One-Step Fabrication of Robust Superhydrophobic Steel Surfaces with

Jun 24, 2019 - Immediately after laser texturing, the rough surfaces exhibit ..... the superhydrophobic surfaces still retain their superhydrophobicit...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 25586−25594

One-Step Fabrication of Robust Superhydrophobic Steel Surfaces with Mechanical Durability, Thermal Stability, and Anti-icing Function Haipeng Wang,† Meijin He,‡ Huan Liu,*,‡ and Yingchun Guan*,†,§,∥

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School of Mechanical Engineering and Automation, ‡Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, and §National Engineering Laboratory of Additive Manufacturing for Large Metallic Components, Beihang University, Beijing 100191, P. R. China ∥ Hefei Innovation Research Institute, Beihang University, Hefei 230013, P. R. China S Supporting Information *

ABSTRACT: Superhydrophobic metallic materials have drawn broad research interest because of promising applications in various fields. The mechanical stability of superhydrophobic surfaces is currently a major concern limiting their practical applications. Herein, we developed a simple method to fabricate robust superhydrophobic surfaces on stainless steel via direct ultrafast laser microprocessing. Of note is that the fabricated superhydrophobic surfaces can withstand mechanical abrasion against an 800 grit SiC sandpaper for 2.3 m at an applied pressure of 5.5 kPa without losing superhydrophobicity. It is proposed that the robust superhydrophobicity may be attributable to the formation of unique hierarchical micro-/nanostructures and a nonpolar carbon layer on the surface. The hierarchical structures are composed of laser-created micropillars and ablation-induced nanoparticles. The fabricated surfaces exhibit good thermal stability and still show superhydrophobicity after thermal treatment at 100 °C for 120 min, which is related to the inorganic nature of metallic materials. An excellent anti-icing property is achieved on the fabricated surfaces with the water droplets on it retaining the liquid state for over 500 min at −8.5 ± 0.5 °C, which benefits from the obtained superhydrophobicity, based on classic nucleation theory and the heat transfer between the rough solid surface and water droplet. We envision that the presented method provides a facile and effective route to fabricate large-area superhydrophobic surfaces with robust mechanical stability and excellent anti-icing property. KEYWORDS: superhydrophobic, stainless steel, hierarchical structures, mechanical stability, anti-icing performance

energy materials (e.g., fluorinated silane).8,16,17 However, it has been revealed that these low surface energy chemical coatings are susceptible to damage, leading to the destruction of surface integrity and the loss of surface superhydrophobicity.18,19 The poor mechanical stability greatly limits the practical applicability of chemically modified superhydrophobic surfaces because mechanical abrasion can easily damage their surface features, resulting in degradation of the functional performance of superhydrophobic surfaces.18−21 Several approaches have been proposed to improve the mechanical abrasion resistance of the superhydrophobic surfaces.20,21 Zhu et al. fabricated superhydrophobic metal/ polymer surfaces by incorporating Ag particles in polymer coating, making the surface still exhibit superhydrophobicity after abrasion for 3 m with a 1500 grit sandpaper.20 She et al.

INTRODUCTION Considerable research efforts have been devoted to the fabrication of superhydrophobic surfaces on metals1−3 because of practical/engineering demanding requirements such as selfcleaning,4 anti-icing property/icephobicity,5 drag reduction,6 and heat-transfer enhancement.7 A rough structure in conjunction with low surface energy is prerequisite to fabricate superhydrophobic surfaces on metals.8 Hierarchical micro-/ nanostructures can be achieved by various micro-/nanofabrication techniques, represented by two classical approaches, namely, top-down approaches (e.g., lithography, plasma treatment, laser treatment)9−11 and bottom-up approaches (processes involving self-assembly and selforganization).12,13 Particularly, laser surface texturing has attracted extensive interest and produced various wellcontrolled microstructures on metallic materials because of a precise and flexible process.3,11,14,15 In general, a superhydrophobic surface can be prepared by laser surface texturing and subsequent surface chemical modification with low surface © 2019 American Chemical Society

