ContentslistsavailableatScienceDirect
Progress
in
Organic
Coatings
jou rn al h om ep ag e :w w w . e l s e v i e r . c o m / l o c a t e / p o r g c o a t
Influence
of
the
coating
method
on
the
formation
of
superhydrophobic
silicone–urea
surfaces
modified
with
fumed
silica
nanoparticles
Cagla
Kosak
Söz,
Emel
Yilgör,
Iskender
Yilgör
∗KUYTAMSurfaceScienceandTechnologyCenter,ChemistryDepartment,KocUniversity,Istanbul,Turkey
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received18December2014
Receivedinrevisedform10March2015 Accepted11March2015
Availableonline7April2015 Keywords: Superhydrophobicsurfaces Fumedsilica Spraycoating Spincoating
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b
s
t
r
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c
t
Effectofthecoatingmethodontheformationofsuperhydrophobicpolydimethylsiloxane–urea copoly-mer(TPSC)surfaces,modifiedbytheincorporationofhydrophobicfumed silicananoparticleswas investigated.Fourdifferentcoatingmethodsemployedwere:(i)layer-by-layerspin-coatingof hydropho-bicfumedsilicadispersedinanorganicsolventontoTPSCfilms,(ii)spin-coatingofsilica–polymermixture ontoaglasssubstrate,(iii)spraycoatingofsilica/polymermixturebyanair-brushontoaglasssubstrate, and(iv)directcoatingofsilica–polymermixturebyadoctorbladeontoaglasssubstrate.Influenceof thecoatingmethod,compositionofthepolymer/silicamixtureandthenumberofsilicalayersapplied onthetopographyandwettingbehaviorofthesurfacesweredetermined.Surfacesobtainedwere char-acterizedbyscanningelectronmicroscopy(SEM),whitelightinterferometry(WLI)andadvancingand recedingwatercontactanglemeasurements.Itwasdemonstratedthatsuperhydrophobicsurfacescould beobtainedbyallmethods.Surfacesobtaineddisplayedhierarchicalmicro-nanostructuresand super-hydrophobicbehaviorwithstaticandadvancingwatercontactangleswellabove150◦andfairlylow contactanglehysteresisvalues.
©2015ElsevierB.V.Allrightsreserved.
1. Introduction
Preparation and characterization of superhydrophobic poly-mericsurfacesand coatingshave beeninvestigated extensively duringthepast15years,followingthedetaileddescriptionofthe surfacestructuresandhierarchicalmicroandnanotopographiesof alargenumberofnaturalplantleaves[1,2].Amajorreasonforsuch aremarkableinterestinsuperhydrophobiccoatingsandsurfaces hasbeentheirinterestingcombinationofproperties,suchasthe self-cleaning,anti-fouling,stain-resistantandice-repellant behav-iors[3–5],whichenablepotentialapplicationsinavarietyoffields, whichincludepaintsandcoatings,textiles,exteriorglasswindows, rooftops,windshields,solarpanels,aircraftwingsandwind tur-bineblades[3,4,6,7].Aswelldocumentedintheliterature,wetting behaviorofasurfaceismainlycontrolledbytwoparameters,which are;(i)thesurfacechemicalstructureandcomposition,and(ii)the surfacetopographyorroughness[8–10].Whennaturally occur-ringsuperhydrophobicplantsurfaces,suchasalotusleafsurfaceis examinedunderascanningelectronmicroscope,itisseenthatthe
∗ Correspondingauthor.Tel.:+902123381418;fax:+902123381559. E-mailaddress:iyilgor@ku.edu.tr(I.Yilgör).
surfaceiscoveredbyirregularlydistributed,micron-sized protru-sionscalledpapilla,whichalsodisplayafurynanoscaleroughness [1,2,10].Whensuchamicro-nanodualsurfaceroughnessis com-binedwiththeinherenthydrophobicityofthewaxylayeronthe leaf,theyprovidethelotusleafitssuperhydrophobicity,withstatic watercontactanglevaluesabove150◦andcontactanglehysteresis valuesbelow10◦[11–17].Similarsuperhydrophobicsurfaceswith micro-nano hierarchicalstructuresare alsoobserved invarious insects,suchasthebutterflywings,whichdisplaytwokeyperiodic structures [18–21].Theindividual shingle-like epidermal scales whichcomprisethewingsofbutterfliesareabout40×80meach andthemicro-reliefoftheraisedridgescoveringeachwingscale, eachbetween1.0and1.5m[22].Thecontactanglesofthewater dropletsonthebutterflywingsurfacearemeasuredtobehigher than150◦[23],whichrollsfreelywhentheincliningangleislarger than3◦,thuskeepingthebutterflywingsurfacecleanofdustand otherdebris.Awiderangeofsyntheticmaterialswith superhy-drophobicsurfaces,basedonpolymers,ceramics,metalsorhybrid compositesystemshavebeenpreparedbytemplatingthe natu-ralsystemssuchaslotusleaf,riceleaf,butterflywings[18,19]and others[24,25].
