1 Introduction

Water pollution has significant detrimental effects on both aquatic and terrestrial organisms. Marine systems, including rivers, lakes, and oceans, are vital in supporting human livelihoods by facilitating food chains and influencing weather patterns. Pollution in these water bodies can have severe consequences for various life forms. Recognizing the global importance of this issue, the United Nations has identified it as a priority objective. The UN urges nations to prioritize the preservation of clean environments by giving equal attention to conserve both land and water resources. This approach is crucial for ensuring the sustainability and well-being of ecosystems and human societies. Water pollution is caused by various factors, such as oil spills and the frequent release of industrial oily effluent [1, 2]. Notably, oil spills have resulted in significant adverse effects on marine life. For example, in 2010, the Gulf of Mexico experienced an inadvertent oil spill, releasing 134 million gallons of oil, which impacted around 2100 km of the American Gulf Coast from Texas to Florida. This incident was described as “the worst environmental disaster America has ever faced” by former US President “Mr. Obama” [3]. Another similar oil spill incident occurred on 15 January 2022 in Peru, where approximately 6000 barrels of oil were spilt over 700 hectares of water, threatening aquatic plants and animals. Ultimately, oil spills in oceans and seas cause a potential risk to climate stability, environmental problems, and loss of ecology [4, 5]. Therefore, to tackle such a problem, different conventional methods like chemical coagulation, biological treatment, centrifugation, physical adsorption, and skimming were explored by researchers [6, 7]. These approaches are critical for reducing the environmental effects of oil spills and preserving the integrity of natural ecosystems. Despite their ubiquitous application, these technologies exhibit notable limitations, such as high operational costs, excessive energy requirements, low separation efficacy, and consequential environmental deterioration due to the release of toxic gas emissions into the atmosphere [8, 9].

Consequently, scientists have developed novel approaches that are economically viable and highly efficient to surmount these constraints for the effective separation of oil from water [10]. However, oil spillage remains a menace for researchers and is still considered one of the most significant factors of water pollution [11]. There has been growing interest in developing material with enhanced wettability for oil–water separation [12, 13]. Moreover, most of the techniques to separate oil from water were inspired by natural entities like lotus leaves [14, 15], butterfly wings [16, 17], peanut leaves [18], red rose petals [19, 20], water strider legs [21], and fish scales [22, 23]. These natural entities possess low surface energy, which resists wetting, encouraging researchers to develop superhydrophobicity in engineering materials [24, 25]. Superhydrophobic surfaces exhibit unique surface properties, characterized by a WCA greater than 150° and a sliding angle smaller than 10°. The superhydrophobicity of the surface is dependent on both surface roughness and chemistry. The surface with hierarchical micro–nano-structures minimizes the adhesion forces, allowing water droplets to slide off the surface, contributing to the superhydrophobic behaviour [26]. The chemical composition of the surface with low surface energy materials minimizes the interaction between the surface and water molecules, promoting water repellency. Combining appropriate surface roughness with hydrophobic chemistry can synergistically enhance the overall superhydrophobicity [27,28,29,30,31].

This review article highlights recent advancements in OWS using porous super-wetting materials, primarily emphasizing the 3D-printed polymeric materials. One of the primary novelties of this work is to emphasize the limited application of low-cost additively manufactured environmentally friendly mesh that can replace the currently available metallic mesh. Most research considers metallic meshes as a filter primarily used for oil–water separation. Still, these meshes are expensive, complex to fabricate, and have limited design flexibility. This paper gives insight into the versatility of polymer-based 3D-printed mesh compared to conventional metallic mesh, their transformative capabilities, and adaptability with emerging manufacturing technologies. The report begins with the significance of water and the necessity for technological advances in the separation process. Furthermore, two methods (superhydrophobicity-superoleophobicity or superoleophobicity-superhydrophilicity) are explored briefly. The discussion concludes with a concise overview of current challenges and prospects in the fields of OWS based on recent findings. This review paper provides vivid insight into various 3D-printed polymeric membranes for OWS. Currently, most reviews primarily focus on additive manufacturing (AM) techniques in mechanical, aerospace, medical, and tissue engineering domains. However, the application of 3D-printed membranes and their potential in diverse OWS scenarios have not been extensively explored. Considering the mentioned research gap, the current contributions of AM technologies, highlighting the most recent developments and potential applications of polymer-based AM technologies for OWS, are explored briefly.

In summary, previous work illustrates that traditional OWS methods often rely on simple geometries and materials with limited customization capabilities. In recent works utilizing 3D printing technology, the approach to OWS has been revolutionized through innovative design, advanced materials, and enhanced functionality. These integrated features enhanced efficacy, durability, and reliability in the separation process. The creative work on OWS leveraging 3D printing technology represents a paradigm shift from conventional methods towards highly customizable, highly efficient, and sustainable solutions with advanced functionalities and improved performance. 3D-printed products designed using eco-friendly and recyclable materials, aligning with sustainable goals. Moreover, these separation devices with enhanced efficacy and performance contribute to reduced resource consumption and environmental impact over their lifecycle.