Received: April 19, 2019 Accepted: June 24, 2019 Published: June 24, 2019 25586

DOI: 10.1021/acsami.9b06865 ACS Appl. Mater. Interfaces 2019, 11, 25586−25594

Research Article

ACS Applied Materials & Interfaces enhanced the abrasion resistance of superhydrophobic magnesium alloy surfaces by electrodeposition of nickel and chemical modification. The fabricated surfaces retained superhydrophobicity after mechanical abrasion for 700 mm with an 800 grit sandpaper.21 These methods are normally time-consuming, have a complex fabricating process, and show limited industrial potential applications, although the fabricated superhydrophobic surfaces show improved abrasion resistance to some extent. Icephobicity/anti-icing property is of great significance to cold environment devices, such as aerofoils, power towers, and radars.22 Once ice forms, these devices may fail to work normally or even be damaged. The fabrication of icephobic surfaces has aroused much interest of researchers since the 1950s.23 Recently, a biomimetic micro-/nanostructured surface composed of microratchets and ZnO nanohairs achieved superhydrophobic and icephobic properties with a 7 μL droplet on the surface taking ∼120 min to freeze at −10 °C.24 Zhan et al. prepared superhydrophobic surfaces on polytetrafluoroethylene by laser microprocessing. The obtained surfaces exhibited icephobicity to a 5 μL droplet even when the surface temperature was lowered to −25 °C.25 Wang et al. carried out anti-icing tests on the fabricated superhydrophobic steel surfaces via a water-dripping process at −20 °C. When a 50 μL droplet fell onto the superhydrophobic surface, the droplet rolled off easily without freezing within such short time.26 However, it is still a great challenge to fabricate more robust and efficient icephobic surfaces. In this study, we provide a facile one-step methodology to fabricate robust superhydrophobic surfaces on stainless steel. The superhydrophobic surface was prepared by direct laser texturing on stainless steel and has shown durable self-cleaning property. Laser texturing-induced dense hierarchical micro-/ nanostructures on stainless steel endow surfaces strong abrasion resistance and durable superhydrophobicity. The tunable water adhesion on superhydrophobic surfaces can be achieved by controlling the groove pitch of the laser-created microgrid structures. Moreover, the fabricated superhydrophobic surfaces exhibit both good thermal stability and excellent anti-icing performance.

Figure 1. Obtained robust superhydrophobicity on laser-textured stainless steel surfaces. (a) Laser-fabricated regular micropillar structures on stainless steel with a groove pitch of 60 μm; (b) hierarchical surface morphology of the created micropillar structure with a groove pitch of 60 μm; (c) water droplets on the laser-textured surface; (d) superhydrophobicity of the created hierarchical surface and experimental snapshot showing the dynamic falling−bouncing process of the falling water droplets on the prepared superhydrophobic surface; (e) squeezing process of a water droplet by using two prepared samples with superhydrophobic surfaces.

process, the generated active magnetite Fe3O4−δ (0 < δ < 1) catalyzes the decomposition of CO2 in air into monoxide and finally zero valence carbon, and the oxygen anions transfer into lattice vacancies of the substrate to form stoichiometric Fe3O4.29,30 After laser processing, the decomposition of CO2 continues over time and the produced nonpolar carbons finally cover the rough surfaces. As indicated by X-ray photoelectron spectroscopy (XPS) results in Figures S2 and S3, the relative amount of carbon on superhydrophobic surfaces is much higher than that on the initial surface. The chemical compositions on superhydrophobic surfaces are almost solely made up of iron oxides and carbons. The produced nonpolar zero valence carbons accumulate on the rough surface and result in the decrease of surface energy.11 These hydrophobic nonpolar carbons confine a three-phase (solid−air−liquid) contact line (TCL) to prevent water from immersing the rough surfaces.31 Together with the amplification effect of rough micro-/nanostructures, the laser-textured surfaces become superhydrophobic. To prove the fact that the generated Fe3O4−δ catalyzes the decomposition of CO2 and finally results in the translation of surface wettability, comparison experiments have been carried out by putting the laser-treated samples into a closed container containing CO2 immediately after laser processing. Dynamic changes of the surface contact

