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Applied Surface Science 355 (2015) 1238–1244

Contents lists available at ScienceDirect

Applied Surface Science

journa l h om epa ge: www.elsev ier .com/ locate /apsusc

acile fabrication of biomimetic superhydrophobic surface withnti-frosting on stainless steel substrate

an Liua, Yuan Baia, Jingfu Jinb, Limei Tiana,∗, Zhiwu Hana, Luquan Rena

Key Laboratory of Bionic Engineering(Ministry of Education), Jilin University, Changchun 130022, PR ChinaCollege of Biological and Agricultural Engineering, Jilin University, Changchun 130022, PR China

r t i c l e i n f o

rticle history:eceived 25 April 2015eceived in revised form 28 June 2015ccepted 4 August 2015vailable online 6 August 2015

eywords:

a b s t r a c t

Inspired by typical plant surfaces with super-hydrophobic character such as lotus leaves and rose petals,a superhydrophobic surface was achieved successfully by a chemical immersion process. Here, 304 SS(stainless steel) was used as substrates and a micro-nano hierarchical structure was obtained by chemicaletching with a mixed solution containing ferric chloride. The results showed that the water contactangle (WAC) decreased obviously due to surface morphology changing after chemical etching process.However, we obtained a superhydrophobic surface with a WAC of 158.3 ± 2.8◦ after modification by DTS

iomimeticuperhydrophobictainless steelnti-frostinghemical etching

(CH3(CH2)11Si(OCH3)3). Furthermore, the superhydrophobic surface showed an excellent anti-frostingcharacter compared to pure staining steel. The surface morphology, chemical composition and wettabilityare characterized by means of SEM, XPS and water contact angle measurements. This method couldprovide a facile, low-cost and stable route to fabricate a large-area superhydrophobic surface with anti-frosting for application in various environments including in humid condition.

. Introduction

In nature, various plants and insects, including lotus leaves1–4], rose petals [5], and legs of water striders [6,7], exhibit thenusual phenomenon of wettability. Wettability is one of the most

mportant characters of material surfaces from both fundamentalnd practical aspects [8], which is governed by both the chemi-al composition and the geometrical microstructure of the surface9–13]. Superhydrophobic surfaces, with a static contact angle (CA)igher than 150◦ and low contact angle hysteresis [14], have beenxtensively studied and possess very important applications in pre-enting ice/frost [15,16], microfluidics [17–19], collecting water20,21], anti-corrosion [22,23] and self-cleaning [24–26] etc.

Stainless steel as a common metallic materials, for its superiororrosion resistance and decorative function, has been employedor applications in various fields, including petrochemical, con-truction, maritime and aviation industries.[27] However, icend frost formation and accumulation on substance surfacesre known to cause serious problems, such as hindering theperation and impairing the efficiency of infrastructural compo-

ents and machines, including aircrafts, ships, electrical powerlants, transportations telecoms equipment and aeronautics andstronautics.[28] In recent decades, efforts have been made to

∗ Corresponding author.

ttp://dx.doi.org/10.1016/j.apsusc.2015.08.027169-4332/© 2015 Elsevier B.V. All rights reserved.

© 2015 Elsevier B.V. All rights reserved.

look for anti-ice/frost materials which can effectively retard andprevent ice/frost formation on cold surfaces. Relevant resultshave confirmed that the super-hydrophobic materials are promis-ing candidates [29]. However, the highest water contact anglesreported on smooth, low energy surfaces are in the range of 120◦

[14]. Inspired from the lotus leaf, researchers have demonstratedthe wettability of solid surface is determined by both the chemi-cal compositions and geometrical microstructures of the surfaces[1,30–34]. Therefore, a rough surface texture is necessary for thepreparation of super-hydrophobic surface. Yan et al. [35] system-atically researched the characteristics of the surface patterns basedon nanoparticles and the formed wettability. Conventionally, twoapproaches can be used to develop the superhydrophobic surface:one is to fabricate surface with micro-nano hierarchical structureson low surface-energy materials; the other is to modify rough solidsurface by low surface-energy molecules.

