AppliedSurfaceScience379(2016)55–65
ContentslistsavailableatScienceDirect
Applied
Surface
Science
j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c
The
effect
of
polar
end
of
long-chain
fluorocarbon
oligomers
in
promoting
the
superamphiphobic
property
over
multi-scale
rough
Al
alloy
surfaces
Zubayda
S.
Saifaldeen
a,b,
Khedir
R.
Khedir
a,b,∗,
Merve
T.
Camci
c,
Ahmet
Ucar
c,
Sefik
Suzer
c,
Tansel
Karabacak
aaDepartmentofPhysicsandAstronomy,UniversityofArkansasatLittleRock,LittleRock,AR72204,UnitedStates bDepartmentofPhysics,CollegeofSciences,UniversityofDuhok,Duhok,KurdistanRegion,Iraq
cDepartmentofChemistry,BilkentUniversity,Ankara06800,Turkey
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:Received27December2015
Receivedinrevisedform18March2016 Accepted8April2016
Availableonline9April2016
Keywords: Aluminumalloy Micro-nanostructure Superamphiphobic Fluorocarbonoligomer Polarend Conformality Chemicalstability
a
b
s
t
r
a
c
t
Roughstructureswithre-entrantpropertyandtheirsubsequentsurfaceenergyreductionwith
long-chainfluorocarbonoligomersarebothcriticalindevelopingsuperamphiphobic(SAP,i.e.bothsuper
hydrophobicandsuperoleophobic)surfaces.However,morphologyofthelow-surfaceenergylayeron
aroughre-entrantsubstratecanstronglydependonthefluorocarbonoligomersused.Inthisstudy,
theeffectofpolarendofdifferentkindsoflong-chainfluorocarbonoligomersinpromotinga
self-assembledmonolayerwithclosepackedmoleculesandrobustadhesiononmulti-scaleroughAlalloy
surfaceswasinvestigated.HierarchicalAlalloysurfaceswithmicrogroovesandnanograssstructures
weredevelopedbyasimplecombinationofone-directionalmechanicalsandingandposttreatment
inboilingde-ionizedwater(DIW).Threetypesoflong-chainfluorocarbonoligomersof1H,1H,2H,
2H-perfluorodecyltriethoxysilane(PFDTS),1H,1H,2H,2H-perfluorodecyltrichlorosilane(PFDCS),and
perfluorooctanoicacid(PFOA)werechemicallyvaporizedontotheseroughAlalloysurfaces.ThePFDCS
exhibitedthelowestsurfacefreeenergyoflessthan10mN/m.Thecontactangleandslidingangle
mea-surementsforwater,ethyleneglycol,andpeanutoilverifiedtheSAPpropertyofhierarchicalrough
Alalloysurfacestreatedwithalkylsilaneoligomers(PFDTS,PFDCS).However,thehierarchicalsurfaces
treatedwithfluorocarbonoligomerwithpolaracidictail(PFOA)showedhighlyamphiphobicproperties
butcouldnotreachthethresholdforSAP.ChemicalstabilityofthehierarchicalAlalloysurfacestreated
withthefluorocarbonoligomerswastestedundertheharshconditionsofultra-sonicationinacetoneand
annealingathightemperatureafterdifferenttreatmenttimes.Contactanglemeasurementsrevealedthe
robustnessofthealkylsilaneoligomersanddeteriorationofthePFOAcoatingparticularlyforlowsurface
tensionliquids.Therobustadhesionandclose-packingofthealkylsilanemoleculesaswellastheir
verti-calorientationwithexposureofmoreCF3groupsinsteadofCF2groupsduetothepolarsilane-basedtail
arebelievedtobethemainreasonsbehindtheirimprovedchemicalstability.Theselectionof
fluorocar-bonoligomerwithproperpolartailwhichcanpromoteaself-assembledmonolayerwithclose-packed
moleculescouldmakeitpossibleforutilizingshorterfluorocarbonoligomers,whichisenvironmentally
favorable,todevelophighsurfaceenergymaterialswithSAPproperties.
©2016ElsevierB.V.Allrightsreserved.
1. Introduction
Thediscoveryoflotusleafwithsuperhydrophobic(SHP) prop-erty[1],exhibiting watercontactangle (CA)of morethan150◦ andsliding angleof lessthan 10◦,more thana decadeago has
∗ Correspondingauthorat:DepartmentofPhysics,CollegeofSciences,University ofDuhok,Duhok,KurdistanRegion,Iraq.
E-mailaddresses:krkhedir@ualr.edu,khedir.ramazan@uod.ac(K.R.Khedir).
inspired both academia and industry todeveloping bioinspired artificialsurfaces possessingsuch propertyfor a wide range of potential technological applications [2,3]. This property of the lotus leaf, which is also known as “lotus effect”, is due to its surface’shierarchicalroughnessincorporatingmicro-and nano-scalestructurescoveredwitharelativelylowsurfaceenergywaxy material.However,SHPsurfacesmaynotbeabletoexhibitlotus effecttowardorganicliquidswithlowsurfacetensionoflessthan about40mNm−1comparedtothewater’shighsurfacetensionof 72mNm−1.Asurfacewithlotuseffecttowardorganicliquidssuch
http://dx.doi.org/10.1016/j.apsusc.2016.04.050 0169-4332/©2016ElsevierB.V.Allrightsreserved.
asoilsisknownasthesuperoelophobic(SOP)surface.Lotusleaf,in spiteofitsSHPproperty,failstorepellowsurfacetensionorganic liquidsandthereforeisnotSOP[4,5].Fortechnological applica-tionssuchasself-cleaningandanti-fouling,surfacesdemonstrating boththeSHPandSOPproperty,whichisknownsuperamphiphobic (SAP),arehighlydesired.MoredetailsaboutSAPsurfacesandtheir potentialapplicationscanbefoundinthefollowingreviewarticles [6,7].
