Fabrication and characterization of durable superhydrophobic and superoleophobic surfaces on stainless steel mesh substrates

Introduction. This study explores the fabrication of durable superhydrophobic and superoleophobic surfaces on stainless steel mesh, inspired by natural structures like lotus leaves. Achieving superoleophobicity, especially with enhanced durability, is challenging due to the lower surface tension of oils. Methodology. This novel technique involves using Perfluorooctyltriethoxysilane (PFOTES) and silicon dioxide nanoparticles to create re-entrant structures, low surface energy, and high roughness. This cost-effective approach ensures simplicity without requiring expensive equipment. Results. The resulting surfaces exhibit remarkable superoleophobic properties, with hexadecane and soybean oil contact angles reaching 170° and 163.8°, respectively. Scanning electron microscopy confirms successful fabrication, and wear abrasion tests demonstrate mechanical durability, with contact angles remaining high even after cyclic loading and sandpaper abrasion. Conclusion. This study presents a pioneering, cost-effective method for fabricating durable superoleophobic surfaces on stainless steel mesh. These surfaces hold promise for applications in self-cleaning coatings and oil-repellent materials.


Introduction
Superhydrophobic surfaces, inspired by natural structures like lotus leaves and insect wings, [1,2]) have been extensively engineered.These surfaces exhibit water droplets with contact angles exceeding 150°and low contact angle hysteresis ranging from 2°to 10° [3,4].In contrast, superoleophobic surfaces, repelling oils, pose greater challenges due to the lower surface tension of oils [5][6][7].This property of oils renders the construction of oilrepellent surfaces considerably more challenging than achieving superhydrophobicity.Achieving superoleophobicity requires precise control over adhesive and cohesive forces, along with considerations for mechanical durability [4].
Contact angle equilibrium, introduced by T Young in 1805, is crucial, denoting the angle formed between the liquid's surface and the contact surface.Young's equation mathematically describes the solid surface's surface free energy [7]: The wetting state on a rough solid surface can be categorized as either the Cassie-Baxter state or the Wenzel state (figures 1(d) and (e)) [6,8] (figures 1(d) and (e)).In the Cassie-Baxter state, air trapped under the water droplet in the grooves forms a solid-air fraction, reducing liquid droplet attachment (figure 1).Conversely, the Wenzel state occurs when the liquid infiltrates the surface's indentations and protrusions, enhancing wettability due to an increased contact area.Surface roughness plays a pivotal role in determining the contact angle hysteresis (CAH) of superhydrophobic surfaces [9], characterizing the ease of droplet displacement.
CAH is crucial in assessing droplet displacement ease on a solid surface, with the static contact angle significantly influencing surface wettability and adhesiveness The surface tension of testing materials also influences the achieved contact angle, making higher angles more challenging with lower surface tension oils.
However, these techniques are often limited by their complexity, the need for expensive equipment, or protracted procedures.The strength of this work lies in the durability of fabricated surfaces on stainless steel mesh substrates.By employing a combination of metal etching, dip coating, and spray coating techniques, hierarchical structures were successfully developed, enhancing surface roughness, and promoting desirable properties.The use of PFOTES, PFDTES and silicon dioxide nanoparticles in the fabrication process contributed to the achievement of superhydrophobic and superoleophobic characteristics.The detailed experimental section provides a clear understanding of the materials, synthesis methods, and testing procedures, contributing to the reproducibility of the results.The inclusion of specific contact angle measurements, SEM analysis, and wear abrasion testing demonstrates a rigorous characterization of the fabricated surfaces, reinforcing the reliability of the findings..00 were all bought from bought from Sigma Aldrich as well.Hexadecane and soybean oils from Sigma Aldrich as well were selected for evaluating the experiments due to their low surface tension values (17.90 and 32.9 mN m −1 , respectively).The equipment used for testing was SEM (ZEISS MERLIN Compact SEM), Goniometer (Drop Meter Element A-60, 0.5 mm flat syringe tip) and Mirka waterproof sandpaper with grit range from P600 to P10,000 sized 230 × 280mm.

Experimental section
Before chemical modification, all substrates underwent cleaning with isopropyl alcohol (IPA) and deionized water, followed by extraction with toluene to remove any potential impurities.This experimental setup involved various weights of nano-silica and fluoroalkylsilane to achieve superhydrophobicity and superoleophobicity.