By now, a variety of biomimetic surfaces with superhydropho-bicity have been fabricated based on the combination of surfacemicro-nano structures and chemical compositions by using manydifferent synthetic methods, including sandblasting [36,37], sol–gelmethods [38,39], chemical vapor deposition [40,41], electrochem-ical deposition [42,43], chemical etching [44,45] and laser surface

treatment [46,47] etc. Recently, some achievements on the creationand characterization of stable superhydrophobic surfaces on stain-less steel have been made. Bizi et al. [46] obtained a multi-scalecorrugated structure by femtosecond laser, making transformation

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Y. Liu et al. / Applied Surfac

rom hydrophilicity to hydrophobicity on the surface of AISI 316LS. Han et al. [42] fabricated lotus-leaf-like superhydrophobic metalurfaces by a simple electrochemical reaction on steel sheets withulfur gas, and a subsequent fluorosilane treatment. Liang et al.43] prepared superhydrophobic nickel films on SS316L substratesy a combined electrodeposition and fluorinated modificationpproach. However, most approaches involve severe conditions,ncluding dangerous chemicals, low processing efficiency, complexevices and high processing cost. To solve the aforementioned dis-dvantages, a facile, highly effective, and low-cost approach wasdopted to fabricate the surperhydrophobic surface on SS substrate.

In our paper, we present a simple method to fabricate super-ydrophobic surfaces on 304 SS with micro-nano structure whichere then modified by DTS. Furthermore, the anti-frosting behavior

f superhydrophobic SS surfaces was investigated, which indicatedhat the frosting of superhydrophobic surface was greatly retardedompared with pristine SS surface.

. Experimental

.1. Chemical etching

SS304 sheets (0.08 wt% C, 18.17 wt% Cr, 9.3wt% Ni, 1.54 wt Mn, 0.8 wt%Si, and the balance iron) with the size of0 mm × 20 mm × 1 mm were polished with 500# and 800# sand-apers in turn, and then cleaned with acetone in an ultrasonic bathor 15 min. Samples of SS were immersed in a solution mixture oferric trichloride aqueous solution (FeCl3, 1.65 mol/L), hydrochlo-ic acid (HCl, 37%) and hydrogen peroxide (H2O2, 30%) (15:1:1,ol%) for 20 min. Here the (FHH FeCl3 + HCl + H2O2 (15:1:1, vol%))olution was used in this work as etching solution. After etchingreatment, samples were again rinsed with DI and dried in atmo-phere condition. The treated samples desiccated in an oven atoom temperature (26 ◦C) for 15 min.

.2. Hydrophobic film modification

In order to obtain superhydrophobic surface, DTS was used toodify SS film. The samples was placed in a modified solution pre-

ared by adding 60 �L DTS to 40 mL Toluene (C6H5CH3) for 60 min,nd then desiccated in an oven at 30 ◦C.

.3. Characterization

The obtained samples were characterized by scanning elec-ron microscopy (SEM) (JBM-7500F, Japan Electronic). The surfacehemical composition was examined by X-ray photoelectron spec-roscopy (XPS, SPECS XR50). The contact angles were measuredith a contact angle meter (JC2000A Powereach, China), where

�L droplets were placed at three different places of the surfaceor investigation and the average value was taken as the contactngle used to measure the static water contact angle on the SS film.

.4. Experiments of anti-frosting property

The experimental apparatus was composed of temperature con-rol, image acquisition and data collection systems. The schematiciagram of the experimental setup is shown in Fig. 1. This apparatus

ncluded a thermostatic bath, K-type thermocouple, data acquisi-ion (DAQ11625, Quatronix, China), a microscope (MZDM0745, MT,hina) and a computer etc. The frosting processes were monitored

nd collected by a CCD camera (73X11H, Mintron, China). The sur-ace temperature of the samples on the cold plate (40 mm × 40

m × 3 mm) was precisely maintained in a range of 20–35 ◦C. Theemperature was measured by a K-type thermocouples which was

Fig. 1. The schematic representation of the experimental setup.

connected with the cold plate and the measurement error was±1.0%.