It hasbeen experimentallyshown thatfor a solid substrate torepelanyspecificliquidpossessingsurfacetension,surface energyofthesubstrateoflessthanabout/4isrequired[8].The lowestknownsurfaceenergymaterialsinthedescendingorderare CH>CH2>CHF>CF2>CF3.Theoretically,thelowestsurfaceenergy materialhasbeenreportedsofarisforasmoothsurface homoge-nouslycoatedwiththeclosepacked CF3moleculeswithasurface energyof6mN/m[9,10].InordertoimpartSAPpropertytosolid surfaces,anoptimalsurfaceroughnesswithre-entrantproperty followedby itscoatingwithultra-lowsurface energymaterials suchaslong-chainfluorocarbonoligomersisrequired[6,7,11–16]. Lowsurfacetensionorganicliquidssuchasoilswithsurface ten-sionoflessthan40mN/mcanstilleasilypenetratealltheabove mentionedlowsurfaceenergymaterialsandhomogenouslywet thesurface.Inordertoavoidthis,inadditiontotheultra-low sur-faceenergyofalong-chainfluorocarbonoligomer,itsmolecules alsoneedtodenselycoattheunderlyingre-entrantsubstrate.Such a closepackingcan preventtheoilsfrom penetratingdownto thehighsurface energy substrate[9]while re-entrant topogra-phycanstimulatethelotuseffect.Fluorocarbongroups(CF2,CF3), inadditiontotheirnon-polarity,promotesthelowestnon-polar dispersiveinteraction.
Inthepast,varioustechniquesandmaterialshavebeenused toengineerre-entrantroughstructuresintheshapeofoverhang structures [5,17], hierarchical micro- and nano-scale structures [11,16,18–20],nanoparticles[4,21,22],andinterconnectedfibers with knots [5,23]. It has been experimentally proven that the surfacechemistrymodificationofroughsurfaceswithre-entrant structureswithalong-chainfluorocarboniscrucialinfacilitating SAPproperty,[11,13,16,24].Zhaoetal.[24]showedthattextured siliconsurfacewithare-entrantpropertymaintainedCAsofmore than150◦ for low surfacetensionorganicliquid ofhexadecane onlyafterthestructureswerecoatedwiththelong-chain fluorocar-bonoligomersofC8H4Cl3F13Si(FOTS).However,thesametextured surfacesafterbeingcoatedwithPTFEandthelong-chainC18 hydro-carbonexhibitedhexadecaneCAsof121and0◦,respectively.Meng etal.[13]reportedSAPzincplatesshowedwaterandoilCAsof around158and155◦,respectively,afterimmersioninthe long-chainfluorocarbonofnonadecafluorodecanoicacid(C10HF19O2). Meanwhile, in the same study, it was also shown that theZn platestreatedwithpentadecafluorodecanoicacid(C8HF15O2)also showedSAPpropertiesafterchangingthereactionconditionsthat ledtotheformationofroughstructureswithre-entrantproperty (SupportinginformationFig.S6oftheirarticle).
Fromtheabovementionedstudiesonecanconcludethatfor highsurface energy rough structures withre-entrant property needto betreated with a long-chain fluorocarbon oligomer of carbonnumber nC≥8 andfluorinenumber nF≥15in orderto achieveSAPproperty.Meanwhile,foraroughre-entrantpolymeric filmthatismadeofafluorocarbonmaterial,SAPpropertycanbe obtainedbyusingashortnon-polarfluorinatedtail.Bellangeretal. [11]reportedSAPsurfacesbyelectrodepositionoffluorinated 3,4-ethylenedioxypyrrole(EDOP)withashortfluorinetailofF-hexyl withwaterandhexadecane,whichshowedCAsofhigherthan157 and152◦,respectively.However,filmspreparedbythe electrode-positionofEDOP(F-octyl)failedtoshowSOPpropertyinspiteofits longerfluorocarbonlength,mainlyduetotheabsenceofre-entrant property.
Fig. 1.A schematic representation of long-chain fluorocarbon oligomers of 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (PFDTS), 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane(PFDCS),andperfluorooctanoicacid(PFOA).
Long-chain fluorocarbon oligomers possess both nonpolar (hydrophobic)andpolar(hydrophilic)tails.Thepolartailwhich islinkedtothesubstratecanbeeithersilane-basedoracid-based groups.Thehydrophobictailwithfluorocarbonmolecules com-ingoutofthesurfaceisresponsibleforrepellingtheliquids.In previousstudiesontheimpactofsurfacechemistryin develop-ing SAP surfaces, most of the attention hasbeen given tothe structureofnon-polartail(fluorocarbonend)suchastheeffectof chainlengthonrepellinglowsurfacetensionorganicliquids. How-ever,theroleofpolartailinpromotingarobustadhesiontothe substrateaswellasclose-packingofthefluorocarbonmolecules withverticalorientationisalsocrucial.Inaddition,theeffectof multi-scaleroughstructuresontheorientationofthefluorocarbon moleculeshasnotbeenaddressedyet.Inthiswork,weinvestigated thewettingpropertiesofhierarchicallyroughAlalloysubstrates afterbeing coatedwithtwo different alkylsilaneoligomers 1H, 1H,2H,2H-perfluorodecyltriethoxysilane(PFDTS)and1H,1H,2H, 2H-perfluorodecyltrichlorosilane(PFDCS)aswellaswithanacidic fluorocarbon oligomer perfluorooctanoic acid (PFOA).All these oligomerspossessednon-polartailsofnC≥8,whichhelped dis-tinguishingtheeffectofpolartailonwettingproperties(Fig.1). Contactangleandslidingangleresponseofthesampleswas ana-lyzedbyusingthreedifferentsurfacetensionprobeliquidsofwater, ethyleneglycol,andpeanutoil.
