Surface Repellency beyond Hydrophobicity: A Review on the Latest Innovations in Superomniphobic Surfaces

Abstract

Superhydrophobic surfaces have long faced challenges in repelling low-surface-tension liquids like oil and alcohol, limiting their practical applications. Over the past few years, researchers have been actively looking for new alternatives to overcome this issue. Recently, superomniphobic surfaces have attracted significant interest due to their ability to repel both high- and low-surface-tension liquids. Compared with superhydrophobic surfaces, superomniphobic surfaces provide enhanced liquid repellency, making them more suitable for industrial and real-world applications. This Review explores the recent advancements in the fabrication of superomniphobic surfaces. Three basic wetting principles, Young’s, Wenzel’s, and Cassie–Baxter’s equations, are discussed. The vital role of low surface energy and high surface roughness of hierarchical and re-entrant structures in achieving a steady Cassie–Baxter state that has a low contact area between the solid surface and liquid droplet is emphasized. Additionally, a comprehensive description of various fabrication techniques, characterizations, and practical applications of superomniphobic surfaces is provided. Finally, the challenges and future prospects regarding this research area are addressed. This comprehensive review aims to inspire researchers to refine and enhance current development methods of superomniphobic surfaces and stimulate further exploration in the research field.

1. Introduction

Liquid-repelling properties of surfaces, including hydrophobicity and omniphobicity, have received great attention from researchers for various applications. These include self-cleaning,^1,2 nonfouling,³ drag reduction,^4,5 metal corrosion,⁶⁻⁸ anti-icing of glass,⁹⁻¹¹ membrane distillation,¹² membrane gas absorption,^13,14 and even biological applications.^15,16 Surface antiwetting refers to a surface’s ability to reduce the adhesive force and contact area when liquid droplets are in contact with a solid surface.¹⁷ This characteristic is quantitatively characterized by contact angle goniometry that measures the angle formed at the interface where liquid, air, and the solid surface come in contact, which was mentioned in Young’s contact angle theory.¹⁸

In nature, some organisms possess liquid-repellent surfaces to survive in the aqueous environment. These natural surfaces serve as an inspirational source for researchers to mimic their surface features in synthetic materials. A very famous example is the lotus leaf (Figure 1(a)), which exhibits excellent water repellency and self-cleaning properties, also known as the lotus effect. The surface of a lotus leaf is covered with microscale hierarchical bumps (Figure 1(b)) and nanoscale hydrophobic waxes (Figure 1(c)), which prevent water droplets (Figure 1(d)) and contaminants like dust and dirt particles from adhering to the leaf’s surface. When water droplets roll off the hydrophobic leaf surface, they bring away the contaminants, leaving the leaf clean again.^19,20 Similarly, insects such as dragonflies and butterflies possess nanopillars and a thin layer of hydrophobic wax on their wings that allow water droplets to roll off, preventing the weight of droplets on the wings and facilitating flight in wet environments.²¹

Figure 1.

Image and morphological images of a lotus leaf surface: (a) a lotus leaf in nature, (b) the microstructure of a lotus leaf, (c) the nanostructure of a lotus leaf, (d) the microstructure of an annealed lotus leaf, (e) the nanostructure of an annealed lotus leaf, (f) a droplet placed on an untreated lotus leaf, and (g) a droplet placed on an annealed lotus leaf and then tilted to an angle of 90°. Scale bars: (b, d) 10 μm and (c, e) 3 μm. Reprinted with permission from ref (22). Copyright 2016 Biotribology.

A hydrophobic surface is classified as a surface with a contact angle with water of more than 90°, whereas a superhydrophobic surface is known as a surface with a contact angle with water of more than 150°.²³ Similarly, a superoleophobic surface repels oil with a contact angle higher than 150°. The term “oleophobic” specifically pertains to the repellency of surfaces to oils, while “omniphobic” and “amphiphobic” are employed to denote repellency to a broader range of liquids, including those with lower surface tension than oils. Due to the similarities, the meanings of the terms overlap, causing confusion between “omniphobic” and “amphiphobic”.^24,25 To clearly identify them, the prefix “omni-” conveys a more comprehensive meaning from the Greek word, encompassing all liquid types, in contrast to “amphi-”, which comes from the Greek word for “both”, suggesting a more limited scope pertaining primarily to oils and water.^24,26 Besides contact angle, surface antiwettability can also be evaluated through parameters such as contact angle hysteresis, which represents the difference between advancing and receding contact angles,²⁷ and sliding angle,²⁸ which measures the tilt required for liquid droplets to roll off a surface. To achieve superomniphobicity, a surface should also achieve a sliding angle or contact hysteresis less than 5°.²⁹⁻³¹

Numerous studies and review papers regarding the fabrication and applications of hydrophobic and superhydrophobic materials have been published in the past decades to address surface wetting.^28,32−36 Nevertheless, the surface wetting challenges persist due to the high sensitivity to organic contamination and low-surface-tension liquids.³⁷⁻³⁹ Although superhydrophobic surfaces exhibit water contact angles of more than 150°, they often perform poorly with low-surface-tension solutions or impurities. Another reason behind surface wetting problems of hydrophobic surfaces could be due to their large pore size and low resistance to chemicals.⁴⁰⁻⁴² To address this limitation, researchers have also designed oil-repelling surfaces known as superoleophobic surfaces.⁴³⁻⁴⁸ However, starting from 2009, Liu et al.⁴⁹ developed a superoleophobic and hydrophilic surface inspired by fish scales. Chu et al.,⁵⁰ Brown et al.,⁵¹ and Yang et al.⁵² fabricated surfaces with a similar concept, and the resulting surfaces showed great potential for operation under the water-wetted state, such as liquid/oil separation.^51,52 Furthermore, superamphiphobic surfaces⁵³⁻⁵⁷ that can repel both water and oil were also designed, but their liquid repelling performances are slightly limited for liquids that have lower surface tension compared to oils.

Therefore, the focus of liquid-repelling surface development has gradually shifted toward the synthesis of omniphobic and superomniphobic surfaces that reject all types of liquid. Currently, the fabrication of superomniphobic surfaces commonly involves depositing micro- and nanoscale hierarchical structures on a surface. Given the similarities in the methods and concepts behind superomniphobic and superhydrophobic surfaces, a superomniphobic surface is also able to repel higher surface tension liquids such as water. Existing research highlights that the combination of rough hierarchical structures and ultralow surface energy is key to superomniphobicity.⁵⁸

This Review addresses recent advancements in the fabrication of superomniphobic surfaces and real-life applications. Section 2 covers the basic concepts such as liquid droplet behavior, including Young’s equation, Wenzel’s equation, and Cassie–Baxter’s equation, and factors contributing to superomniphobicity, such as hierarchical structured surfaces and surface energy. In section 3, two main categories of fabrication methods for superomniphobic surfaces, including direct processing method (section 3.1) and existing surface modification method (section 3.2), are reviewed. Section 4 presents the up-to-date applications of superomniphobic surfaces, including direct contact membrane distillation (section 4.1), chemical shielding (section 4.2), corrosion-protective coating (section 4.3), directional droplet movement (section 4.4), and CO₂ absorption with the gas–liquid contacting process (section 4.5). Finally, research gaps, future research directions, and conclusions are addressed to summarize the paper.

2. Basic Concepts for Superomniphobic Surface Design

This section presents the basic concepts to be considered while designing superomniphobic surfaces, including liquid droplet behavior on a surface as described by Young’s equation, Wenzel’s equation, and the Cassie–Baxter equation. Additionally, this section highlights the importance of low surface energy and high surface roughness, particularly the hierarchical structure and re-entrant structure, in enhancing liquid repellency.

2.1. Young’s Equation

When a liquid droplet contacts a solid surface, it may either leave the surface or permeate through the solid surface and cause wetting. The contact angle, which measures the angle formed between the tangent of the liquid surface and solid surface at the three-phase contact line, is a quantitative measure for antiwetting properties.