RESULTS AND DISCUSSION Regular microgrid structures have been fabricated on the stainless steel surface by laser scanning in two perpendicular directions, as shown in Figure 1a,b. Microgrid structures are composed of intersecting microgrooves of 20 μm in width. The laser ablation-induced plumes lead to the generation of numerous nanosized particles.27,28 The average diameter of these particles is less than 500 nm as shown in Figure S1. These particles redeposit onto lateral unprocessed surfaces during laser processing, agglomerating into rough microaggregates and stacking on unprocessed surfaces (Figure 1a). The initial stainless steel surface shows a contact angle of 87.2° as shown in Figure 1d. Immediately after laser texturing, the rough surfaces exhibit superhydrophilicity because of the intrinsic hydrophilicity of polar iron oxides and the amplification of rough solid surfaces,29,30 which has been confirmed by the rapid spreading of water droplets on surfaces. After exposure to ambient air for 24 h, the textured surfaces become superhydrophobic with a contact angle of over 150°, as shown in Figure 1c,d, and then gradually stabilize over time. The change of surface wettability is related to the decomposition of carbon dioxide in air.30 In the laser texturing 25587

DOI: 10.1021/acsami.9b06865 ACS Appl. Mater. Interfaces 2019, 11, 25586−25594

Research Article

ACS Applied Materials & Interfaces

Figure 2. Surface morphologies and wettability performance of laser-created hierarchical micro-/nanostructures on the stainless steel surface. (a−c) SEM images of hierarchical micro-/nanostructures with groove pitches of 80, 140, and 240 μm, respectively; (d−f) confocal microscopy images of hierarchical micro-/nanostructures with a groove pitch of 80, 140, and 240 μm, respectively; (g) contact angles on superhydrophobic surfaces as a function of structural period; (h) force−distance curves recorded before and after water droplet contacting the superhydrophobic surface with the pitch of 60, 120, and 140 μm, respectively; (i) evolution of adhesive force and area fraction of the solid−liquid interface with increasing groove pitch.

because of their instability on the surfaces (see Video S2).35 The contaminants on the surfaces are carried away by water droplets along the rolling trace. Even after exposure to air for 120 days, the fabricated superhydrophobic surfaces still exhibit good self-cleaning performance for both soils and pencil shavings (Figure S4). On the basis of the wettability difference between laser-textured surfaces and the unprocessed surface, directional fluidic channels have been fabricated by selective laser texturing for controlling the liquid flows (Figure S4b). Water prefers to flow along the channels formed by an unprocessed hydrophilic zone (Video S3). Superhydrophobic surfaces with different textures have been fabricated by controlling the groove pitch of the laser-created microgrid structures (Figures 1 and 2). Figure 2a−c shows scanning electron microscopy (SEM) images of laserfabricated typical microgrid structures with groove pitches of 80, 140, and 240 μm, respectively. The laser ablation-induced nanoparticles mainly redeposit on the lateral edges of grooves, agglomerate into microaggregates, and densely stack on unprocessed surfaces, leading to the formation of microscale protrusions. The central area of the unprocessed surface is covered by redeposited particles (top-right inset in Figure 2a,b). As the groove pitch increases, the amount of nanoparticles redeposited on the surface decreases. The laser-fabricated hierarchical structures mainly consist of four different micro-/nanostructures, including a regular array of micropillars with a groove pitch ranging from 60 to 300 μm, rough microprotrusions sized at tens of micrometers, microaggregates, and discrete nanoparticles covered on top surfaces of micropillars. Figure 2d−f shows three-dimensional (3D) surface morphologies and cross sections of the fabricated hierarchical structures. When the groove pitch is 60 μm, the

angle have been recorded periodically. Results indicate that the CO2 atmosphere accelerates the transition of the fabricated surfaces from superhydrophilic to superhydrophobic state, and the transition period is shortened from 24 h to less than 18 h. To illustrate the wetting behavior of the droplet on the superhydrophobic surface, Figure 1d shows the dynamic process of continuous dropping, contacting, and rebouncing of the falling water droplets (12 μL) on the superhydrophobic surface (also see Video S1). As the droplet hits the surface, the impacting kinetic energy is transferred to the surface energy that enables the droplet to fully rebounce from the surface (t = 0.18 s).32 The droplet retains its spherical shape after the first hit and then experiences a free falling−rebouncing process again and again (t = 0.18−0.25 s), and then it finally rolls away from the surface (t > 0.25 s) or remains on the surface with a spherical shape. Furthermore, squeezing test has been carried out using two superhydrophobic samples as shown in Figure 1e. A water droplet is set on a superhydrophobic surface B (Figure 1e) and shows a spherical shape. Another sample with superhydrophobic surface A (Figure 1e) is used to squeeze the water droplet, and then the droplet becomes an ellipsoid (Figures 1 and 3) from a spherical shape rather than wetting the rough surface. When superhydrophobic surface A moves away, the water droplet recovers its initial spherical shape (Figure 1(e-4)). This indicates that the contact between the water droplet and rough superhydrophobic solid surface is Cassie’s state rather than Wenzel’s state with a wet-contact mode.33,34 The produced superhydrophobic surfaces exhibit durable self-cleaning performance (Figure S4). When dripping water droplets onto superhydrophobic surfaces, the soils or pencil shavings covered on the surfaces are absorbed by the droplets, and then the droplets roll away from the surfaces 25588