2. Materialsandmethod
2.1. Developingmulti-scaleroughAlalloysurfaces
Multi-scaleroughaluminumalloysurfaceswereproducedby firstcuttinganAlalloysheetinto2×3cmpieces.Then,the sub-strates werecleaned for removal of contaminations and native oxidelayerbywet-mechanicalpolishingwithtwodifferent ultra-fine sandpapers of 2500and 3000 grits.This was followed by ultra-sonicationinacetonefor5minandfinallyrinsingwithDIW. ThenanoscaleroughAlalloysurfacewasdevelopedbythe immer-sion of the pre-cleaned flat substrate in boiling DIW for one minute.ThemicroscaleroughAlalloywithgrove-likestructures wasproducedbyone-directionallysandingofthepre-cleanedAl alloysubstratesusingsiliconcarbidesandpaperof60gritwithan associatedparticlediameterof∼500m.Thehierarchical micro-nano-scaleAlalloysurfacewasproducedbysimplyimmersingthe
Z.S.Saifaldeenetal./AppliedSurfaceScience379(2016)55–65 57
Fig.2.SEMimagesofroughAlalloysurfacesof(a)top-viewnanograss,(b)tilted-viewnanograss,(c)microgrooves,and(d)hierarchicalmicrogrooves-nanograssstructures.
microscaleroughAlalloyinboilingDIWforoneminute.Surface morphologyoftheflatandmulti-scaleroughAlalloysurfaceswas characterizedusingscanningelectronmicroscopy(SEM).
2.2. Surfacechemistrymodification
InordertoreducethesurfaceenergyofflatandroughAlalloy samples,threedifferenttypesoflong-chainfluorocarbonoligomers of PFDTS,PFDCS, and PFOA (Fig.1)were chemically vaporized ontosmoothandroughenedAlalloysubstrates.Forthispurpose,a precursorwaspreparedbymixingafluorocarbonoligomerwith hexane. Thevolumeratioof each oligomertohexane was pre-optimizedby usingdifferentratiosaswellasusinga magnetic stirrertoobtainahighlyhomogenousprecursor.Priortosurface chemistrymodification,sampleswerefirstplacedinsideanoven underatemperatureof90◦Cfor2h.Next,forthelowsurfaceenergy self-assembledmonolayercoating,samplesandprecursorsolution containingtheselectedfluorocarbonoligomerwere placedin a sealedcontainer(withouthavinganyphysicalcontact)insidean ovenwhichwassetto90◦Cunderambientpressure.Precursor moleculeswereevaporatedandarrivedatthesubstratesurface throughrandom gasphase collisionwithin thecontainer. After remaininginside theovenfor 5h, chemicallymodifiedsurfaces wereremovedandleftovernightunderambientconditionsforrest ofthechemicalbondingprocesstobecompleted.Thesameprocess wasfollowedfor allofthepristine flatand roughAlalloy sur-facestobechemicallymodifiedbyeachofthethreefluorocarbon oligomers.
2.3. Wettingcharacterization
Wettingcharacterizationofthepreparedsurfaceswascarried outbyusingacontactangle(CA)measurementsystem (VCAOp-tima).5lliquiddropletsofwater,ethyleneglycol,andpeanutoil weregentlydispensedoverthesurfacesfortheirCAmeasurements.
Slidingangle(SA)wasmeasuredusingacustomizedsetupby dis-pensing25lliquiddroplets.Tiltangleofthesubstrateatwhich theliquiddropletbeginstoslideisdefinedastheSA.Foraccuracy, CAandSAmeasurementswererepeatedthreetimesandaveraged foreachsample.
2.4. Surfacefreeenergy
Surfacefreeenergymeasurementsforthethreefluorocarbon oligomerscoatedonsmoothAlalloysurfaceswerecarriedoutby theliquidprobemethod.Threeconventionalliquidswater, ethy-lene glycol, and diiodomithane were utilized to determine the oligomers’totalsurfacefreeenergyaswellastheirenergy com-ponents.
2.5. Surfacechemistryandcoveragecharacterization
Inordertoextractinformationaboutthesurfacechemistryand coverageof differenttypesof oligomersonroughAlalloy sub-strates,X-rayPhotoelectronSpectroscopy(XPS)wasutilizedusing aThermoFisherK-alphaelectronspectrometerwitha monochro-matic AlK␣ X-ray source that hasan energy of 1486.6eV.The homogeneity ofthe coatingswas investigated usingaerial two dimensional(2-D)spectralmapping,whichwasacquiredbyX-ray spotwithadiameterof100monconsecutivepointswith inter-valsof100m,andrecordedinthesnapshotmodewithinF1s,C1s, O1sandAl2pregions.
2.6. Surfacechemistrystability
Stabilityorrobustnessoftheself-assembledmonolayersofthe fluorocarbonoligomersonhierarchicallyroughAlalloysurfaces wastestedunderharshconditions.Thepristinesurfaceswere ultra-sonicated inacetone for varioustime periods of 5,15, 30, and 60min. Anothersetof samples werealsoannealed under high
Fig.3. ArepresentativecorelevelC1sXPSspectrumofthefluorocarbonoligomer coatingsinvestigatedinthisstudy.
temperature of 200◦C for various time periods of 2, 4, 6, and 8hinambientpressure.Aftereachtreatmenttimeofboth ultra-sonicationin acetoneand annealingprocess, CAmeasurements werecarried outforwater,ethyleneglycol,andpeanut oil.The sameprocesswasrepeatedfortheentirehierarchicalroughAlalloy surfaces,whichwerechemicallymodifiedwiththethreedifferent typesoffluorocarbonoligomers.
3. Resultsanddiscussion 3.1. Surfacemorphology
SurfacemorphologiesoftheroughAlalloysurfacescaptured bySEM are shown in Fig.2. Fig.2a and bexhibit the top and tilted-viewsofthenanograssstructuresobtainedbyimmersionof thepolished(flat)AlalloysheetinboilingDIW.Theanalysisof topviewandtiltedviewSEMimagesshowedthatthe nanostruc-tureshavethelateralsizeofaround10nmandaverticalheight of about 200nm. Nanostructures also formed interlinked thin sheetsresultinginaporousstructurewithanapproximatepore diameterof∼100nm.Microgrooves,showninFig.2c,developed by one-directional mechanical sanding possess the anisotropic microgrooveswithanaveragechannelwidthofaround10mand overallroughness(Ra)of3.3±0.2m.