2 Different methods of filtering oily water

This section overviews different methods for filtering oily water, such as gas flotation, gravity settling, chemical coagulation, adsorption, membrane filtration, electrochemical, and centrifugation [32,33,34]. The chemical processes frequently need highly skilled operators, substantial operating expenses, and continuous process monitoring control [35]. Oil pollution droplets can be floated in water using gas flotation techniques such as sparging or dissolved gas floatation. Gas flotation systems create agglomerates; gas bubbles adhere to dispersed oil droplets when injected. Researchers have made significant progress in utilizing surfactants as a promising approach to enhance the extraction of oil droplets from water. The gas floatation approach is superior for oil concentrations below 1000 mg/L [33]. Although centrifugation consumes a significant amount of energy, it successfully segregates oil from an oil–water mixture (OWM), mainly when the densities of the two liquids are similar [36]. Coagulation is a very adaptable technology utilized extensively for oily wastewater. This technique dissolves emulsified oils by combining colloids and suspended particles to form bigger flocs that may be removed from the system [37]. Even though the coagulation approaches are effective, the wastewater’s composition impacts the coagulant’s concentration, and this expensive approach leads to the generation of secondary pollutants that menace aquatic species. As a result, it is recommended to utilize electrocoagulation to enhance coagulation processes by generating separation force. The electric field accelerates coalescence and the movement of water particles towards the electrodes, whereas gravity forces are more efficient at removing bigger agglomerates. Adsorption is a cheaper, more efficient, and space-intensive approach for OWS. Conventional absorbers used in the OWS process, such as wool, zeolites, and activated carbon, may have various issues, including inadequate selectivity, low wettability, and recycling concerns [38]. The choice of method depends on factors such as the type and concentration of the oil, the volume of water, regulatory requirements, and economic considerations. So, it is crucial to conduct a thorough assessment of the specific conditions and requirements before selecting a filtration method for oily water treatment. Table 1 illustrates the summary of the different methods for filtering oily water. In the subsequent section, the investigation of wettability and understanding of surface phenomena is discussed briefly.

Table 1 Comparison between different methods of filtering oily water
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3 Membrane wettability phenomena and their function

The wettability of a membrane is an essential aspect that impacts diverse phenomena and functionalities in membrane-based processes. Wettability refers to the ability of a liquid to spread and adhere to the surface of a solid material. It is a critical property influencing various phenomena and functionalities in different applications, particularly in materials science, chemistry, and engineering. The degree of wettability is determined by the balance between cohesive forces within the liquid and adhesive forces between the liquid and the solid surface. The wetting properties of the surface depend upon factors such as surface roughness, chemical composition, surface energy, and the nature of the liquid play crucial roles in determining the wettability of a surface [39, 40]. Understanding and controlling wettability is essential in designing materials and surfaces with specific characteristics, such as superhydrophobicity or enhanced adhesion. Surface treatments can alter the wetting behaviour of materials for particular applications in creating self-cleaning surfaces, water-repellent coatings, and adhesion-promoting treatments. The contact angle technique is utilized to evaluate the wettability of these surfaces [41, 42].

3.1 Contact angle method

The contact angle method is used to assess the wettability of a solid surface by a liquid. It measures the angle formed at the intersection of the liquid–air interface and the solid surface. The contact angle is crucial to explaining how well a liquid spread and adheres to a solid material. In wettability, a higher contact angle indicates a less wettable surface (hydrophobic or oleophobic), while a low contact angle suggests a more wettable surface (hydrophilic or oleophilic). This method is widely employed in various scientific and industrial fields, including materials science, surface engineering, and biomaterials research. It helps researchers and engineers understand the wetting properties of surfaces, contributing to the development of products with specific functionalities, such as self-cleaning surfaces, water-repellent coatings, and adhesion-promoting treatments. The sessile drop method is the most prominent for measuring contact angles in determining wettability [43, 44]. It entails measuring the angle between the liquid–solid interface and a tangent line drawn at the point of contact. According to Young’s equation, the wetting regime may be separated into three groups based on the contact angle: water-wet, intermediate-wet, and oil-wet states. Conversely, a large contact angle indicates poor wetting, with the liquid-forming droplets exhibiting minimal surface coverage. Figure 1 shows that a surface is hydrophilic when the WCA is less than 90° and hydrophobic when it is more than 90° [45].