Theoreticaltreatmentoftheeffectof thetopographyonthe wetting behaviorofsurfaceshasbeenprovided byWenzel[26] http://dx.doi.org/10.1016/j.porgcoat.2015.03.015
andCassieandBaxter[27].Wenzelassumedcompletewettingofa roughsurfacebythedropletandmodifiedthecontactangle mea-suredbyintroducingaroughnessfactor(r),definedastheratioof theactualareaofaroughsurfacetoitsprojectedgeometricarea, whichhasavaluegreaterthan1.CassieandBaxterassumedthe apparentcontactangleonaroughsurfacetobetheweighted aver-ageofthecontactanglesonthesolidandairsurfaces.Theydefined (f)tobethesurfacefractionontopoftheprotrusionsand(1−f) onairpockets[28].WhenCassie–BaxterandWenzelrelationships arecombined,ageneralequationgivenbelow,whichprovidesthe apparentcontactanglesmeasuredona roughsurface (R)as a functionofthecontactanglemeasuredonasmoothsurface(), roughness(r)andsurfacevoidfraction(f)isobtained.Thisequation clearlyindicatesthatforaninherentlyhydrophobicandflatsurface withacontactanglegreaterthan90◦,increasedsurfaceroughness willleadtomuchhighercontactangles.Modifiedversionsofthis equationexplainingthecontactanglebehaviorofroughsurfaces havealsobeenreported[29–32].
cosR=r·f·cos+f−1
Asaresult oftheremarkableacademicand industrial inter-estin thesuperhydrophobicity,alargenumberofmethods and processes have been developed for the preparation of super-hydrophobicpolymersurfacesshowing hierarchicalmicro/nano roughnesses.Theseincludelayer-by-layerdeposition[33], electro-spinning[34],microphaseseparation[28,35,36],etching[37,38], spin-coatingordip-coating[28,39,40],sol–gelsynthesis[10,41], templating [20,21,28,36,42], spraying [5,43] and many others [10,16,28,37,38,44]. Although a wide range of approaches have beenproposedforthepreparationofsuperhydrophobicpolymer surfaces,mostofthemare polymerspecific,fairlycomplex and involveseveralsteps.Recentlywereportedafairlysimplemethod, whichwasbasedonthespincoatingoffumedsilicadispersions inanorganicsolventontopolymersurfacesforthepreparation ofpolymericmaterialswithcontrolledwettability,from superhy-drophilictosuperhydrophobic[39,40].Themethodwasapplicable toawiderangeofpolymericmaterials,thermoplasticorthermoset. In this study we investigated the utilization of more prac-tical coating methods for thepreparation of superhydrophobic silicone–ureacopolymer surfaces,whichincludedspray coating using an airbrush and direct coating using a doctor blade, in additiontothespin-coating process.Topographyand the aver-ageroughnessofthesuperhydrophobicpolymersurfacesobtained werecharacterizedbyfieldemissionscanningelectronmicroscopy (FESEM)and whitelightinterferometry(WLI). Static,advancing andrecedingwatercontactangle(CA)measurementswerealso performedtodemonstratetheformationofsuperhydrophobic sur-faces.Our results indicate that: (i) the extent of surface silica coverageandthedistancebetweensilicaparticles,(ii)averagesizes ofthesilicaparticlesoragglomerates,(iii)presenceofmicro-nano hierarchicalstructures,and(iv)theaveragesurfaceroughness,play criticalrolesinobtainingsuperhydrophobicsurfaces.
2. Experimental
2.1. Materials
Segmentedthermoplastic polydimethylsiloxane-urea copoly-mer(GeniomerTPSC140)(TPSC)withaPDMScontentofabout 92%by weightand thehydrophobicfumed silica(HDKH2000) weresuppliedbyWackerChemie,Munich,Germany[45].Primary particlesizeforthehydrophobicsilicaisreportedtobe5–30nm, whichincreasesto100–250nmafteraggregation.Thespecific sur-faceareais170–230m2/g[45].Reagentgradeisopropanol(IPA),
tetrahydrofuran(THF)andtoluenewereobtainedfromMerckand wereusedasreceived.