3. Results and discussion

3.1. Microstructure

To obtain rough microstructure on SS surface, the sampleswere chemically etched for 20 min in FHH solution with 0.5 mol/L,1 mol/L, 1.5 mol/L, 2 mol/L and 2.5 mol/L FeCl3, respectively. Duringthe FHH etch process, SS samples change from their well-known,shiny silver appearance to black due to the added surface roughnessand changes in chemical surface composition. Through the rede-position of metal chloride and oxides, roughness is created withmicro/nano scale on SS surface. Fig. 2 shows the SEM images ofan as-prepared surface. It can be found that FeCl3, as an etchingagent, plays a major role in the forming process of the hierar-chical structure. With the reaction going on, the SS surface wasgradually etched and irregularly-shaped micro-nano islands andconcave pits were formed in various sizes on the specimen sur-face at different FeCl3 solution concentration. As shown in Fig. 2a,the micro-scale pits structure appeared on the surface of SS, whichwas arising from the lack of iron on SS surface after FeCl3 etching. Asthe etching concentration was increased, the pits become deeperand smaller, constituting a rough micro-nano multiscale structureon the surface. The micro-nano structure gradually increased withthe increasing of FeCl3 solution concentration, as shown in Fig. 2band c. When further increasing the concentration of ferric chlo-ride, the solution concentration reached 2 mol and the rough microstructures were relatively small and more intensive. The struc-tured surfaces were of a combination of a coarse microstructureand submicrometer-sized structures, as shows in Fig. 2d, and thestructure was lamellar films interlaced whose structure can trapa large amount of air. However, as the FeCl3 density is contin-uously increased and reaches 2.5 mol/L, it can be found that thehierarchical structure on the SS surface is destroyed.

We further analyzed the reasons for the formation of micro-nano structures on SS surface. In the etching process, besides FeCl3concentration, there are a number of other factors determiningthe SS morphology, such as etching time, etchant composition, thecharacteristics of the metal being etched, and so forth. The reactionmechanism of stainless steel in etching solution can be representedas:

2FeCl3 + Fe → 3FeCl2 (1)

As is known to all, there are many microscopic defects and faultsbetween the crystals of stainless steel. When the etching agent

1240 Y. Liu et al. / Applied Surface Science 355 (2015) 1238–1244

eCl3 concentration for (a–e) is 0.5, 1, 1.5, 2 and 2.5 mol/L, respectively.

rsocmTFhtiissfjiwmwsas

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Fig. 3. SEM images of the stainless steel surface under different etching time for (a)10 and (b) 20 min, respectively. The inset of (b) shows the high magnifications of

Fig. 2. SEM images of the stainless steel surface under different F

eaches a certain concentration, the stainless steel with micro-copic defects and faults priority dissolves and the concentrationf ferrous ion increases in the etching solution. As the etching pro-eeds, Fe2+ increases and deposition on the surface being etchedakes penetration of the etchant to the etched surface difficult.

he effect of HCl was studied by keeping the concentrations ofeCl3 and H2O2 in the etching solution constant. By addition ofydrochloric acid, the solubility of ferrous ion in the etchant solu-ion can be increased and therefore results in a slight increasen the etch rate of SS surface. Furthermore, the addition of H2O2s needed to oxidize Fe2+ into Fe3+, keeping the etching reactionustained. As shown in Fig. 3, after etching treatment with FHHolution for 10 and 20 min respectively, there is no significant dif-erence in the morphology of sample surfaces and the contact angleust increased from 154◦ to 158◦ after DTS modification. The exper-ment results show that at the initial stage of etching reaction,

hich is very intense and accompanied by a lot of bubbles, theicro-nano structure appeared and gradually increased. However,ith the decrease of the ion concentration, the etch rate greatly

lowed down when the etching time exceeded 10 min. The resultslso indicate that to obtain micro-nanostructure on stainless steelubstrates, an etchant consisting of FeCl3, HCl and H2O2 are needed.

.2. Chemical composition

XPS is used to test the chemical composition of the as-preparedurfaces modified by DTS after etching. Fig. 4a and b show the XPSurvey spectrum of the chemical composition on the modified sur-aces. It reveals the presence of C, Si and O on the as-preparedurfaces. The modified surface shows significant peaks at 284.7 eVnd 102.75 eV corresponding to the C1s peak and Si 2p peak. Theomposition of Si, C increased remarkably compared to the sub-trate, implying that the SS surface has been covered with silanelm. As a result, the obtained surfaces were able to possess bothicro-nano hierarchical structure and low surface energy at the

ame time, thus providing basic conditions for superhydropho-icity. Furthermore, the molecules grafted SS substrates throughhe surface reaction of the hydrolysis silane species with the func-

ional groups (Fe–OH) of metallic surface to form self-assembled

onolayer (SAM) with low surface energy as shown in Fig. 5. Theormation mechanism of SAMs is as follows: first, the hydrolysiseaction is initiated between methoxy groups (–OCH3) and water

the stainless steel etched by 2 mol FeCl3 solution.

