Fabrication of Durable Superhydrophobic Surfaces with a Mesh Structure and Drag Reduction by Chemical Etching Technology

Abstract

Superhydrophobic surfaces are critical in the marine industry because ships and underwater vehicles are constantly exposed to hydrodynamic friction and biofouling during operation, which can negatively affect their efficiency and increase operating costs. To address these challenges, this study proposes a straightforward method for fabricating stable superhydrophobic surfaces. By modifying nano-copper oxide on a microstructure substrate, a coating exhibiting exceptional hydrophobicity, designated as 100-SHB, was successfully developed. The 100-SHB has a water contact angle of about 163.0° and a sliding angle of about 2.0°, which is highly repulsive to water droplet impact. Furthermore, 100-SHB maintained its superhydrophobic properties under rigorous testing, including water puncture resistance, sandpaper abrasion, and ultrasonic damage tests. The incorporation of a lithography-based network structure further enhanced the mechanical stability of the surface, highlighting its robustness. In ship model experiments, the surface demonstrated a remarkable drag reduction rate of 64.2%. This environmentally friendly, simple, and scalable fabrication method represents a significant advancement toward practical implementation in the marine industry and holds promise for expanding applications in non-wetting-related fields.

1. Introduction

Nowadays, the development of the maritime industry is rapid and drives the growth of the national economy, but the resistance of ships and underwater vehicles when moving will further affect the overall performance of ships and underwater vehicles, and even the frictional resistance on the surface of some ships can account for more than 70% of their total resistance [1,2]. A surface is referred to as a superhydrophobic surface when the angle of contact (CA) is greater than 150° and the slope of the sliding angle (SA) is lower than 10°. This surface can effectively reduce flow resistance, and its drag reduction effect is dependent on the fluid state, surface energy size, surface microstructure arrangement, and surface stability [3,4]. Among them, the surface microstructure [5] is the main influencing factor, because a superhydrophobic surface can bind the air film layer in water [6], transform part of the solid–liquid contact interface [7] into a gas–liquid contact interface [8], and generate velocity slip on the gas–liquid interface [9] to reduce the flow resistance, and there is a certain distance between the flow of liquid on the superhydrophobic surface [10] and the solid surface, thereby reducing the frictional resistance caused by direct contact.

Inspired by the surface properties of natural superhydrophobic organisms such as the lotus leaf [11] and water strider leg [12], micro-nano structures can be constructed on metal surfaces to achieve excellent superhydrophobicity, and there are various methods for constructing superhydrophobic surfaces on metal surfaces [13], including chemical etching [14], laser etching [15], physical vapor deposition (PVD) [16], chemical vapor deposition (CVD) [17], sol-gel [18], spraying [19], the template method [20], and so on. For example, Zhu et al. [21] studied a superhydrophobic surface constructed on a Cu substrate by molding, oxidation, and fluorination modification, and although the fabrication process was relatively simple, the structure of his sample meant that the super-biphobic surface prepared with this expensive fluorine-containing toxic reagent could not meet the high efficiency standards required by modern large-scale industrial production. Although laser etching has high precision, high stability, and good flexibility, it cannot achieve the production of large quantities, so it is mostly used in the consumer electronics industry, such as silver paste etching and ITO film etching in the touch screen industry. In addition to the fact that chemical vapor deposition (CVD) cannot be produced on a large scale, chemical vapor deposition (CVD) also needs to be carried out at high temperatures, which some substrates cannot withstand, and the production process may produce toxic gases, which is dangerous. The sol-gel method has high requirements for the compatibility of raw materials and the production process is complex and time-consuming, so it is not suitable for large-scale production. The superhydrophobic surfaces prepared in this study are not only environmentally friendly, but also simple and can be mass-produced.