DOI: 10.1021/acsami.9b06865 ACS Appl. Mater. Interfaces 2019, 11, 25586−25594

Research Article

ACS Applied Materials & Interfaces unprocessed surfaces are completely covered by rough microaggregates, leading to a regular array of micropillars (Figure 1a,b). As the pitch increases to 80 μm, an irregular concave micropit is generated on the micropillar top surface. The formed microprotrusions on top surfaces of micropillars show a size of more than 25 μm in height and 23 μm in width (inset in Figure 2d). As the pitch increases to more than 140 μm, much less nanoparticles redeposit onto unprocessed surfaces during laser ablation, resulting in a significant decrease of the height of micropillars (Figure 2b,c,e,f) and a thin layer of nanoparticles covering on the top surface. Simultaneously, microprotrusions become smaller (around 17 μm in height and 20 μm in width), and the dimension of the concave micropit on the top surface incrementally increases with increasing groove pitch. Wettability evolution with the increasing groove pitch of the fabricated hierarchical surfaces is characterized by contact angle and adhesive force measurements (Figure 2g−i). When the pitch increases to more than 160 μm, the contact angle on the surfaces shows a slight decrease from 156.3 ± 2.1° to 152.9 ± 3.0°. Contact angle as a function of droplet size is shown in Figure S5. As the droplet size increases, the droplet deforms from a spherical shape to an ellipsoid because of the effect of gravity (Figure S5b), and the contact angle gradually decreases to less than 150° when the droplet size exceeds 25 μL. Figure 2h shows the recorded force−distance curves as a water droplet gradually approaches and retreats from superhydrophobic surfaces. The fabricated superhydrophobic surfaces show strong adhesion to water droplet with adhesive force ranging from 41.7 ± 1.0 to 72.3 ± 8.4 μN, as shown in Figure 2i. For a superhydrophobic surface with Cassie impregnating wetting state, water can wet the rough solid surface but does not completely impregnate surface micro-/nanostructures.36 The convex micro-/nanostructures on rough solid surfaces can prevent water from filling the valleys between convexities. The trapped air between the rough solid surface and water droplet leads to very small liquid−solid interface and a large number of micro-/nanoscale air pockets.3 On the one hand, the area fraction of the liquid−solid interface can be calculated through the Cassie−Baxter equation37,38 cos θCB = f1 cos θ1 + f2 cos θ2


f1 + f2 = 1


Figure 3. (a) Strong adhesive force arises when the water droplet is drawn away from the rough hierarchical superhydrophobic surface because of the generated negative pressure by volume change of the sealed air in micropits; (b) schematic diagram of the three-phase contact state between the water droplet and rough solid surface with increasing groove pitch.