Fig. 1
figure 1

The Contact angles of different wetting properties [45]

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The sessile drop method is well known for its ease of use and efficacy, especially when evaluating flat surfaces [46, 47]. However, real-world surfaces are often rough, heterogeneous, and porous, which poses challenges in applying this method [47, 48]. The superhydrophobic surface exhibits advantageous properties owing to its uneven and rough topography. The surface features micro–nano-structures and nano-enclosed pores, introducing height variations that result in air bubbles’ entrapment. The Wenzel model is a theoretical framework used to describe the wetting behaviour of a liquid on a rough or textured surface. The Wenzel model is applicable for homogenous surfaces, as shown in Eq. (1). Scientists have researched the chemical aspects of non-stick and anti-adhesion coatings, employing the Cassie–Wenzel hypothesis, as shown in Eq. (2) [49]. Understanding and controlling membrane wettability is critical for designing membranes with desired functionalities. By manipulating wettability, researchers and engineers can improve membrane performance in filtration, antifouling, and selectivity for various other applications. The mechanism and factors that affect the membrane design for OWS application are discussed briefly in a subsequent section.

$$ \cos {\uptheta }_{{\text{w}}} = r\cos {\uptheta } $$
(1)

where θw = Wenzel angle of contact, θ = Young’s angle of contact, and r = surface roughness factor. For heterogeneous surfaces, Cassie and Baxter’s model is applicable by Eq. (2):

$$ \cos {\uptheta } = f_{1} \cos {\uptheta }_{1} + f_{2} \cos {\uptheta }_{2} $$
(2)

where θ = angle of contact, f1 = ratio of solid to liquid contact area, and f2 = contact area ratio with air packets that confine the inner side of surface cavities.

4 Mechanism of OWS and design strategies of membrane

Superhydrophobic surfaces have unique surface properties that make them extremely adaptable to various applications such as self-cleaning, anti-fogging, antifouling, and material drag reduction [50, 51]. Numerous strategies have been employed to address the challenges associated with oil–water mixture (OWM). These methodologies aim to manage diverse repercussions by utilizing materials designed to selectively facilitate the passage of one wetting liquid while impeding the flow of another. The effectiveness of this separation is contingent upon the mechanism of oil and water blocking. Optimal selection of a separating medium is crucial for allowing the preferential passage of one wetting liquid while impeding the flow of another, typically achieved through gravity-driven processes. In OWM separation, hydrophobic or superhydrophobic surfaces are commonly utilized to prevent water permeation. These surfaces, characterized by low surface energy and distinctive micro- or nanostructures, repel water. Water passage is hindered by incorporating such features into porous materials, while oil can selectively flow through the pores.

On the contrary, a surface exhibiting hydrophilic or superhydrophilic properties is suitable for effectively repelling oil and maintaining separation from water. The choice of surface characteristics plays a crucial role in achieving efficient and selective separation of OWM. Size-sieving and demulsification are two essential processes involved in isolating emulsified OWM, are shown in Fig. (2). Size-sieving is based on the difference in molecule sizes between water and oil, and materials with superhydrophobic-superoleophobic (SHSO) properties have been used to separate oil from water. Conversely, demulsification is an intriguing separation approach in which certain chemicals help break the emulsion [52, 53]. The membrane’s separation design depends on pore size and breakthrough pressure, allowing efficient separation depending on these characteristics.

Fig. 2
figure 2

Mechanism of oil–water separation [52]

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4.1 Critical factor affecting membrane for OWS

In the systematic design of porous materials for OWS, two critical physical parameters affect surface structure: pore size (porosity) and breakthrough pressure. The surface structure, precisely the pores’ size and distribution, influences the material’s ability to selectively adsorb or repel certain substances, such as oil or water. Porosity determines the overall volume of the material available for fluid interaction and permeation. Additionally, the breakthrough pressure represents the point at which the applied pressure exceeds the material’s capacity to retain the desired fluid, leading to potential leakage or compromised separation performance. By carefully considering and optimizing these physical parameters, researchers can enhance the design of porous materials for efficient OWS applications.

4.1.1 Pore size

The size of the membrane pores primarily determines the separation performance for OWS. The bubble pressure test is the standard technique for measuring membrane pore sizes. It works on the capillary action of liquid in the membrane pores. This test provides information about the most prominent pores in a membrane and helps characterize microfiltration and ultrafiltration membranes. It aids in quality control and ensures that membranes are suitable for specific OWS filtration applications. Other methods for determining membrane pore size include computer tomography, gas adsorption, porosimeter testing, and mean-flow pore size methods. Computer tomography is a medical imaging technique that uses X-ray tools for visualizing the internal structure and characteristics of the membranes including details about pore size, shapes, and distribution. This information is crucial for understanding and optimizing the performance of membranes in OWS applications. Another technique is gas adsorption used to characterize membranes’ surface area, porosity, and pore size distribution. It involves exposing a material to a gas and measuring the amount of gas adsorbed as a function of relative pressure. It provides information for understanding the structural properties of membranes and is often used in research and quality control processes related to membranes. Porosimeter testing measures the pore volume and pore size distribution of porous membranes. It is based on the principle that the liquid volume is displaced to the membranes’ pores. It provides information crucial for understanding the structural properties of membranes in the applications of OWS. Another method is mean-flow pore size techniques to determine the average pore size of the membranes. These above methods help understand the structural properties of membranes and are commonly used in research, development, and quality control in OWS. It assesses the flow of liquid through a membrane at varying pressures and results in information to calculate the average pore size of the membranes. Furthermore, it should be noted that the membrane’s pores exhibit irregular shapes and sizes, including their pore size and surface area [54, 55]. Finally, understanding and controlling the mean pore size of a membrane is crucial aspects of membrane technology. It allows for optimizing separation processes by tailoring membranes to the specific requirements of the intended application, ensuring efficient and precise filtration based on their pore sizes. Understanding pore size is essential in establishing an environment that optimizes the precision of particle size information throughout the separation procedure.