2.2. Preparationofsuperhydrophobicsilicone–ureacopolymer surfaces
Methodsusedforthepreparationofsuperhydrophobicpolymer surfacesthroughtheuseofhydrophobicfumedsilica(HDKH2000) areexplainedindetailbelow.
2.2.1. SpincoatingofsilicadispersedinTHFontotheTPSCsurface Thefirstmethodwaslayer-by-layerspincoatingofsilica par-ticles onto TPSC surface from a dispersionin THF, which was explainedindetailpreviously[39,40].TPSCwasdissolvedinIPA (15%byweight),whereasthehydrophobicsilicawasdispersedin THFataconcentrationof0.5%byweightandwassubjectedto ultra-soundsonicationatafrequencyof35kHzonaSonorexRK255H typebathfor10h.Dynamiclightscattering(DLS)measurementson hydrophobicsilicasuspensionsinTHF(10mg/mL)indicatedfairly homogeneousdistributionofthesilicananoparticles,witha num-beraveragesizedistributionof44±9nm.Spincoatingwasapplied onaModel7600SpinCoaterbySpecialtyCoatingSystems,Inc., Indianapolis,IN,USA.
Glassslides(20×20×0.15mm),cleanedbywipingwithIPAand THFwereusedasthesubstrateforspincoatedTPSCfilms,which hadathicknessofabout20–30m.8–10dropsof0.5%byweight silicadispersioninTHFwerethenplacedontothepolymerfilm andspincoatedafterwaitingfor1mintoallowthesurfacewetting andefficientpenetrationofsilicaparticlesintothepolymerfilm. Thisstepwasrepeatedseveraltimestoachieveoptimum cover-age.Toimprovethedurability,thesilicacontainingsurfaceswere finallyspincoatedwithathinlayerofTPSCfilm.Spincoatingwas performedat1000rpmfor70s.Allsamplesweredriedatroom temperatureovernightandtheninavacuumovenatroom tem-peratureuntilconstantweightandwerekeptinsealedcontainers untilfurthertesting.
2.2.2. SpincoatingofTPSC/silicadispersionsonaglasssubstrate TPSCwasdissolvedinIPAataconcentrationof0.5%byweight. Tothispolymersolutionsilicaparticleswereaddedatdifferent amountstoobtainTPSC/silicaratiosof1/4,1/7and1/10(byweight). Mixtureswerestirredvigorouslyfor30minbyamagneticstirrer andthenweresonicatedfor30mintoobtainhomogeneous disper-sions.DLSmeasurementsonTPSC/silica(1/10)mixturescontaining 40mg/mLsilicaindicatednumberaverageparticlesizedistribution of272.0±24.7nm,whichwasstableforseveralhours.TPSC/silica dispersionswerespincoatedontoglassslidesat1000rpm.Spin coatingstepwasrepeatedtoapplysuccessivelayers.Sampleswere driedatroomtemperatureovernightandtheninavacuumoven at40◦Cfor24h.
2.2.3. CoatingofTPSC/silicadispersionsonaglasssubstrateusing adoctorblade
TPSC/silica (1/10) (byweight) dispersion preparedin IPA as described before wascoated on a glass substrate using a doc-torblade witha gaugethickness of200m.Coatingwasdried overnightatroomtemperatureandtheninavacuumovenat40◦C for24h.
2.2.4. SpraycoatingofTPSC/silicadispersionswithanairbrush SpraycoatingwasappliedbyusingaMaxH2000modelCora airbrushpainterwithanozzlediameterof0.8mm,pressurizedby aBlackandDeckercompressor.TPSC/silica(1/10)(byweight) mix-turepreparedinIPAwastransferredintothetankoftheairbrush andthemixturewassprayedontotheglassslidesunder differ-entconditions.Thecoatingparametersincludedthetankpressure,
distancebetweennozzleandthesubstrateandthedurationofthe sprayingprocess.
2.3. Characterizationmethods
Dynamiclightscattering(DLS)measurementsonsilica disper-sions wereperformedonMalvernZetaSizerNano-SInstrument withtheNano-Ssoftware.Glasscuvetteswithsquareapertures wereusedassampleholders.Transparenciesofthesampleswere determinedinthevisibleregion,usingaSchimadzuModel3600 UV–vis-NIRspectrophotometeragainstairasthereference.