The earliest mathematical model to determine contact angle is Young’s equation. Young’s equation can be applied ideally on a flat surface. According to Young’s relation, liquid permeates the surface texture until the local contact angle for the three-phase boundary equals the equilibrium contact angle (illustrated in Figure 2(a)). The liquid–surface interface in this instance consists of two phases: the liquid–solid interface and the liquid–vapor interface. As a result, the contributions from these two interfaces are combined to determine the apparent contact angle.⁵⁹ Young’s law also explains the negative relationship between contact angle and solid surface energy, as expressed in eq 1:¹⁸

1

γ_SV, γ_SL, and γ_LV are the interfacial tension of the solid–vapor, solid–liquid, and liquid–vapor interfaces, respectively. However, in realistic cases, solid surfaces are rough and not chemically homogeneous, and it is too complex to estimate contact angle through Young’s equation. Therefore, Young’s equation is only applicable for smooth and nontextured surfaces.

Figure 2.

Liquid droplet behavior on the surface for (a) Young’s model, (b) Wenzel’s model, and (c) Cassie’s model.

2.2. Wenzel’s Equation

When a drop of liquid comes in contact with a rough texture on a surface, it will either fully permeate through the ridges on the surface and enter a fully wetted Wenzel state or remain partially suspended on the surface with air pockets trapped beneath, entering a partially wetted Cassie–Baxter state.⁶⁰⁻⁶²

The Wenzel model is used to describe a surface where droplets pierce the rough structures and produce significant adhesive forces between liquid droplets and the solid surface⁵⁹ (as shown in Figure 2(b)). Wenzel’s equation describes wetting on textured surfaces that take surface roughness into account, in which any change in surface roughness directly amplifies the wettability of a surface.⁶³ The contact angle that is expressed by the Wenzel model is defined as shown in eq 2:⁶⁴

2

Referring to eq 2, θ_w represents the contact angle on a rough surface, while r represents the surface roughness factor that is calculated by the ratio of the actual surface area to the projected horizontal area. The surface roughness factor, r, equals 1 if the surface is smooth. But realistically, r will be higher than 1. Specifically, surface roughness increases the contact angle when the flat surface’s contact angle is larger than 90° and decreases the contact angle when the contact angle is less than 90°.⁶⁵ However, under surfaces that have very high roughness, air bubbles tend to get trapped underneath liquid droplets at the rough structures and form a composite interface of solid, liquid, and air. The phenomenon is better explained with Cassie–Baxter’s equation.

2.3. Cassie–Baxter’s Equation

In situations where air bubbles are trapped below a liquid droplet, the relationship between the solid and gas fraction, or the area percentage of the liquid–solid and liquid–vapor interfaces, and the apparent contact angles of a suspended liquid have all been described using the Cassie–Baxter model.⁶⁶ According to the Cassie–Baxter study, when liquids penetrate a heterogeneous surface, the gaps between protrusions on the surface are not completely wet (as illustrated in Figure 2(c)). Instead, air bubbles remain to be contained in their own pockets beneath the liquid droplet.⁵⁸ For the Cassie–Baxter model, the contact angle can be determined by eq 3.^67,68

3

From the equation, the apparent contact angle, θ_c, is the sum of contact angles in different phases. θ₁ and θ₂ are the contact angles of a liquid on a smooth solid surface and an ideal air surface, respectively. f_SL represents the percentage of solid contact surface to total contact area, while f_LV represents the percentage of gas–liquid contact surface to total contact area. When f equals 0, the liquid droplet does not contact with the surface, while the liquid droplet fully contacts when f equals 1. Compared to the Wenzel state, the Cassie–Baxter state provides a higher contact angle for both high- and low-surface-tension liquids as liquid droplets do not penetrate through the surface like the Wenzel state.

2.4. Surface Energy

Referring to Young’s equation, the contact angle θ is inversely proportional to the surface tension of solid surface γ_SV, which is expressed in the unit of Nm^–1 that stands for surface energy per unit area.¹⁸ Surface energy and surface roughness have a significant impact in determining a solid surface’s wettability.⁶⁹ Surface energy analyses analyze the breakdown of intermolecular bonds caused by the development of a surface. Surfaces with very high surface energy tend to have contact angles smaller than those of surfaces with very low surface energy. This implies that great wetting resistance is influenced by a low surface energy. Therefore, surfaces with low surface energy should be prioritized when designing super-repellant surfaces to attain high performance.

The surface energies of various functional groups on a surface are determined to be in the following order: CF₃, CF₂H, CF₂, CH₃, and CH₂.^58,70 Fluorine is one of the most electronegative elements due to its small atom size and weak attractive interactions. Weak van der Waals forces between fluorinated chains in fluorocarbons resulted in low cohesive energy and surface tension.⁷¹ In past decades, the majority of antiwetting surfaces were obtained using fluorinated materials.^{43,47,56,72−77} Other than that, Darmanin and Guittard⁷⁸ pointed out that fluorinated tails are involved in the formation of surface structuration besides reducing surface energy. They also highlighted that the length of the fluorinated tails determines the abundance of the surface structuration. Based on that, Golovin et al.,⁷⁹ Wang et al.,⁸⁰ Zhang and Seegar,⁸¹ and Jin et al.⁸² showed that fluorinated and perfluorinated substances, such as fluorinated monomers, copolymers, and other fluorinated precursors, are applicable for developing antiwetting surfaces that can repel liquids with very low surface tension, such as toluene, hexadecane, and cyclohexane.

2.5. Surface Roughness

A hierarchical structure possesses multiple roughness scales like microscale and nanoscale particles on the surface, granting the surface impermeability to liquids.⁸³ Past studies⁸⁴⁻⁸⁸ have highlighted those hierarchical structures play an important role in increasing a surface’s contact angle and decreasing the sliding angle and contact angle hysteresis. A texture with a finer length scale is frequently stacked on top of a texture with a coarser length scale that is layered beneath it on hierarchically created surfaces;⁸⁹ commonly, nanoscale particles are stacked on top of microscale structures. The nanostructure helps sustain a high liquid pressure, whereas the microstructure reduces the contact area.⁹⁰ The synergy between the micro- and nanoroughness stabilizes the antiwetting state by reducing the adhesion between the droplets and the surface and enlarging the energy barrier between the Cassie–Baxter and Wenzel states.⁹¹ The apparent contact angle of a liquid droplet on a hierarchically structured surface is typically greater than that on a smooth surface because the droplet is kept in the Cassie–Baxter state at all length scales^58,92 due to the air pocket that is trapped underneath a hierarchically constructed surface.

In addition to adding different scales of hierarchical structures on surfaces, surfaces with exceptional resistance to wetting can be designed using surface patterns such as re-entrant surface curvature.⁹³ According to past research works,^45,76,94,95 re-entrant curved-shaped microstructure coating formed during the construction of surface achieves excellent oleophobicity. The features of springtails’ cuticles inspired the researchers, especially in developing superomniphobic surfaces. Small soil-dwelling arthropods called springtails have cuticles with surfaces that are inherently omniphobic and exhibit both self-cleaning and pressure resistance to impacts from liquid droplets.⁹⁶ Because they breathe via their skin, the shape of their cuticles has evolved to prevent wetting by water and other organic liquids, a trait that is essential to their survival. The cuticle’s hierarchical structure displayed in Figure 3 consists of primary granules in the form of mushroom-shaped (re-entrant shape) nanostructures and secondary granules in the form of microscale grooves.⁹⁶⁻⁹⁸ According to the study conducted by Hensel and his team,⁹⁹ the mushroom-shaped re-entrant structure can hold low-surface-tension liquid droplets by providing a stable pinning point. Replicas of Tetrodontophora bielanensis (T. bielanensis) were fabricated by casting 0.5 wt % Irgacure 651 (photoinitiator) in polyethylene glycol diacrylate (PEGda) precursor solution onto a prepatterned MD40 mold, followed by settling in vacuum conditions of 3 × 10^–3 hPa for 12 h. Subsequently, PEGda was exposed to UV irradiation for 5 min under nitrogen atmosphere for cross-linking. After cross-linking and demolding, the PEGda replicas possessed the entire superficial comb-like granular structure of T. bielanensis. At the nanoscopic scale, the primary granules were interconnected by ridges that extended across the whole surface, creating nanoscopic cavities with an average diameter of approximately 300 nm. After being coated with hydrophobic amorphous Teflon (Teflon AF) with a surface energy of about 13.6 mN m^–1, the polymer replicas showed high contact angles reaching up to 150° with both high surface tension water (γ = 72.3 mN m^–1) and low-surface-tension hexadecane (γ = 27.5 mN m^–1).

