investigatedwww

12015Wiley-VCHVerlagGmbH&Co.
KGaA,Weinheimwileyonlinelibrary.
comwww.
MaterialsViews.
comInterfaceManipulationforPrintingThree-DimensionalMicrostructuresUnderMagneticGuidingLibinWang,FengyuLi,MinuanKuang,MengGao,JingxiaWang,YuHuang,LeiJiang,andYanlinSong*surfaces,diverse3Dmicrostructuresincludinghat,cone,pillar,andspindleareformed.
Therelationshipbetween3Dmorphologyandinterfacepropertiesisclaried.
Accord-ingly,accurate-positionedandoriented-patterned3Darraysarefacilelyprinted,demonstratinghighcontrollabilityandlarge-scalefabricationofuniform3Dmicrostructures.
Theas-prepared3Dmicrostructuresandarrayshavegreatpotentialinactuators,[29,30]sensors,[31,32]andphononiccrystalsstudy.
[33]Thisadvancein3Dfabricationtechnologybasedoninterfacemanipulationwillpresentsignicantinsightandpromisingapplicationsincontrollable3Dmanufacturing.
Figure1aillustratestheschemetoachieve3Dmicrostruc-turesontheliquid–solidcompositesurfaceguidedbyacubicmagnet.
Magneticink(M-ink)dropletsareprintedontothesurfaceanddeformedfromhemisphericaltopillaredmicro-structurewithdropletedgeretracing.
Theorientationofthe3Dmicrostructurescanbealteredwithinawidetiltinganglebydeectingthemagnet,enablingasymmetric3Dmanu-facturing.
Theliquid–solidcompositesurfaceisconstructedbyswellingpolydimethylsiloxane(PDMS)withsiliconeoil(Figure1b).
SwellingcausesthePDMSchainstobeunfoldedandcladdedofoil,formingacompositesurfaceexposingacertainamountofliquid,whichsuppressestheresistanceandfavorstheslidingofdroplet.
Thedynamicdewettabilityofthesurfacecanbeaccuratelycontrolledbyswellingratio(denedastheratioofabsorbedsiliconeoilandPDMS),whichessentiallyalternatestheamountofliquidcomponent(FiguresS1a–candS2,SupportingInformation).
Particularly,recedingangle(θR)ofthesurfacepresentsneadjustabilitywiththeswellingratio(Figure1c).
Asaresult,theM-inkdropletspossesscontrollableretracingbehavior(FigureS1d,SupportingInformation).
Theformationschemeofthe3DmicrostructureisdepictedinFigure1d.
PulledbymagneticforceFmanddistortedbyresistanceforceFr,thedropletisdeformedintoconicshapewithfastreducedinstantaneouscontactangle(CA).
Whenitislessthantherecedingangle,thedropletedgestartstoretrace.
Withthedropletheight-eninganddiameterreducing,theinstantaneousCAincreasesgraduallytoreachtherecedinganglevalueandtheraiseddropletwillnotbedrawnawayduetothesubstrateresistance.
Eventually,a3Dhigh-aspect-ratiomicrostructureisformed,andthebaseangleofthe3DstructureonsubstrateisequaltoitsM-inkrecedingangle.
AtypicaldeformationprocessforoneM-inkdropletonthecompositesurfacewithrecedingangleofca.
95°wascapturedbyaCCD-camera(Figure1e).
ThedynamicevolutionsshowthatthedropletisheighteningDOI:10.
1002/smll.
201403355ControllabilityDr.
L.
Wang,Dr.
F.
Li,Dr.
M.
Kuang,Dr.
M.
Gao,Dr.
J.
Wang,Dr.
Y.
Huang,Prof.
L.
Jiang,Prof.
Y.
SongBeijingNationalLaboratoryforMolecularSciences(BNLMS)KeyLaboratoryofGreenPrintingKeyLaboratoryofOrganicSolidsInstituteofChemistryChineseAcademyofSciencesBeijing100190,P.
R.
ChinaE-mail:ylsong@iccas.
ac.
cnDr.
L.
Wang,Dr.
M.
Kuang,Dr.
M.