Staticwatercontact anglemeasurementswereconductedat roomtemperature(23±2◦C)onaDataphysicsOCA35 goniome-terequippedwithSCA20software,whichprovidedtheelectronic controlof thedeviceparameters andmeasurement ofthe con-tactangles.Anaverageofatleast10contactanglereadingswere takenforeachsampleusing5Ldeionized,tripledistilledwater droplets.Contactanglehysteresismeasurementswereconducted bydynamicsessiledropmethodbyplacinga0.5Lwaterdroplet ontothesurfacefromsyringetipandgraduallyincreasingto5L. Duringthemeasurementoftheadvancingangle,thevolumeofthe sessiledropwasincreasedfrom5Lto25Latarateof0.2mL/s and thehighest angle achievedwasaccepted asthe advancing angle.Then,thevolumeofthewaterdropletwasdecreasedfrom 25Lto5Lwiththesamerate.Thelowestanglewasacceptedas therecedingcontactangleafterthecontactlinebetweenthewater dropletandthesurfacestartedtodecreasewithasatisfactorydrop shape.
Afield-emissionscanningelectronmicroscope(FESEM)(Zeiss UltraPlusScanningElectronMicroscope)operatedat2–10kVwas usedtoinvestigatethecoatedsurfaces.PriortoFESEMstudy, sam-pleswerecoatedwitha2–3nmlayerofgoldtominimizecharging. Surfacetopographiesandaverageroughnessvaluesofthesilica coatedsurfaceswereinvestigatedbyWhiteLightInterferometry (WLI)onaBrukerContourGTMotion3DMicroscopeandNon Con-tactSurfaceProfilerattheverticalscanninginterferometry(VSI) mode.UsingWLIfeaturesizesfromsubnanometertomillimeter rangecaneasilybemeasured.InVSImodeaveragesurface rough-nesseswithheight discontinuitiesbetween150nm and several mmcanbepreciselydetermined.Atleast10surfacemapswith dimensionsof47×63m2wereobtainedfromdifferentpartson thesampletocalculatetheaverageroughnessvalues.
3. Resultsanddiscussion
Althoughthesuperhydrophobicbehaviorofroughsurfaceshave beentheoreticallyformulatedbyWenzel[26]andCassieandBaxter [27]over 70 yearsago, preparation,characterization and appli-cationsofsuperhydrophobicsurfaceshave receivedwidespread attentiononlyduringthepast10–15yearsfollowingseveral pub-lications reporting the characteristic surface morphologies and superhydrophobicproperties of a largenumber of plants [1,2]. Asa classical example, thesurface of the lotus leafwas found to beconsisting of cone type protrusions with base diameters of 5–15m, heights of 10–50m and aspect ratios in 0.7–10 range. Theyare irregularly distributed ontheleaf surface with distances betweeneach other rangingfrom 10 to100m.The conesalsodisplayednanometersized hairyfeatures,which are reported to be critical in achieving superhydrophobic surfaces withlowcontactanglehysteresis[11–17].Someofthe pioneer-ingworkinthefieldwasperformedbyMcCarthyandco-workers [8,9,11–14,46–58] and other groups [4,10,15–17,28,41,59–74], whichincludedexperimentalstudiesonthepreparationand char-acterizationofsuperhydrophobicsurfacesandcriticalevaluation ofthetheoreticalfoundations.Alargenumberofexcellentreview
articles were also published recently, which provide detailed experimentaland theoretical information onvarious aspectsof superhydrophobicsurfaces[4,10,28,36–38,44,62,63,75].
In this report we discuss our studies on the development of superhydrophobicsilicone–ureacopolymersurfacesbyusing hydrophobicfumedsilicaandvariouscoatingtechniques.Effect of;(i)thecoatingmethod,(ii)TPSC/silicaratiointhecoating mix-tureand(iii)thenumberofsilicalayersapplied,ontheformationof thesuperhydrophobicsurfaceswereinvestigated.Surface topogra-phies,includingtheextentofsurfacecoveragebysilicaparticles, theirsize,distributionandaveragesurfaceroughnessvalueswere determinedbyvarioustechniques.Formationof superhydropho-bicsurfaceswasdemonstratedbyadvancingandrecedingwater contactanglemeasurementsandcontactanglehysteresis(CAH). Beforebeginningourdiscussions,itisimportanttonotethat vir-ginTPSCcoatedsurfacesdisplaya staticwatercontactangleof 110.0±1.0◦andcontactanglehysteresisof22.0±2.0◦,fairly sim-ilar tothat ofthe crosslinkedsilicone rubber.Thesevalues are independentofthecoatingmethodused.