Figure 3.

Surface morphological image of primary and secondary granules present on the springtail surface with bristles and a unique nanoscopic comb pattern. Reprinted with permission from ref (98). Copyright 2011 PloS one.

In short, a re-entrant structure can repel a droplet because it has a larger top and a smaller bottom, as demonstrated in Figure 4. Simple “re-entrant” designs, like inverse trapezoidal microstructures, initially displayed enhanced omniphobicity but were unable to maintain a persistent hydrophobic condition.³⁸ Therefore, re-entrant texture offers a bonus that enables low-surface-tension liquid repellency, but the re-entrant texture itself might be insufficient for the formation of a stable Cassie–Baxter state while designing superomniphobic surfaces.^100,101 Additional considerations are necessary when designing superomniphobic surfaces.

Figure 4.

Geometries of the re-entrant structure.

3. Fabrication Techniques to Achieve Superomniphobicity

As mentioned in the previous section, a superomniphobic surface must possess a high apparent contact angle of 150° or greater and low contact angle hysteresis or a sliding angle of 5° or lower with all types of liquid, specifically for liquids that have a surface tension lower than oil. The contact angle hysteresis or sliding angle that determines the difficulty for liquid droplets to roll off from a surface was sometimes not considered in past research. Jin et al.¹⁰² emphasized that both high contact angle (CA) and low CA hysteresis are critical indictors to determine effective liquid repellency of a solid surface. However, it is possible for a surface to have a high CA and high CA hysteresis at the same time, which is known as the “petal effect”. Erbil¹⁰³ pointed out that measuring only CA is not adequate and measuring both advancing and receding contact angles is necessary, as the magnitude often indicates the surface configurations such as roughness, shape, and size. Therefore, comprehensive evaluation of both CA and CA hysteresis is essential.

In this section, the fabrication techniques to achieve superomniphobicity are described and discussed. As is well-known, the combination of appropriate surface chemistry and surface roughness is needed to improve surface antiwetting properties. Hence, a superomniphobic surface can be created by reducing the surface energy and roughening the surface. There are two main approaches to fabricating superomniphobic surfaces. The first approach is directly fabricating a rough surface using a low-surface-energy base material without further chemical treatment, whereas the second approach is further modifying the fabricated existing surface by generating multiscale surface roughness and surface functionalization using materials that have low-surface-energy functional groups (mainly fluorinated chemicals). It should be pointed out that some of the superomniphobic surfaces included in this section are not truly superomniphobic due to the incomplete liquid repellency test.

3.1. Direct Surface Fabrication for Superomniphobicity

A surface with excellent liquid repellency can be obtained by entrapping additives such as nanoparticles directly onto the surface. This can be accomplished by blending these additives into a polymeric dope solution for the phase inversion process and the electrospinning process. Phase inversion processes such as combining non-solvent-induced phase separation (NIPS) and vapor-induced phase separation (VIPS)^23,104 and the electrospinning process^{39,76,105−107} are popular choices for the fabrication of a polymeric membrane as a base material for superomniphobic surfaces. After the initial membrane fabrication, further modifications are applied to enhance the liquid repellency. While direct processing methods have been found to be successful for creating superhydrophobic polymeric membranes,^108−110 achieving superomniphobicity is more challenging because low-surface-tension liquids wet the surfaces more easily compared to water.¹¹¹

Phase inversion is one of the most popular techniques for creating polymeric porous membranes with complex structures because of its simplicity and flexibility.¹¹² Phase inversion occurs via the physical phase transition from a homogeneous polymer solution’s initial liquid state to a solid state. Therefore, the choice of polymer and additives used in the dope solution is crucial, as it affects the membrane’s structure, characteristics, and chemical interactions¹¹³ during phase inversion membrane formation. Pang et al.¹¹⁴ fabricated a superomniphobic polymeric membrane by adding liquid-repelling additives like silica nanoparticles (SiO₂) and hexadecyltrimethoxysilane (HDTMS) during the preparation of homogeneous PVDF dope solution before conducting the dry-jet wet-spinning phase inversion process. First, defluorination and oxygenation reactions occurred in PVDF in a mixture of N-methylpyrrolidone (NMP), ammonia, water, and SiO₂ at 60 °C to introduce oxygen-containing functional groups such as hydroxyl groups. The hydroxyl groups on PVDF chains and SiO₂ will form −O– bonds, resulting in the formation of a stable PVDF-SiO₂ dope solution. HDTMS was added in the following step and grafted onto PVDF-SiO₂ through reactions with the hydroxyl groups of PVDF-SiO₂. In the morphological image of the surface shown in Figure 5, the formation of lumps was discovered on the spherulite structure of the PVDF-SiO₂-HDTMS membrane after coating with SiO₂ compared to the honeycomb spherulite structure of PVDF-HFTMS. The fabricated membrane had contact angles of 160 ± 0.2° and 158 ± 0.2° with water and DEA, respectively.

Figure 5.

SEM morphological image of hollow fiber membrane surfaces of (a) PVDF-SiO₂–HDTMS and (b) PVDF-HDTMS: (1) outer surface, (2) cross-section, and (3) inner surface. Reprinted with permission from ref (114). Copyright 2023 Scientific Reports.

On the other hand, electrospinning offers another simple and flexible method to produce continuous hollow fibers with diameters ranging from micrometer to nanometer scale. Using the principles of electrohydrodynamics, the electrospinning process uses a high-voltage electric field to electrify liquid droplets of the polymer dope solution to produce a jet in a spinneret, and the jet then experiences elongation and stretching, which causes fibers to develop and solidify.¹¹⁵ Electrospinning is one of the more adaptable processes for substrate creation due to its simplicity of fabrication, availability of enhanced surface area in relation to volume, ability to alter fiber composition, and ability to control morphology.¹¹⁶ Pan et al.¹¹⁷ used the electrospinning method to produce a uniform layer of polydimethylsiloxane (PDMS) + 50 wt % F-POSS (Figure 6(a and b)) on the surface of stainless steel wire meshes. The coated surface obtained a low solid surface energy due to the preferential segregation of F-POSS molecules. In a solid blend consisting of multiple components, the component with the lowest energy migrated to the surface to minimize the free surface of the system. The electrospun coating provided a hierarchical structure that consisted of two different length scale particles, and the re-entrant structure was also discovered at both length scales, shown in Figure 6(c). Referring to Figure 6(d), the surface successfully achieved a high apparent contact angle and a very low roll-off angle of 2° with different low-surface-tension Newtonian liquid droplets, even those with extremely low surface tension (γ < 25 mN m^–1). As depicted in Figure 6(e) and Figure 6(f), the surface also possessed outstanding wetting and chemical resistance that enabled liquid jets, including organic acid and base, to bounce and roll off the surface without causing damage.

Figure 6.

(a) SEM image of the hierarchically structured surface illustrating the electrospun coating of cross-linked PDMS + 50 wt % fluorodecyl POSS on a stainless steel wire mesh 70. (b) Elemental mapping of fluorine on the hierarchically structured surface. (c) SEM image illustrating the re-entrant curvature of the electrospun texture. (d) Roll-off angles for various Newtonian liquids. The inset shows an ethanol droplet rolling on the surface at a roll-off angle of ω = 2°. (e) Droplets of various low-surface-tension Newtonian liquids show very high contact angles. (f) Jets of different Newtonian liquids shown in (e) bouncing on the surface. Reprinted with permission from ref (117). Copyright 2012 Journal of American Chemical Society.

3.2. Existing Surface Modification for Superomniphobicity

Surface modification of the surface can be done physically, chemically, or using a combination of both methods. Physical modification does not involve forming chemical bonds to change the chemical composition of the surface. Instead, the low-surface-energy additives are applied on the surface through a physical interaction. Physical modification is straightforward and effective, as no chemical reactions are to be considered. On its downside, the adhesion of coating layers coated on surfaces is weaker and the modification effects may not last as long as those achieved through chemical modifications.¹¹⁸ The most common and widely used example of a physical modification is surface coating. Various commonly used surface coating techniques, such as dip-coating, spin-coating, spray-coating, and electrospraying, are applied to attain a uniform and stable distribution of nanoparticles on membrane surfaces.