GaoSchoolofChemistryandChemicalEngineeringUniversityoftheChineseAcademyofSciencesBeijing100049,P.
R.
China3Dmicrostructures[1,2]hasarousedgreatinterestandbeenappliedinintegratedelectronics,tissueengineering,high-efcientcollectorsandsensors,photonicmaterials,etc.
[3–6]Various3Dfabricationtechniqueshavebeenproposedindecades,includingself-assemblyofin-planeunits,[7,8]layer-by-layerprinting,[9,10]directwriting,[11–14]andsoon.
Althoughthesestrategiesprovideexiblefabricationplatform,thecontrollabilityofprecise3Dmicrostructuresremainsasig-nicantchallenge.
Thefundamentalproblemsinvolvethemanipulationoftheliquidwetting,dewetting,[15,16]coales-cence,[17]transport,[18,19]andreshaping,[20,21]whichinuencetheformationofthe3Darchitectures.
Someeffortshavebeeninvestedtocontroldropletsbyappropriateinterfacedesign.
Forinstance,dropletsweretransformedtospecic3Dmicro-structuresbasedonpyroelectrodynamic-inducedliquid–airinterfacedeformation.
[22,23]Moreover,externallydrivenmagneticdropletswereseparatedintocomplicatedpatternonsuperhydrophobicsurfaces.
[24]However,therelationshipof3Dmorphologyandinterfacialpropertiesisstillunclear,whichlimitstheoperabilityofdroplets,resultinginpoor3Dcontrollability.
Hence,exploringtheinuenceofinterfacialpropertiesuponnal3Dmorphologyofprinteddropletswillhavebroadtheoreticalandtechnologicalimplications.
Herewepresentacontrollablestrategytoprintprecise3Dmicrostructuresvia2Dinterfacemanipulationofdrop-letsonsurfaceswithtunabledynamicdewettingproper-tiesundermagneticguiding.
Forthispurpose,aliquid–solidcompositesurfacewithtunabledynamicdewettability[25,26]andremarkableslipproperty[27,28]isconstructed,wheredropletspossessescontrollablestick-slipbehavioranddefor-mationperformance.
Bytuningthedynamicdewettability,especiallytherecedingangleofprinteddropletsonsuchsmall2015,DOI:10.
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2014033552www.
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KGaA,Weinheimwww.
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comanditsedgeisretracinguntilthebaseangleapproachestoaround90°,forminga3Dpillarstructure.
Thecorrespondingshapeparametervariationofdiverse3Dmicrostructuresonunswollen,partial-swollenandsaturated-swollenPDMSwithdifferentrecedingangleswereinvestigated,demonstratingvariousdropletretracingperformancesandcontrollable3Dmorphologies(FiguresS3,S4andMoviesS1–3,SupportingInformation).
Figure2showsscanningelectronmicroscopy(SEM)imagesofcontrolled3Dmicrostructuresandprintedarraysondifferentsurfaces.
OnunswollenPDMSwithlargeCAhysteresisandsmallrecedingangle(θRinedforthestickingofthedropletedge(Figure2a).
Ithasbaseangleofaround30°,andthebasediameterapproximatetothatoftheinitiallyprinteddropletofca.
120m.
Whereas,onthecompositesurfacessmall2015,DOI:10.
1002/smll.
201403355Figure1.
Interfacemanipulationprincipleforprinting3Dmicrostructures.
a)Schematictoachieve3Dmicrostructuresbyprintingdropletsonliquid–solidcompositesurfaceundermagnet;theorientationofthemicrostructurescanbecontrolledbymagneticguiding.
b)ConstructionofthecompositesurfacebyswellingPDMSwithsiliconeoil.
c)Relevanceofrecedingangleandswellingratioofthesurface,demonstratingtunableinterfaceproperties.
d)Illustrationofthedropletdeformationandretractionprocess,whichdependsontherecedingangleofthesurface.
e)SequenceofapillarstructureformedonswollenPDMSsurfacewithrecedingangleofca.
95°.
Scalebar,500m.
Figure2.
