proportion of liquid-solid contact area (including the wetted area inside the roughness) to the entire composite interface projection area, and
fg represents the liquid-air contact fraction. As shown in
Figure 4(1), when
θY>90°, the vertical structure allows water and other liquids to present the Cassie state. If
θY<90°, reentrant microscopic structures are needed to suspend oil and organic solvents, preventing them from entering gaps and wetting the surface (
Figure 4(2)). In an ideal environment (without positive liquid pressure), a reentrant topology with
θY≈0° will also make the liquid suspended. However, there always exists pressure in real environments (such as hydrostatic pressure, Laplace pressure, environmental disturbances). When
θY≈0°, because surface tension aligns parallel to the horizontal overhang, there is almost no vertical component to suspend the liquid. Once pushed into the gap, the liquid will spontaneously spread. As shown in
Figure 4(3), when the liquid contacts the top of the dual-reentrant microstructure, it continues to descend along the sidewalls of the vertical overhang and stops at the upper end of the bottom of the vertical overhang, where surface tension begins pointing upward. Therefore,
θY≈0° can also provide stronger anti-wetting properties and maintain the liquid in a suspended state.
The construction of micro-nano multi-scale rough structures can be achieved through either bottom-up or top-down approaches. The bottom-up approach refers to the gradual accumulation of atoms, molecules, or clusters, such as the sol-gel method, self-assembly technology, chemical vapor deposition (CVD), and electrospinning technology. The top-down approach involves preparing by removing part of the material surface through crushing or grinding, with commonly used methods including etching, nanoimprinting, and template methods. Wong et al.
[15] utilized CVD technology to expose the surface of SiO
2 nanoparticles to hydrocarbon or dimethylsiloxane variant vapors to achieve denser surface functionalization, which can result in super-repellency to liquids with surface tension ranging from 32 to 33 mN/m. Zhang et al.
[16] developed an organosilane/Al₂O₃ nanocomposite coating through the hydrolysis and condensation of TEOS and hexadecyltrimethoxysilane (HDTMS) with the addition of aluminum oxide (Al₂O₃) nanoparticles. The coated substrate surface exhibited excellent liquid repellency, with the contact angle
θ for ethylene glycol being greater than 158° and the sliding angle
α less than 5°.
For porous substrates such as fabrics and paper, oleophobicity mainly refers to the ability to prevent oil droplets from penetrating under Laplace pressure, which primarily depends on fiber geometry, pore size, and factors like solid surface energy and liquid surface tension. Cheng et al
[17] used dimethyldichlorosilane as a monomer to prepare polydimethylsiloxane (PDMS) brush-like coatings on fabrics via a catalyst-free, solvent-free CVD process, with oil repellency rated at level 5 according to the GB/T 19977-2014 standard (
Fig. 5(1)). Jiang et al
[18] treated fabrics with TEOS sol-gel solution to introduce silanol groups. Subsequently, 1,3-dichlorotetramethyldisiloxane (DCTMDSO) was deposited onto the fabric through the CVD process, utilizing the hydrolysis and condensation of bifunctional chlorosilanes to successfully create a fluorine-free PDMS oil-repellent coating on the fabric surface, with
θ values for castor oil and hexadecane on the treated fabric surface being 119.0° and 81.4°, respectively. Shabanian et al
[19] added a secondary texture with shorter length to the fabric fibers and treated the fabric with silanols and siloxanes, as shown in
Fig. 5(2), demonstrating lyophobic effects for rapeseed oil, olive oil, and castor oil on the treated fabric surface. The research group's findings indicate that compared to the influence of yarn diameter in fabrics, yarn spacing plays a critical role in forming a stable non-wetting interface, and they developed a PDMS-based coating that imparts oleophobicity to fabrics, showing significant repellence towards low surface tension liquids like ethanol
[20]. Yang et al
[21] grafted silicone nanoparticles (SiNPs) onto the surface of quartz fibers functionalized with 3-aminopropyltriethoxysilane (APTES) through electrostatic attraction to form a multilayer structure, using PDMS as a low surface energy material. Results showed that the
θ of a 28 mN/m ethanol-water mixture on this surface was approximately 120°. Du et al
[22] coated sulfonated nanofibrillated cellulose (SNFC) on the surface of paper to fill voids, then immersed it in a chitosan (CTS) aqueous solution, and finally coated the paper with hydrophobic polymer-modified polyvinyl alcohol. The
θ of castor oil on the paper after three-step coating treatment was 103.9°. Yi et al