Synthesis of superhydrophobic surface
The initial series of experiments focused on fabricating superhydrophobic surfaces.To accomplish this, 1g of PFOTES was mixed with 100 ml of absolute ethanol and stirred magnetically for 20 min at room temperature (25 °C) to enable hydrolysis of the fluoroalkylsilane.nano-silica was then added to the stirring solution and left for 1 h to achieve a homogeneous suspension.The suspension was subsequently sprayed onto the substrates using a spray gun at 20 psi with a pressure of 0.6 MPa and a distance of approximately 15-20 cm (figure 2).The treated surfaces demonstrated superhydrophobicity once they were dried in an oven at 80 °C for 2 h.The reaction of PFOTES with silicon dioxide nanoparticles on a stainless-steel mesh substrate involves the hydrolysis and condensation of PFOTES, forming a silane-based coating on the substrate.Here's a simplified representation of the reaction: Silane Based Coating Byproducts 2

Hydrolysis and Condensation
During the reaction, the ethoxysilane groups (−OCH2CH3) in PFOTES undergo hydrolysis, producing silanol (−OH) groups.Subsequently, condensation reactions occur between these silanol groups and the surface hydroxyl groups of silicon dioxide nanoparticles.The condensation reactions lead to the formation of siloxane (-Si-O -Si) bonds, creating a cross-linked silane-based coating on the stainless-steel mesh substrate.

Synthesis of superoleophobic surface
To achieve superoleophobicity, a similar methodology was employed due to the simplicity of the experimental technique.In this case, a hierarchical or re-entrant structure was introduced to attain the Cassie-Baxter state.The re-entrant structure was formed by conducting metal-assisted chemical Etching (MacEtch) before applying the PFOTES and nano-silica dispersion to the substrate.MacEtch process involved preparing an etching solution I containing HCl (37 wt%), H 2 O 2 (30 wt%), and H 3 PO 4 (85 wt%) for the acidic etching of the steel substrate.Additionally, a solution II was prepared by mixing deionized water and FeCl 3 .6H 2 O under magnetic stirring until a homogeneous solution was obtained.The mixtures I and II were then combined in a beaker and left to form a suspension for 20 min in an airtight environment.Steel mesh substrates were immersed in this suspension for varying durations of 20 min, 40 min, and 1 h to achieve three distinct levels of hierarchical structures on the steel, which were subsequently dried at 80 °C for 1 h.The cylindrical structures formed by MacEtch served as the basis for the mechanical durability of the fabricated surfaces (figure 3).
Continuing from the previous experimental technique, another solution III was prepared by mixing TEOS and 1 g of PFDTES in a beaker containing 100 ml of ethanol.The solution was magnetically stirred under sealed conditions for an hour.Finally, the three steel mesh samples with hierarchical structures were immersed in the suspension and dried in an oven at 80 °C for 2 h.The nanostructures formed on these samples were assessed for contact angle and observed under scanning electron microscopy (SEM).The sample immersed in the MacEtch solution for 60 min exhibited the best results in terms of the nanostructure formed on the steel mesh.illustrates the analysis of metal-etched steel immediately after the hierarchical structure formation to observe the contact angle for soybean oil and evaluate the surface roughness.

Comparison of amount of SiO 2 for oil-repellency
To understand the effect of nano-silica in the solution, 1 g, 2 g, and 3 g of nano-silica (15 nm) were added to PFDTES and 99 g of ethanol.The substrates were soaked in the suspension for 30 min before being dried in the oven for dip coating.The surfaces fabricated with solutions A, B, and C exhibited remarkably similar hydrophobic performances, but solution C demon strated the highest contact angle with soybean oil, making it ideal for further testing (table 1).Solution C was used throughout the experiments unless specified.The resulting surface coating provided the desired mechanical durability, as evidenced by the retention of the hierarchical structure and oil-repellent nature even after repetitive mechanical and wear abrasion tests.Table 1 displays the effects of ethanol, an alkylfluorosilane co-polymer, and nano-silica on the contact angles of soybean oil and hexadecane on a coated surface.When 1g of nano-silica were added to PFDTES, the contact angles were 137°a nd 148°for soybean oil and hexadecane, respectively.Concentrations of nano-silica increased to 2g, resulting in contact angles of 155°for soybean oil and 160°for hexadecane.When the concentration of nano-silica was increased to 3g, the greatest contact angles were observed.The angles for soybean oil were 161°and 170°for hexadecane.The improved water and oil repellent characteristics of the coated surface when exposed to various liquids relate to the amount of nano-silica, according to these findings.
Even though hexadecane has a lower viscosity than soybean oil and has a higher surface tension, the higher contact angles that were observed with hexadecane on the coated surface can be attributed to a variety of chemical interactions, intermolecular forces, surface roughness, and the impact of nano-silica.This shows the complexity of contact angle behavior.After the fabrication process the resultant film is both superhydrophobic  and superoleophobic as shown in figure 6. Thickness of the film is 100 μm which shows excellent mechanical durability after wear abrasion testing discussed in the following sections.