Meanwhile, chemical modification creates a strong intermolecular force of chemical bonding between the surface material and surface coating additives, providing durable modification effects, and the coating layers become resistant to peeling off. Plasma treatment, the water/solvothermal method, sol–gel, chemical bath deposition (CBD), and chemical vapor deposition (CVD) are examples of chemical modification. However, some of these methods, such as plasma treatment and the water/solvothermal method, require expensive and complicated equipment setups,¹¹⁹ making them less economically viable for large-scale production in industry.

In Table 1, surface modification techniques are classified into the surface roughening method and fluorination and discussed separately. To achieve superomniphobicity, most researchers will combine surface roughening and surface fluorination approaches.

Table 1. Surface Modification to Achieve Superomniphobicity.

surface modification category	modification technique	description	ref
surface roughening	surface functionalization with silica nanoparticles	1. Commonly done by spray-coating, dip-coating and spin-coating	(23, 120−124)
		2. Silica nanoparticles are well-known to be easily available and inexpensive
		3. Greater binding strength with positively charged substrate due to electrostatic interaction120
		4. Heat treatment after coating process improves stability by forming cross-linking121
	micromolding	1. Done by pouring or spraying low-surface-energy polymer solutions such as PDMS into a mold with micropillars	(125−129)
		2. Micropillars with uniform radius and spacing ratio can be fabricated on a silicon mold with photolithography125
	etching	1. Before reactive ion etching, the rapid thermal annealing process is used to dewet a metal film on a glass substrate into metal nanoparticles	(130)
		2. After reactive ion etching and removal of metal nanoparticles, monolithically integrated nanopillars are found on the substrate
		3. The geometry of the nanopillars can be controlled based on different process condition
	hydro/solvo-thermal method	1. Involves heterogeneous chemical reaction in a high temperature and pressure autoclave	(106, 132)
		2. Creating rough structures on the surface via combined effects of thermal energy and solvent
		3. Nanomaterials are not needed
		4. The main challenge is expensive equipment setup, especially for autoclaves131
	unidirectional rubbing	1. Done by unidirectionally rubbing dry powder of silica nanoparticles on a PDMS-coated substrate with another PDMS substrate	(133)
		2. Heat treatment after rubbing to form cross-linkage
		3. Has potential in large-scale production133
prefluorination	alkaline treatment	1. Immersion in NaOH solution	(23, 105, 134, 135)
		2. Hydroxide ions destroy hydrogen bond and contribute to the increment of active hydroxyl groups
	plasma treatment	1. Oxygen plasma treatment is usually used	(122, 126, 129)
		2. Reactive oxygen plasma species reacts with the treated surface, producing oxygen-containing groups such as hydroxyl groups136
surface fluorination	chemical vapor deposition (CVD)	1. The substrate and fluorinated chemicals are placed inside a vacuum oven	(120, 121, 126, 137, 138)
		2. Chemical vapor is deposited on the substrate, introducing fluorine functional groups (−CF3)
	grafting	1. Done by immersion in fluorinated chemicals or spraying fluorinated chemicals on a substrate	(134, 139−142)
		2. Hydrolysis–condensation reaction introduces low surface energy fluorine functional groups on the surface

Zhang et al.¹³⁵ successfully fabricated a superomniphobic PVDF membrane with an oil contact angle of 160.5° by combining grafting and dip-coating methods. They started by electrospinning 8 wt % polyvinylidene fluoride (PVDF) to develop the membrane surface base. Then, the membrane was treated with a NaOH solution for hydroxylation. The membrane with hydroxyl groups was dipped in a silica precursor solution that contained 0.3 wt % tetraethyl orthosilicate (TEOS), 1.0 wt % water, and 0.3 wt % NH₄OH and ethanol to improve surface roughness by attaching SiO₂. The membrane surface was then dip-coated in 1% v/v 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS) solution for 24 h to undergo fluorination. According to Figure 7(A1), the pristine PVDF sample (PVDF-P) displayed nanofibers with cylindrical geometry, demonstrating the formation of primary microscale rough structures on the membrane surface. In contrast, the surface morphology of the FAS fluorinated PVDF sample (PVDF-S) shown in Figure 7(A2) did not show a noticeable change, as surface fluorination primarily reduced the surface energy without altering the structure of the surface. As displayed in Figure 7(A3), the SiO₂-coated and fluorinated PVDF membrane exhibited hierarchical microscale roughness from nanofibers and nanoscale roughness from SiO₂ nanoparticles. As observed by Zhang et al., the deposition of SiO₂ in PVDF-O enhanced the surface roughness of the other membrane as well from 201 to 237 nm. After testing the membrane wettability, PVDF-O stood out from the other synthesized membranes, achieving an in-air water CA and oil CA of above 150° and a water sliding angle (SA) of 10°, indicating its superomniphobicity. Zhang et al. also fabricated a Janus membrane by additionally spraying multiwall carbon nanotubes (MWCNTs) on the membrane surface for cross-linking. The Janus PVDF-O membrane overcame the weakness of oleophilicity under water and displayed an oil contact angle above 150° after being immersed under water. The Janus membranes maintained their hierarchically structured surfaces with microscale and nanoscale roughness (referring to Figure 7(A6)) and successfully sustained high permeate flux and a 99.9% salt rejection rate throughout the whole 8 h membrane distillation testing period.

Figure 7.

Surface and cross-sectional morphologies of (A1 and B1) #PVDF-P, (A2 and B2) #PVDF-S, (A3 and B3) #PVDF-O, (A4 and B4) #Janus-P, (A5 and B5) #Janus-S, and (A6 and B6) #Janus-O. Reprinted with permission from ref (135). Copyright 2023 Desalination.

Brown and Bhushan¹²⁰ applied a layer-by-layer approach to synthesize a surface with 4 layers of coatings on a glass substrate that possessed superomniphobicity, mechanical durability, self-cleaning, and antismudge properties. Positively charged polydiallyldimethylammonium chloride (PDDA) was selected as the base layer of the coating due to its great binding strength with the glass substrate and negatively charged silica nanoparticles. The specific molecular weight of 100 000–200 000 was selected to achieve an optimum balance between mechanical strength and viscosity for ease of silica nanoparticle deposition. After depositing the PDDA base layer, untreated hydrophilic silica nanoparticles in acetone were spray-coated as the second layer to enhance the surface roughness and mechanical strength, particularly in terms of hardness¹⁴³ and wear resistance.¹⁴⁴ Another PDDA layer was deposited as the third layer to stabilize the silica nanoparticles. The final layer, a layer of fluorosilane (FL), was deposited via chemical vapor deposition (CVD) to reduce the surface free energy via surface fluorination. The four-layered coating contributed to a total thickness of 625 nm, 200 nm for first PDDA base layer, 350 nm for SiO₂ nanoparticle roughness layer, 50 nm for the second PDDA layer, and finally 25 nm for the final fluorosilane layer. As observed by Brown and Bhushan, the flat PDDA/FL coating was hydrophobic (water CA of 109 ± 1°) and slightly oil-repelling (oil CA of 83 ± 1°). On the other hand, the four-layered composite coatings successfully achieved superomniphobicity by having contact angles of more than 155° and sliding angles of less than 9° with octane, decane, dodecane, tetradecane, hexadecane, and water (γ = 21–72 mN m^–1). Brown and Bhushan assessed the mechanical durability of the surfaces using the AFM wear test and ball-on-flat tribometer experiment. For the AFM wear tests, a borosilicate ball with a 15 μm radius was mounted on a rectangular cantilever with a nominal spring constant of 7.4 N/m (resonant frequency f = 150 kHz, all-in-one). Wear was applied to 50 × 50 μm² areas for a single cycle at a load of 10 μN, allowing the worn regions to be imaged within the scanning limits of AFM. Macrowear experiments followed an established ball-on-flat tribometer procedure. A 3 mm diameter sapphire ball was fixed in a stationary holder, and a normal load of 10 mN was applied to the surface. The tribometer was set to a reciprocating motion with a stroke length of 6 mm and an average linear speed of 1 mm/s. Optical microscopy using a CCD camera (Nikon Optihot-2) was used to observe the changes in the surfaces after the tribometer experiment. Flat PPDA/FL coating showed significant and observable damage after both AFM and tribometer experiments. Meanwhile, the layer-by-layer composite surface demonstrated excellent mechanical durability by surviving the AFM wear experiment without any observable defects and the tribometer experiment with only minimal burnishing to the coatings (Figure 8). This indicated that the strong SiO₂ nanoparticle layer located beneath the PDDA/FL layers contributed to enhancing the coating’s durability while the PDDA binder layers anchored the particles to the glass substrate through electrostatic bonding with its opposite electrostatic charge. The self-cleaning properties of the coatings were further evaluated by pouring a stream of water droplets on the coatings contaminated with silicon carbide. Most of the silicon carbide particles were effectively removed as water droplets rolled across the liquid repellent coatings, demonstrating the coatings’ self-cleaning properties.