[23] coated paper with cellulose nanofibril (CNF) suspension and then sprayed PDMS and cellulose nanofibril microparticles (CNFmp), successfully preparing paper-based materials with excellent oil resistance, where the
θ of castor oil could reach 91.8°. Among them, the CNF layer can fill surface pores, enhance barrier properties, and its hydrophilicity helps prevent oil contact, while the rough-structured CNFmp and PDMS coating can further improve the oil resistance of the paper. Wen et al
[24] crosslinked 2,2,6,6-tetramethylpiperidinyloxy radical (TEMPO)-oxidized cellulose nanofibers (TOCNF) with citric acid (CA) to generate carboxyl-rich CA-CNF, followed by amidation modification with octadecylamine (ODA) to impart hydrophobicity to CNF. The
θ of castor oil on the prepared modified cellulose nanofibers (ODA-CA-CNF) surface reached 130.6°, and when ODA-CA-CNF was sprayed on paper, the
θ of castor oil still reached 118.4°, exhibiting excellent oleophobic performance.
Table 1 shows relevant examples of fluorine-free anti-oil surfaces prepared based on the synergistic effect of low-surface-energy compounds and micro-nano multi-scale structures.
To date, the fluorine-free anti-oil contamination technology on solid surfaces has made some progress through constructing micro-nano multi-scale structures and optimizing fiber morphology. However, how to achieve better superoleophobic effects on the surface of solid substrates by utilizing multi-scale rough structures still requires further research and exploration. Overall, the development direction of fluorine-free anti-oil contamination surfaces on solids lies in designing diverse multi-scale rough structures to enhance oleophobic performance and simplify the preparation process, thereby effectively addressing challenges such as insufficient oleophobic performance.
3.2 Liquid Fluorine-Free Anti-Oil Surface
A liquid surface refers to a surface formed by replacing the air layer in a structure with a liquid substance, utilizing a liquid-liquid interface instead of a solid-liquid interface, mainly including types such as slippery liquid-infused porous surfaces (SLIPS) and hydrogels. This kind of surface can effectively reduce the adhesion force of liquids, allowing various polar and non-polar liquids to slide on it with a lower α/Δθ.
3.2.1 SLIPS
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 SiO
2 nanoparticles, and prepared flat films through thermal compression. During the preparation process, polyethylene and SiO
2 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 (TiO
2) 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.
3.2.2 Fluorine-Free Anti-Oil Stain Gel
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.
3.3 "Fluid-like" Fluorine-Free Anti-Oil Surface
A "liquid-like" surface refers to grafting a layer of liquid-like low surface energy oligomers polymers or composite coatings on a flat substrate. This kind of surface possesses characteristics similar to liquids retaining the high mobility of oligomers or polymers thereby improving the sliding behavior of external test liquids and effectively reducing the value of α /Δθ.
3.3.1 Monolayer Membrane "Liquid-like" Surface
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 Δ
θn-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).
3.3.2 Polymer Brush "Liquid-like" Surface
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 —NH
2, and vinyl —HC=CH
2, 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 (
Tg = -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 (H
2N-PDMS-NH
2) 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 TiO
2, aluminum oxide Al
2O
3, and nickel oxide NiO), but only PDMS grafted onto Si-based substrates can make nonpolar liquids exhibit a Δ
θ 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 Δ
θ. 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=CH
2. 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 H
2N-PDMS-NH
2 solutions. Various low-surface-tension liquids could slide on the surface with extremely low Δ
θ, 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.
3.3.3 Composite Coating "Liquid-like" Surface
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-NH
2, 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 NH
2-PDMS on a semi-crosslinked PU coating; NH
2-PDMS would penetrate from the top into the PU matrix, and its —NH
2 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, —CH
3 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:
FLa =
FLaFront +
FLaRear. 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,
FLaFront =
Dc(-
γcos
θa + 2
γH/
L), where
Dc is the droplet contact diameter,
γ is the liquid surface tension, and
θa 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,
FLaRear =
Dc(
γcosθ
r + 2
γsin
θrH/
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 SiO
2 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.