3.1. SpincoatingofsilicadispersedinTHFontotheTPSCsurface SurfacemodificationofTPSCthroughlayer-by-layerspin coat-ingbyusingasilicadispersioninTHFhasalreadybeendiscussed indetailinourpreviouspublications[39,40,76].Asaresulthere wewillonly provideexamplesregardingthetopography, aver-ageroughnessandwatercontactanglebehaviorofthesurfaces obtainedbythistechniqueasareferenceinordertocomparethem withthetopographyand propertiesofthesurfacesobtainedby othercoatingtechniques.Fig.1a–cprovidestheSEMimagesof theTPSCsurfacesspincoatedwithdifferentnumberoflayersof hydrophobicfumedsilica.Sinceverydilutesilicadispersion(0.5% byweightinTHF)isused,surfacecoverageofparticlesand sur-faceroughnessincreaseslowlyascanbeseenthroughFig.1a–c. TPSCsurfacebecomessuperhydrophobicafterthreelayersof coat-ingasindicated bythevaluesofthestaticwatercontactangles displayedonFig.1a–c[76].Averagesurfaceroughnessofthevirgin TPSCcoatingis6.3±1.1nm,whereastheroughnessvaluesfor sam-plescoatedwithone,threeandsevenlayersofsilicaare76.6±14.4, 124.3±28.6and208.0±70.4nm,respectively[76].
3.2. SpincoatingofTPSC/silicadispersionsonaglasssubstrate In ordertosimplifytheprocessand obtainrobust, superhy-drophobicsurfacesafterminimumnumberofcoatinglayersand possibly withonly onelayer,in this processsilica particlesare premixedwiththesilicone–ureacopolymerindiluteIPAsolutions (2–5%byweightsolids)toobtainTPSC/silicaratiosof1/4,1/7and 1/10(byweight).Solutionswerethenspincoatedonglass sub-strates,toobtainfilmswiththicknessesin20–30mrange.SEM imagesofTPSC/silica(1/10)coatedglasssurfacesasafunctionof thenumberoflayersofcoatingappliedarereproducedinFig.2.Due tothehigherconcentrationofthesilicadispersionused,afteronly onelayerofcoatingthesurfacesobtaineddisplayedfairly homoge-neousanddensesilicacoverageandsuperhydrophobicproperties withcontact angleswellabove 160◦ asdisplayed oneach SEM image.Asexpected, applicationof successivelayersof coatings increasedthedensityofthesurfacesilicacoverage.Averagesizesof theagglomeratedsilicaparticlesalsoincreasedslightly.Asshown inFig.3silicaagglomeratesdisplayednanoroughnessandall sur-faceswerehighlysuperhydrophobicwithverysmallcontactangle hysteresisvaluesasshownonTable1.
Two-dimensionalsurfacetopographyimagesandaverage sur-faceroughnessvaluesofsilicacoatedTPSCsurfaceswereobtained usingaWhiteLightInterferometer(WLI).WLIimagesofthe sur-facesspin coated withsilicaare provided in Fig.4a and b and
Fig.1.SEMimagesofhydrophobicfumedsilicacoatedTPSCsurfaces,(a)onelayer,(b)threelayersand(c)sevenlayersofsilicaand(d)expandedwievofanagglomerated micronsizedsilicaparticledisplayingnanostructure.
roughnessprofilesalongXandYaxesaregiveninFig.4candd. Aver-ageheightsofsilicaagglomeratesweregenerallyaround1–2m withmaximumheightsreachingtoabout4masshowninFig.4c andd.
Table1providestheaveragesurfaceroughnessvaluesandstatic watercontactanglesasafunctionoftheTPSC/silicaratioandthe
numberofcoatinglayersapplied.AscaneasilybeseenfromTable1, evenafteronelayerofcoatingwith(1/4)mixture,asurfacewith anaverageroughnessvalueabove100nmisobtained, whichis aroundthethresholdroughnessvaluetoobtaina superhydropho-bicsurfacebasedonourpreviousstudiesfortheTPSCsystem[76]. AssummarizedonTable1,astheTPSC/silicaratiointhecoating
Fig.2.SEMimagesofTPSC/silica(1/10)spin-coatedglasssurfaces,(a)onelayer,(b)twolayers,(c)threelayersand(d)fourlayersofspin-coatings.Scalebaris50mforall samples.
Table1
AveragesurfaceroughnessvaluesandstaticwatercontactanglesasafunctionoftheTPSC/Silicaratio(byweight)andnumberofcoatinglayersapplied.