microaggregates on the micropillar top surface (Figure 3b). The open air pockets are connected to the atmosphere, causing high contact angle on rough solid surfaces and contributing little to surface adhesive force, whereas the nanoscale sealed air pockets between microaggregates form a closed system and generate negative pressure when the droplet moves. Although the adhesive force induced by the negative pressure of each nanoscale sealed air pocket is small, the total adhesive force is high because of the large number of the sealed air pockets. In this case, surface adhesive force mainly derives from the generated negative pressure by nanoscale sealed air pockets between microaggregates and the van der Waals interactions between the rough solid surface and water molecules.36 As the groove pitch increases to more than 80 μm, micropits are generated on micropillar top surfaces. A new closed system is formed by the microscale sealed air pockets in micropits and negative pressure is generated when the droplet moves. As illustrated in Figure 3a,b, the air−liquid interface is curved from concave into convex when the droplet is drawn upward.33,39 Compared to nanoscale sealed air pockets, these microscale sealed air pockets produce higher adhesive force, and the induced adhesive force increases with increasing micropit size.36 It should be noted that as the groove pitch increases, microprotrusions become smaller and nanoscale sealed air pockets between microaggregates contribute less to surface adhesive force because of the reduction of nanoscale sealed air pockets. Ultimately, the total surface adhesive force incrementally increases with an increasing groove pitch from 80 to 120 μm. Therefore, as the groove pitch increases from 60 to 120 μm, the increase of adhesive force is mainly attributed to the sealed micro-/nanoscale air pockets between the water droplet and rough solid surface. As the pitch further increases, both surface adhesive force and area fraction of the liquid− solid interface first decrease and then increase with increasing groove pitch, as indicated in Figure 2i. In this situation, the variation of surface adhesive force is mainly attributed to the changes of area fraction of the liquid−solid interface. The strong adhesion of superhydrophobic surfaces to water droplet shows potential applications in liquid transportation and analysis of liquid samples.40 Robustness of superhydrophobic surfaces is a major concern in many outdoor applications. In this work, scratch tests were

where θCB is the contact angle of water on the prepared rough superhydrophobic surface, f1 and f 2 are the area fractions of the solid−liquid and gas−liquid interface, respectively, and θ1 and θ2 are the contact angle of water on the unprocessed solid surface and in air, respectively. According to the measured θCB, the area fraction of the liquid−solid interface of water on the superhydrophobic surface is shown in Figure 2i. As the groove pitch increases from 60 to 120 μm, the area fraction of the liquid−solid interface fluctuates at around 8.5%, while adhesive force on superhydrophobic surfaces incrementally increases from 41.7 ± 1.0 to 72.3 ± 8.4 μN. On the other hand, there generally exist two kinds of the trapped air pockets: air pockets in open state (continuous with the atmosphere) and air pockets in sealed state. As the droplet moves on the superhydrophobic surface, the sealed air pockets generate negative pressure because of the volume change as illustrated in Figure 3a. In the case of a small groove pitch (e.g., 60 μm), there exist microscale open air pockets between micropillars and nanoscale sealed air pockets between 25589

DOI: 10.1021/acsami.9b06865 ACS Appl. Mater. Interfaces 2019, 11, 25586−25594

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ACS Applied Materials & Interfaces

Figure 4. (a) Evolution of contact angle on superhydrophobic surfaces with increasing abrasion length at the applied pressure of 5.5 kPa; (b) adhesive force of superhydrophobic surfaces before and after abrasion for 4.7 m; (c−e) SEM images of superhydrophobic surfaces after abrasion for 4.7 m: (c) pitch of 80 μm, (d) pitch of 140 μm, and (e) pitch of 240 μm.

Figure 5. (a) Thermal stability of the fabricated superhydrophobic surfaces on stainless steel; (b) EDS results before and after thermal treatment for 150 min; and (c) anti-icing performance of the fabricated superhydrophobic surfaces on stainless steel in natural environment at −8.5 ± 0.5 °C.

polymer−organoclay films,41 magnesium alloy,21 or silicon surface.42 Adhesive force of superhydrophobic surfaces was also measured after abrasion tests for 4.7 m, as shown in Figure 4b. After abrasion tests, the rough surfaces show much higher adhesive force than those before abrasion. After abrasion tests, arrays of micropillars are retained (Figure 4c−e), which is important for hydrophobic surfaces with high contact angles. For the fabricated hierarchical structures with small groove pitch (80 and 140 μm), the microaggregates stacked on micropillars are severely destructed and removed to adjacent grooves during abrasion tests, as

conducted to evaluate the mechanical stability and durability of the fabricated superhydrophobic surfaces. Figure 4a shows evolution of contact angle with increasing abrasion length. After abrasion for more than 2.3 m, superhydrophobic surfaces gradually become hydrophobic with contact angle less than 150°. Even after abrasion for 4.7 m, surfaces still exhibit high hydrophobicity with contact angle above 145°. These results demonstrate the excellent mechanical abrasion resistance of the fabricated superhydrophobic surfaces, which is much better than other reported superhydrophobic surfaces produced on polymer coating,18 metal/polymer composite coating,20 25590