4 Application of Fluorine-Free Anti-Oil Stain Surface
Fluorine-free anti-oil stain surfaces can achieve multiple functions such as oil repellency, stain resistance, moisture resistance, corrosion resistance, anti-graffiti, fingerprint resistance, anti-adhesion, wind resistance, and anti-icing. They can be widely applied in many fields such as fabric cleaning, fingerprint treatment, food preservation, biomedical analysis, indoor cleaning, anti-aging of sensor devices, information transmission, liquid transportation, and oil-water separation.
① Fabric cleaning: The application of oil-repellent and stain-resistant technology in the post-finishing of textile materials can reduce the loss of manpower and material resources. In addition, fluorine-free environmentally friendly textiles are beneficial to human physical and mental health, and can provide a more comfortable and convenient experience for the wearer. The fluorine-free finishing agent prepared by Shabanian et al
[19] can effectively resist various liquid stains with environmental protection characteristics when applied to the surface of jacket fabrics after treatment. The 5-level oil-repellent fabric developed by Cheng et al
[17] can meet the needs of daily production and living. It is expected that fluorine-free oil-resistant textiles will be widely used in daily wear, hospital protective clothing, military combat clothing, space suits, diving suits, and surveying suits, among other fields.
② Fingerprint Treatment: The application of fluorine-free anti-oil stain coatings on windows, goggles, windshields, and building exteriors can effectively reduce noise, prevent fingerprints, pollution, dust, fog, and freezing, possessing excellent self-cleaning functions and ease of cleaning. The hydrogel developed by Xu et al
[39]still exhibited oleophobicity to rapeseed oil after being fully immersed in liquid nitrogen for 1 minute, and when applied to pipelines, it can reduce resistance; when applied to glasses and goggles, it can be used for anti-fouling and anti-fogging. As shown in
Fig. 20(1~4), compared with untreated glass, the imprints such as fingerprints are shallower on the Zr:BSA hybrid surface developed by Masheder et al
[68]and are easier to clean.
③Food preservation: Applying fluorine-free anti-oil coatings to food packaging bags and cling films can isolate external liquids, thereby significantly slowing down the spoilage of food. The coating possesses environmentally friendly characteristics and will not cause harm to the human body. As shown in
Fig. 20(5), the fluorine-free anti-oil coating developed by Zhao et al
[57] forms a barrier effect on paper-based surfaces, which can prevent various liquids from penetrating. Its application in the field of food packaging and transportation can extend the storage time of food.
④Biomedical analysis: Liu et al
[44] studied the "liquid-like" surface of PDMS and found that due to its low Δ
θ, the organic precursor droplets deposited on the surface would shrink during evaporation, causing the droplet profile to become smooth and smaller, achieving a high-density dot distribution of organic liquids. At the same time, the solute concentration in the precursor also increased accordingly, leading to a condensation-enrichment process, forming surface patterns with high density and high resolution fixed fluorescent molecules on precursors with lower initial concentrations (
Fig. 20(6)), which can be used to expand surface functionalization analysis and biomedical applications.
⑤ Indoor cleaning: Using fluorine-free anti-oil stain coatings in areas inside the room that are prone to getting dirty can significantly reduce the manpower required for cleaning, providing a comfortable home experience. As shown in
Figure 21, the LESS fluorine-free anti-stain coating developed by Wang et al
[48], when applied to toilet bowls, demonstrates superior cleaning effectiveness on feces compared to the advanced commercial coatings currently available on the market.
⑥Anti-aging of sensor devices: Components treated with fluorine-free anti-oil contamination technology can significantly reduce stain adhesion, lower the aging rate of devices, and possess characteristics such as moisture resistance, anti-humidity, and corrosion resistance. This technology can be widely applied in outdoor devices as well as chips, optical components, engines/pumps, biochips, and microreactor systems, among other fields. As shown in
Figure 22, the fluorine-free anti-oil contamination coating developed by Rabnawaz et al.