Formation of hierarchical structures
When using an MacEtch solution containing HCl, H 2 O 2 , and H 3 PO 4 , along with a suspension of FeCl 3 .6H 2 O, over a steel mesh substrate, several reactions and processes take place that contribute to form hierarchical structures.First, the acidic nature of the etching solution, particularly HCl, promoted the dissolution of the metal surface of the steel mesh substrate.HCl reacted with the metal, causing metal ions to be released into the solution.Secondly, the presence of HCl in the MacEtch solution resulted in the formation of metal chloride compounds.The metal ions released from the steel substrate reacted with HCl, forming metal chloride complexes in solution.Thirdly, the combination of H 3 PO 4 and H 2 O 2 in the MacEtch solution facilitated oxidation reactions on the steel surface.The metal ions released from the steel mesh can undergo oxidation, resulting in the formation of metal oxide compounds.Furthermore, the MacEtch process, aided by the acidic solution and suspension of FeCl 3 .6H 2 O, lead to the removal of material from the steel mesh surface.This removal created irregularities and surface roughness, contributing to the desired re-entrant structure and enhanced surface area.Finally, H 3 PO 4 present in the etching solution also passivated the steel surface.It formed a thin, protective phosphate layer that inhibited further corrosion of the steel substrate, adding to the cause of self-cleaning.
Overall, the combination of an acidic etching solution and FeCl 3 .6H 2 O suspension over a steel mesh substrate resulted in the dissolution of metal, the formation of metal chlorides and oxides, surface roughening, and surface passivation.These processes and reactions were crucial for creating the desired re-entrant structure and enhancing the surface properties of the steel mesh, ultimately leading to improved superoleophobicity.The cylindrically etched structures are few micrometers in height (4 to 6 μm).

Surface analysis via SEM
Scanning electron microscopy SEM (ZEISS MERLIN Compact SEM) was used for topographical analysis.The specifications of SEM for analysis of the experiment are as follows.The scanning electron microscope magnification range was 100X-5000X.Its accelerating voltages were between 5-10kV since it had gold coating surface by metal sputter, and the resolution of the microscope varied from 1 μm to 100 μm.The sample size of the substrate for testing was 100 mm×100 mm, and the samples were mounted at the height of 55 mm.The surface analysis from SEM is presented through figures 4-6.
The metal-etched surface of the substrate was initially examined to evaluate the effect of MacEtch on surface roughness and contact angle.As shown in figure 4, the MacEtch process resulted in the development of a hierarchical structure, which significantly influenced the surface roughness and facilitated the formation of the Cassie-Baxter state.Consequently, the contact angle for water increased to 125°, indicating inherent hydrophobicity.
Following the MacEtch process, the stainless-steel mesh substrate was dip-coated in the PFOTES and nanosilica solution for 60 min, and SEM analysis was performed to assess the contact angle and surface roughness.Figure 5 illustrates the higher surface roughness observed, along with an increased contact angle, resulting in an overall improvement in water repellency.The contact angle rose to 137.5°, indicating enhanced water repellent properties.
To achieve superoleophobicity, the substrate underwent additional processes of dip coating and spray coating to deposit a higher concentration of hydrolyzed fluorides on the surface.This approach aimed to optimize both surface roughness and contact angle.As a result, the contact angle achieved in this methodology increased to 167°(figure 6), indicating a significant advancement towards superoleophobicity.The SEM analysis provided valuable insights into the surface morphology, roughness, and contact angles of the fabricated surfaces.These findings confirmed the successful development of hierarchical structures and the effectiveness of the coating techniques in enhancing the desired surface properties.