DOI: 10.1021/acsami.9b06865 ACS Appl. Mater. Interfaces 2019, 11, 25586−25594

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ACS Applied Materials & Interfaces

lost its transparency, and transformed into a frozen state. Both the ice bulk on the initial surface and the water droplets on superhydrophobic surfaces gradually shrunk over time because of the sublimation of ice bulk and the vaporization of water droplets. After 320 min, the ice bulk on the initial surface completely disappeared, while the droplets on superhydrophobic surfaces retained transparent liquid state with ellipsoid shapes until the droplets were completely evaporated after cooling for 500 min, as shown in Figure 5c. The fabricated superhydrophobic surfaces on stainless steel exhibit a better anti-icing property than those previously reported icephobic surfaces.24−26,47,48 The excellent surface superhydrophobicity at subzero environment is responsible for the anti-icing property based on classic nucleation theory and the heat transfer between superhydrophobic surfaces and water droplets.49,50 According to classical nucleation theory, at the same temperature, the free-energy barrier for heterogeneous nucleation is much lower than that for homogeneous nucleation. The lower the temperature, the lower the free-energy barrier and the faster the nucleation rate. Upon the cold surface, heterogeneous nucleation mainly occurs near the liquid−solid interface, and the free-energy barrier, ΔG, for the formation of a liquid nucleus is closely related to the intrinsic surface wettability and can be estimated by51,52

presented in Figure 4c,d. In the case of the groove pitch of 240 μm, both microaggregates and nanoparticles covered on micropillars have been almost completely removed (Figure 4e), leading to the exposure of the initial steel surface. Table S1 summarizes chemical compositions of superhydrophobic surfaces before and after abrasion tests. After abrasion, the relative amount (atom %) of carbon and oxygen shows slight decrease while that of iron increases from 9.66 to 17.61% because of the exposure of initial surface and more nonstoichiometric oxygen-deficient iron oxides inside microaggregates and nanoparticles.30,43 Severe abrasion results in a larger liquid−solid interface between the droplet and rough solid surface. The Cassie’s state between the droplet and rough solid surface is damaged and partially replaced by Wenzel’s state with a wet-contact mode,43,44 leading to notable deterioration of surface nonwetting property and increase of surface adhesive force. To elucidate the effect of thermal treatment on surface superhydrophobicity, we heated superhydrophobic surfaces at 100 ± 5 °C and recorded dynamic behaviors of water droplet on the heated surfaces, as shown in Figure 5a. After being treated for 120 min, the superhydrophobic surfaces still retain their superhydrophobicity, showing good thermal stability. When the water droplet is set on the heated surface, hightemperature-induced bubbles at the liquid−solid interface result in the instability of the droplet. Then the droplet vibrates and rolls away from the surface or shrinks in volume as shown in Figure 5a. After being treated for more than 150 min, the fabricated surfaces lose their superhydrophobicity and become superhydrophilic as presented in Figure 5a. As soon as the droplet comes in contact with the heated surface, the droplet immediately spreads, evaporates, and rapidly disappears on the surface. Figure 5b shows energy-dispersive X-ray spectroscopy (EDS) spectra and elemental concentrations (atom %) on the fabricated surfaces before and after thermal treatment for 150 min. After treatment, carbon concentration on the surface decreases from 13.45 to 6.63%, and oxygen concentration decreases from 56.22 to 50.55%. In contrast, all the metal elemental concentrations increase after thermal treatment. The changes of surface elemental concentrations mainly result from the reduction reactions between metal oxides on the solid surface and reducing substances (carbons on the solid surface and hydrogen and carbon monoxide in air) during thermal treatment.45,46 The reactions lead to the reduction of carbons on the solid surface, and more iron and iron oxides are exposed to air. The intrinsic hydrophilicity of the polar iron oxides is amplified by rough solid surfaces. Thus, the prepared superhydrophobic surfaces become superhydrophilic. Anti-icing performance of the prepared superhydrophobic surfaces has been explored in natural environment with an ambient temperature of −8.5 ± 1 °C and a humidity of around 30%. For comparison, a sample with initial surface was used as control. A water droplet of 15 μL was used for anti-icing tests. All the samples were completely cooled for 1 h prior to antiicing tests. At such a cooling state, the initial surface exhibits smaller contact angle (Figure 5c) as compared to that at room temperature (25 °C, Figure 1d), whereas superhydrophobic surfaces present superhydrophobicity with the water droplet showing a transparent ellipsoid shape (Figure 5c). To estimate the anti-icing performance of the superhydrophobic surfaces, we monitored evolution of water droplets on surfaces for the entire duration of experiments. After cooling for 70 min, the transparent droplet on the initial surface rapidly crystallized,

