robust technique allowing the manufacture of superoleophobic (omniphobic) metallic surfaces

DOI: 10.1002/adem.201300561

Robust Technique Allowing the Manufacture ofSuperoleophobic (Omniphobic) Metallic Surfaces**

By Anton Starostin, Viktor Valtsifer, Vladimir Strelnikov, Edward Bormashenko,* Roman Grynyov,

Yelena Bormashenko and Alexander Gladkikh

Omniphobic metallic surfaces obtained with a robust two-stage process are reported. The surfacesdemonstrate high apparent contact angles accompanied with a low contact angle hysteresis for avariety of liquids, including water, diidomethane, canola, castor, silicone oils, and crude petroleumoil. The superoleophobicity was achieved by fluorination of the nano-rough aluminum surfacesunder treatment with perfluorononanoic acid, as established by TOF-SIMS spectrometry. Thestability of the Cassie wetting regime was investigated. The critical surface tension of a liquidcorresponding to the onset of wetting transitions was established experimentally.

1. Introduction

The discovery of natural surfaces demonstrating pro-nounced water repellence (the so-called “lotus effect”)stimulated extended theoretical and experimental researchof wetting phenomena occurring on rough surfaces. It wasdemonstrated that a diversity of biological objects, includingplants, butterfly and bird wings, and water-strider legs aredistinguished by an apparent contact angle (APCA) close toits extreme value of 180.[1] The paradigm of wetting ofchemically heterogeneous and rough surfaces is well under-stood today.[2,3] The APCA characterizing the wetting ofrough surfaces depends on both the topography and thechemical composition of the surface.[4,5] Wetting occurring onheterogeneous surfaces is described by the Cassie–Baxtermodel.[2] The Cassie–Baxter equation applied to a solidsurface comprised of pores (see Figure 1A) yields for the

APCA u*:

cos u� ¼ � 1þ f Sðcos u þ 1Þ ð1Þ

where fS and 1 � fS are the fractions of the solid and airsurfaces underneath the drop respectively, and u is the Youngcontact angle, established for a flat, homogenous, non-deformable solid surface.[2] It was shown both experimentallyand theoretically that the hierarchical multi-scale topographyof a relief strengthens water repellency of surfaces andincreases the APCA.[6] In this case, the modified Cassie–Baxterequation considering the hierarchical topography of the reliefshould be applied.[6]

The wetting of rough, chemically homogenous surfaces isgoverned by the Wenzel model.[2b] According to the Wenzelmodel, the surface roughness r, defined as the ratio of the realsurface in contact with liquid to its projection onto thehorizontal plane, always magnifies the underlying wettingproperties (see Figure 1B). Both hydrophilic and hydrophobicproperties are strengthened by surface textures. The apparentWenzel contact angle is given by Equation 2:

cos u� ¼ r cos u ð2Þ

One more wetting state has been introduced.[7] This is theCassie impregnating state, depicted in Figure 1C. In this case,liquid penetrates into grooves of the solid and the drop findsitself on a substrate viewed as a patchwork of solid and liquidsurfaces (solid “islands” ahead of the drop are dry, as shownin Figure 1C). The pure Cassie and Wenzel wetting regimesare rare in occurrence; the so-called mixed wetting, depictedin Figure 1D and described by more complicated equations, isoften observed.[8] Rigorous thermodynamic grounding ofEquations 1 and 2 has been reported recently.[7–9]

*[*] A. Starostin, V. Valtsifer, V. StrelnikovInstitute of Technical Chemistry of Ural Division of RussianAcademy of Science, Perm, RussiaE. Bormashenko, R. Grynyov, Y. BormashenkoPhysics Faculty, Ariel University, P.O.B. 3, 40700, Ariel,IsraelE-mail: [email protected]. GladkikhWolfson Applied Materials Research Center, Tel AvivUniversity, Tel Aviv 69978, Israel

[**] The work was financially supported by the Foundation forBasic Research (Grant Nos. 13-03-96112 and 14-03-96009),the Foundation for Assistance to Small Innovative Enter-prisesin the Scientific and Technical Field (Agreement No.0001799) and of Ministry of Education of Perm Region(Agreement No.C-26/203 of 09.12.2011).