Figure 8.

(a) Surface height maps and sample surface profiles (locations indicated by arrows) before and after the AFM wear experiment with a 15 μm radius borosilicate ball at a load of 10 μN for flat and optimized layer-by-layer composite coatings. RMS roughness values are displayed. (b) Optical micrographs before and after wear experiments using ball-on-flat tribometer at 10 mN for flat and layer-by-layer composite coatings. Reprinted with permission from ref (120). Copyright 2015 Journal of Colloid and Interface Science.

Ezazi et al.¹²¹ simplified the method by spraying a mixture of hydroxypropyl cellulose (HPC), glyoxal, and SiO₂ nanoparticles (ratio of 98:1:1) on a glass substrate, followed by heat treatment at 50 °C for 15 min. During the heat treatment, hydrolysis–condensation of hydroxyl groups of HPC and SiO₂ with glyoxal led to the formation of a cross-linked composite of HPC-SiO₂. Glyoxal acted as a cross-linking agent to promote the formation of cross-linkages between HPC and SiO₂ to produce a rough re-entrant surface. To reduce the surface energy of the film, the researcher then employed the CVD method to deposit 1H,1H,2H,2H-perfluorodecyltrichlorosilane (F-silane) at 130 °C for 60 min. According to the SEM image of the resulting HPC-SiO₂ film (Figure 9(a)), micro- and nanoscale roughness hierarchical structures with re-entrant curvatures were discovered. The film exhibited very high CAs of larger than 155° and SAs of lower than 6° with different liquids that have surface tensions ranging from 21 to 85 mN m^–1 (Figure 9(b)). On top of that, the CA for the 80:20 ethanol/water mixture droplet remained nearly unchanged during evaporation of the droplet, supporting that the produced HPC-SiO₂ film is capable of forming a robust solid–liquid–air composite interface. Additionally, the film displayed self-repairing properties that recovered almost completely from physical damage caused by a razor blade after exposure to water vapor or ethanol vapor. A scratch with a width of approximately 60.3 ± 2.1 μm was engraved onto the HPC-SiO₂ film, which had a thickness of around 100.1 ± 3.2 μm, using a razor blade until the underlying glass substrate was exposed. Upon exposure to water vapor at a flow rate of approximately 0.5 mL/min at room temperature, the scratch began to narrow and completely disappeared after about 10 s. To ensure the restoration of both hierarchical structures and low-surface-energy coatings in the damaged region, a liquid droplet was placed on the repaired area to measure its roll-off angle. The HPC-SiO₂ film displayed nearly complete recovery of its superomniphobicity after comparing the roll-off angle with the undamaged surface. The ability of the HPC-SiO₂ film to repair damage and recover its mechanical hardness was due to the reversible hydrogen bonds between the hydroxyl groups in the HPC. These bonds could dissociate upon exposure to water molecules and subsequently reform in the absence of water vapor. This mechanism allowed the HPC-SiO₂ film to undergo repeated damage-repair cycles. Figure 9(c) also shows that the HPC-SiO₂ film is versatile, as it showed superomniphobicity after being coated on substrates made of different materials such as stainless steel, polyester, and ceramic–resin composite or even without substrate. The versatility was attributed to the dialdehyde chemistry of glyoxal, which strongly anchored to virtually any substrate through an acetalization reaction, forming a covalent acetal linkage with the underlying substrate.

Figure 9.

(a) SEM image showing the surface of an HPC-SiO2 film that exhibits a hierarchical surface texture. The scale bar represents 20 μm. The inset demonstrates a high-magnification SEM image of nanoscale SiO2 particles with re-entrant curvatures. The scale bar represents 1 μm. (b) Measured advancing and receding apparent contact angles, as well as roll-off angles for liquids with a broad range of surface tension values on an HPC-SiO2 film. (c) Photographs of four droplets including ethanol (dyed blue, γ = 22.1 mN m^–1), n-octane (dyed gray, γ = 21.2 mN m^–1), n-heptane (dyed red, γ = 20.1 mN m^–1), and n-hexane (dyed yellow, γ = 18.4 mN m^–1) on an HPC-SiO2 film fabricated on (i) stainless steel, (ii) polyester fabric, (iii) ceramic–resin composite, and (iv) freestanding HPC-SiO2 film. The scale bar is 3 mm. Reprinted with permission from ref (121). Copyright 2022 ACS Materials Au.

In surface modification, the solvent-thermal induced roughening method is an interesting approach that utilizes the combined effects of thermal energy and solvent to create rough structures on a surface rather than relying on typical nanoparticle coatings.¹⁴⁵ Qing et al.¹⁰⁶ used electrospinning to fabricate a membrane as the surface base, followed by solvent-thermal-induced roughening and CVD for multiscale roughness creation and surface fluorination, respectively. Before the hydrothermal treatment was conducted at 150 °C for 4 h, the electrospun PVDF membrane and solvent-thermal treatment solution consisting of mixtures of water, hydrochloric acid, and n-pentanol was added into a Teflon-lined autoclave. After the process, the shells of PVDF fibers undergo thermal expansion and deformation, forming rough fins on the surface of the fibers.¹⁴⁵ To further decrease the surface energy of the surface, surface fluorination was carried out in 2 steps. Without causing significant differences in surface morphologies (depicted in Figure 10(b and c)), the surface was immersed in Tris-HCl buffer solution that contained dopamine hydrochloride as the first step to produce hydroxyl groups on the membrane surface, and the membrane was named as PDA/STIR-PVDF. After that, the hydroxyl groups were reacted with fluoroalkylsilane during the CVD process, and the resulting membrane was called fluorinated PFTS/PDA/STIR-PVDF. Additional roughness was not identified on the surface after the surface fluorination process with fluoroalkylsilane, as shown in Figure 10(d). Without a nanoparticle coating on the surface, the PFTS/PDA/STIR-PVDF membrane displayed superomniphobic properties with a water contact angle of 173.2° and a mineral oil contact angle of 153.8°. The liquid entry pressure (LEP), which is the critical pressure at which liquid starts to penetrate the membrane pores, was evaluated. The LEP of the original PVDF membrane was 83 ± 3 kPa, while the LEP of the fluorinated PFTS/PDA/STIR-PVDF membrane was significantly improved to 216 ± 29 kPa. The improvement in the LEP can be attributed to the reduced mean pore size and improved superomniphobicity of the treated membrane, indicating better resistance to wetting and operational stability.

Figure 10.

SEM images showing the top view of (a) the pristine PVDF membrane, (b) the STIR-treated membrane, (c) the PDA activated PDA/STIR-PVDF membrane, and (d) the fluorinated PFTS/PDA/STIR-PVDF membrane. Reprinted with permission from ref (106). Copyright 2020 Journal of Membrane Science.

