Fabrication and drag reduction of superhydrophobic surface on steel substrates

Materials

The materials used in this study include acetone, ethanol, hydrochloric acid (HCl), ferric trichloride (FeCl₃), zinc acetate (Zn(CH₃COO)₂·2H₂O), zinc nitrate (Zn(NO₃)₂·6H₂O), 1,3-diaminopropane (DAP, Aldrich), fluorinated silane (FAS, 1H,1H,2H,2H-perfluorooctadecyltrichlorosilane), hexamethylenetetramine (C₆H₁₂N₄), sodium chloride (NaCl), and steel foils of 0.2-mm thickness. Deionised water is used for washing throughout the experiment.

Sample preparation

The steel foils were cut into pieces of 2 × 5 cm², polished mechanically, and cleaned ultrasonically with acetone and ethanol in sequence to remove grease. Micro/nanoscale hierarchical structures of the steel surfaces were produced simply through electrochemical and hydrothermal synthesis, as follows. The microscale structures on the steel surface were fabricated through electrochemical processes. A steel foil and a graphite plate were used as an anode and a cathode, respectively. The steel foil was immersed in a mixture solution of 0.5 M FeCl₃ and 0.2 M HCl at 25°C for a period under a constant voltage of 4 V. This formed a microtextured surface on the steel foil. After the reaction, the sample was rinsed thoroughly with deionised water and dried in air.

A two-step process was employed to prepare ZnO nanowires: the preformation of a thin ZnO seed layer, followed by the hydrothermal growth of ZnO. In the first step, the substrate was immersed into a 5-mM zinc acetate ethanol solution and heated for 20 min at 300°C in air. In the second step, three solutions were mixed to grow ZnO nanowires: 20 mM Zn(NO₃)₂, 20 mM C₆H₁₂N₄, and a DAP solution. The concentration of DAP was varied from 0 to 100 mM (in steps of 0, 20, 40, 60, and 100 mM) to study its effect on the morphology and thickness of the film [27]. After the three reactants were completely dissolved in deionised water, the mixed solution was poured into an autoclavable bottle. The pretreated substrates were submerged vertically, and the bottle was sealed and immersed in a thermal bath for 8 h at 95°C. The substrates were taken out from the bottle when the ZnO film deposition was complete. The substrates were washed with deionised water and dried at room temperature. Finally, these samples were immersed in an ethanol solution of 0.5 wt-% FAS for 30 min and dried in a drying oven at 120°C for 60 min.

Characterisation and property test

The surface morphology of the as-prepared surface was analysed using a field-emission scanning electron microscope (FE-SEM, TESCAN VEGA).The element distributions of the sample were determined by energy-dispersive X-ray spectroscopy (EDS). The CAs and sliding angles (SAs) of water droplets were measured with an optical contact angle meter system (Data Physics Instrument GmbH, Germany) at temperature of 25°C. The CAs and SAs were obtained by averaging five measurement results at different positions. SAs were determined by slowly tilting the sample stage until the droplet started to move. The drag reduction rate was measured with a self-designed liquid spraying system. A polarisation curve was used to test the corrosion resistance of the untreated steel surface and the as-prepared surface. The Tafel polarisation curves of the untreated steel surface and superhydrophobic steel surface were measured in a solution of 3.5 wt-% NaCl.

Formation of micro/nanoscale hierarchical structures

To obtain rough microstructure surfaces, steel samples were electrochemically etched in a mixture solution of 0.5 M FeCl₃ and 0.2 M HCl at 25°C for 0.2, 2, 4, and 8 min. During the electrochemical etching process, the steel samples changed from shiny silver to grey owing to an increase in surface roughness and changes in the chemical composition of the surface. The surface morphologies of electrochemically etched steel surface were investigated by SEM. Figure 1 shows the SEM images of an as-prepared surface. It can be found that FeCl₃, as an etching agent, plays a significant role in the formation of the hierarchical structure.

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Figure 1

SEM images of steel electrochemically etched in mixture solution of 0.5 M FeCl₃ and 0.2 M HCl at 25°C at different times: (a) 0.2 min, (b) 2 min, (c) 4 min, and (d) 8 min.

Figure 1(a) shows that the surface is flat after electrochemical etching for 0.2 min. Although there are some shallow craters on the surface, no microstructures are formed. When the etching time increased to 2 min, there were some cavities, and concave structures appeared successively on the surface. As shown in Figure 1(b), the diameter of some craters was approximately 2 μm. When the electrochemical etching time increased to 4 min, the steel surface was gradually etched, and irregularly shaped micro/nano islands and concave pits of various sizes formed on the sample surface. In Figure 1(c), the diameter of the holes was approximately 16 μm. However, when the steel surface was etched for 8 min, the surface appeared to contain some lamellar interlaced structures with micrometer and submicron scales.

The reasons for the formation of micro/nano structures on the steel surface were further analysed. The reaction mechanism of steel in an etching solution can be described as (1) As the etch proceeds, the concentration of Fe²⁺ increases and deposits on the surface; therefore, it becomes difficult for the etchant to penetrate the etched surface. As the concentration of HCl increases, the solubility of the ferrous ion in the etchant solution also increases, which results in a slight increase in the etch rate of the steel surface.

We grew ZnO nanorods on the etched steel surface (etched for 4 min) to obtain micro/nanoscale hierarchical structures [16]. Figure 2 shows changes in the morphology on steel with an increase of the DAP concentration. As shown in Figure 2(a), when the DAP concentration is 0 mM, ZnO nanorods are densely located on the steel surface. We can observe the formation of well-defined columns with mean diameters of 150–200 nm. Most of the nanorods grow vertically on the substrate surface. They have a hexagonal cross section. When the concentration of DAP increases to 20 mM, the ZnO nanorods convert to nanowires, and the density of the nanowires is even.