Superhydrophobic surfaces also have good antifouling [22], self-cleaning [23], anti-corrosion [24], and anti-icing [25] properties, so they have a wide range of applications in drag reduction [26], oil-water separation [27], and biomedicine [28]. In practical applications, superhydrophobic surfaces have been applied to the surface drag reduction of underwater vehicles [29], surface modification of micropores in petroleum reservoirs, and water injection resistance reduction technologies. Wang [30] et al. found that by preparing superhydrophobic coatings on the surface of model ships, the drag reduction rate of superhydrophobic surfaces can reach 28.7% compared with that of ordinary surfaces, which enabled model ships to realize drag reduction. However, achieving excellent mechanical properties and long-lasting stability of superhydrophobic surfaces are still a big challenge in terms of the current development. Therefore, it is imperative to create some low-cost, easy-to-use, effective, and eco-friendly techniques for creating superhydrophobic surfaces.

In this study, a superhydrophobic surface with exceptional liquid repellency, wear resistance, water penetration resistance, and sonic shear damage resistance was fabricated using simple, cost-effective, and environmentally friendly methods. Stearic acid was used as the hydrophobic agent and a combination of chemical etching and hydrothermal techniques was employed to construct the superhydrophobic surface on a copper substrate, simultaneously enhancing both the superhydrophobicity and durability of the substrate. The results indicated showed that after 30 min of hydrothermal treatment and 5 h of stearic acid modification, a plate-like micron structure was formed on the substrate surface, capable of trapping a significant amount of air. At this stage, the 100-SHB demonstrated optimal hydrophobicity, with a water contact angle of 163°. Furthermore, after 10 cycles of abrasion testing with sandpaper, the contact angle remained at 151°, confirming the excellent impact resistance and wear resistance of 100-SHB.

2. Experimental

2.1. Materials

The copper sheet (25 mm × 25 mm), stainless steel mesh (100 mesh), and drainage sink (160 mm × 1000 mm) were purchased from local material markets. Model boats were sourced from a local toy store. Shanghai Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) provided the nitric acid, while Chengdu Boret Chemical Technology Co., Ltd. (Chengdu, China) provided the ammonium persulfate, sodium hydroxide, stearic acid, and anhydrous ethanol. The supplier of photoresist and developer was Suzhou Futuo Scientific Instrument Co., Ltd. (Suzhou, China). All of the analytical-grade compounds used in the studies were used exactly as supplied, requiring no additional purification.

2.2. Fabrication of Superhydrophobic Surface

The copper sheet was ultrasonically cleaned in anhydrous ethanol for 10 min, then air-dried naturally. A 100-mesh stainless steel pattern was transferred onto the copper surface using a photolithography machine. The sample was then etched in nitric acid for 25 s. To remove any residual acidic substances and excess photoresist, the sample was ultrasonically cleaned in anhydrous ethanol. Next, the above sample was heated for 30 min at 90 °C in a mixed solution of sodium hydroxide and ammonium persulfate at different concentrations, as shown in Figure 1a. Afterward, the sample was rinsed with deionized water and dried. Finally, the sample was treated with an ethanol solution of stearic acid at a volume fraction of 0.23% for 5 h to achieve a superhydrophobic surface. For convenience, the samples prepared using this method are referred to as 100-SHB.

2.3. Characterization

The CA between the samples and deionized water was measured by a CA meter (JC 2000 C1, Zhongchen Digtal Technology Apparatus Co., Ltd., Shanghai, China) and the liquid was added by drops with a micro sampler. To determine the average value, the CA was measured at five distinct places, and the volume of each drop of liquid was about 8 μL. The materials’ surface microscopic morphology was examined using a cold field emission scanning electron microscope (FESEM, JSM-6610 LV, JEOL Electronics Co., Ltd., Tokyo, Japan). FTIR Nicolet 8700 Fourier Transform Infrared Spectroscopy was used to examine the chemical composition of the sample surface. The samples were ultrasonically damaged and their durability was determined by an ultrasonic machine (40 kHz, 120 w). For the above tests, average data were obtained after repeating the experiment with three parallel samples.