Controllabilityof3Dmicrostructuresbyinkjetprintingonsurfaceswithtunabledewettability.
a–e)Multiplemicrostructuresshapedfromhat,cone,andspindleonsurfaceswithincreasedM-inkrecedingangles.
f)HatarrayonunswollenPDMSwithrecedinganglebelow30°.
g)Conearrayonpartial-swollenPDMS(swellingratioofca.
0.
45)withrecedingangleofca.
85°.
h)Spindlearrayonsaturated-swollenPDMS(swellingratioofca.
1.
6)withrecedinganglebeyond90°.
Scalebars,100m;viewangles,60°.
3www.
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KGaA,Weinheimwww.
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comofpartial-swollenPDMSwithM-inkθRofca.
75°,85°,and95°(swellingratiosofca.
0.
29,0.
45,and1.
43),conicmicro-structureswithdifferentbaseanglesanddecreaseddiametersweregeneratedduetotheedgeretractionofvariantdegrees(Figure2b–d).
Specically,the3DmicrostructurespresentpillaredshapewhenθRisaround90°.
Asexcessiveslidingofthedropedgeoccurredonsaturated-swollenPDMS(swellingratioofca.
1.
6)withnegligibleCAhysteresisandlargeM-inkθRofca.
105°,aspindlemicrostructurewithbaseangleofabout105°wasachieved(Figure2e).
Thebaseandmaximummiddlediameterofthespindleis28mand49m,respectively.
TheSEManalysisshowsthattheinterfacialdewettingpropertiesdirectlyinuencethe3Dmorphologyofthemicrostructures;indeedthebaseanglevaluescorre-spondwellwiththerecedinganglevalues,andthediametersdecreasewiththeenhancingofdropletretraction.
Remark-ably,typical3Dmicrostructurearraysofhat,cone,andspindlewereeasilypreparedbyinkjetprinting(Figure2f–h).
Comparingtotheirregulararrayconsistedofrandom3Dmicrostructuresongeneralsurface(FigureS5,SupportingInformation),thearrayspreparedbyinterfacemanipulationoncompositesurfacespossessmuchmoreuniform3Dmicro-structures,achievingexcellentcontrollabilityandlarge-scalefabricationfor3Dprintingtechnique.
Astherecedinganglesofsurfacescontrolthedropletretractionandthe3Dmorphologies,weillustratethefor-mationoftypical3Dmicrostructuresontheliquid–solidcompositesurfaceswithdifferentinterfacialproperties(Figure3a).
(i)Onsolidsurfacewithextremelysmallrecedingangle(unswollenPDMS,θRinterfacialresist-ance,deformingtohat-likestructure.
(ii)Onliquid–solidcom-positesurfacewithincreasedrecedingangle(partial-swollenPDMSwithdifferentswellingratios),thedropletedgeisretracingcontinuouslybecausethedecreasedinstantaneousCAcanreachtherecedinganglereadily.
Forrecedinganglesrangingin30°–90°,the3Dstructurespossessconiccongura-tionsofserialbaseangles.
Particularly,the3Dpillarstructurecanbeobtainedonsurfacewithrecedinganglearound90°.
(iii)OnsurfacewithrecedingangleapproachingtoitsstaticCA(saturated-swollenPDMS,θR>90°),thedropletedgeiseasiertoslide,thusspindlearchitectureisformed.
M-inkdropletpresentsellipticaldeformationundertheactionofaperpendicularmagneticeld(FigureS6,SupportingInformation).
ThedeformeddropletissubjectedtobothmagneticforceFmandresistanceforceFr.
Onlyifthemagneticforceisnotsufcienttoovercometheresist-anceforceFm≤Fr,stable3Dstructurescanbeachievedwithoutthewholedropletdrawnaway(gravityforceisneg-ligiblefornanoliterdroplets.
[24]Frisalignedwiththeverticaldirection,whosevalueisrelatedtothebasediameterdofthe3DmicrostructureandtherecedingangleθRofthesur-face.
[34]Inthiscase,forconstantmagneticforceFmactingonthejetteddroplets,[35]therelevanceofbasediameterdandrecedingangleθRcanbedescribedas(detailsinSupportingInformation)θ≥24.
4/sindRRegardingthecriticalconditionforavailable3Dmicro-structures,acurvecorrespondingtodc=24.