TPSC/Silica:1/4 TPSC/Silica:1/7 TPSC/Silica:1/10
Ra(nm) CA(◦) Ra(nm) CA(◦) CAH(◦) Ra(nm) CA(◦) CAH(◦)
Onelayer 127±34 144.5±1.1 140±31 168.0±0.7 6.6 195±72 167.7±0.3 7.0
Twolayers 142±72 163.4±0.4 172±8 168.1±1.1 2.4 222±110 168.1±0.9 3.0
Threelayers 141±64 165.0±0.8 190±29 167.4±1.5 2.1 207±45 165.8±1.2 2.1
Fourlayers 178±42 167.5±0.9 189±38 169.7±0.8 4.3 237±40 164.0±1.0 2.4
Fig.3. SEMimageofthenanostructuredisplayedbythesilicaagglomeratesonthree layersofTPSC/silica(1/10)spincoatedglasssurface.
solutionincreasesto(1/7)and(1/10)orthenumberofcoating
lay-ersappliedisincreased,theaverageroughnessvalues obtained
alsoincrease,asexpected.Except for thesurface obtainedbya
singlelayercoatingofthe(1/4)mixture,whichdisplayedastatic
watercontactangleof144.5◦,allsurfacesdisplayedveryhighwater
contactanglesbetween163and170◦andwatercontactangle
hys-teresisvalueswellbelow10◦,clearlyindicatingtheformationof
superhydrophobicsurfaces.
3.3. CoatingofTPSC/silicadispersionsonaglasssubstrateusinga
doctorblade
Conventionally,doctorbladetechniquehasbeenanimportant
methodforthepreparationofthinpolymericfilmsandcoatingson
varioussubstratesfromsolution.Itisafairlysimpletechniqueand
providesprecisecontrolofthefilmthicknessandiswidelyused
bothforresearchandcommercialapplications,especiallyforthe
directortransfercoatingofwovenandnon-wovenfabrics.
InthisstudyaTPSC/silica(1/10)mixturewaspreparedinIPA
andwascoatedonacleanglasssurfaceusingadoctorbladewith
agapthicknessof200m,whichprovidedadrycoatingthickness
ofabout5m.SEMimagesofthecoatedsurfacesareprovidedin
Fig.5atvariousmagnificationstoprovidethedetailsofthe topog-raphyandthehierarchicalmicro-nanostructuresobtained.Fig.5a clearlyshowsveryhomogeneousdistributionofthesilica parti-clesonthesurfacewithparticlesizesranginginafairlynarrow rangeofabout2–5m,asshownonFig.5b.Silicaparticlesare wellembeddedintothepolymer(Fig.5c),providingarobustand durablesurface.Thesurfaceofthesilicaparticlesclearlydisplays
Fig.5.SurfaceSEMimagesofthedoctorbladecoatedTPSC/silicafilmsatvariousmagnificationsprovidingthedetailsofthetopographyandmicro-nanostructuresformed. (a)500×,(b)2000×,(c)20,000×and(d)50,000×.
ahairynanostructure(Fig.5d),similartothelotusleaforother naturalsurfaces.
WLIimageofthefilmsurfaceisprovidedinFig.6a,together withtheroughnessprofilesalongX(Fig.6b)andY(Fig.6c)axes. Inadditiontoafairlynarrowparticlesizeandhomogeneous dis-tribution,averageheightsofthesilicaagglomeratesarealsoina verynarrowrangearound1–2m.Averagesurfaceroughness(Ra) wasdeterminedtobe168±57nmbyWLIstudies.Formationof asuperhydrophobicsurfacewasdemonstratedbyastaticwater contactangleof161.0±1.2◦ andacontactanglehysteresisvalue below10◦.
3.4. SpraycoatingofTPSC/silicadispersionswithanairbrush Spraycoatingisoneofthemostversatileandpracticalcoating methods,whichcanbeusedonsmallorlargeareasandonflator intricatesurfacessuccessfully.InthisstudyweutilizedTPSC/silica (1/10)dispersioninIPAandspraycoated2×2cm2glassslides. Dur-ingthecoatingprocessweinvestigatedtheinfluenceofpressure, distancebetweennozzleandsubstrateandspraytimeontheextent ofsurfacesilicacoverage,topography,averageroughnessandstatic watercontactanglesandcontactanglehysteresisofthesurfaces obtained.Allofthesamplesdiscussedherewerepreparedusing atankpressureof2barandnozzletosampledistanceof20cm. Theonlyvariablewasthespraytime,whichwas1,2and3s.SEM imagesofsilicamodifiedTPSCsurfacesobtainedatdifferentspray timesarereproducedinFig.7.