3. Results and Discussion

3.1. Morphology and Composition

The copper surface was patterned with a network of concave pit structures using photolithography and a 100-mesh stainless steel mesh. The pit structures, resembling “field roads”, had an approximate length and width (l) of 250 μm, with a spacing (r) of 50 μm, as shown in Figure 1b. The pits had a height (h) of about 30 μm, with the concave regions resembling “fields” and the raised regions resembling “field embankments.” The entire pit structure was covered with nanostructures, as depicted in Figure 1c,d. These nanostructures were created by immersing the copper sheet in a mixed solution of sodium hydroxide and ammonium persulfate, followed by a hydrothermal reaction, leading to a preliminary nanorough surface. The CA measurements clearly show that the prepared 100-SHB exhibits excellent hydrophobicity, with a WCA of 163°. Generally, the key factors influencing 100-SHB properties are the high surface roughness and the inherent low surface energy of the material. These characteristics are achieved by chemically etching the copper surface to form plate-like CuO nanostructures and using stearic acid to reduce surface energy. The plate-like CuO nanostructures not only enhance roughness but also improve mechanical robustness. The long-chain fatty acid groups (-CH₂-)nCOOH in stearic acid interact with oxygen atoms in CuO, repelling water molecules, thereby reducing surface energy and enhancing the hydrophobicity of the entire system, resulting in a stable superhydrophobic surface on the 100-SHB.

After the hydrothermal reaction on the copper substrate, the 100-SHB surface exhibits a uniform morphology, and high-magnification SEM images clearly show an even distribution of cross-aligned plate-like nanostructures which impart excellent hydrophobicity to the surface. The 100-SHB without stearic acid modification is referred to as W-SHB. In the droplet impact test, the hydrophilicity of a clean copper surface is shown in Figure 2a, where the droplet immediately wets the copper surface upon gravity-induced fall. Similarly, the W-SHB surface shows strong water adhesion after the droplet falls, as shown in Figure 2b. However, on the 100-SHB surface, the droplet exhibits noticeable rebound under multiple impacts from gravity and retains its impermeability (Figure 2c). The experimental results demonstrate that the 100-SHB surface exhibits low water adhesion, with droplets rolling off the surface after multiple rebounds without any signs of wetting.

CA tests on both the W-SHB before modification and the 100-SHB after modification reveal that the W-SHB has a CA of 119°, which does not qualify as superhydrophobic, whereas the 100-SHB exhibits a superhydrophobic state with a CA of 163°, as shown in Figure 3a,b. Figure 3c presents the FTIR spectra of CuO nanoparticles before and after stearic acid treatment, measured in the 400 to 4000 cm⁻¹ wavenumber range. The IR peaks at 504.70 cm⁻¹ and 607.04 cm⁻¹ correspond to the vibrational modes of CuO [31]. The symmetric stretching vibrations of C=O and C-H are responsible for the two infrared peaks seen at 1630 cm⁻¹ and 2850 cm⁻¹, respectively. Compared to the unmodified CuO, the modified CuO exhibits two extra peaks at 2840.06 cm⁻¹ and 2916.06 cm⁻¹, which are attributed to the -CH₂- groups’ stretching vibrations, indicating successful modification of CuO by stearic acid. Additionally, a broad peak at 3441.81 cm⁻¹ is related to the symmetric stretching vibration of O-H, which is attributed to the absorption of moisture from the atmosphere [32].

3.2. Liquid Repellency

On the 100-SHB surface, the contact between water and the solid surface is primarily mediated by a thin layer of air, referred to as an “air cushion” or “air film” [33]. This structure allows the water droplet to mostly contact the air rather than directly contacting the solid surface, thereby creating gas cavities that reduce drag underwater [34]. Furthermore, when the 100-SHB surface interacts with water, the surface tension increases the buoyancy of the water droplet on the 100-SHB surface (Figure 4a–c). It is clearly observed that even when the modified sample has a density greater than that of the liquid, it can still float on water, whereas the unmodified hydrophilic sample and the hydrophilic copper sheet directly sink into the water (Figure 4d).

In addition, due to the water-repellent property of the 100-SHB surface and the influence of surface tension, the 100-SHB surface can support weight underwater. A twisted liquid–air interface develops at the 100-SHB blade angle as the weight increases. A theoretical model of the water–air–solid interface contact (Figure 4e–g) further clarifies the mechanism by which the buoyancy of the 100-SHB surface is enhanced. At the air–water interface, the tension between the two surfaces usually points into the liquid and is perpendicular to the interface, or along the tangential direction. After modification, the 100-SHB wedge angle exhibits excellent water-repellent properties, increasing the tension surface area, and the direction of the interfacial tension points upward. However, because the unmodified sample is hydrophilic and has a WCA smaller than 90°, when the unmodified 100-SHB is immersed in water, the liquid tension surface at the 100-SHB wedge angle is depressed and the surface pressure points downwards.