Wear abrasion testing
The wear abrasion tests used the ISO 4586 standard to evaluate the resistance of the superoleophobic surfaces to abrasive forces.The samples had been subjected to controlled wear cycles and examined for any signs of degradation, such as changes in contact angle.The test results provided valuable insights into the mechanical robustness and long-term stability of the fabricated surfaces, further validating their suitability for real-world applications.
For evaluating the mechanical durability, a wear abrasion test ISO 4586 was conducted as depicted in (figure 7).The test involved placing weights on top of sandpaper, which was in direct contact with the fabricated surface.Weights ranging from 100g to 500g were used in 50g increments.Sandpaper with varying grit sizes, ranging from P10,000 to P600, was employed to induce mechanical wear and tear on the surface.The samples underwent cyclic loading at the rate of 4 cm s −1 for 30 min, and afterward, the contact angles were measured again.The contact angle measured after the cyclic loading were used to draw the graph in figure 8.The trend depicted in the figure 8 demonstrates the exceptional durability of the fabricated surface under wear and abrasion tests, effectively addressing the durability issues associated with superoleophobic surfaces, as outlined in the problem statement.

Contact angle analysis via goniometer
Goniometer (Drop Meter Element A-60, 0.5 mm flat syringe tip) was used to analyze the contact angle values.For this texting procedure, 7 μl droplet of testing liquids (deionized water and low surface tension oils, soybean and hexadecane) were placed on the substrate using a 0.5 mm flat syringe while mounted on the rotatable square platform.The contact angle was observed at ambient temperature.

Cleaned, dry and untreated surface contact angle
Initially, the contact angles of water and hexadecane were observed on a clean and dry stainless-steel mesh substrate, yielding values of 80.8°and 75.1°, respectively.These measurements served as the baseline for further analysis, aiming to achieve contact angles above 150°(figure 9).

Superhydrophobic surface contact angle
To validate the superhydrophobicity of the treated surface, contact angle measurements were taken before and after conducting wear abrasion tests.The figure 10 presented in the results section demonstrates the mechanical durability of the surface, achieved through dip coating in the PFOTES and nano-silica dispersion for 60 min, on the steel mesh substrate.

Superoleophobic surface contact angle
Similarly, contact angle measurements were performed before and after wear abrasion tests to assess the superoleophobicity of the treated surface.The figure 11 provided in the results section illustrates the mechanical durability of the surface achieved through a combination of dip coating and spray coating in the PFOTES and nano-silica dispersion for 60 min, applied to the steel mesh substrate.The obtained contact angles for hexadecane and soybean oil, prior to wear abrasion testing, were 170°and 163.8°, respectively.After the wear abrasion tests, the contact angles decreased to 165.9°and 161.0°for hexadecane and soybean oil, indicating a significant advancement towards achieving superoleophobicity.

Conclusion
In conclusion, this study aimed to fabricate superhydrophobic and superoleophobic surfaces using a combination of chemical modification and surface coating techniques.The introduction highlighted the importance of such surfaces in various applications, including self-cleaning coatings, anti-fouling materials, and  oil-repellent surfaces.The methodology involved the use of metal etching, dip coating, and spray coating processes to create hierarchical structures and deposit hydrophobic and hydrophobic coatings on stainless steel mesh substrates.
The results obtained from scanning electron microscopy (SEM) analysis demonstrated the successful formation of hierarchical structures on the metal-etched surfaces.The SEM images revealed the presence of reentrant structures, which are known to enhance surface roughness and promote superhydrophobicity and superoleophobicity.The contact angle measurements obtained through the goniometer analysis confirmed the hydrophobic and oleophobic nature of the fabricated surfaces.The contact angles for water and hexadecane on the cleaned and dry stainless-steel mesh were observed to be 80.8°and 75.1°, respectively, providing a baseline for further improvements.
The dip coating and spray coating processes using PFOTES, and nano-silica dispersion led to significant enhancements in surface properties.The contact angle measurements demonstrated a remarkable increase in contact angles, with values reaching 170°and 163.8°for hexadecane and soybean oil, respectively.These findings indicate the successful achievement of superoleophobic surfaces.The wear abrasion tests revealed the mechanical durability of the fabricated surfaces, as the contact angles remained relatively high even after exposure to cyclic loading and sandpaper abrasion.
Overall, the combination of metal etching, dip coating, and spray coating techniques proved to be effective in fabricating durable superhydrophobic and superoleophobic surfaces on stainless steel mesh substrates.The fabricated surfaces exhibited excellent repellence towards water and low surface tension oils, making them promising candidates for applications requiring self-cleaning, anti-fouling, and oil-repellent properties.
Future research can focus on further optimizing the fabrication process, exploring alternative materials, and investigating the long-term durability and performance of these surfaces under real-world conditions.Additionally, additional characterization techniques such as x-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) can provide valuable insights into the chemical composition and bonding mechanisms present on the fabricated surfaces.In conclusion, this study contributes to the advancement of superhydrophobic and superoleophobic surface fabrication techniques, paving the way for their practical implementation in a wide range of industrial and everyday applications.