(H2O) to form silanols (Si–OH) (Eq. (2)), and then the silanols react

with the surface functional groups of hydroxyl (–OH) on SS surfaceto form a self-assembled film (Eq. (3)). In addition, the silanols canalso bond with other silanol by forming a siloxane bond (Si–O–Si)

Y. Liu et al. / Applied Surface Science 355 (2015) 1238–1244 1241

Fig. 4. XPS spectrum of the as-prepared superhydrophobic stainless steel surface surface of (a) full-spectrum and (b) Si 2p.

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ig. 5. Schematic illustrating the formation of DTS self-assembled monolayers ontainless steel.

Eq. (4)). After modified with self-assembled films, the hydrophilicS surface becomes hydrophobic.

Si–OCH3 �Hydrolysis

Si–OH + CH3OH (2)

SiOH + FeOH �Condensation

Si–O–Fe ≡ +H2O (3)

SiOH + SiOH � ≡ Si–O–Si ≡ +H2O (4)

.3. Wettability

Fig. 6 shows the CA of SS etched with FHH solution and theollowing modification with DTS. The samples are hydrophilicCA < 20◦) without the DTS coating. The CA reached 117.6 ± 2◦ after0 min etch reaction in 0.5 M FeCl3 and DTS modification. As thetchant concentration increased to 1 M, the CA grew to 134.2 ± 2.2◦

nd when the etchant concentration reached 1.5 and 2 M, the

A rose to 142.9 ± 2◦ and 158.3 ± 2.8◦ after DTS modification andA hysteresis was less than 3 ± 1.2◦. Therefore, we can infer thathe micro–nano structures with low surface energy contributedreatly to the superhydrophobic property because the air is trapped

Fig. 6. Contact angles at different FeCl3 concentration.

between the droplets and the hierarchical structure of the coatingsurface. Afterwards, with further increase of FeCl3 concentration,the excessive etch of SS resulted in the destroying the complex hier-archical structure, and therefore the contact angle values decreasedslightly to 154.5 ± 3.2◦. Fig. 6 indicates that as FeCl3 concentrationwas raised from 0.5 to 2 mol of FHH solution, the density of the sur-face microstructures gradually changed, and the superhydrophobicproperty was slightly enhanced.

3.4. Anti-frosting properties

In order to reveal the anti-frosting property of superhydropho-bic surface, a study on the frost formation on plain SS surface andsuperhydrophobic SS surface with a size of 20 mm × 20 mm werecarried out at room temperature of 14 ◦C, relative humidity of 60%and cold surface temperature of −22 ◦C. Fig. 6a–d shows opticalphotographs of frost formation on plain SS with a CA of 66◦ at dif-ferent time. As a comparison, the superhydrophobic SS surface witha CA of 158.3◦ was obtained by etching process and then modifiedwith DTS (Fig. 7e–h). In Fig. 7 the frost-free regions are separatedfrom the regions covered by frost with red lines. It was found that

the frost size and distribution were completely different from eachother on the superhydrophobic surface and the plain SS surface. Thestarting time for the observable frost crystal was also different. Thefrost particles of several microns formed on plain SS surfaces after

1242 Y. Liu et al. / Applied Surface Scien

Fst

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ig. 7. The optical photographs of frost formation on (a–d) plain stainless steelurface and (e–h) superhydrophobic surface with contact angle of 158.3◦ at roomemperature 14 ◦C, humidity 60% and cold surface −22 ◦C with magnification of ×4.5.

nly 30 s (the bright areas in Fig. 7b) and the surface was fully cov-red by a continuous frost layer only after 90 s (Fig. 7d). In contrast,lmost no frost is found on the superhydrophobic surface even after0 s and frost firstly nucleates and grows at such areas as markedy the red line in Fig. 7f–g. Comparing the frost and frost-free arean superhydrophobic surfaces, it shows that the area covered withrost on these surfaces increases with the increase of time. Whats more, once the water droplets on the superhydrophobic sur-

ace were frozen, the frost particles tend to grow on the top ofhe original frost layer with sizes from a few microns to tens of

icrons. About 90 s later, the surface is covered almost completelyy frost on plain SS surface (Fig. 7d) while only part of the area is

ig. 8. Schematic illustrations of a water droplet on the as-prepared surface. (a) The modith a superhydrophobic surface.