ΔG =

16πγSL 3Tslf 2 3HSL 2(Tslf − Tinterface)2



where s(θC) =

1 3 1 + cos θC − cos3 θC 2 4 4


In equation, γSL is the ice-water interfacial energy, Tslf is the temperature of the freezing front, HSL is the latent heat of freezing, θC is the contact angle of ice on the substrate surface in ambient water, Tinterface is the temperature in the interface, and s(θC) accounts for the effect of heterogeneous nucleation and ranges from 0 to 1. θC = 0° means that the formation of critical ice embryo no longer requires additional energy and the nucleation process only relies on diffusion. Conversely, θC = 180° means that the nucleation process is equivalent to a homogeneous ice nucleation and the presence of ice nucleus does not reduce the Gibbs formation energy at all.53 Hence, the larger contact angle leads to a larger ΔG and a smaller nucleation rate. In other words, the larger contact angle makes the occurrence of nucleation become difficult. On the other hand, the Cassie’s state between rough superhydrophobic surfaces and water droplets indicates the existence of trapped air pockets between rough surfaces and water droplets. These air pockets reduce the heat transfer between water droplets and surfaces during the cooling process.15 On the basis of Fourier’s law, the heat transfer between solid surfaces and water droplets is proportional to the liquid−solid area fraction.50,54 As a result, the superhydrophobicity of the fabricated surfaces leads to great retardation of the icing process.

CONCLUSIONS In summary, we provide a facile methodology to fabricate robust superhydrophobic surfaces on stainless steel by a onestep laser surface texturing process. Hierarchical micro-/ nanostructures have been fabricated on stainless steel. The robust microgrid structure and microprotrusions impart great 25591

DOI: 10.1021/acsami.9b06865 ACS Appl. Mater. Interfaces 2019, 11, 25586−25594

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ACS Applied Materials & Interfaces

plastic dropper, which dripped water droplets of 5−60 μL on the surfaces (see Video S2). A contact thermometer (IKA ETS-D5, Germany) was used to heat and control sample surface temperature. Dynamic changes of the water droplet on the as-prepared surface were recorded at an elevated temperature using a camera. High-temperature surface wettability was tested by dripping the water droplet (around 12 μL) on the heated surface and recorded by a camera. The experiments were carried out indoors with an ambient temperature maintaining at 26 ± 0.1 °C. Anti-icing tests were conducted in natural environment with an ambient temperature maintaining at −8.5 ± 1 °C and humidity at 30%. Water droplet of 15 μL was used for antiicing experiments.

mechanical stability and durable self-cleaning property to the fabricated superhydrophobic surfaces. After abrasion for 2.3 m with an applied pressure of 5.5 kPa, the fabricated surfaces still retain superhydrophobicity. The obtained superhydrophobic surfaces show strong adhesion to water droplet because of the existence of sealed air pockets between hierarchical solid surfaces and the water droplet as well as the van der Waals interactions between water and the rough solid surface. The fabricated surfaces retained superhydrophobicity for more than 120 min at 100 ± 5 °C, indicating good thermal stability. An excellent anti-icing property was achieved on the fabricated hierarchical surfaces. At a temperature of −8.5 ± 1 °C in natural environment, the water droplets on superhydrophobic surfaces retained transparent liquid state with ellipsoid shapes for more than 500 min until the droplets were completely evaporated. For comparison, the water droplet on the initial surface completely froze after exposure for 70 min. The reported facile method offers an effective strategy for extending the life span of superhydrophobic surfaces and allows the surfaces to have more practical applications in the future.