Figure 2 .
Figure 2. (a) Pictorial representation of the experimental methodology described to prepare superhydrophobic surface.The fluorate polymer contains amounts of fluoric group -CF2 and -CF3, leading to lower surface energy and low surface tension.(b) PFOTES chemical structure.

Figure 3 .
Figure 3. Visually depiction of the hierarchical structure formed on the stainless-steel mesh surface after the MacEtch process, showcasing the key characteristics that support the Cassie-Baxter state and contribute to the surface's superhydrophobic properties (a) MacEtch solution (b) Stainless steel substrate (c) 2D hierarchical structures (d) 3D hierarchical structure.

Figure 4 .
Figure 4. Analysis of MacEtch prepared surface under SEM (a) Resolution value 500X, scale 10 μm (b) Resolution value 1KX scale 10 μm (c) Resolution value 2KX scale 2 μm (d) Resolution value 100X scale 100 μm.This surface was assessed for contact angle with water.The output contact angle was little more than 125°.

Figure 5 .
Figure 5. Analysis of Metal Etched and dip coated Surface under SEM (a) Resolution value 100X scale 100 μm (b) Resolution value 500X scale 10 μm (c) Resolution value 5KX scale 1 μm (d) Resolution value 1KX scale 1 μm.The surface was tested against water and the maximum contact angle observed was 137.5°.The cylindrically etched structures are few micrometers in height (4 to 6 μm).

Figure 6 .
Figure 6.Analysis of Spray coated, and Dip coated substrate after Metal Etched Prepared Surface under SEM (a) Resolution Value 100X scale 100 μm (b) Resolution Value 5KX scale 1 μm.The surface was tested against hexadecane and soybean oils and the maximum repeatable contact angle observed was 167°.Thickness of the film is 100 μm.

Figure 7 .
Figure 7. Schematics of wear abrasion test.The sliding weight includes both horizontal and vertical placement of the substrate.

Figure 8 .
Figure 8. Wear and Abrasion Test for steel mesh with 500g weight P10,000 sandpaper for 10 cm distance.Testing material includes low surface tension materials, hexadecane.The contact angle stays above the 150°line after the testing which displays the higher mechanical durability of surface manufactured.

Figure 9 .
Figure 9. Measurement of Contact angle of (a)Hexadecane (b) water on stainless steel mesh surface.The low surface tension materials used during testing procedure include hexadecane and soybean oil.

Figure 10 .
Figure 10.Measurement of contact angle of water on the fabricated water-repellent surface.Right after drying out the surface water contact angle is (a) 155.4°(b)157.6°.After wear abrasion test (c)154.1°(d)152.3°onstainless steel mesh surface.All of the images represent more than 150°before and after testing process proving durability of fabricated superhydrophobic surface.

Figure 11 .
Figure 11.Measurement of contact angle of hexadecane and soybean oil on the fabricated oil-repellent surface on stainless steel mesh substrate.The oil contact angle for hexadecane (a) before and (b) after wear abrasion testing.For soybean oil contact angle (c)before and (d)after wear abrasion testing.All of the images represent more than 150°before and after testing process proving durability of fabricated superoleophobic surface.Thickness of the film is 100 μm.

Table 1 .
Summary of Contact angle for different amounts of Silicon Dioxide and different Oils used for testing.