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High APCAs are necessary but not sufficient for a truehydrophobicity of a surface attended with self-cleaningproperties. Such a surface must also be characterized by alow contact angle hysteresis and high stability of the Cassiewetting state preventing Cassie–Wenzel transitions, convert-ing the wetting regime from low adhesion to a “sticky” one.[10]

The quantitative characterization of “sticky” wetting withcentrifugal adhesion balance was reported recently.[10g]

Very different experimental techniques have been reported,which allow the manufacture of superhydrophobic surfa-ces.[11] However, preparing reliefs demonstrating highlystable Cassie wetting regime and pronounced oleo-repellencyremains the challenging experimental task. Oils are usuallycharacterized by surface tensions much lower than that ofwater, and strong liquid–solid interactions; thus, theypenetrate easily into grooves constituting the relief, promot-ing sticky Wenzel-like or Cassie impregnating wetting. Anumber of sophisticated experimental techniques werereported, which allowed the manufacture of oleophobicsurfaces.[12] However, the development of a robust industri-ally applicable technique giving rise to superoleophobicsurfaces remains attractive for researchers. Our paper reportsa robust technique resulting in simultaneously hydrophobicand oleophobic metallic surfaces.

2. Experimental Section

Superoleophobic metallic surfaces were prepared asfollows. Polished aluminum plates with a thickness of200 mm were cleaned thoroughly with ethanol and acetone.The cleansed Al plates were immersed for 5 min in a 5% watersolution of hydrochloric acid (HCl was supplied by AlfaAesar). Etching of the Al plate by the HCl gave rise to thenano-rough relief depicted in Figure 2. After etching, thesamples were dried during 10 min at the temperature of100 °C.

Dried micro-rough Al plates were immersed for 30 min in asolution of perfluorononanoic acid, 97% C9HF17O2 (suppliedby Alfa Aesar). Then the hydrophobized plates were driedduring 30 min under the temperature of 80 °C.

Wetting properties of the surfaces were established for thefollowing liquids: de-ionized water, diiodomethane, ormethylene iodide (MI, CH2I2, supplied by Sigma–Aldrich);dimethylformamide (DMF, (CH3)2NC(O)H, supplied byBiolab); polydimethylsiloxane (PDMS, (C2H6OSi)n, silicone

Fig. 1. Wetting regimes: (A) the Cassie air trapping, (B) the Wenzel, (C) the Cassieimpregnating, and (D) mixed.

Fig. 2. SEM image of the micro-rough Al plate. (A) Scale bar is 5mm. (B) Scale bar is 1mm.

A. Starostin et al./Robust Technique Allowing Manufacture of Superoleophobic Surfaces

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oil for melting point and boiling point apparatuses, suppliedby Aldrich); edible oil canola, castor oil (supplied by VitamedIsrael); turpentine mineral spirit (supplied by Maoz, Israel);crude oil (petroleum), supplied by Givot Olam Petroleum.

Contact angles (static, receding, and advancing) weremeasured by a Ram�e-Hart Advanced Goniometer Model 500-F1. The advancing and receding contact angles weremeasured by the tilted plate method. Measurements weremade on both sides of the drop and were averaged. A series of10 experiments was carried out for every aforementionedliquid.

Sliding angles were established for 30 mL droplets with thetilted plate method, as a minimal inclination angle of a surfaceat which a drop starts to slip downwards the plate.

Surface composition was analyzed by time-of-flightsecondary ion mass spectrometry (TOF-SIMS) using aPhysical Electronics TRIFT II instrument. The analysis wasperformed with a 15 KeV Gaþ primary ion beam andcollecting positive secondary ions.