It is noteworthy that, aside from the top portion of the 100-SHB that does not contact the water surface, the gravitational drainage force and the vertical surface tension in other areas are the main reasons for the increased buoyancy of the 100-SHB surface. Therefore, surface tension plays a critical role in the interaction between the 100-SHB surface and water [35]. Since the contact area between the water droplet and the surface is minimal, surface tension keeps the droplet at a high contact angle, thus reducing adhesion to the surface. A two-dimensional model analysis of the interaction between the 100-SHB surface and water (Figure 4e) can be used to describe the surface tension on the SHB surface as follows:

$$\gamma ={\displaystyle \frac{2h}{R}}\rho gh$$

(1)

in this model, $R$ refers to the radius of the tension surface and $h$ is the vertical distance from the apex of the tension surface to the SHB wedge angle. Additionally, when an external force, aligned with the direction of gravity, is applied to the 100-SHB surface, it is observed that the tension surface increases in size (Figure 4f). However, when the applied external force exceeds the buoyant force, the supporting force generated by surface tension no longer exists, causing the 100-SHB surface to eventually submerge in the water (Figure 4g).

This observation demonstrates the delicate balance between the external force, buoyancy, and surface tension. When the external force is small enough, the buoyant force, sustained by the surface tension, supports the 100-SHB surface above the water. As the external force increases and surpasses the buoyant force, the 100-SHB surface no longer maintains its position, leading to submersion. This dynamic interaction between surface tension and external forces further emphasizes the importance of surface characteristics, such as hydrophobicity and surface roughness, in maintaining buoyancy on superhydrophobic surfaces.

The results highlight that surface tension plays a pivotal role in regulating the interaction between the 100-SHB surface and water. By manipulating the tension surface and applying controlled external forces, the behavior of the 100-SHB surface can be finely tuned to either float or sink, depending on the specific conditions.

3.3. Friction Resistance

Because superhydrophobic surfaces are vulnerable to structural damage and chemical property loss under mechanical wear, their mechanical resilience is one of the most important metrics for assessing coating quality. By varying the reaction solution’s concentration throughout the hydrothermal process, the stability of the micro-nano structures on the samples’ surfaces can be affected. For instance, the sample T-SHB was obtained by reacting with a solution of 0.03 mol/L persulfate of ammonium and 0.5 mol/L sodium hydroxide. By doubling the concentrations of both reactants, the sample F-SHB was prepared, and by doubling the concentrations again, the sample S-SHB was synthesized.

A sandpaper abrasion experiment was used to evaluate the superhydrophobic surfaces’ mechanical durability. The results showed that the T-SHB, F-SHB, and S-SHB samples lost their superhydrophobic properties after the fourth, seventh, and eighth cycles, respectively. This degradation was attributed to the damage of the nanostructures on the surface due to repeated wear, as shown in Figure 5b.

To further improve the damage resistance of the S-SHB surface, a network of concave pit structures was fabricated on the copper surface using photolithography with a 100-mesh stainless steel mesh. The S-SHB sample served as the base for constructing the 100-SHB, as illustrated in Figure 5a. In the sandpaper abrasion test, each friction cycle was defined by a 10 cm friction distance, with the sample placed on 1200-grit sandpaper under a load of 100 g. After each cycle, the changes in CA and SA were measured. After four cycles, T-SHB exhibited a CA of less than 150° and an SA greater than 10°, while the 100-SHB maintained a CA above 150° and an SA of less than 10° after ten cycles, as shown in Figure 5c.

This remarkable performance can be attributed to the raised “field embankment” structures that protected the nanostructures on the concave regions during the abrasion, preserving the superhydrophobicity of the surface. Furthermore, these results demonstrate that the SHB surface can maintain excellent superhydrophobicity and exhibit strong mechanical durability under mechanical wear, offering a high level of resistance to damage.