SEMimagesprovidedinFig.7clearlyindicateafairly homo-geneous coverage of the TPSC surfaces with silica aggregates, regardlessofthe spraytime. Coatingthickness obtainedfor 2s spraytimeisabout35m.Averagediametersofthesilica aggre-gatesaregenerallyin2–20mrange.Asthespraytimeincreases there seem tobea slight increase in the averageparticlesize. Moreinterestinglytheaveragedistancebetweensilica agglomer-atesseemtodecreasewithanincreaseinthespraytime.Aswill bediscussedlater onindetail, this seemstohave asignificant effectonthecontactanglehysteresisbehaviorofthesurfaces.Fora
betterunderstandingofthenatureofthesurfacestructuresformed, highermagnificationSEMimagesofthesamplecoatedfor2sare providedinFig.8.AscanbeseeninFig.8a,silicaagglomeratesare homogeneouslydistributedandwellembeddedintothepolymer matrix,indicatingtheformationofadurablesurface.Thedistance betweensilicaaggregatesseemtobebetween1and20m.When closelyexamined,thehairysurfacenanostructureofthesilica par-ticlescanalsoclearlybeseen,asprovidedinFig.8b.Thisclearly demonstratestheformationofahierarchicalsurfacestructurewith micro-nanofeatures,whichwasshowntobecriticalinobtaining superhydrophobicbehavior[1,2,10].
2Dand3DWLIimagesandroughnessprofilesofthesurface obtainedfor 2sspray coated sample are reproduced in Fig. 9. 47×63m22Dand3DimagesgiveninFig.9aandbshowafairly goodsurfacecoverageandabroaddistributionofsilica agglom-erateswithparticlesizesin2–20mrange.Heightsofthelarge silicaagglomeratesarearound5m,whicharemuchhigherthan thoseobtainedbydoctorbladecoatingtechnique. Formationof suchlargesilicaagglomeratesresultsinanincreaseintheaverage surfaceroughnessfromabout170nmforthedoctorbladecoated sampletoabove300nmforthespraycoatedsamplesasgivenin Table2.Interestingly,theroughnessprofilesofthe47×63m2 surfaceprovidedinFig.9canddclearlyshowsthepresenceoffairly smoothvalleysbetweensilicaagglomerates.
Table2providesasummaryoftheeffectofspraytimeonthe averagesurfaceroughness,staticwatercontactanglesandcontact anglehysteresis(CAH)forTPSC/silica(1/10)coatingsperformed underapressureof2barandatanozzletosubstratedistanceof 20cm.
As provided in Table 2, all samples display average surface roughnessvaluesbetween300and400nm,whichisfairlyhigh. Averagesurfaceroughnessvaluesofthesamplesshowasignificant increasewithspraytime,whichmaybeexpected.Ascanclearlybe seeninTable2,allsurfacesdisplayveryhighstaticwatercontact anglesabove160◦,indicatingtheformationofsuperhydrophobic surfaces.Contactanglehysteresis(CAH)valuesforsamplesS2and S3are4.8and2.9◦respectively,stronglysupportingtheformation
Fig.6.47×63m2WLIimageof(a)thedoctorbladecoatedTPSC/silica(1/10)surfaceandtheroughnessprofilesalong(b)Xand(c)Yaxes.
Fig.8. SurfaceSEMimagesoftheTPSC/silica(1/10)samplespraycoatedfor2s,showingtheformationofmicro-nanohierarchicalsurfacestructures.
Fig.9.(a)2Dand(b)3D47×63m2WLIimagesofthe2sTPSC/silica(1/10)spraycoatedsamplesurfaceandtheroughnessprofilesalong(c)Xand(d)Yaxes.
of truly superhydrophobic surfaces. Interestingly, although the staticwatercontact angleforsample S1is above160◦,itsCAH valueis26◦andveryhigh.WhentheSEMimagesprovidedinFig.7 iscloselyexaminedwebelievethisismainlyduetothepresence ofalargenumberofsmallersizedsilicaagglomeratesonsampleS1 surface,togetherwiththelongerdistancesbetweenthe agglom-erates.Webelievethesmallersilicaagglomeratesseparatedfrom each other by longerdistances are not able tohold the water
dropletsonthetopoftheprotrusionswithanaircushionunder themasinthecaseofCassie–Baxterregimeschematicallyshown inFig.10a.AsthedropletsgetlargerduringCAHmeasurements, theyseem topenetrate through theagglomerates and wetthe surface,therebychangingthemechanismfromCassie–Baxterto theWenzelregime(Fig.10b),whichresultsinadecreaseinthe recedingwatercontactangleandanincreaseintheCAH.Detailed studiesinordertobetterunderstandthisbehaviorisunderway.In Table2
Effectofspraytimeontheaveragesurfaceroughness,staticwatercontactanglesandcontactanglehysteresisvaluesonspraycoatedTPSC/silicasurfacesperformedunder 2barpressureandatanozzletosubstratedistanceof20cm.
