Superhydrophobic surfaces created by a one-step solution-immersion process and their drag-reduction effect on water

Abstract

A simple one-step process was developed to fabricate a superhydrophobic surface on copper alloy substrates, which was applied by immersing the sheets in a solution containing sodium hydroxide (NaOH), ammonium persulphate ((NH₄)₂S₂O₈) and fluoroalkyl-silane (FAS-17) molecules. The consequent surface was characterized by contact angle measurements, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The water contact angle (CA) and sliding angle (SA) on the resulting surface were ∼158° and ∼4°, respectively. Here, the stability of the obtained surfaces was measured by a wearing test. The damaged coatings can be quickly repaired by a one-step solution-immersion process. Subsequently, the liquid/solid friction on the distinct wetting surface was studied by using a water spraying system. By comparing the friction on a normal surface and the as-prepared surface at different flow velocities, we found that superhydrophobic coatings exhibited an excellent drag reduction of ∼40% at a low wall shear rate and ∼20% at high wall shear rate. Based on these results, we attribute the superhydrophobic drag-reducing results to the plastron effect, which is characterized by surface wetting and roughness. This study not only presented a simple one-step method to fabricate a superhydrophobic surface on the copper alloy substrates but also showed the drag-reducing effect on the obtained surfaces for engineering applications.

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1. Introduction

In nature, lotus leaves exhibit the unusual properties of superhydrophobicity, displaying a water contact angle (CA) greater than 150° and sliding angle (SA) lower than 10°. It appears that both binary structures and low surface energy materials lead to the repellent character at these surface.¹ Based on current understanding of the key elements of surface energy and roughness on natural surfaces, many methods of fabricating superhydrophobic surfaces (SHS) have been proposed in the literature.^2–5 Artificial superhydrophobic surfaces are generally produced via a two-step process where rough structures are created on different substrates (such as glass, metal and polymer). Then the surfaces are modified using a low surface energy coating. Because of their characteristic water-repellency, the superhydrophobic surfaces have attracted considerable interest in various fields, such as the self-cleaning, anti-icing and reduction of frictional drag.^6,7

A number of researchers have begun to study methods of producing the superhydrophobic surfaces, including anodization, electrodeposition and chemical vapor deposition.^8–10 Copper alloy is an important metal in the chemical and microelectronics industries due to their high thermal and electrical conductivity. Jiang reported that superhydrophobic surface on copper substrates can be created in a one-step process by immersing the copper substrates in an ethanol solution of n-tetradecanoic acid. Over 5 days, the flowerlike cluster coating formed on the copper substrates, and the surface appeared superhydrophobic character.¹¹ Historically, superhydrophobic surfaces may be applied in various fields. Wang showed that use of the superhydrophobic character can achieve a high-heat transfer coefficient.¹² With the important development of numerous micro/nanoelectromechanical systems (MEMS/NEMS), the surfaces with low adhesion and friction have been investigated by several researchers. Thus far, many investigators have studied superhydrophobic drag-reducing effect to reduce solid/liquid friction.^13–15 During these studies, Zhao found that the friction could be reduced up to 9% on the superhydrophobic surfaces.¹⁶ In Daniello, ∼50% drag reduction was observed.¹⁷

However, the traditional two-step procedure of fabricating superhydrophobic surfaces may be limited by certain factors, such as complex operations, severe conditions and long time of preparation. Few studies have attempted to fabricate superhydrophobic surface on copper substrates using a one-step method which presents a simple and quick feature. Meanwhile, it has been shown that surface morphology can affect the drag-reducing effect. To date, no research has been performed on superhydrophobic drag-reduction in the nanoribbon structures. From an academic essential of view, the mechanisms behind the drag-reducing phenomenon are still largely unknown. It is therefore of interest to be better understanding the drag mechanisms.