Moreover, Fig. 2d shows a magnified SEM of hierarchically roughAlalloysurfacethatincorporatesmicrogroovescoveredby nanograssstructures.Theimagerevealstheconformalcoverageof microgrooveswithnanograss.Presenceofnanograssstructureson theinnerwallsofmicrogroovesprovidesare-entrantpropertyto thesurface’soveralltexture.Inaddition,thesurfacealsopossesses ahigherdegreeofroughnesscomparedtotheindividualcasesof microgroovesandnanograsssurfaces[16].
3.2. Fluorocarbons’surfacechemistry
Fig.3showsarepresentativecorelevelC1sspectrumofthe flu-orocarbonoligomercoatingsinvestigatedinthisstudyafterthey wereevaporatedonroughAlsubstrates.Theprofilesinvolve four-differentphotoelectronpeakscorrespondingto CH(284.85eV), CF (289.01eV), CF2(291.48eV) and CF3(294.01eV) groups, whichareconsistentwiththetheoreticalmolecularstructuresof theoligomersillustratedinFig.1.
3.3. Fluorocarbons’surfacefreeenergies
Itisvitaltodeterminethesurfacefreeenergiesofthe fluoro-carbonoligomerschemicallyvaporizedontothepolishedAlalloy
Table1
Calculatedvaluesofsurfacefreeenergyalongwithitspolarandnonpolar compo-nentsforthefluorocarbonoligomersofthisstudy.
FluorocarbonOligomer d
s(mN/m) ␥+s (mN/m) ␥−s (mN/m) s(mN/m)
PFDTS 7.96 0.27 1.06 10.27
PFDCS 6.36 0.13 1.23 8.7
PFOA 7.62 0.03 1.16 9.81
surfaces.Surfacefreeenergycomponentsoflow surfaceenergy solids(s)andliquids(l)areresolvedintobothnon-polar
(Lifshitz-vanderWaals)dispersiveenergy(d)andpolarenergy (±) with
bothcomponentsofLewisacid(+)andLewisbase(−),according tothefollowingformulas:
s=sd+2(s++s−) 1/2 (1) l=ld+2(l++l−) 1/2 (2) Thesurfacefreeenergycomponents(d
s,s+,s−)ofthe
fluo-rocarbonoligomersofthis studyweredeterminedbyusingthe measurementsofforwater,ethyleneglycol,anddiiodomethane (zero-polar liquid), and contributing them in the Van Oss-Chaudhury-Good’sequation: l(1+cos)=2(ld d s) 1/2 +2[(s+l−) 1/2 +(l+s−) 1 2] (3)
MeasuredCAsofthethreeprobeliquidsalongwiththeir sur-facetensionsandassociatedpolarandnonpolarcomponentswere substitutedintoEq.(3)todeterminethesurfacefreeenergy compo-nentsofeachfluorocarbonoligomer.Thesofeachfluorocarbon oligomerwasdeterminedbysubstitutingtheirobtainedsurface freeenergycomponentsintoEq.(1).Thevaluesofsandd
s,s+,s− forthethreefluorocarbonoligomersarelistedinTable1.Itcan benoticedthatthesofthethreefluorocarbonoligomersarein therangeof8–10mN/m,whichareingoodagreementwiththe reportedvaluesof(9–11mN/m)forfluorocarbonoligomerswith (CnF2n+1)andn=8–10coatedongoldsubstrate.Amongthethree fluorocarbonoligomers,PFDCSshowedthelowestsof8.7mN/m while PFDTSwiththehighest s of10.27mN/m. In spite ofits longestcarbonchainlength,PFDTSdidnotshowthelowestsurface energyafterbeingvaporcoatedontothesurfacesofflatAlalloy. Furthermore,itcanalsobenoticedthatmostofthethree fluoro-carbonoligomers’siscontributedbytheirnonpolarcomponents ofd,whichisexpectedfromalowsurfaceenergyoligomers.From thetwopolarcomponents,valuesofLewisbase−componentare relativelymuchhigherthanvaluesofLewisacid+componentfor allthefluorocarbonoligomerswestudied.
CAvaluesfortheliquidswithdifferentsurfacetensions mea-suredontheflatandroughAlalloysurfacescoatedwiththethree differentfluorocarbonoligomersareplottedinFig.4.Thevalues ofCAforwater,ethyleneglycol,andpeanutoildecreasewiththe decreaseintheliquid’ssurfacetensionforallthethree fluorocar-bonoligomers(PFDTS,PFDCS,andPFOA).ThistrendofCAsversus theliquid’ssurfacetensionisconsistentwiththeYoung’smodel (=cos−1[s−sl/l]),whereslrepresentsthesurfaceenergyat thesolid-liquidinterface.Theequationshowsthatthespreading tendencyofaliquiddropletwhilebeingdepositedonasolidsurface increaseswiththedecreaseofliquid’ssurfacetensionandtherefore exhibitsalowerCA.
Meanwhile,asshowninFig.4,fluorocarbonoligomerswe stud-ieddemonstrateddifferentbehaviorinrepellingliquidsdepending onboththesurfaceroughnessandsurfacechemistryofthe sub-strate.ForflatAlalloysamples(Fig.4a),thesurfacecoatedwith PFDCSshowedhighestCAsof125,115,and89◦forwater,ethylene glycol,andpeanutoil,respectively.TheseCAsareabout5◦and14◦ higherforwater,8◦and13◦higherforethyleneglycol,and5◦and 18◦higherforpeanutoilcomparedtotheCAsobservedontheflat
Z.S.Saifaldeenetal./AppliedSurfaceScience379(2016)55–65 59
Fig.4.Contactangle(CA)profilesforthewater,ethyleneglycol,andpeanutoilonthesurfacesof(a)flat,(b)microgrooves,(c)nanograss,and(d)hierarchical microgrooves-nanograssAlalloysubstratesthatwerecoatedwiththethreedifferentfluorocarbonmaterials(PFDTS,PFDCS,PFOA).
surfacescoatedwithPFOAandPFDTS,respectively.Thisisingood agreementwiththecalculatedvaluesofthesurfacefreeenergiesof thefluorocarbonoligomerslistedinTable1.InthecaseofAlalloy surfacewithmicrogrooves(Fig.4b),PFDCSand PFOAstillshow higherCAsofaround5◦forallthethreeliquidscomparedtothe surfacecoatedwiththePFDTS,inspiteofthePFDTS’longercarbon chain.