4.1.2 Breakthrough pressure

The critical pressure (Pc), also referred to as the breakthrough pressure (Pb), holds significant importance as a fundamental physical property in membrane design. It is commonly known as the liquid entry pressure (LEP) and is considered the maximum pressure that must be applied to a membrane before liquid seeps into the pores. The determination of breakthrough pressure often involves observing the initial droplet formation on the membrane, and the widely used Young–Laplace method is employed to measure this phenomenon [56, 57]. Researchers, including Franken et al., Kim, and Harriott-Zha, have developed models with the same objective. These models aim to determine the breakthrough pressure (Pc) of cylindrical structures possessing small, uniformly sized pores. The breakthrough pressure obtained by Eq. 3 is shown below:

$$ \Delta P_{{\text{c}}} = \frac{{ - 2\gamma_{L} \cos \theta_{L} }}{{r_{{\text{P}}} }} $$
(3)

where \({\upgamma }_{L}\)= surface tension of the fluid and rp = maximum pore radius of the membrane.

As mentioned earlier, Young’s equation is limited to ideal surfaces and cannot be extended to actual surfaces, such as the rough surface of a membrane. In contrast, the Laplace–Young equation accounts for the influence of membrane wettability, which can significantly affect the pressure-driven membrane filtration process. Consequently, the expression governing surface wettability and breakthrough pressure may require modification. The fabrication of membranes using various techniques has been considered to separate oil from water effectively. These processes involve developing membrane materials with tailored properties and structures to enhance their OWS capabilities.

5 Fabrication of metallic membrane filter and its application

In recent years, substantial scientific steps have been undertaken to explore the potential applications of metallic meshes in the field of OWS. The advantages of metallic mesh are their ease of access, cost-effectiveness, and porous mesh structure, enabling a high flux rate and ensuring optimal separation efficacy. Various metallic meshes, such as stainless steel, copper, and others, have been used for separation due to the surface characteristics and affinity of materials. The surface properties of mesh material can be enhanced by hydrophobic or hydrophilic polymeric coating on their surface. The applications of polymeric coating on metallic mesh substrate for OWS are explored briefly in a further section.

5.1 Fabrication of polymeric coating on a metallic substrate for OWS

Scientists have successfully constructed metallic membranes for OWS by using changes in interfacial energy between oil and water [58]. These membranes are made of mesh and have hydrophobic or hydrophilic properties, allowing them to operate as “oil-removing” or “water-removing” materials. However, it should be noted that these meshes are only suited for use in mild environments since they are prone to corrosion and are vulnerable to harm from acidic, alkaline, or salt solutions [59]. In the subsequent section, polymer coating on stainless-steel and copper mesh material is discussed briefly for OWS applications. Researchers have been interested in using metallic mesh for OWS since the publication of Feng et al. [60] in 2004. In their study, the authors established a uniform dispersion of polytetrafluoroethylene (PTFE) particles (30 wt. %), polyvinyl acetate (PVAC) as an adhesive (10 wt. %), sodium dodecyl benzene sulphonate (SDBS) as a surfactant (2 wt. %), and thinner sprays (50 wt. %) to coat stainless-steel (SS) mesh. The stainless-steel mesh was cleaned before applying the coating solution by spraying and curing the sample at 350 °C. The coated mesh has a WCA of 156.2° and a sliding angle 4°. This proposed technique, however, has received criticism because of its poor thermal and mechanical stability. Nonetheless, it has been observed that PTFE-coated SS meshes exhibit superhydrophobicity characteristics, resulting in good OWS performance. PTFE inherently possesses hydrophobic properties, with a WCA ranging from 98° to 112°. Notably, PTFE’s exceptional chemical resistance can make dissolving in a solvent for electrospinning applications challenging [61].