In this paper, we first demonstrate that the superhydrophobic surfaces can be prepared by a simple one-step solution-immersions process. Following, a drag-reducing effect on the obtained substrates is studied via the water spraying system. We experimentally demonstrate that a drag reduction can be achieved on the as-prepared surfaces. The proposed method could be useful in preparing superhydrophobic surfaces on copper alloy substrates in industry. We hope that the findings could provide a better understanding of superhydrophobic drag-reducing properties.

2. Experimental details

2.1 Materials

The available copper ally sheets were obtained from Shenzhen materials company. The fluoroalkyl-silane (FAS-17) was used from Sigma-Aldrich. Sodium hydroxide (NaOH) and ammonium persulphate ((NH₄)₂S₂O₈) were used as received. Ethanol and acetone were distilled before using. Deionized water (16.7 MΩ cm) was purified using a ModuPure system.

2.2 Preparation of superhydrophobic surface

Inspired by the previous reports,¹⁰ the copper alloy sheets were first cleaned with acetone, ethanol and deionized water, successively. Once clean, the sheets were simply immersed in beakers containing a mixture of 2.5 M NaOH, 0.13 M (NH₄)₂S₂O₈ and 500 mM fluoroalkyl-silane (FAS-17) for 15 min to produce superhydrophobic surfaces at room temperature. Then the copper alloy sheets were rinsed with deionized water. Finally, the sheets were dried under a vacuum at 80 °C for 1 h.

2.3 Surface characterization

Water contact angles of 4 μL water for all samples were measured and analyzed using a contact angle meter (JC2000C1, Shanghai Zhongchen Digital Technic Apparatus Co., Ltd.) at ambient temperature. Sliding angles of 4 μL were measured by a conventional tilting sheet method. The values of contact angle are averages of five measurements made for different areas of the surfaces. The surface morphology of the samples was observed on a scanning electron microscope (TESCAN VEGA3). The compositions of the samples were characterized by an axis ultra X-ray photoelectron spectrometer (XPS) and the binding energy 285.0 eV of C1s in hydrocarbon was used as a reference.

2.4 Experiments for drag measurement

To evaluate the drag-reducing effect of the proposed superhydrophobic surfaces, the friction at the solid/liquid interface was measured by the water spraying system. Fig. 1 shows the schematic of the experiment. The sensor was fixed by two sliding pairs. The pairs were mounted on the side wall that could be moved up and down to adjust the sensor in the water level to ensure the same test condition. A power source was used to power sensor. The samples were cut to the same size (20 mm × 30 mm) and bond to bottom of the sensor during each test. The nozzle (caliber: 30 mm × 15 mm) fabricated by copper pipe was use connected to pipeline to restrict flow. The constant flow rate was provided by a water pump. The different rate of flow was obtained by a flowmeter.

Fig. 1 Schematics of visualization for the drag-reducing test.

The experimental samples were tested by the different rate of flow. Based on the principle of bridge balance, the sensor measured the deflecting of stain slice produced by the drag and converted this variation to electrical output. The output signal of sensor was collected by a voltmeter, and then computed by a computer.

3. Results and discussion

3.1 Surface structures and compositions

Fig. 2 shows SEM images of the prepared surfaces. A copper alloy plate was immersed in a solution, including NaOH and (NH₄)₂S₂O₈ in the presence of FAS-17 molecules at an FAS-17/NaOH ration of 0.5 for 3–19 min. Nanoribbon structures are created on the substrate, as shown in Fig. 1. When the immersion time is short (7 min or less), small nanoribbons can be observed (Fig. 1a and b). Increasing the immersion time to 15 min, the nanoribbons grow bigger and longer, as shown in Fig. 2c. The nanoribbons are 70–130 nm thick, 1–2 μm, and several micrometers long. For immersion times to 19 min, microflower structures form on the nanoribbon structures, increasing the surface roughness. Fig. 1 also shows the water contact angles and sliding angles on the prepared surfaces. These results suggest that the immersion time changes the surface morphology which affects superhydrophobic behavior. The process of produced super-hydrophobic surface may be generated by the following reaction equations:¹⁸