Next, Kim et al.¹²⁶ fabricated a superomniphobic membrane by developing doubly re-entrant mushroom-like micropillars (D-MP) without involving perfluorocarbon coatings, which are not environmentally friendly. To fabricate the micropillar structures, Kim employed micromolding techniques with nonsolvent-induced phase separation, coupled with metal evaporation for surface modification. First, PDMS elastomer with a curing agent at a weight ratio of 5:1 was poured on a photoresistant mold and cured at 80 °C for 2 h to create the mushroom structure micropillars. After the surface was peeled off the mold, the micropillar (MP) structure was coated with an aluminum (Al) layer at a thickness of 50 nm via metal evaporation at vacuum conditions (10^–6 Torr). Due to the compressive residual stress of Al on the PDMS-based structures, the cap edges of the MPs became bent after the Al deposition, which led to the formation of mushroom-like doubly re-entrant structures that are identical to springtail skin on the surface. Figure 11(a) presents optical images comparing deionized (DI) water (γ = 72 mN m^–1) and a 50% ethanol solution (γ = ∼28 mN m^–1) on the MP and D-MP structures. Although both structures retained super-repellency against DI water, the MP structure was fully wetted by the ethanol solution, while the D-MP structure maintained its super-repellency against the ethanol solution. As illustrated in Figure 11(b), the wetting behaviors of the MP and D-MP structures differs significantly. Due to the low surface tension of the ethanol solution, a contact line cannot be formed beneath the cap, causing the ethanol to penetrate the cavities and fully wet the surface. Unlike the typical superomniphobic surfaces that use low-surface-energy perfluorocarbon materials to repel low-surface-tension liquids, the resulting D-MP surface was able to maintain stable Cassie–Baxter state with CAs of more than 150° and low SAs with different liquid droplets with only a microscale structure on the surface. This was attributed to the mushroom-like doubly re-entrant structures that provided a stable pinning point for the liquid droplets beneath the downward-facing edges of the cap, creating a negative cap angle and an air pocket between the surface and liquid droplets. As a result, a contact line can be established between the ethanol solution and the D-MP structure beneath the downward cap edges, despite the liquid’s low surface tension.

Figure 11.

Superomniphobicity of the D-MP structure. (a) Optical images of the DI water and ethanol solution droplets with a volume of 2 μL on the MP and D-MP structures. (b) Schematic images of the different wetting mechanisms of the MP and D-MP structures. Δp is the pressure difference between the liquid and the air. (c) Photograph of DI water (red), vegetable oil (sky blue), and ethanol (transparent) droplets on the D-MP patterned surface. (d) CAs and SAs of various liquid droplets on the MP, F-MP, and D-MP structures. Reprinted with permission from ref (126). Copyright 2019 ACS Applied Materials & Interfaces.

On top of comparing the D-MP surface with the pristine MP surface, the researchers also compared the D-MP surface with the fluorinated MP surface named the F-MP surface. To fabricate the F-MP structure, oxygen plasma treatment was conducted on the MP-patterned surface at 100 W for 20 s. Following this, 1H,1H,2H,2H-perfluorooctyltrichlorosilane (perfluorocarbon material) was applied via chemical vapor deposition for 12 h in a vacuum chamber. From the results of the contact angle measurement displayed in Figure 11(d), the D-MP surface could maintain a CA of more than 150° against liquid droplets with surface tension as low as 14.8 mN m^–1, whereas the MP surface and F-MP surface could not repel liquid droplets with surface tension above 32 and 25 mN m^–1 ,respectively. Impressively, the D-MP surface retained its excellent liquid repellency even after undergoing high strain, including 1000 repeated cycles of stretching, bending, twisting, and exposure to oxygen plasma treatment.

4. Applications of Superomniphobic Surfaces

The superhydrophobic and superoleophobic surfaces lack the versatility and durability required for industrial applications compared with superomniphobic surfaces that can delay or prevent surface wetting to the greatest extent with any liquid. A superomniphobic surface possesses a contact angle of more than 150° with any liquid and has greater potential for commercialized uses, including self-cleaning,^121,146,147 nonfouling,^117,148,149 antismudge,^120,123 stain-free clothing,^117,142,150 drag reduction,¹⁵¹ corrosion prevention,¹⁵² liquid separation,¹⁵³ directional droplet movement,^129,137 gas absorption with a gas–liquid contacting process,^111,114 and liquid removal after icing and melting.¹⁵⁴Table 2 summarizes the current applications of superomniphobic surfaces and their respective research focus demonstrated by the researchers.

Table 2. Applications and Respective Research Focus of Superomniphobic Surfaces.

applications	type of surface	research focus	ref
direct contact membrane distillation (DCMD)	● Fluorinated PVDF surface coated with SiO2 nanoparticles	● Development of superomniphobicity while maintaining high water flux	(12, 23, 105, 106, 132, 134, 135, 155)
	● Hydrothermal treated PVDF fluorinated with PFTS	● Oily wastewater separation
	● Hydrothermal treated Co3O4 HF fluorinated with FAS	● Wastewater with surfactant separation
	● DBP, DOP and SiO2 mixed matrix PVDF-HFP membrane	● Comparison of performance between superhydrophobic and superomniphobic with the stability of long-term MD operation, surface wetting ratio, and water recovery ratio
chemical shielding	● F-POSS cross-linked PDMS surface	● Liquid repellency test with 35 Newtonian liquids and 25 non-Newtonian liquid	(117, 133)
		● Chemical resistance test with concentrated acid, base, and organic solvent
corrosion-protective coating	● Epoxidized soybean oil, F-epoxy, citric acid, and SiO2 nanoparticles on a metal substrate	● Overcoming the limitations of conventional corrosion-protective coating	(152)
		● Corrosion resistance test with potentiodynamic polarization method
		● Evaluation of corrosion potential, corrosion current density, and corrosion rate with conventional paint and unmodified surface
directional droplet movement	● PTFE coated with PFDTS	● Evaluation of needed inclination to transport different liquid droplets along inclined tracks and designated tracks without external forces	(137, 156)
	● PDMS coated with SiO2 nanoparticles and PFDTS	● Development of a photoresponsive surface that allows droplet movement with near-infrared ray
CO2 absorption using gas–liquid contact process	● Mixed matrix PVDF membrane with SiO2 NP coated with HDTMS	● Overcoming pore wetting problem faced by the superhydrophobic membrane in gas–liquid contact process	(111, 114)
		● Evaluation of CO2 absorption flux within 20 days by comparing with unmodified surface and superhydrophobic surface

4.1. Direct Contact Membrane Distillation (DCMD)

The current focus of superomniphobic surface application is direct contact membrane distillation (DCMD), especially in separation of oily wastewater or wastewater with surfactants like sodium dodecyl sulfate (SDS)^{12,23,105,106,132,134,135,155} which often hinder the performance of surfaces during membrane distillation (MD) by diffusing into surface pores and cause pore wetting. Due to the presence of micro- and nanoscale hierarchical structures on superomniphobic surfaces, using superomniphobic membranes for DCMD had successfully improved the long-term stability by maintaining a steady high rejection rate and water flux throughout the whole oil wastewater and surfactant wastewater MD duration ranging from 4.3¹³² to over 24 h.²³

The superomniphobic surface fabricated by Kharraz et al.²³ had only 0.109 liquid droplet to solid membrane surface contact fraction. A dual-layered macro-corrugated membrane denoted as CM was first fabricated with a combination of NIPS and VIPS for each layer. The CM was fabricated in two layers because incorporating a spacer imprinting step (macrocorrugations step) into the fabrication of a single-layer membrane led to the formation of large pores, which are unsuitable for MD applications. The first layer was fabricated using NIPS to achieve a small mean pore size with a controlled pore size distribution. Meanwhile, the second layer was fabricated using VIPS to introduce macrocorrugations and attain the desired liquid repellency and surface roughness. The negatively charged CM was further treated with 7.5 M NaOH and 3% v/v APTES/ethanol solution to introduce positively charged amine functional groups. After that, the amino-functionalized CM was coated with SiNPs through electrostatic interaction, followed by fluorination via immersion in 1% v/v FAS/n-hexane solution. The spacer imprinting step formed the first macrolevel of roughness for the hierarchical structure, increasing the surface area by 42% compared to the commercial membrane. On the other hand, PVDF spherulites formed via exposure to humidity during VIPS and coating of SiNPs contributed to microlevel and nanolevel roughness, respectively. The resulting low liquid droplet to solid membrane surface contact fraction showed good correlation with the contact angle and sliding angle measurements. The membrane successfully achieved stable water and oil CAs of 160.8 ± 2.3° and 154.3 ± 1.9°, respectively, and a sliding angle of 3.8 ± 1.3°. Two separate DCMD experiments were conducted. In the first experiment, SDS surfactants were incrementally added to the NaCl solution feed every 1.5 h until the SDS dosage reached 1.0 mM. In the second experiment, 1% v/v oil in water emulsion was dosed into the 1 M NaCl/0.05 mM SDS aqueous feed solution every 2 h, gradually increasing the oil concentration in the feed solution to 400 ppm. The superomniphobic membrane exhibited stable salt rejection and water flux throughout an extended period of 24 h in both experiments with minimal foulant deposition. The combination of macrocorrugations, micromorphology, and nanopatterns with a stable fluorinated SiNP coating produced a robust superomniphobic membrane capable of maintaining a stable Cassie–Baxter state even under challenging DCMD operating conditions.