Since then, several researchers have worked together to create wettable membranes that may be used for OWS. Qin et al. [62] altered Feng’s experimental method by utilizing PTFE suspension, adding polypropylene sulphide (PPS), and obtaining a similar WCA of 156°. Several researchers used immersion techniques to provide the metallic mesh with SHSO characteristics by altering their surfaces with stearic acid (CH3 (CH2)16COOH). Researchers employed stearic acid to construct SHSO features on SS mesh in various works using nanoparticles such as Mg (OH)2, Cu crystals, and ZnO. In another study, Khosravi et al. [63] created a superhydrophobic surface on SS mesh by deposing polypyrrole and carbon soot nanospheres on the mesh surface, followed by surface modification with stearic acid and obtained a separation efficacy of 99% after 50 cycles. Guo et al.[64] reported that epoxy/hexadecyltrimethoxysilane halloysite nanotubes (HDTMS-HNTs) were sprayed onto SS mesh to create a superhydrophobic halloysite-based mesh that can successfully separate a variety of OWM with a separation efficacy of over 98.6%. The HNTs treated with Hexadecyltrimethoxysilane (HDTMS) enhance the mesh’s surface roughness and superhydrophobic characteristics. The mesh maintains a static WCA of 154° and sliding angles of 1.5° even after 25 separation cycles. In another study, Guo et al. [65] applied a polyhedral oligomeric silsesquioxane (POSS) hybrid acrylic polymer coating on SS mesh for OWS applications. The coated mesh exhibits a WCA of 153° and a sliding angle of 4.5° after undergoing 25 separation cycles with a separation efficacy of 99%. In other works, Zhang et al. [66] developed an SHSO surface on SS mesh by immersion technique to grow a hierarchical ZnO micro–nano-structure for OWS. The coated mesh revealed a WCA of 156° and a separation efficacy of 95% after ten separation cycles. These functionalized membranes perform admirably in harsh operational scenarios such as corrosive environments, acidic and basic environments, and saline solutions. Zhang et al. [67] developed an efficient and robust superhydrophilic coating for OWS on SS mesh. Figure 3 shows SEM images of both coated and uncoated mesh, along with XRD and EDS analysis. In Fig. 3 a–b, the uncoated mesh displays interwoven pure steel wires to form a regular reticulated surface. In contrast, the coated mesh shown in Fig. 3 c–e exhibits an abundance of microstructure particles characterized by irregular cube-like structures that facilitate the creation of a unique wettability surface. On the surface of the mesh, as shown in Fig. 3 f–i, an EDS revealed the presence of only Mn, Co, and O elements that conform coating of MnCo2O4 on SS mesh. The MnCO2O4-SSM (Manganese Cobalt Oxide-Stainless-Steel Mesh) achieves exceptional antifouling properties with a WCA of 156°, separation efficacy of 99.9%, and high flux rate of 63 Lm−2 h−1. The MnCO2O4-SSM exhibits superior recycling stability, maintaining its performance after 30 separation cycles.

Fig. 3
figure 3

SEM images of SS mesh at various magnifications: both a–b pure SSM, c–e SSM coated with MnCo2O4, f XRD patterns of MnCo2O4-SSM, and g–i EDS mapping of the MnCo2O4-SSM [67]

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Using stainless-steel mesh (SSM) as an OWS substrate has significant implications for treating and reusing petrochemical effluents. Table 2. highlights the stainless-steel mesh substrates employed in OWS, indicating their excellent separation efficacy and recycling stability, ensuring long-term performance through several cycles. Extensive research has also been undertaken on using superhydrophobic coatings on metallic mesh for OWS, particularly copper mesh. These coatings attempt to improve metallic meshes’ separation efficacy and performance by generating a superhydrophobic surface. The extensive exploration of superhydrophobic coatings on metallic mesh, particularly copper mesh, advances OWS technologies and broadens the materials suitable for such applications. [68, 69]

Table 2 Summary of stainless-steel mesh substrate for OWS
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Cao et al. [70] created micro–nano-hierarchical structures on the copper mesh using electrodeposition and immersion procedures. The coated mesh exhibits a WCA of 152.4°, a sliding angle of 12.6°, and a high oil flux rate of 4507 Lm−2 h−1 with 90% separation efficacy. Figure 4 shows SEM images of copper mesh surfaces before and after coating. Figure 4 a–c shows that the bare copper mesh surfaces exhibit relatively smooth characteristics. Conversely, Fig. 4 d–e reveals the presence of rough micro–nano-wire structures on the coated surface, indicating the successful implementation of the electrodeposition method. The coating on the mesh surface shows a remarkable increase in hydrophobicity.