3. Results and Discussion

3.1. Wetting Properties of Al Plates Treated with thePerfluorononanoic Acid

Superhydrophobic and superoleophobic metallic surfacesmanufactured under sophisticated experimental techniqueswere reported recently by various groups.[13] Our paperreports the simple two-stage process giving rise to omni-phobic metallic surfaces. The Al surface resulting from thechemical etching carried out at the first stage of the processdemonstrates complete wetting (the apparent water contactangle is close to zero). Thus, it is reasonable to suggest thatWenzel (homogeneous) wetting or the Cassie impregnatingwetting occurred at the etched Al surface depicted in Figure2.[2b,4,7,14] It noteworthy that it is not simple to distinguishbetween the Wenzel and the Cassie impregnating wettingregimes that cause complete wetting on inherently hydro-philic surfaces.[14] Obviously, nano-rough relief itself,

depicted in Figure 2, did not endow the aluminium surfacewith omniphobic properties.

The wetting regime changed dramatically after treatmentof the etched Al surface with the perfluorononanoic acid asdescribed in the Section 2. The surface gained pronouncedomniphobic properties, illustrated by Figure 3. High APCAswere registered for a diversity of liquids (see Table 1),including edible canola and castor oils, diiodomethane, andsilicone oil. High APCAs observed for water and canola oil aredisplayed in Figure 3. Data related to APCAs, contact anglehysteresis, and sliding angles established for various liquidsare summarized in Table 1.

3.2. Study of the Stability of the Cassie Wetting Regimewith Water/Alcohol Solutions

It is recognized from the data presented in Table 1 that thereported metallic surface demonstrated distinct Cassiewetting for certain liquids (water, MI, canola, and siliconeoils), whereas the same surface demonstrated moderateoleophobicity for crude petroleum and Castor oils (therelatively high sliding angles established for 30 mL dropletsare noteworthy). High-stick Wenzel-like wetting was ob-served for DMF and turpentine. It should be stressed that thehigh APCA does not guarantee the oil-repellence of a surface.Low contact angle hysteresis and high stability of the Cassiewetting state are necessary for providing easy sliding ofdroplets accompanied by self-cleaning properties of thesurface.[14] It should be mentioned that it is more difficultto manufacture surfaces repelling liquids possessing lowvalues of surface tension. It was suggested that there exists acertain critical value of the surface tension gc of a liquid atwhich the Cassie wetting is not already observed; and adroplet demonstrates the high sticky Wenzel wetting.[15]

For the experimental establishment of gc, several groupsexploited water/alcohol solutions, possessing controllablesurface tension.[15] Droplets of water/alcohol solutions weredeposited gently on the studied surface, and the APCA wasmeasured. The concentration of ethanol in the droplets was

Fig. 3. Images of 8ml water (A) and edible canola oil (B) droplets deposited on the omniphobic metallic surface.


DOI: 10.1002/adem.201300561 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.aem-journal.com 3ADVANCED ENGINEERING MATERIALS 2014,

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gradually increased, and as expected the APCA decreased. Ata certain concentration of ethanol, an abrupt change in theAPCA was observed, indicating the onset of the wettingCassie–Wenzel transition. The surface tensions of the water/ethanol solutions displayed in Figure 4 were taken fromexisting literature.[16]

Figure 5 displays the dependence of the APCA establishedfor various concentrations of water/ethanol solutions on thesurface tension of the water/alcohol solutions. The abruptdecrease of the apparent contact angle was registered at

gc � 40– 45 mJm2, corresponding to the onset of the Cassie–Wenzel transition.[14] Let us compare this result with theexperimentally established oil repellency of reported surfaces,summarized in Table 1 (surface tensions of oils used in theinvestigation were extracted from ref.[17]). It is recognized thatour surfaces repelled water and MI, which demonstratesurface tensions higher than gc. The APCAs established forwater and MI were high and the sliding angles were low. Atthe same time, high stick wetting was observed for DMF andturpentine possessing surface tension lower than gc. Themoderate oil repellency of the surfaces with respect to theCastor oil (demonstrating a surface tension close to gc) is alsodiscernible.