4/sinθRispre-sented(Figure3b),wheredcsuggeststhecriticalbasediameterofthe3Dmicrostructures.
IntheexperimentedθRrangedfrom0°to90°onthecompositePDMSsurfaces,3Dstructuresshowmorphologiesfromhattoconewithdecreaseddc.
Particularly,high-aspect-ratio3Dmicrostruc-turecanbeachievedatrecedinganglesaround90°.
WhenθRisabove90°,3Dspindlestructureswouldbeobtained.
ForθRapproaching180°,thedropletstendtobedrawnawayduetotheirtinysurfaceresistance;[35]3Dmicrostructureswithlargedccanonlybeobtainedwithmilddropletdeformationandretraction.
InFigure3b,thegreenregionpredictsthefeasiblemorphology(diameterandbaseangle)for3Dmicrostruc-tures,whilethepinkregionrepresentstheparameterspacewherethedropletwillbedrawnaway.
small2015,DOI:10.
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201403355Figure3.
Interfacemanipulationsketchandcapacityforprintingdesirable3Dmicrostructures.
a)Modelsoftypical3Dmicrostructures(i)hat,(ii)cone,and(iii)spindleformedonsurfaceswithdifferentinterfacialproperties.
b)Theoreticalcurveandexperimentalresultspredictingthemorphologyofthe3Dmicrostructures.
Thesolidcurveshowsthecriticalrelevancebetweenthebasediameterof3Dmicrostructuresandtherecedingangleofcorrespondingsurfaces.
Thegreenregionconnesthefeasibleparametersforstable3Dmicrostructures,whilethepinkregionindicatesthatthedropletwoulddeviateorbedrawnaway.
Thesquaresandcrossesrepresenttheexperimentalresultsofavailableandunavailable3Dmicrostructures,respectively.
Thethreeregionsseparatedbydottedlinescorrespondtothethreetypical3Dmicrostructuresina).
4www.
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comcommunications2015Wiley-VCHVerlagGmbH&Co.
KGaA,Weinheimwww.
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comToverifytheanalysis,wedidstatisticsonthebasedia-metersoftheas-printed3Dmicrostructuresandtherecedinganglesofrelevantsurfaces.
Controlledmicrostructurearrayscanbepreparedwithparameterinthecurve(squaresingreenregion),whilethe3Dmicrostructureswouldeitherdeviatefromtheirdesignedpositionsorbedrawnawayasthemagneticforceislargerthantheresistanceforce(crossesinpinkregion,FigureS9,SupportingInformation).
Thethreeregionsseparatedbydottedlinescorrespondtothethreetypical3DmicrostructuresinFigure3a,whicharedenedashat,cone,andspindle.
Manipulatingthedropletretractionanddeformationbytuningthe2Dinterfacialpropertiesleadstodiverse3Dmicrostructures,andtheexploredrelevancebetween3Dmorphologyanddynamicdewettabilityofsur-facemakesitpossibletopreciselydesignandcontrolarbi-trary3Dmicrostructures.
Interestingly,thisstrategyalsoallowsversatilefabricationofasymmetricmicrostructuresatavarietyoftiltingangles,pavingthewaytomanufactureawidepaletteofcomplex3Dmicrostructures.
Thesetiltedmicrostructuresarecreatedbylaterallymovingandrotatingthemagnet;theorientationandtiltingangleoftheresulted3Dmicrostructurescorrespondtothedirectionanddisplacementsofthemagnet(Figure4aandMovieS4,SupportingInformation).
Thetiltedpillarmicro-structureisaffectedbyboththemagneticforceandtheresist-anceforce.
Onlyiftheresistanceforceisstrongerthanthemagneticforce,thepillardropletisinclinedbyamagneticanisotropictorquewithoutslippingonthesurface.
Asaresult,tilted3Dmicrostructuresofdiversefeaturesandtiltingangleswereobtained(Figure4b–e).
Moreover,synchronized-tiltedpillararrayswithcertainangleswerefacilelyprinted(Figure4fandFigureS10,SupportingInformation).