Fig. 4
figure 4

a–c SEM images of bare copper mesh and d–f coated copper mesh [70]

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In other works, Liu et al. [71] coated the copper mesh for OWS applications using the electrodeposition process. The coating on the mesh surface shows a WCA of 155.5° with a separation performance of 93% for various OWM. These remarkable surface properties, including high WCA and complete oil wetting, were retained after ten separation cycles. In another study, Zhang et al. [72] employed the CVD technique to coat copper mesh with silica particles for OWS applications. After being modified with hexamethyldisilane (HMDS), the PSCCM-coated membrane shows SHSO properties. The coated mesh exhibits 98% separation efficacy even after 300 separation cycles with a gravity-driven system.

Similarly, Khosravi et al. [73] earlier used SS mesh and, in a further study, used copper mesh for OWS. In his research work, authors have used a hydrothermal approach for the first time to develop a superhydrophobic CuxS/Cu (x = 1, 2) mesh, followed by modification with stearic acid for OWS. The coated mesh exhibits a WCA of 160° ± 1° with a separation efficacy of 99.9% even after 100 cycles. The developed superhydrophobic CuxS/Cu mesh has significant potential for in situ OWS and is easily accessible for collecting organic contaminants and spilt oil. In recent work, Luo et al. [74] produced extremely hydrophilic nickel nanoparticles with a core–shell shape for coating the copper mesh by electrodeposition method. The developed mesh exhibits an OCA of 155°, with a separation efficacy of 98% after seven cycles.

In a further study, You et al. [75] reported coating on copper mesh with deposition of Zn–ZnO to develop a highly hydrophilic structure. The electrodeposited copper mesh featured a flower-like hierarchical structure with underwater OCA of 155.6° and separation efficacy of 99%. The durability and anti-corrosive properties of the copper mesh make it suitable for treating complex industrial oily wastewater. The separation device based on this copper mesh holds promising potential for addressing challenging environmental pollution issues related to oil contamination. Lastly, the primary approach employed in the utilization of copper mesh for OWS involves the creation of rough surfaces, leading to exceptional wettability.

These modified meshes exhibit surface patterns reminiscent of natural structures observed on water skippers and lotus leaves. Graphene oxide, carbon nanotubes, and dopamine enhance the copper mesh’s mechanical and photocatalytic performance [76, 77]. Table 3 illustrates the summary of the copper mesh substrate for OWS. Researchers have further turned to polymeric mesh fabricated using additive manufacturing techniques to overcome the corrosion associated with metallic mesh filters used for OWS. These polymeric meshes offer excellent corrosion resistance and can efficiently separate oil and water phases. Polymeric meshes with higher durability, chemical resistance, and superior separation efficacy may be created through additive manufacturing, making them well suited for OWS applications.

Table 3 Summary of copper mesh substrate for OWS
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6 Application of additively manufactured membrane

Additive manufacturing, also known as “3D printing”, produces 3D objects by depositing materials in a layer-by-layer mode with the help of CAD design. It helps fabricate customizable products and intricate structures in a single step with nil material wastage. Moreover, it has the benefits of accessibility, flexibility, speed, sustainability, and risk reduction. It is one of the fastest-growing techniques and proves an alternative to traditional subtractive manufacturing due to its outstanding efficacy in terms of precision, performance, cost, and time [78, 79]. Hideo Kodama first developed the concept of 3D printing in 1981, where a photopolymer-based rapid prototyping method was used to build a 3D model using ultraviolet (UV) light. Since then, several innovative additive manufacturing methods have been developed rapidly, which include stereolithography (SLA), fused deposition modelling (FDM), and selective laser sintering (SLS). It has been noticed that this technology has increased in the past few decades, and most of the work has been published in tissue engineering, biomedical, aerospace, etc. A variety of thermoplastic polymers like acrylonitrile butadiene styrene (ABS), polystyrene (PS), polycarbonate (PC), nylon, high-density polyethylene (HDPE), polylactic acid (PLA), etc., were used in 3D printing processes like FDM and SLS for part fabrication. Still, 3D printing in membrane separation is a new and challenging research field for OWS. Figure 5 shows an exponential rise in the number of publications related to 3D printing from 2007 to 2022. Still, there have been few publications in recent years on OWS using additive manufacturing. Therefore, scientific communities have shown keen interest in developing membranes using additive manufacturing for OWS in recent years.

Fig. 5
figure 5

Publication trends from 2007–2022 in 3D printing and 3D printing membrane [80]

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6.1 Fabrication of polymeric coating on the 3D-printed membrane for OWS

The quantity of water increases as the world population grows, but water quality is declining steadily. As explained earlier, the primary source of water pollution is frequent oil spill accidents during excavation, extraction, and transportation, posing significant environmental challenges. The primary concern is effectively separating oil from the water after such incidents. Earlier, various conventional methods were used to clean water, but they were not much efficient. Contrary to traditional manufacturing techniques, 3DP has been employed to develop nearly perfect porous membrane structures with the required OWS characteristics. This approach empowers the creation of intricate shapes and customizable structures, enabling the design and production of efficient separation devices tailored to specific applications. The utilization of additive manufacturing techniques in OWS has demonstrated the potential to achieve enhanced separation efficacy, improved selectivity, and reduced costs compared to traditional manufacturing methods. By changing the design flexibility offered by additive manufacturing, it becomes possible to fabricate complex porous structures, customize surface properties, and optimize flow paths within separation devices.