Next, Chiao et al.¹⁰⁵ demonstrated a comparison of MD performance between pristine PVDF, hydroxyl-functionalized PVDF-O, and superomniphobic PVDF-F using real produced water that has both organic and ionic solutes. PVDF-O was prepared by hydrolyzing PVDF-HFP powder with a NaOH solution before electrospinning. To prepare PVDF-F, an additional fluorination step was taken on PVDF-O via chemical vapor deposition using trichloro(1H, 1H, 2H, 2H-heptadecafluorodecyl)silane (TFS). PVDF and PVDF-O experienced fouling after 8h of MD operations. In contrast, PVDF-F sustained 12 h of MD operation with only a 15% reduction of water flux and a further reduction of another 25% after 19 h of continuous operation. To study the nature of fouling, the PVDF-F membrane was cleaned with distilled water and reused for three consecutive MD cycles. After 24 h of MD operation, followed by washing with DI water, an 80% recovery of water flux was observed. In the subsequent 24 h of operation, only a 10% reduction in water flux occurred. The findings suggested that the PVDF-F membrane can be easily regenerated and reused, as no further permanent damage was observed after the slight irreversible fouling during the initial cycle.

Li et al.¹³⁴ demonstrated different MD tests treating multicomponent shale gas wastewater. During the test, wastewater was only supplied at the beginning of the test, meaning that solution concentration will rise continuously as water permeates through membrane pores, leaving over waste substances that promote membrane fouling. Pristine PVDF membranes experienced pore wetting and only had a water recovery ratio of 10%. Meanwhile, the modified superomniphobic PVDF membrane completed the MD operation in the shortest duration of 1050 min and maintained 70.7% of its initial water flux. After the MD test, the resulting membrane had a relatively cleaner surface compared to other tested membranes, with only a small amount of related elements detected (Ca, Na, Mg, and Cl). This highlighted the potential of superomniphobic membranes for water reclamation from practical hypersaline wastewater.

4.2. Chemical Shielding

The application of superomniphobic surfaces for chemical shielding offers a simple yet effective solution for protecting surfaces against a wide range of liquids. Pan et al.¹¹⁷ conducted comprehensive tests to evaluate the liquid repellency of their fabricated superomniphobic cross-linked PDMS surface, demonstrating its ability to repel 35 different Newtonian liquids and 25 different non-Newtonian liquids, as well as different organic solvents like toluene and chloroform that are known as effective solvents for a cross-linked PDMS surface. This highlights its potential for applications in environments in which exposure to chemicals is a concern. The surface also showed excellent chemical resistance to concentrated hydrochloric acid and concentrated sodium hydroxide when compared to unmodified surfaces. The air pocket layer present on the modified surface resulting from F-POSS particles remained stable throughout the whole duration of exposure to a wide variety of liquids. From the results, the modified surface did not show any damage, proving that the surface is effective in being used for chemical shielding.

Similar findings were also obtained by Wu et al.,¹³³ who developed a facile unidirectional rubbing method to apply SiO₂ coatings with the assistance of heat treatment. First, a monolayer silica array was acquired by unidirectionally rubbing a dry powder of silica spheres on a PDMS-coated substrate using another PDMS substrate. Thermal treatment of temperature above 220 °C was conducted on the substrate for 72 h after assembling silica spheres on the surface. Subsequently, an epoxy-based photoresist, SU-8, was infiltrated into the silica template and cured to form a film after exposure to UV radiation for 1 min. After the silica template was removed, the film was modified with PFOTS via CVD, and a single-layered and ordered re-entrant structure with excellent liquid-repellent properties was obtained. On top of testing superomniphobicity through CA and SA, Wu further assessed the chemical stability of the synthesized films by immersing the films in strong acid, alkaline, and concentrated salt solutions. The films did not lose their excellent liquid repellency even after being immersed in 1 M HCl, 1 M NaOH, and 1 M NaCl for 96 h. In short, the excellent wetting resistance alongside multifunctional properties of superomniphobic surfaces, such as chemical resistance, offer great protection and enhancement for materials for windows, lenses, mobile phones, laptops, cameras, solar panels, and optical devices.

4.3. Corrosion-Protective Coating

Ezazi et al.¹⁵² developed a new alternative for a corrosion-protective surface coating to overcome the limitations of conventional corrosion-protective coatings, which are unable to repel low-surface-tension liquids and exhibit bad tolerance to mechanical stress. In the research work, the facile spray method was used to separately spray a solution of epoxidized soybean oil (ESO), 3-perfluorooctly-1,2-epoxypropane (F-epoxy), a citric acid mixture, and a solution of SiO₂ on a metal substrate. The high fluorine content in F-epoxy was used to reduce surface energy, whereas the SiO₂ coating increased the surface roughness. The surface’s superomniphobicity is primarily influenced by the composition of F-epoxy in the blend. The optimum composition of 5 wt % F-epoxy content could achieve a surface energy as low as 11.3 mN m^–1, and the surface displayed CAs more than 150° and SAs less than 10° with liquids with surface tensions equivalent to or larger than 25.3 mN m^–1. The researcher demonstrated a corrosion resistance test with the potentiodynamic polarization method. The corrosion potential (E_corr) and corrosion current density (I_corr) of the superomniphobic blend, unmodified ESO blend, and conventional paint in NaCl solution were compared. The superomniphobic blend displayed more positive E_corr readings of −81.8 mV than those of −142 and −201 mV of the other two tested surfaces, indicating that the superomniphobic blend had a lower corrosion probability. Other than that, the superomniphobic coating possessed a significantly smaller diffused mass of NaCl solutions (surface energy of 28.5 mN m^–1) compared to the other two tested coatings, showing that the resulting superomniphobic coating could sustain significantly longer than conventional coatings in low-surface-tension corrosive liquid. After estimating the corrosion rate of studied coatings, the superomniphobic coating only had approximately 20% corrosion rate of conventional coatings due to the air pocket layer that acts as a barrier to separate the metal substrate surface and low-surface-tension corrosive liquid. As a result, such anticorrosion superomniphobic coatings would be a suitable and sustainable material for rush inhibitors, pipelines, and marine heat exchangers.