Cu + 4NaOH + (NH₄)₂S₂O₈ → Cu(OH)₂ + 2Na₂SO₄ + 2NH₃↑ + 2H₂O (1)

Cu(OH)₂ + 2OH⁻ → Cu(OH)₄²⁻ ↔ CuO + 2OH⁻ + H₂O (2)

Fig. 2 SEM images of the sample surface after immersion in the solution of 2.5 M NaOH, 0.13 M (NH₄)₂S₂O₈ and 500 mM fluoroalkyl-silane (FAS-17) for (a) 3, (b) 7, (c) 15 and (d) 19 min (inset shows a magnified SEM images as well as a water drop image).

Fig. 3 shows the survey XPS spectra of high resolution C1s core level spectrum acquired from the copper alloy surface immersed in the solution with 15 min. The XPS C1s spectrum shows that there are four types of carbon in the sample besides the carbon source from the detection, which are fitted to the –CF₃ (293.82 eV), –CF₂ (291.22 eV), –CH₂–CF₂ (288.82 eV) and C–C (285 eV) groups. It is well-known that the –CF₃ group has the lowest surface energy, and the –CF₂ is the next lowest. It also indicates that low surface energy the –CF₃ and –CF₂ components comprise the outermost surface, contributing to fabricate superhydrophobic surfaces. The binding energies reported here are consistent with previous reports.¹⁹ The –CF₃ and –CF₂ concentrations from the C1s spectra of the surface were 5.92 and 41.71%.

Fig. 3 High-resolution C1s core level spectrum treated copper surface.

Theoretically, one reason for the surface with low energy groups may be that formation of copper hydroxide combined the FAS-17 molecules. During the hydrolysis process, the C₂H₅ component was removed from the FAS-17 molecules. Meanwhile, the silicon bonds with oxygen in the surface, leading C–F groups to orient outward from the surface.²¹ These findings suggest that the substrate was covered with the low surface energy film, leading to superhydrophobic properties.

3.2 Surface wettability

The surface wettability of the as-prepared surfaces was researched. Water contact angles increased between 3 min and 20 min of immersion time, reaching a peak of ∼158°, as shown in Fig. 4. These result shows that the proposed one-step method can be used to fabricate superhydrophobic surface.

Fig. 4 Water contact angle measured on copper alloy surfaces treated with different time of immersion.

The wetting behavior of a surface is dependent on both surface chemistry an surface topography. It can be explained by two distinct models by Wenzel given in eqn (3) and Cassie–Baxter given in eqn (4)^20,21

cos θ^c = r cos θ (3)

cos θ^c = −1 + λ_s(1 + cos θ) (4)

where θ is the equilibrium contact angle and θ^c is apparent contact angle for a droplet on a surface, r is the roughness factor and defined as the ration of surface area over the apparent surface area, and λ_s is the fraction of solid/liquid contact. The Cassie–Wenzel transition is introduced in previous research.

From a theoretical essential of opinion, higher surface roughness and lower surface energy are the two important requirements for the fabrication of superhydrophobic surface.^22,23 Understanding of the high rms roughness can increase the water contact angle. The rms roughness increase when immersion time is prolonged, as shown in Fig. 2. However, Fig. 4 shows that water contact angle decrease after 15 min. These results agree with the findings of Saleema, et al.¹⁰ A possible explanation for this is that a part of CF₂ groups on the surface was replaced by hydroxide groups as long immersion time, leading to increase the surface energy.

Some researchers also reported several superhydrophobic surfaces on copper substrates by a two-step procedures: first, rough structures were created via techniques such as electric brush-plating and electrodeposition, then the formation of a rough binary structure on the surface was modified with low surface energy molecules.²² This research investigated the effect of a one-step immersion method to fabricate superhydrophobic surface on copper alloy surface. It seems that both surface roughness and low surface energy are formed on the copper ally substrates by a one-step process. The immersion time appears to affect the surface wettability by changing the surface morphology and energy.