Additionally, additive manufacturing allows for incorporating functional features such as hydrophobic or hydrophilic coatings, precise control over pore sizes, and size-sieving capabilities within the separation devices. These features can enhance the performance and adaptability of the separation process, facilitating the effective removal of oil from contaminated water. Researchers have proposed various materials, including polymers and ceramics, to coat substrate surfaces with micro–nano-scale superhydrophobic structures, thereby enhancing the OWS properties [81]. Lv et al. [82] have used 3D-printed superhydrophobic structures at microscopic and nano levels for OWS. The mesh structure was 3D printed with polydimethylsiloxane (PDMS) ink containing hydrophobic nano-silica for the OWS. The incorporation of nano-silica in PDMS solution improves the printability as well as the mechanical characteristics of the sample. The 3D-printed membrane of pore size 0.37 mm has a high oil flux of 23,700 LMH. The printed membrane has a WCA of 158° with a separation efficacy of 99.6%. The 3D printing method incorporates the superhydrophobic surface into the porous framework, thus eliminating the weaker interfacial adhesion problem that occurred with conventionally prepared superhydrophobic membranes. The superhydrophobicity of the membrane was created by “coating on a mesh structure”, adjusting the ink rheology, and generating the desired porous structure. In another study, a 3D-printed porous structure was prepared using PLA filament via the FDM technique. The samples were coated using methyl-ethyl-ketone (MEK) solution etching with hydrophobic nano-silica (HN-SiO2) and low surface energy fluorosilane modification at different wettability for OWS. The cylindrical sample was designed by UG NX 12.0 software, having three kinds of holes containing side lengths of 0.8, 1.0, and 1.2 mm for OWS fabricated, as shown in Fig. 6. The outcomes demonstrate OWS with a side length of 1.0 mm has better separation efficacy with a high flux rate [83].

Fig. 6
figure 6

a 3D model of a cylindrical OWS designed by UG NX12.0 software and b Porous bottom surface c OWS using 3D-printed cylindrical membrane [83]

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Similarly, Xin et al. [84] fabricated a 3D porous film via the FDM technique and immersed it in acetone to develop a flower-like surface. Furthermore, the structure was dipped in dopamine buffer containing polystyrene nanospheres to create superhydrophobic behaviour. It was observed that after dip coating surface exhibits a WCA of 151.7°, low water adhesion force of 21.8 μN, maximum efficacy of 99.4% with a pore size of 250 μm, and a high flux of 60 kLm−2 h−1. Li et al. [85] have used the direct inkjet writing (DIW) technique to create a superhydrophilic and underwater superoleophobic membrane with an ordered porous structure for OWS. The ink fabricating the membrane was a solid-like solution made of cellulose acetate (CA), poly-(vinyl alcohol), and silica nanoparticles. The fabrications of 3D-printed porous membranes are shown in Fig. 7. It was observed that after 50 cycles, a separation efficacy that was driven by gravity was found to be approximately 99.0%, with WCA in the air of about 18.14 ± 2.61° and OCA underwater of about 159.14 ± 0.59°, thus showing superhydrophilic and superoleophobic characteristics for OWS.

Fig. 7
figure 7

Preparation of 3D-printed composite membrane [85]

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In another study, Lee et al. [86] used FDM 3D printing with PLA filament to fabricate a mesh structure. They coated it with silica nanoparticles through a dip coating technique to create a superhydrophobic surface. The line and grid pattern were 3D printed, as shown in Fig. 8. As shown in Fig. 8, a and c show uncoated bare samples, and it can be noticed that water droplets wet the surface. As shown in Fig. 8 b and d, dip-coated samples were water droplets rolled on the surface, showing superhydrophobic characteristics that can be used for OWS applications.

Fig. 8
figure 8

Optical microscopic image of the water droplet on a surface having 1.0 mm of step size a–b uncoated samples and c–d dip-coated samples [86]

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Yang et al. [87] fabricated eggbeater heads with superhydrophobic micro-scale artificial hairs inspired by salvinia molesta leaf. The structure was aided with the 3DP technique for adjustable hydrophobicity and mechanical endurance of the microstructure, which has exhibited 99.9% OWS. Similarly, in another study, 3D-printed polysulfone (PSU) membranes were covered with candle soot, and it was observed that the structure exhibited separation efficacy greater than 99% for a mixture of hexane/water [88]. Xing et al. [89] reported the fabrication of 3D-printed superhydrophobic PLA packing for OWS. The superhydrophobicity of the sample was achieved through solvent etching and nanoparticle decoration. The 3D-printed PLA model was designed as a hollow cylinder, as shown in Fig. 9. It was noted that after acetone etching, the packing surface turned opaque, indicating a rough structure.