However,fornanograssandhierarchicalAlalloysamples(Fig.4c andd),thesurfacescoatedwithalkylsilaneoligomersPFDTSand PFDCSshowedsignificantlyhigherCAscomparedtothesurfaces coatedwithPFOA.Ingeneral,nanograssandhierarchicalsurfaces coatedwithPFDTSexhibitedrelativelygreaterCAscomparedto PFDCS,whichhadhigherCAsonflatandmicrogrooveAlalloy sur-faces.Fornanograssstructures(Fig.4c),thesurfacescoatedwith thetwoalkylsilanesshowedrelativelycomparableCAvaluesfor thethreeliquids,whichwerehighermorethan10◦forwaterand 5◦forpeanutoilcomparedtothosewithPFOA.Inthecaseof hier-archicalAlalloysamples(Fig.4d),thesurfacescoatedwithPFDTS andPFDCSshowedsimilarCAsofaround158◦forwaterandabout 154◦and152◦forethyleneglycolandpeanutoil,whichwereabout 10◦highercomparedtotheCAsofsamesurfacescoatedwithPFOA. ComparingthewettingpropertiesofflatandmicrogroovedAl alloysurfacescoatedwiththefluorocarbonoligomersofthisstudy, itisobservedthatPFDTSresultedinthelowestCAsinspiteofits longestcarbonchain-lengthof16comparedto10and8forPFDCS andPFOA,respectively.Ontheotherhand,theseflatandmicroscale roughAlalloysurfacespresentedhighestCAswhencoatedwiththe
PFDCS,althoughitdidnothavethelongestcarbonchain-length, butthesamefluorocarbonchain-length(nonpolartail)ofPFDTS. ThisisbelievedtobeduetothatPFDCS’spolartailchlorosilane (SiCl3)mighthaveprovidedamoreclose-packedself-assembled monolayerwithverticalorientationofitsmoleculestothe sub-strate,whichresultsinthelowestsurfacefreeenergyamongthe fluorocarbonsweinvestigated.Inaddition,PFOAshowedhigher wettingrepellencycomparedtoPFDTSinspiteofPFOA’s signifi-cantlyshortercarbonchainandonelessCF2group.
However, in the case of Al alloy samples with nanograss and hierarchical microgrooves-nanograss structures, the two alkylsilanes of PFDTS and PFDCS showed significantly higher CAs for the entire range of the liquids’ surface tensions. This might due to the potential difference in surface coverage of the fluorocarbon oligomers used in this study. Because we used an evaporation method to coat the samples with these oligomers, themolecular weight of a given fluorocarbon oligomer can affect themean-free-pathof its molecules in the vapor phase, which can in turninfluence theflux distribution
on the substrate. The molecules of a more uniform
random flux formed by small mean-free-paths can coat the roughsurfacesmoreuniformly(i.e.conformally)inaprocess sim-ilartothatofchemicalvapordeposition;while,ontheotherhand, it mightsufferfromnon-uniformcoverage duetoa shadowing effectwhenthefluxismoredirectional[25].Fig.5showsthe2-D XPSareaprofilesofPFOA,PFDTSandPFDCScoatedhierarchically roughAlsamples.Theseareaprofilesconsistoftwoaxes(xandy)
Fig.5.2-DXPSareaprofilesofPFOA,PFDTS,andPFDCScoatedonhierarchicallyroughAlsubstrates.
thatshowthepositionsinmillimeter-scaleandanadditionalcolor barcorrespondingtothepeak intensity,computedasthe area. Sincealloftheoligomerscontain CF2groupsintheirstructure, theareaprofilesobtainedbyanalysisoftheC1speak observed around 291.5eV and theF1s peak should becorrelated in the intensitymanner.Moreover,theareaprofilesrecordedfromAl2p andO1sregionswhichrevealinformationaboutthemorphology of underlying Al alloy substrates (i.e. brightercolor for higher surfacepoints)arealsoexpectedtobecorrelatedwitheachother. InadditiontotheseexpectedcorrespondencesbetweenC1sand F1speaksplusAl2pand O1speaks,thesignalintensitycoming fromfluorocarbonmoleculeswasobservedtobematchedtothat ofAlsignalintensitycomingfromthesubstrateinthesemapping profiles,eventhoughthesignalintensityarisingfromthe under-lyingsubstrateshouldbeobservedtodecreaseupontheoverlayer depositionontop.Inotherwords,theintensityprofilesofallofthe oligomerscloselyfollowedthetopographyofthesubstrate.These resultsindicatethatalloftheoligomersinvestigatedinthisstudy conformally coated the rough Al substrates, which eliminates
thehypothesisofdifferentsurfacecoverageduetothepossible differenceintheirfluxdistributionduringevaporation.
Another and more likely origin of higher CAs observed for PFDTS and PFDCS on nanograss and hierarchical substrates is that higher oxidized surface (alumina) area of these samples can provide more favorable bonding sites towards alkylsilanes due to the presence of higher rate of hydroxyl ( OH) groups comparedtoflatandmicrogroovedAlalloysurfaceswithlower rate of native OH groups. In addition, the nanoscale rough-ness of the nanograss structures could have also helped in facilitating more close-packing of alkylsilanes molecules and consequent vertical orientation of their molecules. Sizes of the oligomer molecules of this study are in the nanoscale range of around 5–10nm [26], which can cause orientation ofthemoleculebeaffectedbythenanoscaleroughnessofthe alu-minananograssstructureshavinglateralsizesofabout10–15nm. Inaddition,alkylsilaneoligomersPFDTSandPFDCSbothexhibited similarCAvaluesinspiteofthedifferenceintheircarbonchain length.Alltheseindicatetothesignificantroleofthepolartailof fluorocarbonoligomerintermsofprovidingclose-packing
mono-Z.S.Saifaldeenetal./AppliedSurfaceScience379(2016)55–65 61
Fig.6.Slidingangle(SA)profilesofwater,ethyleneglycol,andpeanutoilonthe hierarchicalmicrogrooves-nanograssstructurescoatedwithdifferentlong-chain fluorocarbons(PFDTS,PFDCS,PFOA).
layerandthewaythatitsmoleculesareoriented.Consequently,the robustnessandsurfacefreeenergyofthesubstratescoatedwiththe fluorocarbonoligomerscanbeassociatedwithallthesefactors.