S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06865. SEM images of laser ablation-induced nanoparticles, XPS spectra of C 1s and full spectrum of initial and asprepared superhydrophobic surfaces on stainless steel, durable self-cleaning performance of superhydrophobic surfaces and the fabricated directional fluidic channels on stainless steel, contact angle of superhydrophobic surfaces as a function of droplet size, and elemental content of the prepared superhydrophobic surface before and after abrasion tests (PDF) Dynamic behaviors of the falling water droplets on the superhydrophobic surface (AVI) Self-cleaning for soils (AVI) Self-cleaning for pencil shavings (AVI) Directional water flow on fluidic channels created by selective laser texturing on stainless steel (AVI) Dynamic behaviors of water droplets on as-prepared superhydrophobic surfaces after thermal treatment for 120 min (AVI) Dynamic behaviors of water droplets on as-prepared superhydrophobic surfaces after thermal treatment for 150 min (AVI)


Fabrication of Hierarchical Superhydrophobic Surfaces. Commercial 302 stainless steel plates of 25 mm × 25 mm × 1 mm size were used in this work with an average surface roughness of Ra 0.255 μm. The samples were processed by a picosecond laser system (Time Bandwidth; Duetto) which produces 10.3 ps pulses at a repetition rate of 50 kHz with a central wavelength of 1064 nm. The laser beam has a Gaussian profile with a TEM00 (M2 < 1.3) spatial mode and is directed to the sample surface using a galvanometric scanner with a telecentric f-theta lens. The beam was focused to a spot size of around 30 μm. Laser fluence was fixed at 5.13 J/mm2. The optimized laser scanning speed of 10 mm/s and the scanning time of 15 were adopted. Before laser treatment, stainless steel samples were sequentially sanded with SiC papers up to 2000 grid and ultrasonically cleaned using deionized water. Laser surface texturing was carried out by laser scanning in two perpendicular directions with a laser scanning pitch varying from 60 to 300 μm, producing microgrid textures on the surface. Experiments were performed in the air atmosphere. After laser texturing, the as-prepared surfaces were cleaned by blowing compressed dry air and then exposed to ambient air. Surface Morphology and Wettability Characterization. SEM (JSM-6701F) and a 3D laser scanning microscope (VK-X100, Keyence) were used for surface morphology characterizations. XPS (Thermo ESCALAB 250XI) and EDS coupled with SEM were used to analyze surface elemental composition. Contact angle measurements were conducted using an OCA15EC system (DataPhysics, Germany) under atmospheric condition. A syringe with a capillary tip was used to produce a 6 μL droplet of distilled deionized water onto the sample surface. Contact angle was determined by analyzing droplet images using software SCA202. The adhesive force was measured using a high-sensitivity microelectromechanical balance system (DCAT21, DataPhysics, Germany). A 3 μL water droplet was suspended with a metal ring and controlled to contact with the asprepared surface and then to leave at a constant speed of 0.05 mm/s. The force versus the moving distance during the whole process was recorded. Superhydrophobic Surface Performance. The mechanical stability of the prepared surfaces was evaluated by the scratch test using a homemade scratch tester. Scratch tests were carried out in one direction with an 800 grit SiC sandpaper as the abrasion surface. The surfaces were tested facing this abrasion surface with an applied pressure of 5.5 kPa and varied distance. The common dry soils and the prepared fine pencil shavings were taken as contaminants and used for characterizing the self-cleaning performance of the prepared surfaces. Self-cleaning tests were manually conducted by using a


Corresponding Authors

*E-mail: [email protected] (H.L.). *E-mail: [email protected] (Y.G.). ORCID

Huan Liu: 0000-0001-9009-7122 Yingchun Guan: 0000-0002-6897-6064 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially Support by the National Key Research and Development Program of China under grant 2018YFB1107400, 2018YFB1107700, and 2016YFB1102503; National Key Basic Research Program of China with grant number 2015CB059900; and National Natural Science Foundation of China (51705013, 21622302, and 21872002). We thank Dr. W. Feng and Prof. H. Zheng from Singapore 25592

DOI: 10.1021/acsami.9b06865 ACS Appl. Mater. Interfaces 2019, 11, 25586−25594

Research Article

ACS Applied Materials & Interfaces

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Institute of Manufacturing Technology for their help with laser processing.


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DOI: 10.1021/acsami.9b06865 ACS Appl. Mater. Interfaces 2019, 11, 25586−25594

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DOI: 10.1021/acsami.9b06865 ACS Appl. Mater. Interfaces 2019, 11, 25586−25594