Fig. 9
figure 9

Optical image of a and c additively manufactured PLA packing and b superhydrophobic PLA packing [89]

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The surface morphology of the packing is shown in Fig. 10. Figure 10 a and c illustrates a highly uneven surface with a denser micro-scale spherical rose petal structure after etching. Figure 10 b and d shows that micro/nano-hierarchical surface structures were observed after nanoparticle decorations. It was noted that PLA packing after the coating has a WCA of 150° and a water adhesion force of 22 μN. The maximum separation efficacy of 95% was obtained by exhibiting a high flux rate of 75 kLm−2 h−1. Table 4 illustrates the summary of additively manufactured membranes for OWS.

Fig. 10
figure 10

Surface of PLA packing. by scanning electron microscopy a–b etched PLA packing and PS NP decorated PLA packing, respectively, and c–d respective magnified images [89]

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Table 4 Summary of additively manufactured membrane for OWS
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7 Conclusion

The fundamental theories, recent advancements in materials, and design strategies for stratified and emulsified OWM are summarized. In each segment, a particular focus is dedicated to comparing separation efficiency and membrane flux rates in real-world industrial applications of oil–water separation. The advantages of additive manufacturing for the mesh design are summarized and compared with other metallic mesh for the OWS field. In comparison to other techniques like nano-imprinting, the application of a 3D-printed coating does not affect the size of the membrane surface nor decrease the number of pores on the surface. Moreover, 3D printing offers advantages such as speed, simplicity, cost-effectiveness, and excellent performance with micro-scale materials used in treating oily water. Although 3D printing has already appeared in many industries, integrating 3D printing with other fabrication processes is anticipated to enhance OWS’s efficacy, transform it into an environmentally friendly product, and reduce its overall cost. This review also draws the following conclusions:

  1. 1)

    Superhydrophobic surfaces require nano- or micro-scale structures with low surface energy. In recent reports, 3D printing has been utilized to fabricate both micro-scale structures on the surface and nano-scale structures with low surface energy.

  2. 2)

    Among the most utilized surface modifications, silanes and chemical compounds like (thiols, stearic, lauric, and oleic acids) with extended functional groups are widely employed for surface modifications to enhance the performance of SHSO surface. However, the fluorine atoms in the functional groups may cause ecological problems.

  3. 3)

    The application of external forces and exposure to harsh conditions, including hot water, acidic solutions, brine, and alkaline solutions, can cause damage to the micro–nano-surface structures responsible for membrane superhydrophobicity. Consequently, guidelines are urgently needed to assess the long-term stability and durability of (SHSO) membranes under severe operating conditions.

  4. 4)

    The 3D-printed membrane exhibited notable mechanical stability, reusability, and high efficacy, making it suitable for a wide range of OWS applications.

The review findings indicate the potential customization of coatings on additive-manufactured membranes to improve separation efficacy and various mechanical properties. Additionally, a diverse selection of biodegradable materials suitable for 3D printing is available, exhibiting specific improvements in properties. Furthermore, applying polymeric composite coatings on printed membranes has exhibited superior efficacy in separation processes. This review provides valuable insights for researchers in this field. It guides additive manufacturing industries, enabling them to understand the benefits of coatings on 3D-printed membranes and integrate the aforementioned findings into their work.

8 Future perspective

Possible improvements in wettability through modifications in the membrane for OWS open various opportunities for multiple applications, such as denser fluid separation and separation of fat cells in the blood. Further, there are still a few challenges in scientific and industrial communities, pointing to future research as follows:

  1. 1)

    The temperature, acidity, and alkalinity of actual oily effluent vary with each oil field and refinery. As a result, the resistance and stability of specific wetting materials should also be considered. The materials with cognitive responses are more suited for OWS in adverse environments.

  2. 2)

    River effluents or oily waste river water usually contain viscous oil and sand. The oil with high viscosity adheres to the surface of the 3D-printed mesh, and it is difficult to remove and thus reduces the efficacy after repeated use. Moreover, oily sand particles block the mesh, reducing separation efficacy.

  3. 3)

    The design and control of pore size from nanometres to microns are not accurate enough. Hence, further investigation on surface structure and OWS wettability must be studied.

  4. 4)

    The single super-wettability of separating material has certain limits in the application for separating stratified and emulsified OWM, whereas switchable wettability displays outstanding separation feasibility. Further, newer advancements in methods, theories, and the development of unique wettability separating materials are currently undergoing a lot of research.

In short, a 3D-printed material allows low-cost fabrication that integrates separation and purification functions and can be recycled. Despite numerous barriers and drawbacks, the efforts in this field will enable enormous development and innovation in 3D-printed materials for OWS.