3.3 Stability and repairability

To study the mechanical stability of the superhydrophobicity on the obtained substrates, the as-prepared superhydrophobic surface with the size of 15 mm × 15 mm was wear on the abrasive paper (800 mesh) under the 30 g load. Fig. 5a shows the schematic of the experiment. Experimentally, it appears that the surface lost the superhydrophobicity when the sliding distance extends ∼30 cm (Fig. 5b). Traditionally, the artificial superhydrophobic surface is hampered by the stability. To experimentally study the effect of losing superhydrophobicity on the wearing surface, we measured the surface morphology of the damaged surface. It is obviously observed that the nanoribbons were destroyed, and grooves were produced after sliding for 30 cm (Fig. 5b). These results suggest that the wearing process destroyed the surface structures and low energy film. However, the damaged the surface can be rapidly repaired by dipping the surface into solution-immersion again. The surface is covered with nanoribbons structure again, as Fig. 5d. The water contact angle and sliding angle on repaired surface were 155 ± 2° and 5 ± 2°, respectively. Therefore, after the repairing step, the nanoribbons of a copper compound is similar to that on the surface prepared by a one-step solution-immersions process. We assumed that, due to the relatively small size of nanoribbons and micrometer-scale grooves, the repaired surface exhibited higher stability than the previous surface. These results agree with the findings of Ras, et al.²⁴ The easy repairability of this method is expected to apply in the practical application.

Fig. 5 (a) Schematic view for the devices used to test the stability for the surface obtained by one-step solution-immersion process. (b) The as-prepared surface was wear for a certain sliding distance and water contact angle/sliding angle was monitored. SEM images for the treating surface after sand abrasion test (d) and the damaged surface after repairing by dipping into solution again.

3.4 Drag reduction

In order to investigate the drag-reducing property of the as-prepared superhydrophobic surface on the copper alloy substrates, the solid/fluid friction was measured on the different plates by the water spraying system. To experimentally quantify the force of the solid/fluid friction on the different wetting surface, the drag coefficient (C) was applied, which is expressed as:where N, g, ρ, s and I are the value of output (V), acceleration of gravity (9.8 m s⁻²), water density (1000 kg m⁻³), surface of substrate and calibrating value (102.55 mV g⁻¹).

Fig. 6a shows the that drag coefficient curve gained when changing the water flow velocity on different substrates. The three coatings are referred to as normal, S-A and S-B (S-A: water contact angle 152° and rms 0.2 μm, S-B: water contact angle 150° and rms 0.6 μm). As can be seen from the figure, the coefficient curve of normal surface is higher than other two curves of superhydrophobic surface, indicating obvious drag reduction between normal and superhydrophobic surfaces. It also shows the drag coefficient curves are correlated with the flow velocity, which increase with an increase in flow velocity. Based on the common belief, the surface with a high water contact angle leads to a high drag-reduction. However, the curve B with water contact ∼150° was lower than curve A with water contact ∼152°, indicating that the surface wettability does not affect drag-reduction completely. It appears that both the surface wetting and morphology affect drag-reducing effect.

Fig. 6 Results of super-hydrophobic drag test at different flow velocity. (a) The drag coefficient of the different plates. (b) The drag reduction of the different plates for the changing the velocity.

As shown in Fig. 6b, the drag reduction ration reduce when the flow velocity is enhanced. The drag reduction ratio (CR) can be expressed as:

where c_n and c_s are the drag coefficient of the normal surface and superhydrophobic surface. The drag reduction ration on the S-B surface was higher than that on S-A surface under the same flow velocity. During the testing, the drag coefficient of S-B and S-A coatings were ∼44% and ∼35% at low flow velocity. However, when the flow velocity was at about 5 m s⁻¹, the drag coefficient of the S-B and S-A surfaces reduced to ∼20% and ∼15%. These results suggest that flow velocity can affect the drag-reducing effect.