Somewhat surprising was the oil-repellency of reportedmetallic surfaces established for canola, silicone, and crudepetroleum oils. Paradoxically, all these oils possess surfacetensions lower thatgc. This means that use of the criterion ofthe critical surface tension for predicting the stability of theCassie wetting (implying the oil-repellency of a surface) needssome care. This is quite understandable, because the wettingproperties of a surface depend not only on the surface tensionof a liquid but on a triad of interfacial tensions, including thesolid/liquid surface tension, which is regrettably not a well-estimated physical value.[4,5,14]

Now, let us discuss the chemical composition of thereported surfaces. TOF-SIMS study of the superoleophobicmetallic surfaces indicated the presence of a fluorocarbonsurface layer. Figure 6 shows comparative surface massspectra obtained from the Al plate surface before and aftertreatment with the solution of perfluorononanoic acid.Characteristic mass peaks of the fluorocarbon compoundfragments are distinctly observed at the TOF-SIMS spectrumof the perfluorononanoic acid treated, superoleophobic Alsurface. Table 2 allows the identification of the mass peaks offluorocarbon compound fragments.

Hence, it is reasonable to attribute the superoleophobicityof the perfluorononanoic acid treated aluminum surface to thecombination of the hierarchical nano-topography of the reliefdepicted in Figure 2 with the fluorination of the surface.

Table 1. Wettability of reported metallic surfaces established for various liquids.

LiquidSurface tension, g, 10� 3

[mJ m� 2]Apparent contact

angle, �1°Advancing contact

angle, �1°Receding contact

angle, �1°Sliding

angle, �1°

Water[a]

72.0MI

[a]66.98 150 160 126 6

Canola oil[b]

28–30 153 156 135 6Castor oil

[c]

40.4 155 160 99 20Silicone oil

[d]

20 142 158 117 7Crude oil

[e]28–30 135 145 104 15

DMF[a]

35.74 123 126 70 –Turpentine

[f]

21.1 127 144 86 –

[a] Surface tensions are extracted from ref.[17a]

[b] Surface tension is extracted from ref.[17b]

[c] Surface tension is extracted from ref.[17c]

[d] Surface tension is extracted from ref.[17d]

[e] Surface tension is extracted from ref.[17c,e]

Fig. 4. The dependence of the surface tension of water/ethanol solutions on theconcentration of the ethanol.[16]

Fig. 5. The dependence of the apparent contact angle (APCA) on the surface tensionof water/alcohol solutions.



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4. Conclusions

We reported superoleophobic metallic (aluminum) surfa-ces obtained with a robust two-stage technique. The surfacesrepelled water, MI, canola, and silicone oils. Moderateoleophobicity was registered for the Castor and crudepetroleum oils. At the same time, highly sticky wetting wasobserved for DMF and turpentine. The stability of the Cassiewetting was investigated with the use of water/alcoholsolutions allowing the control of the surface tension of adroplet. The critical surface tension of a liquid gc correspond-ing to the onset of wetting transitions was establishedexperimentally. We conclude that the criterion of the criticalsurface tension of a liquid corresponding to the breaking ofthe Cassie wetting state should be used with some care.Canola, silicone, and crude oils possessing surface tensionslower than gc were strongly repelled by the surface. The

observed superoleophobicity was achieved by fluorination ofthe nano-rough aluminum surface under treatment withperfluorononanoic acid, revealed by TOF-SIMS spectrometry.The crude-oil-phobicity of the reported metal surfaces makesthem suitable for exploitation in the petroleum industry.[18]

Received: December 16, 2013Final Version: January 22, 2014

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Fig. 6. TOF-SIMS spectra of perfluorononanoic acid treated (upper) and virgin(lower) Al surfaces.

Table 2. Mass peaks of fluorocarbon compound fragments revealed in TOF-SIMSspectrum of superoleophobic (omniphobic) surfaces.

Compound Mass peak

C 12CF 31CF2 50CF3 69C3F3 93C2F4 100C3F4 112C2F5 119C3F5 131C4F5 143C3F6 150C4F6 162C3F7 169C4F7 181C5F7 193


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robust technique allowing the manufacture of superoleophobic (omniphobic) metallic surfaces

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