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Figure 2

SEM images of steel surface with nanostructures at different DAP concentrations: (a) 0 mM, (b) 20 mM, (c) 40 mM, and (d) 100 mM.

The presence of DAP in the reaction is crucial for producing nanocolumns. We observed that the mean diameter is reduced to 70–100 nm. However, when the concentration of DAP increases to 40 mM, there are only some nanoparticles instead of nanowires sparsely scattered on the surface. When the DAP concentration increases to 100 mM, the surface is relatively flat, with a few nanostructures distributed on it. The DAP solution can effectively suppress homogeneous nucleation and prevent the formation of ZnO particles in the bulk solution. It is also beneficial to the rapid growth of ZnO nanowires on seed substrates. The surface morphology becomes worse as the DAP concentration increases because the DAP solution is not only catalytic and facilitative in the vertical growth of ZnO, but is also corrosive to the ZnO on the surface. Low concentrations of DAP solution can promote the growth of ZnO, and heavy concentrations of ammonium ions suppress ZnO growth. Therefore, the higher the concentration of DAP, the more ZnO is consumed, which leads to a sparse distribution of nanorods.

Surface composition

The corresponding element distributions are determined by EDS. Figure 3(a) shows the ED spectrum of the etched surface, which reveals the presence of Fe, C, and O elements. Figure 3(b) shows the ED spectrum of the ZnO nanowire film. Sharp peaks for O and Zn are observed, and the atomic ratio is 0.91:1 in the spectrum, which is close to the theoretical value (1:1) of ZnO. This confirms that the nanostructure on the steel surface is composed of ZnO. ZnO nanowires are synthesised based on the following reaction: (2) For this reaction, hexamethylenetetramine may be used as the hydroxide source. In the mixture solution, the supersaturation of ZnO or Zn(OH)₂ induces the formation of ZnO nanowires through homogeneous nucleation on the ZnO seed layer of the steel surface [28,29].

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Figure 3

ED spectra of as-prepared surface: (a) electrochemical etched surface and (b) resultant ZnO nanowire film.

Surface wettability

The CAs and SAs are used to define the wettability, which is an important property of a superhydrophobic surface. To evaluate the wettability of the as-prepared samples, the CAs and SAs were measured using a water droplet of 4–5 μL. Here, we measured the CAs and SAs of samples that grew in DAP solution of different concentrations. As shown in Figure 4, the CAs of the as-prepared surfaces are higher than 150° after fluorination. When the concentration of DAP solution is 20 mM L⁻¹, the as-prepared surface has the largest CA (164.9°) and the lowest SA (2.3°). In addition, the surface exerts no apparent adhesive force on the suspended droplet.

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Figure 4

CAs and SAs of as-prepared surface after fluorination at different DAP concentrations: (a) 0 mM, (b) 20 mM, (c) 40 mM, and (d) 100 mM.

Drag reduction on superhydrophobic surface

Drag reduction is a promising application of superhydrophobic surfaces. An experimental setup that was used to measure liquid/solid friction drag was able to estimate the drag reduction property [30]. As shown in Figure 5, the drag reduction ratio of a superhydrophobic surface is approximately 40–50% compared to an untreated steel surface at low velocity. As the micro/nanostructures on the superhydrophobic surface can trap bubbles, the surface can produce slip and reduce the friction resistance. When the flow velocity reaches 4.5 m s⁻¹, the drag reduction ratio of the superhydrophobic surface decreases to 37.7% because higher flow velocities cause faster removal rates of the air layer on the superhydrophobic surface. The disappearance of entrapped gas leads to a decrease in the slip length, which results in less drag reduction on the superhydrophobic surface. Through a drag reduction test, we conclude that as the flow velocity increases, drag reduction is weakened. This is attributed to the morphology of the surface air layer and its depletion by the high velocity shear flow.

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Figure 5

Friction drag vs. velocity of water flowing over surfaces.

Corrosion resistance of superhydrophobic steel surface

Corrosion resistance is another promising application of superhydrophobic surfaces. A polarisation curve is a useful tool for determining the instantaneous corrosion rate of a substrate [31,32]. Saturated potassium chloride solution and a platinum electrode are used as the reference electrode and counter electrode, respectively. In this study, the Tafel polarisation curves are recorded at a sweep rate of 10 mV s⁻¹, and each electrochemical test is conducted three times to ensure good repeatability.

As shown in Figure 6, the Tafel polarisation curves of an untreated steel surface and a superhydrophobic steel surface are measured in a solution of 3.5 wt-% NaCl. The corrosion potential ( ) and corrosion current density (I_corr) derived from the Tafel polarisation curves are listed in Table 1. The corrosion current densities (I_corr) of the untreated steel and superhydrophobic steel are 1.694 × 10⁻⁸ A and 1.549 × 10⁻⁹ A, respectively. This indicates that the corrosion current density decreases by approximately one order of magnitude when the steel surface becomes superhydrophobic. Moreover, the corrosion potential of the superhydrophobic steel surface is −0.475 V, which is higher than that of the untreated steel surface (−0.662 V). These results indicate that superhydrophobic steel has better corrosion resistance than untreated steel.

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Figure 6

Tafel polarisation curves of untreated and superhydrophobic surfaces.

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Table 1

Corrosion potentials (E_corr) and corrosion current densities (I_corr) of untreated and superhydrophobic surfaces in 3.5-wt-% NaCl solution.

Sample	Ecorr (V)	Icorr (A)
Untreated steel surface	−0.662	1.694 × 10−8
Superhydrophobic steel surface	−0.475	1.549 × 10−9