From a theoretical point of view, we believe that superhydrophobic drag-reducing property was due to the “plastron effect”.²⁵ It explained that there is a thin layer of air with the low dynamic viscosity between the water and solid. It is well known that dynamic viscosity of water is larger than that of air. The liquid/air/solid friction on the superhydrophobic surface is smaller than liquid/solid friction on the normal surface. This can be understood by Newton's model, which can be expressed as:

where F is the drag force, λ is the dynamic viscosity of the fluid, S is the wetting area of the substrate, and is the velocity gradient in the fluid. It seems that increasing surface roughness and water contact angle could reduce the solid/liquid friction. The superhydrophobic surface with high surface roughness may capture more air bubbles, which produce a sufficient layer of air close to the solid. Further, we consider that the friction of the superhydrophobic surface in the Cassie model is obvious smaller than the surface in the Wenzel model due to the more air bubbles existing on the Cassie status.²⁶ As shown in Fig. 7, we can theoretically estimate characteristics of the flow on a different wetting surface. On the normal flat surface, the liquid flowed over the solid surface without a layer of air, and the friction is produced by solid/liquid contact (Fig. 7a). When the liquid flowed on the superhydrophobic surface in the Wenzel model, flow partially rids on the sparse layer of air, and friction is produced by solid/liquid/gas contact. We consider the layer of air was sufficient in the Cassie model and friction was mainly produced by solid/gas contact (Fig. 7c).

Fig. 7 Schematics of flow model: (a) flow on the normal surface with no-slip boundary conditions. (b) Flow with a film of air produced by the superhydrophobic surface (state in a Wenzel mode) on the obtained surface. (c) Flow with a film of air generated by the superhydrophobic surface (state in a Cassie mode) on the obtained surface.

In addition to the discussion above, when the flow velocity was enhanced, the drag reduction ratio reduced to ∼20%. We considered that, due to the high wall shear rate, the layer of air was damaged and the liquid could not sufficiently ride the layer of air, leading to increase surface roughness and skin friction. This experimental study was designed to examine the superhydrophobic drag-reducing effect on the as-prepared surface on copper alloy substrates. The findings support that obvious drag-reducing effect on the superhydrophobic surface. Moreover, the experiments indicate that drag-reducing effect was influenced by the surface morphology and wettability. One reason for this effect could be that there is a thin layer of air between solid and liquid, leading to change a slip boundary.²⁷

Experiments have shown that drag-reducing property on the superhydrophobic surface.²⁸ In this work, we studied the drag-reduction on the as-prepared superhydrophobic surface. It is important to note that some reports show that superhydrophobic surface could increase the solid/liquid friction. In contrast to the drag-reducing effect, superhydrophobic surface of the sphere enhanced drag force when the surface was sunk into water.²⁹ However, other experiments have shown that superhydrophobic surface of the sphere was responsible for the drag reduction.³⁰ We consider that different surface morphology and roughness may be the possible reasons for this contrary result. Experiments have also shown that direction of flow on the surface would affect the drag-reduction.³¹ In the future, it would be interesting to fabricate the different surface morphology on the surface to investigate the drag-reducing character.

4. Conclusions

In summary, this paper presents a simple method for fabricating the superhydrophobic surfaces on copper alloy substrates. Use of a one-step process, treated to produce superhydrophobic surfaces, appears simple and rapid advantage, which needs only 15 min to complete a main fabrication. Moreover, the findings showed obvious drag-reducing effect on the obtained surfaces. In contravention of common opinion, surface wetting does not affect drag-reducing effect completely. We consider that both surface roughness and wettability affect superhydrophobic the drag-reducing effect simultaneously. The results of this research might help researchers to under more the drag-reducing effect. With further refinement, we hope this method to produce superhydrophobic surfaces can be used in many filed, such as micro-fluidic systems, surface ships and water pipeline.

Acknowledgements

The authors would like to thank National Basic Research program of China (no. 2012CB934104), National Science Foundation of China (no. 61071037), National Science Foundation of China (no.61474034) and Fundamental Research Funds for the Central Universities (no. HIT.NSRIF.2014040) for financial support.

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