4.4. Directional Droplet Movement

Jang et al.¹⁵⁶ and Wang et al.¹³⁷ demonstrated the modification and application of superomniphobic surfaces in controlling the movement of droplets. The formation of advancing contact angle of a droplet in a specific direction and its receding contact angle in the opposite direction can generate an effective driving force to control the direction of movement for droplets.¹⁵⁷ Jang et al.¹⁵⁶ designed a superomniphobic PTFE surface with polymer molding and fluorination techniques. To prepare a polymer mold, ratchet-like triangular microstructures with a height of 200 μm and a width of 400 μm, featuring base angles of approximately 27° and 90°, respectively, were designed and engraved onto an aluminum substrate to enhance the internal driving force of droplets. Second, SiO₂ with a diameter of 600 nm was deposited as second layer using the Langmuir–Blodgett coating technique, followed by spin coating of a protective layer, contributing a thickness of approximately 50 nm. Further surface modifications of the molded PTFE substrates were performed using oxygen plasma treatment (300 W power at 13.56 MHz) to introduce hydroxyl groups, and immersion in 0.1 vol % PFDTS in n-hexane solution was used for surface fluorination. The research findings showed that strong retention provided by superomniphobicity allowed the movement of tilted liquid droplets without the need for additional forces like vibration, whereas ratchet-like microstructures present on the modified surface enabled droplets to move from left to right. Under a small inclination of only 1.5°, a water droplet on the resulting surface was able to move to the right side at a speed of 1.9 cm/s with no additional forces. To deal with low-surface-tension liquid or a liquid with higher viscosity, a higher tilting angle was required. For example, soybean oil droplets required an inclination of 9.8° to move with a velocity of 0.8 cm/s. In addition, they demonstrated the directional movement of droplets using S-shaped and U-shaped tracks constructed by modified superomniphobic substrate (Figure 12), achieving similar results

Figure 12.

controllable movement of droplets on S-shaped and U-shaped superomniphobic tracks with no additional external force: (a) S-shaped track, (b) U-shaped track, and (c) droplet motion captured at an interval of 150 ms. Reprinted with permission from ref (156). Copyright 2017 ACS Applied Materials & Interfaces.

Wang et al.,¹³⁷ on the other hand, developed a photoresponsive slippery superomiphobic surface by coating a layer of nonvolatile and immiscible Fomblin Y liquid at the final step of surface modifications as a lubricating layer. By using a near-infrared laser to cause temperature changes on the micronanostructured lubricant-infused surface, low-surface-tension liquid droplets such as ethylene glycol could be transported due to temperature imbalance on the surface. The near-infrared laser caused a decrease in the liquid–air-interface tension of the droplet, leading to the shrinking of the heated spot and the generation of an unbalanced Young’s force that propelled the movement of liquid droplets. The strong driving force generated with a near-infrared laser even enabled droplet movement in an uphill direction. Overall, these studies highlight the promising applications of superomniphobic surfaces in directional droplet transport, paving the way for advancements in fields such as microfluidics, lab-on-a-chip devices, and liquid transport systems.

4.5. CO₂ Absorption using Gas–Liquid Contact Process

Geyer et al.¹¹¹ and Pang et al.¹¹⁴ demonstrated significant improvement in the long-term stability of carbon dioxide absorption in the gas–liquid contacting process by replacing the superhydrophobic membrane with a superomniphobic PVDF-SiO₂-HDTMS hollow fiber membrane. Pang et al.¹¹⁴ carried out a CO₂ absorption gas–liquid contacting experiment using mixed inlet gas (CO₂/N₂ = 19:81 % v/v) at a flow rate of 20 mL/min and 1 mol/L DEA at a flow rate of 50 mL/min as an absorbent under moderately alkaline conditions. Based on Figure 13, the unmodified PVDF-PA-8 membrane was completely wetted after 10 days, leading to a notable drop in CO₂ flux of 31.5%, while PVDF-SiO₂-HDTMS withstood wetting throughout the whole 20 days experiment period and had minor flux decrement of only 3% compared to that of 17% of the superhydrophobic PVDF-HDTMS surface. After 20 days of long-term operation, contact angles of PVDF-SiO₂-HDTMS decreased the least compared with the other two tested membranes, proving that the DEA solution used during the gas–liquid contacting process did not bring significant degradation on the resulting superomniphobic surface. Similar findings were found in the work of Geyer et al.,¹¹¹ in which the developed superomniphobic membranes improved the CO₂ capture rates in the gas–liquid contacting process by up to 40%. The findings highlighted the robustness and durability of superomniphobic surfaces in gas–liquid contact applications, showing their potential to improve the longevity of separation processes that involve a low-surface-tension liquid.

Figure 13.

Long-term CO₂ absorption performance of the membrane in the gas–liquid contacting process. Reprinted with permission from ref (114). Copyright 2023 Scientific Reports.

5.0. Conclusion and Future Research Directions

To summarize, this Review highlighted recent advancements in the fabrication of superomniphobic surfaces and their diverse industrial applications. Past literature suggests that achieving superomniphocity requires surfaces to possess sufficiently low surface energy and high surface roughness. A variety of different techniques combining surface roughening and altering surface chemistry were found to be effective in increasing surface roughness at micro- and nanoscale levels and reducing surface energy through the incorporation of low-surface-energy components and functional groups, such as fluorine functional groups. In addition to the exceptional liquid repellency, other attractive surface properties such as self-cleaning, drag reduction, and self-repairing contributed by the multiscale hierarchical structure are valuable and useful for various research and applications, such as stain-free clothing.

Figure 14 outlines the key research gaps and challenges, as well as their respective future research directions. The research gaps and future directions can be summarized as follows:

• So far, the liquid-repelling effects of surface modifications are not permanent, as they lose their effectiveness over time, especially under extreme operating conditions such as mechanical damage, chemical reactions, and UV exposure. Previous studies have developed superomniphobic surfaces that have excellent mechanical durability by surviving through pressure,⁹⁶ wear,¹²⁰ and abrasion¹³⁴ tests. Superomniphobic surfaces that can allow strong chemicals like acids and bases to bounce off the surface were also developed.¹¹⁷ However, to apply the superomniphobic surfaces, it is also important to evaluate the surfaces against immersion by estimating the time required for a superomniphobic surface to lose its superior antiwetting properties.¹⁵⁸ Furthermore, very limited research has been conducted to study the stability of superomniphobic surfaces toward UV radiation. Therefore, this provides future directions to further improve the durability of superomniphobic surfaces.

• Until recent times, achieving superomniphobicity was challenging as it required multiple types of chemicals and complex surface modification techniques. This highlights the need for further exploration of base materials and additives with inherent liquid-repellent properties, reducing the need for extensive modifications.

• Although superomniphobic surfaces have been gaining popularity over the years, they are still facing scale-up difficulties. As of now, most of the surface modification techniques often involve expensive setup or have limited capacity, which are impractical for industrial applications. To address this issue, Vahabi et al.¹⁴⁰ fabricated a superomniphobic film by only spray-coating fluorinated silica nanoparticles on a glass substrate. However, the spray-coated layer could not withstand abrasive and harsh environments.¹⁴⁰ Future research should focus on developing and improving economical modification techniques with large capacity to replace costly methods, such as solvothermal and plasma treatments.

• Long-chain fluorocarbon materials, which can decompose into perfluorooctanoic acids, have been classified as emerging contaminants by the U.S. Environmental Protection Agency.^159,160 While nonfluorinated substances such as silicone and hydrocarbons are slowly being explored as environmentally friendly alternatives for fluorinated substances in some research,^126,161,162 their performance may be slightly inferior compared to fluorinated materials, particularly in repelling extremely low-surface-tension liquid. So far, very few studies have investigated nonfluorinated materials, suggesting a need for further research into greener chemical alternatives.

• Superomniphobic coatings enhance liquid repellency but can compromise other properties such as diffusivity¹⁶³ in membrane distillation applications. More investigations can be conducted about optimizing and balancing the trade-off between superomniphobicity and other properties.

• Most of the previous research focused on membrane distillation applications, and there are numerous potential applications yet to be discovered, such as biomedical devices that prevent biofluid buildup, energy harvesting equipment or electronics protection, and smart military suits, to find out the full potential of superomniphobic surfaces. Apart from that, several potential applications can be extended to superomniphobic surfaces. For instance, superhydrophobic and superamphiphobic surfaces have been found to be successful in triboelectrification or liquid–solid contact electrification.^164−166 During the process, the surface becomes electrically charged through contact or friction with liquid. With the minimized wetting effect and improved durability, superomniphobic surfaces could have enhanced performance in triboelectrification for water energy harvesting, microfluidics, continuous droplets transport, and so on. In addition, superhydrophobic and superamphiphobic surfaces exhibited great performance in controlling/preventing foaming,^167,168 which is prevalent in various industries ranging from chemical manufacturing, oil recovery, and pharmaceutical production to food processing. So far, the effect of superomniphobic surfaces’ enhanced wettability on antifoaming performance has yet to be discovered.

Figure 14.

Summary of research gaps and future research directions.

The authors declare no competing financial interest.