[62] can be used on various substrates. When coated on the surface of a mobile phone, it not only maintains high optical transparency but also exhibits excellent anti-stain performance, effectively delaying the aging process.
⑦Information Transfer: Applying anti-oil stain coatings to books can effectively prevent graffiti and stop pollution from various water-based and oil-based inks. Moreover, anti-oil stain coatings used in conjunction with ink can achieve the function of repeated erasure and reprinting, similar to "invisible ink." As shown in
Figure 23, He et al
[30] applied coatings loaded with thermochromic ink for writing, through which the letters could repeatedly appear and disappear by adjusting the temperature; the patterns are also easy to erase. The coating is not only suitable for daily writing but also can be applied to confidential reports and other information transmission.
⑧ Liquid transport: Applying anti-oil pollution technology to pipelines can effectively control fluid flow and achieve classified treatment functions. This technology is not only suitable for water transport in daily life but also can be widely used in the petroleum industry, thereby significantly saving human and material resources. As shown in
Fig. 24 (1-3), the fluorine-free anti-oil pollution coating prepared by Wu et al
[67] has excellent physical and chemical stability, which enables various substrates to repel crude oil and resist crude oil adhesion, showing good operability in the industry. Therefore, it is suitable for equipment or liquid transport pipelines that are often exposed to a crude oil environment.
⑨ Oil-water separation: As shown in
Fig. 24(4~6), the hydrophilic and oleophobic binary polymer saline hydrogel coating developed by Xu et al
[39] exhibits the ability of self-cleaning of oil and oil-water separation in an aqueous environment, which can maintain efficient separation performance after multiple cycles, showing practical application value.
Table 10 summarizes the applications of multiple fluorine-free oil-resistant surfaces, and the development of new fluorine-free oil-resistant materials will inevitably enhance the quality of life and promote the development of various industries.
5 Conclusion and Prospect
This paper analyzes the mechanism of fluorine-free oil-resistant material surfaces based on wetting theory, further explores the construction methods of fluorine-free oil-resistant surfaces, comprehensively summarizes the research progress in this field, and provides an integrated summary and prospect of its practical application directions.
(1) At present, the fluorine-free anti-oil stain surface is still the focus of research in the field of wettability. Compared with fluorine-containing finishing agents, fluorine-free finishing agents have the advantages of not being easily deposited in the body, being easily degraded, and being harmless to the human body, which makes them a safer and more environmentally friendly choice.
(2) The development of novel fluorine-free finishing technology has become a research hotspot and difficulty in the current oil-repellent finishing. It can be achieved by synergistically using low surface energy substances and micro-nano multi-scale structures, or by utilizing liquid and "liquid-like" surfaces.
(3) Designing multi-scale micro-nano rough structures to enhance the oleophobicity of fluorine-free anti-oil solid surfaces is an important development direction for future anti-oil solid surface technology.
(4) Most of the currently developed fluorine-free anti-oil stain surfaces are liquid and "liquid-like" surfaces, and their significant characteristic is that oil-based liquids can easily slide on the surfaces. From a practical application perspective, although the substrate exhibits good anti-stain performance for low surface energy liquids, the lubricating layer will gradually be consumed and reduced as the usage time increases. How to effectively slow down the loss of the lubricating layer and extend the service life of liquid and "liquid-like" surfaces is the focus of attention in this field. Currently, it is mostly improved by adding other components to form composite coatings to enhance their fastness.
(5) Because the liquid film coating layer has high requirements for the substrate, most "liquid-like" coatings are suitable for smooth and flat substrates. However, through special structural design, rough surfaces can also achieve "liquid-like" properties.
(6) In the textile field, how to further improve the oil resistance of textiles through fluorine-free functional finishing and maintain or repair their oil resistance in extreme environments remains an unsolved problem, which will also become the key focus of future research on fluorine-free oil resistance finishing.
In summary, in recent years, with the continuous growth of environmental protection demands, researchers have been committed to developing fluorine-free oleophobic surfaces, and related preparation technologies have gradually matured. However, when applying fluorine-free oleophobic surfaces to chemical industry production and practical life fields, there are still many shortcomings and challenges, which urgently require further in-depth research and exploration.
National Natural Science Foundation of China(22372087)
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