ce 355 (2015) 1238–1244

covered with frost on the superhydrophobic surface (Fig. 7g). Theresults show that the small droplet size on superhydrophobic sur-face delayed the freezing of droplet, retarded the appearance offrost crystal in the early stage about 60 s under the temperatureof −22 ◦C, and reduced the density of the frost layer growth on thesurface of hydrophobic coating. The frost formation process on coldsurface is dynamic and complex and the reaction process is not easyto control. The main factors affecting frost formation is the temper-ature of cold surface and environmental humidity. The initial stageof frost formation was accompanied by atomization. Therefore, thedroplets formation and growth are not able to be observed at thistemperature. Furthermore, the water droplets on plain SS surfacehas been completely frozen and a large number of small frost sticksgrow on its surface after 30s, and the volume of frozen drops at thisstage is very small, leading to an increase in the density of frostlayer in the later stage.

The superhydrophobic SS surface was horizontally placed on acold plate with the temperature being kept at −22 ◦C for 3 min.Then it was moved out and placed in ambient environment untilthe surface was completely dried. The freezing-thawing processwas repeated 10 times on the same sample, which could still retardthe appearance of frost crystal after 60 s. Moreover, the preparedsample surface still showed superhydrophobicity after exposed tothe air for one month. This stable character is important for thesurface to be applied to achieve practical application.

The profile of water in contact with the as-prepared surface areschematically shown in Fig. 8a and b Water on these surfaces is pri-marily in contact with air pockets trapped in the rough surfaces. Thecontact angle is calculated in terms of the Cassie–Baxter equation[48]:

cos �r = f1 cos �o − f2 (5)

where f1 and f2 are the fractions of the solid surface and air incontact with the liquid, respectively; �r (158.3◦) and �o (66◦) arethe contact angles on the as-prepared surface by Chemical etchingand hydrophobic thin film deposition and on the primary surface,respectively. Given that f1 + f2 = 1, we calculated the correspondingf1 to be 0.05. The low value of f1 implies that only about 5% of thesurface is in direct contact with water. Therefore, the air trappedin the micro–nano structures surface plays an important role inincreasing the contact angle and enhancing the hydrophobicity.

Frost formation on a cold surface is a typical crystal growth pro-cess and the contact angle will certainly affect the frost growthby influencing vapor condensation and nucleation on the cold sur-face. According to classic nucleation theory [49], the nucleation

free-energy barrier (Gc) for the heterogeneous nucleation is:

�Gc = 4�rc2�1�

3f(

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(6)

el of a water droplet on the surface. (b) Cross-sectional profile of water in contact

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=(

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here rc is the critical radius, �lv is the solid–liquid interfacial ten-ion, g is the Gibb’s energy density difference between ice and liquidater and � is the apparent contact angle.

From Eqs. (6) and (7), it can be seen that Gc is proportional to(�), indicating an increase in Gc with the contact angle. Therefore,ncreasing the contact angle of a surface will augment the energyarrier, and thus restrain the frost crystals nucleation. Therefore,he superhydrophobic SS surface is more inclined to restrain frozenroplets and frost growth than the plain surface.

. Conclusion

In summary, we have developed a facile, highly effective, andow-cost two-step methodology to fabricate the biomimetic super-ydrophobic surface on 304 SS. First, the SS surface with binaryicro–nanoscale structures can be obtained by changing the mor-

hology using a chemical etching process with FHH solution,nd then a superhydrophobic surface with a CA for water of58.3 ± 2.8 ◦can be prepared by modifying the surface with low freenergy material of DTS. Furthermore, the as-prepared films havexcellent anti-frosting property. The facile, highly effective, andow-cost approach can be adopted to fabricate the surperhydropho-ic SS surface which may be a good super-hydrophobic engineeringaterial in the industrial applications even in humid condition.

cknowledgements

The authors thank the National Natural Science Foundation ofhina (nos. 51275555, 51475200 and 51325501), Science and Tech-ology Development Project of Jilin Province (no. 20150519007JH)nd Basically Science Research Foundation of Jilin University (no.013ZY09).

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