Kineticsoftheliquiddropletonatiltedsubstratecouldprovide abetterinsightintothehomogeneityandclosepackingofthe fluo-rocarbonoligomersonAlalloysurfaces.Foraliquiddroplettoslide overaroughsurfacecoatedwithafluorocarbonoligomer,ithasto overcometheadhesionforcesattheliquid-solidinterfaceasaresult ofsurface’scontactanglehysteresis(CAH).TheCAHarisesfromthe existenceofvariouslocalizedmicroscopicCAsatthecontactarea duetothephysical(roughness)andchemicalheterogeneitiesofthe surface[17,27–29].Amonolayeroffluorocarbonoligomerenriched withmoreclose-packedandverticallyorientedmoleculescan sig-nificantlyreducethesurface’sCAH.InthecaseofCAHduetothe surfaceroughness,asurfacetexturewithoptimalroughnessand re-entrantpropertycanfacilitatepartialwettingofthesurfacewith unstableliquid-solidcontactline.Foraliquiddroplettorolloffover thesurfacebytiltingit,thedroplethastoovercomethesurface’s CAH.LowerCAHmeanssmallerSAfortheliquiddroplettoroll off.Fig.6showsthevaluesofSAsofthethreeliquidswith vari-oussurfacetensionsoverthehierarchicalAlalloysurfacescoated withthethreedifferentfluorocarbonoligomers.Thesurfacecoated withPFDTSexhibitedthelowestSAsoflessthan10◦ forallthe threeliquidsandtheonlysurfacetoberecognizedastheSAP sur-facewithlotuseffectproperty.Inthecaseofsurfacecoatedwith PFDCS,italsoshowedlowSAsoflessthan10◦,butonlyforthehigh andmoderatesurfacetensionliquidsofwaterandethyleneglycol, respectively.ItshowedsignificantlyhighSAofmorethan30◦for thelowsurfacetensionpeanutoil.Meanwhile,thesurfacecoated withPFOAshowedthehighestSAscomparedtothesurfacescoated withsilane-basedfluorocarbonoligomersofPFDTSandPFDCS.The surfacedemonstratedlotuseffectpropertyonlyforthecaseofhigh surfacetensionliquidofwater.Inthecaseofwater,thesurface coveredwithCF2moleculescanfacilitatelowadhesionand conse-quentlowSA.However,lowSAsofoilsindicatedominantcoverage ofCF3moleculesonthesolidsurface.Accordingly,SAresultsreveal thathierarchicalAlalloysurfacecoatedwithPFDTShasprovided thebestconformityandclose-packingcoating.However,therestill mightbeascenariowherethefluorocarbonoligomer’smolecules tobehorizontallystackedonthesurfacewithouttightlyadhering tothesubstratethroughitsnonpolartail.Therefore,therobustness testofthecoatingmayprovidemoredetailsabouttheadhesion
sta-bilityandclose-packingofthefluorocarbonoligomersofthisstudy. Theseresultsareingoodagreementwiththestudiesconductedby HozumiandMcCarthy[30,31],wheretheyshowednegligibleCAH forsmoothandnano-roughoxidizedAlsurfacescoatedwith long-chainfluorocarbonoligomers(PFDTS)forbothwaterandn-hexane. Theresultswereattributedtothemonolayeroligomercoatingby chemicalvapordepositionandsubsequentumbrella-likecoverage ofthesurfaceswithCF3molecules.
3.4. Workofadhesioncalculations
Thequantificationoftheadhesionpropertiesofthethree fluoro-carbonoligomerscoatedonthehierarchicalroughAlalloysurfaces canbecarriedoutbydeterminingtheirassociatedworkof adhe-sion.Foraliquiddroplettoslideoverthesolidsurface,anapplied forceofequalorgreaterthantheworkofadhesion(Wad)atthe solid-liquidinterfaceis required.AccordingtotheYoung-Dupre relation,theWadbetweenaliquid dropletandaphysicallyand chemicallyhomogenoussolidsurfacecanberepresentedas: Wad=lv
1+cos
(4)wherelvrepresentsthesurfaceenergyattheliquid-vapor inter-face.Foraroughsolidsurfacethatispartiallywettedbyadopting the Cassie state or the metastable Cassie state of wetting, the dropletcontactareaispartiallyincontactwiththesolidsurface. Therefore, according to themodified Young-Dupre relation for determiningWadofroughsurfaces[32,33]:
Wad=lvf
1+cos
(5)Empirically,thesurface’ssolidfractionincontactwiththethree differentliquidscanbedeterminedusingCassierelation,inwhich fisthefunctionofbothassociatedapparentandintrinsicCAs. f= 11+cos∗ +cos (6) Andsubsequently, Wad=lv
1+cos∗ (7)Thevaluesofbothand∗representthemacroscopicpictureof theinterfacialinteractionofwettingmechanismofonlyaroundthe contactline,whichrepresentsameetinglineoftheair-liquid-solid phases,withoutconsideringtheinterfacialinteractionatthe cen-tralcontactareaofthethreephasesofthesolid-liquidinterface. Thus,theaboveequationdeterminesthesolidfractiononlyonthe premierofthecontactareaaroundthecontactline(tripleline). InarecentstudybyBormashenkoandhiscoworker[34],itwas demonstratedthattheliquidCAisonlygovernedbytheadjacent areaaroundthetriplelinewhichisanarrowareaaroundthe pre-mierofthecontactarea.Meanwhile,theadhesionofthedropletto thesurfaceisgovernedbytheentireinterfacialcontactarea.Thus, morepreciseand comprehensivevalues offcanbedetermined byconsideringbothstaticandkineticwettingparameters repre-sentedbytheCAandSAvalues,respectively.Inaddition,thesolid surface’sphysicalandchemicalheterogeneities,whichis quanti-fiedasCAH,causeincreaseintheadhesion.Foraliquiddropletto slideataspecificsurfacetiltingangle˛(SA)duetoitsweightof (Vg),thedropletmustovercomealltheenergybarriermanifested astheWadattheentirecontactareaofthesolid-liquidinterface,as follows[32].
f= 2(Vg)sin˛ wlv
1+cos
(8)Herefisthefunctionofthesurface’swettingpropertiesintheform ofstatic()andkinetics(˛)parameters,inadditiontotheliquid’s physicalproperties(Vg/lv)andthewidthofthecontactarea(w).