Sampleno Pressure(bar) Spraytime(s) Distance(cm) Roughness(Ra)(nm) Av.CA(◦) CAH(◦)
S1 2 1 20 293±85 162.0±1.8 26
S2 2 2 20 375±59 165.7±0.5 4.8
Fig.10.Schematicdescriptionof(a)Cassie–Baxterand(b)Wenzelwettingstates. 60 70 80 90 100 400 500 600 700 800
Transmittance (%)
Wavelength (nm)
Fig.11.Comparisonoftherelativetransparenciesofuncoatedglassslide( ) andTPSC/silicacoatedglasssurfacesbydifferentmethods.Doctorbladecoating(
),spin-coatingofsilicaparticlesonTPSC(···),spin-coatingofTPSC/silica(1/10) ( )andspraycoating( ·· ·· ).
addition,thelongerdistancesbetweenthesilicaparticlesin
sam-pleS1,whencomparedtoS2andS3mayresultindroppinning,
resultinginafairlyhighCAHvalue.
For various applicationsa critical requirement is the
trans-parency of the superhydrophobic coatings. We measured the
transparencies of the glass slides used as the substrate and
TPSC/silicacoatedglassslidesobtainedbyvariouscoatingmethods
describedinthispaper.Percenttransmittanceversuswavelength
curves obtained against air as thereference are reproduced in
Fig.11.Uncoatedglassslidesontheaverageshow85%transparency inthevisibleregion.Itisinterestingtonotethatthedoctorblade coated TPSC/silica film,with a thicknessof about 5m, shows anaverageof90% transparency,which ishigher thantheglass substrate.Silicaspin-coatedTPSCfilmwithathicknessofabout 20m, alsodisplays about85% transparency similar tothat of theglasssubstrate. Spray coatedTPSC/silica (1/10)film, witha thicknessof about 35m displaysa fairly low transparency of around63%at400nm,whichgraduallyincreasesto85%at800nm. SpincoatedTPSC/silica(1/10)filmhasthelowestaverage trans-parencyofaround75%,whichdoesnotchangewithwavelength. Theseresultsindicatethatthetransparencyofthesilicamodified superhydrophobicTPSCfilmsisfairlygood.Asmightbeexpected, transparencyofthecoatingdependsonthefilmthicknessandthe coatingmethodused.
4. Conclusions
Influence of coating methods on the preparation of super-hydrophobicsilicone–urea copolymer (TPSC)surfaces, modified bythe incorporationof hydrophobicfumed silica nanoparticles was investigated. The methods employed were: (i) successive spincoatingofhydrophobicfumedsilicadispersedinanorganic solventonto preformed silicone–ureafilms, (ii)spin coating of silica–polymerdispersionontoaglasssubstrate,(iii)direct coat-ingofsilica–polymermixturebyusingadoctorblade,ontoaglass substrate,and(iv)spraycoatingofsilica–polymerdispersionbyan air-brushontoaglasssubstrate.Inadditiontothemethodused,
influenceoftheconcentrationofthefumedsilicadispersionand thenumberofcoatinglayersappliedonthesurfacetopography, theextentandnatureofthesilicacoverageandaveragesurface roughnessweredetermined.Allsurfacesobtaineddisplayed micro-nanohierarchicalstructuresasobservedbySEM studies.It was demonstratedthatsuperhydrophobicsurfacescouldbeobtained byallmethodsemployed,whichisclearlyindicatedbythestatic andadvancingwatercontactangleswellabove150◦andcontact anglehysteresisvaluesbelow10◦.Twocriticalquestionsregarding thesurfacesformedaretheirdurabilityandtransparency. Dura-bilityorsuperhydrophobicityretentionofthecoatedsurfaceswas determinedbytwosimpletests.Firstoneinvolvedthe measure-mentofthewatercontactangles(CA)andcontactanglehysteresis (CAH)onsamples,whichwerestoredforoverayearunder ambi-entconditions.Inthesecondtestascotchtapewasfirmlypressed ontothecoatedsurfaceandwaspeeledoffafter5sofcontact.CA andCAHvaluesdidnotshowanynoticeable changeafterthese tests,whichindicatedtheformationofdurable,superhydrophobic surfaces.Regardingthetransparencyofthefilms,wehave demon-stratedthatsuperhydrophobicTPSC/silicafilmswiththicknesses uptoabout20mareverytransparent.Formanypractical appli-cationsspraycoatingprovidesasimplerouteforthepreparationof superhydrophobicsurfaces.Ontheotherhand,doctorblade tech-niquecanbeusedtoobtainsuperhydrophobicpolymerfilmsor textilesbyusingconventionaldirectcoatingmethods.
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