3.4. Resistance to Water Jet Penetration

The samples were placed under a faucet for water jet impact testing, with the water pressure error disregarded. In each water impact test, the faucet was set to a fixed angle to ensure a consistent water flow velocity, as depicted in the theoretical model in Figure 6a. A 2 min water flow burst was used as one cycle to conduct water jet impact experiments on T-SHB, F-SHB, and 100-SHB at the same flow velocity. At the end of each waterjet impact test, the sample is placed in a room to dry naturally and the contact angle change is tested. The results indicate that the 100-SHB surface maintained excellent superhydrophobicity after 15 cycles, while T-SHB lost its superhydrophobicity after the 10th cycle. This demonstrates that increasing the concentration of the reaction reagents enhances the water jet penetration resistance of 100-SHB, as shown in Figure 6c.

Moreover, during the impact process, the hydrophobic micro-nano structures on the surface caused the water flow to form a thin water film on the sample’s surface, reducing the direct contact between the water flow and the solid sample, as shown in Figure 6b. This effectively weakened the impact force of the water jet on the 100-SHB surface, further protecting it from damage.

The enhanced durability of 100-SHB against water jet impacts can be attributed to the combined effect of the surface’s high hydrophobicity and the structural integrity of the micro-nano features, which work synergistically to repel water and reduce the forces acting on the surface. These findings underscore the importance of optimizing both surface chemistry and structural features to improve the water resistance and longevity of superhydrophobic surfaces.

3.5. Resistance to Sonic Shear Destructive Testing

To further evaluate the shear damage resistance of the samples, an ultrasonic lateral shear damage test was conducted using an ultrasonic cleaning machine. The samples were submerged completely in water in the ultrasonic cleaner (40 kHz, 120 W), as seen in Figure 7a, and the mechanical waves were acting on the sample’s surface through the water medium. A 2 min cycle was used, and the CA and SA of the surface were measured at different ultrasonic treatment durations to assess the shear resistance.

The results revealed that, although the surface micro-nano structures were damaged after ultrasonic treatment, with CA dropping below 90°, the superhydrophobicity could be restored after re-immersing the samples in stearic acid. As shown in Figure 7b,c, the nanostructure was a standard sheet structure before ultrasonication, and although the sheet structure was damaged after ultrasonication, the surface of the microstructure still existed, so the surface CA of the sample could still reach 160° after re-immersion in the stearic acid solution. This suggests that the ultrasonic treatment did not completely destroy the surface micro-nano structures, allowing the restoration of superhydrophobicity after modification. However, after repeated ultrasonic cycles, when the micro-nano structures were completely destroyed, the superhydrophobicity could not be recovered, even after re-immersion in stearic acid.

Additionally, after 11 ultrasonic cycles, the 100-SHB maintained a CA above 150° (Figure 7f), demonstrating superior stability compared to T-SHS and F-SHB (Figure 7d,e). However, due to the gradual reduction of the nano-copper oxide on the surface of the sample and the exposure of part of the copper substrate due to multiple ultrasonic experimental cycles, the contact angle was measured to be less than 150° after 14 cycles, and finally the superhydrophobicity was completely lost. This further supports the idea that increasing the concentration of the reaction solution enhances the stability and durability of the surface structure.

The experimental results indicate that the 100-SHB sample retained a high contact angle and exhibited excellent shear resistance under ultrasonic damage, making it the preferred choice for subsequent experiments. These findings highlight the significant impact of reaction reagent concentration on improving the durability of superhydrophobic surfaces, particularly in their ability to withstand ultrasonic shear forces.