Fig.7. Thevaluesofworkofadhesion(Wad)andsolidfractionpercentages(f%)versustheliquids’surfacetensionsforthehierarchicalAlalloysurfacescoatedwith
fluorocarbonoligomersofPFDTS,PFDCS,andPFOA.
SubstitutingEq.(5)intotheEq.(2),Wadforaliquiddropletpartially wettingtheroughsolidsurfaceisgivenas:
Wad=
2(Vg)sin˛
w (9)
Fig.7showsthecalculatedvaluesofsolidfractionsandwork ofadhesionsbetweenthethreeliquiddropletsandthe hierarchi-calAlalloysurfacescoatedwithPFDTS,PFDCS,andPFOA.Using themacroscopicvalues ofintrinsicCA()andapparentCA(∗) andutilizingCassierelationEq.(3)todeterminef%(Fig.7a),Eq. (4)resultedincorrespondingWadvaluesplottedin(Fig.7c).The valuesofbothfandWadincreasedwiththedecreaseintheir cor-respondingvalues of ∗, which is consistentwiththe fact that higherCAs istheindicationof havingsmallercontact areaand consequentloweradhesion.However,peanutoildropletinspite ofpossessingthelowestCAandgreatestSAvaluescomparedto both water and ethylene glycol showedthe lowest f and con-sequentminimum Wad for the all three different fluorocarbon oligomers.Thisisinconsistentwiththegeneralunderstandingthat lowersurfacetensionliquidstendtowetlargersolid areasand leadstosubsequentlargeradhesion.Fig.7banddshowthe
val-uesofboth f andWad afterconsideringthekineticsofdroplets motioninadditiontomacroscopicvaluesofintrinsicCAs.Using Eqs.(5)and(6),thetrendsofthecalculatedvaluesofbothf and WadareconsistentwiththecorrespondingvaluesofmeasuredSAs (Fig.6).HigherCAsandlowerSAsisthemanifestationoflessf incontactwiththeliquiddropletsandconsequentlowerWadto overcome.
ThehierarchicalsurfacescoatedwithbothPFDTSandPFDCS oligomersshowedsimilarvaluesoffandconsequentlysimilarWad valuesforwater(Fig.7bandd).For ethyleneglycolandpeanut oil,thesurfacescoatedwithPFDTSexhibitedlowerf aswellas WadcomparedtothesurfacescoatedwithPFDCS.Meanwhile,the surfacescoatedwithPFOAoligomershowedsignificantlyhigher valuesforbothf andWadcomparedtothesurfacescoatedwith thetwosilane-basedfluorocarbonoligomers(PFDTSandPFDCS). Theresultsareconsistentandfollowaverysimilartrendofthe observedvaluesofthesurfaces’SAsdepictedinFig.6.Theseresults indicatethatthewettinganalysisofsolidsurfacescannotbe com-prehensiveandprecisewithoutconsideringbothstatic(,∗)and kinetic(˛)propertiesofliquiddropletsonthesurface.
Z.S.Saifaldeenetal./AppliedSurfaceScience379(2016)55–65 63
Fig.8.CAvaluesofwater,ethyleneglycol,peanutoilonthehierarchicalAlalloysurfacescoatedwithPFDTS(aandb),PFDCS(candd),andPFOA(eandf)afterultra-sonication ofthesurfacesinacetoneforvarioustimeperiodsof0,5,15,30,and60min(a,c,e),andannealingat200◦Cforvarioustimeperiodsof0,2,4,6,and8h(b,d,f).
3.5. Surfacechemistrystability
Robustnessandchemicalstabilityofthefluorocarbonoligomers after being vapor coated on the hierarchical microgrooves-nanograssAlalloysurfacesisofgreatimportanceinassessingtheir molecules’close-packingandrobustadhesiontothesubstrate.A stablecoatingcanbeachievedbythestrongcovalentbondingof oligomermoleculethroughitspolartailtothesurface,whichalso
facilitatesaverticalorientationofthemolecule.Fig.8showsthe CAvaluesofthehierarchicalsurfacescoatedwithPFDTS,PFDCS, andPFOAaftertheirultra-sonicationinacetoneandannealingas afunctionoftime.ForthesurfacescoatedwithPFDTS,after ultra-sonicationinacetone(Fig.8a),thewaterCAsexperiencedalmost nochangeevenafter60minoftreatment.However,amaximum decreaseofabout7–10◦ intheCAsof bothethyleneglycoland peanutoilwasobserved.Theannealingofthesurfacesatambient
conditionsunder 200◦C temperature (Fig. 8b)had a very simi-lareffectonthevalues oftheliquids’ CAs.Thewater CAskept unchangedevenafter8hofthetreatment.Afteraslightdecreasein theCAvaluesofbothethyleneglycolandpeanutoilafter2h,they gotreducedof∼10◦after8hofannealing.Forthesurfacescoated withPFDCS,afterultrasonicationinacetone,waterCAswerestable forthewholetreatmenttime(Fig.8c).However,therewasaslight decreaseoflessthan8◦intheCAvaluesofbothethyleneglycoland peanutoil.Evenaftertheannealingprocessofupto8h,almost nodecreaseintheCAvaluesforanyoftheliquidswasobserved (Fig.8d).ThisshowssignificantrobustnessofthePFDCSunderhigh temperatures.Ontheotherhand,forPFOA,ultra-sonicationand annealingtreatmentscausedsignificantreductionsinCAs,which indicatesubstantialdegradationofthecoating.PFOAsamples suf-feredfrompeanutoilCAsdroppingbyasmuchas70and120◦,after the60minultrasonicationand8hannealingtreatments, respec-tively(Fig.8eandf).Howeverthesurfacesstillmanagedtostay hydrophobicbyadeclineinwaterCAsofaround30and10◦after thesameultrasonicationandannealingtreatments,respectively.
ThechemicalstabilitytestingofthehierarchicalroughAlalloys coatedwithPFDTSandPFDCSverifiedtherobustnessofthe coat-ingsbyretainingtheirstrongrepellencytowardthethreeliquids studied.Strongcovalentbonding through theirpolartails onto thesubstratecouldbethemainreasonbehindtherobustnessof thesecoatings.ItisalsopossiblethatPFDTSandPFDCSmolecules mighthaveachievedamoreverticalorientationthatcanprovide aclose-packedconfiguration.Aproperbondingandclose pack-ingofthemoleculesfacilitateaself-assembledmonolayertobe denselycoatedonthesurface.Inaddition,theverticalorientation alsoallows theoutwardexposureofmore CF3 groupsinstead of CF2 groups.The CF3 grouppossessthelowestsurfacefree energyof6mN/mcomparedtosignificantlyhighersurfacefree energyof16mN/mforthe CF2group[9,10,35].Meanwhile,the surfacescoatedwithPFDCSshowedrelativelybetterrobustness, whichcouldbeanindicationofa betteradhesionand a conse-quentlybetterclosepackingofthePFDCSmolecules.Ontheother hand,deteriorationofthePFOAcoatingonthehierarchicalAlalloy surfacesis anindicationoflow adhesionofitspolartailtothe substrateprovidingmorehorizontalorientationandsubsequently higher CF2presenceonthetopsurface.
Threedimensional(non-vertical)orientationofthe fluorocar-bonoligomerscouldcauselessclosepackingoftheirmoleculesas wellastheoutwardexposureofmore CF2insteadof CF3,lower CF3: CF2ratios.Forhighsurfacetensionliquidssuchaswater, thevariationintheexposureratioof CF3: CF2islessimportant. However,forlowsurfacetensionliquidssuchasoil,ahigher expo-sureratioof CF3isrequiredtoachievesuperoleophobicity[30,31]. Therefore,webelievethataproperadhesionandclosepackingof thefluorocarbonoligomerscouldhelpinutilizingshorter fluoro-carbonmoleculestoachieve superamphiphobicity.Theusageof shortfluorocarbonoligomersfordevelopingSAPsurfacesisalso moreenvironmentallyfriendly.InarecentworkbyParketal.[36], itwasshownthataflatoelophobicsurfacewithlowCAHcanbe obtainedwithanalkylsilanehavinganextremelyshortnonpolar fluorocarbonendofonlyone CF3group.Meanwhile,itshouldbe notedthatthesuperoleophobicsurfacesandslipperyoelophobic flatsurfacesthathavebeenobtainedsofarbyutilizingshortlength fluorocarbonoligomerswerenotdemonstratedsofartoextend toself-assemblymonolayercoatingsofsimilaroligomers.Inthose studies,thesubstrateitselfwasmadeofalowsurfaceenergy flu-orocarbonoligomer[11,36].Inthisscenario,theclosepackingand subsequentexposureofmore CF3 groups isguaranteed. How-ever,forahighsurfaceenergymetallicsubstrate,asinourcase, anextremeclosepackingofthefluorocarbonoligomers’molecules withverticalorientationtopreventthepenetrationoflowsurface tensionoilsismorechallenging.Therefore,moredetailedstudies
areneededtoaddressthepossibilityofobtainingsuperoleophobic surfacesonhighsurfaceenergysubstratesbyutilizingshortlength fluorocarbonoligomers.
4. Conclusions
Threelong-chain fluorocarbonoligomers of alkylsilanes and perfluorooctanioc acid were chemically vaporized onto hierar-chically rough Al alloy surfaces that incorporate micro- and nano-scalefeatures.Thesemulti-scaleroughAlalloysurfaceswere producedbyasimpleandenvironmentalfriendlytechniqueof one-directionalmechanicalsandingandtreatmentinboilingwater.The effectoffluorocarbons’polarendsinfacilitatingaclosepacked self-assembledmonolayerontheAl-alloysurfaceswasinvestigated.In general,thealkylsilaneoligomersshowedhigherliquidrepellency towardbothhighandlowsurfacetensionliquidscomparedtothe oligomerwithacidicpolarend.Inaddition,chemicalstabilitytests revealedtheenhancedrobustnessofthealkysilaneoligomers.A strongerbonding ofthefluorocarbonoligomer byits polarend tothesubstrate maypromotea moreverticalorientationofits moleculesandenhancedexposureof CF3groupsinsteadof CF2 groups.Fordevelopingsurfaceswithstrongrepellencytowardoils, thesurfacesneedtobeenrichedwith CF3groups.Fromthetwo alkylsilaneoligomersstudied,PFDCSshowedabetterchemical sta-bilitycomparedtoPFDTSinspiteofthelongeroverallcarbonchain butsimilarfluorocarbontailofthelaterone.Therefore,itcanbe concludedthatthereisapossibilityofimpartingSAPpropertyto highsurfaceenergysolidsutilizingashortfluorocarbonoligomer. Thiscanbeachievedbycoatingthere-entranttextureofthesolid surfacewithafluorocarbonoligomerwhichpossessapolarendthat canprovidethehighadhesion,closepacking,andvertical orienta-tionofitsmolecules.Consequently,thiscouldfacilitateadominant ratioof CF3 groups tobepresent at theliquid-solid interface insteadofthehighersurface energy CF2 groups.Furthermore, thecalculationsofworkofadhesionofthehierarchicalsurfaces showedthatmorerealisticresultscanbeobtainedbyconsidering bothstaticandkineticwettingmeasurements.
Acknowledgements
This work was supported in part by NASA (grant no.: NNX09AW22A) and NSF(grant no.: EPS-1003970). Theauthors thankUALR Centerfor Integrative NanotechnologySciences for helpingwithSEMmeasurements.
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