3.6. Long-Term Stability Testing

To investigate the long-term stability of 100-SHB under complex environmental conditions, we tested the corrosion resistance of microstructured superhydrophobic surfaces. First, the 100-SHB surface was immersed in a 3.5 wt% NaCl solution, measured every 10 h, then removed and blow-dried, and the contact angle change was measured, with the sample finally losing its superhydrophobicity after 50 h, as shown in Figure 8a. Secondly, the samples were also tested for UV resistance, irradiated under a UV (200 w, 365 nm) lamp, and tested for CA changes every 1 h. Experiments showed that the CA of 100-SHB also completely lost its hydrophobicity at less than 150° after 7 h of UV irradiation, as shown in Figure 8b. Finally, the samples were also subjected to high temperatures: the samples were baked at 200 °C and the CA changes were tested every 1 h. Experiments showed that the CA of 100-SHB was less than 150° after 5 h, completely losing its superhydrophobicity, as shown in Figure 8c. In addition, the sample loses its hydrophobicity when baked at high temperatures, but can be restored after stearic acid immersion again. This is mainly due to the fact that the original hydrophobic layer on the surface was decomposed during the test, and the presence of nanometer CuO on the surface of the sample can restore the superhydrophobic properties of the sample after re-immersion. However, in the final test, the surface nanometer CuO continued to fall off due to the repeated soaking experiments of the sample and finally lost its superhydrophobic properties completely after the fifth test cycle.

3.7. Drag Reduction

The drag reduction performance of the surface was further evaluated through a sailing experiment conducted on a model boat equipped with a fixed-speed gear system. The model boat, which was placed in a 1 m long fixed water channel, was connected to a shaft linked to a gear mechanism inside the gear box. When the gear box is pulled via a tethered line, the model boat is propelled forward by the mechanical energy from the gear system. The gear system is designed with a recoil gear, which stores elastic potential energy during the forward motion. Upon releasing the line, the recoil gear reverses its rotation via the shaft, releasing the stored potential energy, which is then converted into kinetic energy. As seen in Figure 9a, this energy powers the propeller, which moves the boat ahead.

By changing the solid–liquid contact to a solid–gas contact, the drag on the 100-SHB surface is reduced. This conversion reduces friction and promotes boundary slip at the interface due to the presence of gas bubbles. The reduction in friction between the fluid and the surface is facilitated by the air layer that forms between the surface and the water, reducing drag.

Additionally, as illustrated in Figure 9b, the average speed of the model boat during sailing was used to quantitatively assess the drag reduction performance. The relationship can be expressed as

$$D={\displaystyle \frac{V_{100-\mathrm{SHB}\mathrm{boat}}-V_{\mathrm{Copper}\mathrm{boat}}}{V_{\mathrm{Copper}\mathrm{boat}}}}\times 100\%$$

(2)

These include the V_{100-SHB boat}, which is the normal speed of a model boat, and the V_{Copper boat}, which is the average speed of a model ship carrying a copper sheet. As demonstrated in Figure 9b, the model boat with the 100-SHB surface has a greater average velocity than the boat with the copper surface, suggesting that the 100-SHB surface performs exceptionally well in terms of drag reduction.

Further continuous sailing tests were conducted, and it was observed that the drag reduction rate remained relatively stable with no significant change as the number of voyages increased. This suggests that the superhydrophobic surface maintains its effectiveness over time. The final results indicate that the drag reduction rate is largely maintained between 60% and 70%, with a maximum value of 64.2%, as shown in Figure 9c. This further confirms that the 100-SHB surface exhibits outstanding drag reduction effects in water, demonstrating its potential for practical applications in reducing drag in fluid environments.

The stability of the drag reduction effect suggests that the surface modification is durable and capable of maintaining its performance over extended periods. This long-lasting performance makes the 100-SHB-modified surface a promising solution for applications requiring sustained drag reduction, such as in marine engineering and aquatic transportation.

4. Conclusions

A simple and environmentally friendly technique was used to create patterned network structures that enhanced the stability of micro- and nano-structures while maintaining superhydrophobicity. FT-IR analysis was performed to characterize the surface chemistry of CuO, confirming the successful modification of CuO by stearic acid. In addition, the structural changes of the modified CuO were analyzed and the formation process and potential mechanism of superhydrophobicity in functionalized surfaces were elucidated. In long-term stability testing, the 100-SHB surface exhibited excellent superhydrophobic properties and strong resistance to mechanical wear. In addition, in the drag reduction experiment of the ship model, the surface of 100-SHB showed significant drag reduction characteristics, with a drag reduction rate of up to 64.2%. This low-cost, scalable, and fluorine-free surface not only has high mechanical stability, but is also very effective at reducing drag. These characteristics make it ideal for applications such as boats, autonomous underwater vehicles, and submarines.