Ananiso-tropic-tiltedpillarrowpossessedtiltinganglesfrom0°to90°wasachievedbyrotatingthemagnet(Figure4g).
Insummary,manipulationofinterfacialpropertiesadvancesthecapabilityof3Dconstructionbyenablingcon-trolledretractionanddeformationofprinteddroplets.
Wehaveveriedtherelevanceofinterfacialpropertiesandspa-tialmicrostructureparameters,anddemonstratedthatthe3Dmorphologyisdeterministicallydependentonthedynamicdewettabilityofsurface.
Accordingly,precisecontrolonthebaseangle,diameterandheightofthe3Darchitectureswasaccomplishedviamanagingtherecedinganglesofsurfaces,resultinginhat,cone,pillar,andspindlemicrostructures.
Besides,morecomplex3Dmorphologiescanbeattainedbyanisotropicinterfacemanipulationofdroplets(FigureS11,SupportingInformation),forexample,onpatternedsurfacewithdifferentialdewettability.
Wehavealsofabricatedpat-ternedandasymmetric3Dmicrostructurearraysbyinkjetprintingundermagneticguiding,whichenablesversatileandlarge-scale3Dconstruction.
Asageneralapproach,numeroustunableinterfacescouldbeadoptedtosatisfythecontrolofvariousinks(water,oilorsolvents).
Moreover,othertriggers,suchaselectriceld,capillaryforce,andthermostimuli,canguidethedeformationofdroplets,offeringawiderangeoftoolstofabricatediverse3Dmicrostructures.
Furthermore,3Dmicrostructureswithdesiredmorphologiesandelabo-rateoptical/electricalpropertiescouldbeachievedbydevel-opingfunctionalmaterialsasink,providingthisapproachapromisingfutureforon-demandandhighlyprecise3Dmanufacturing.
ExperimentalSectionM-Ink:Magneticink(M-ink)inthisworkwasobtainedbymixingferrouid(BeijingShenranTech.
Co.
,Ltd.
,MFW),aqueouspolyvinylalcohol(PVA1788,Aladdin-reagent)solutionof10wt%,andethyleneglycol(EG)withproportionof1:0.
4:0.
6.
Itcontained17wt%Fe3O4nanoparticleswithdiameterbelow10nm.
Thesurfacetensionandsaturationmagnetizationwere52.
37±0.
026mNm1andca.
100±10Gs,respectively.
CompositeSurfaces:Liquid–solidcompositesurfaceswithtunabledynamicdewettabilitywerepreparedfromPDMSelas-tomerkits(DowCorningSylgard184,30:1precursorandcuringagent)swollenbysiliconeoil(Shin-EtsuChemicalIndustryCo.
,Ltd.
,Japan,KF-5,5cs).
PDMSwerespin-coatedoncleanedglassslides,annealedat80°Cfor1handthenimmersedinsiliconeoilforpredeterminedperiods.
Afterwipingexcessoilonthesurface,theswollenPDMSwithdifferentswellingratioswereusedastun-ablecompositesurfaces.
Inthiswork,swellingratiowasdenedastheratioofabsorbedsiliconeoilandPDMS,andwasmeasuredbyweighingPDMSatdifferentswellingtime.
ThetunabledynamicdewettingpropertiesofswollenPMDSwerecharacterizedbyacontactangle(CA)measurementdevice(OCA20,DataPhysics,Germany)at23°C.
Advancingangleandrecedingangle(θAandθR)weremeasuredas3Lliquidwasaddedandwithdrawndynamicallyfromasurface-bounddroplet.
Contactanglehysteresis(Δθ)wasdenedasthedifferenceofadvancingsmall2015,DOI:10.
1002/smll.
201403355Figure4.
Versatileprintingofasymmetric3Dmicrostructures.
a)Schematicillustrationoftheforcesituationforatiltedpillarunderdeectingmagneticeld.
SEMimagesoftiltedb)hat;c)cone;d)pillar;ande)spindlewithdifferenttiltinganglesandorientations.
Scalebars,50m.
f)Printeddeectivepillararraywithanaccordanttiltingangleof30°.
Insertistheopticalmicrographofapillarwithviewangleof90°.
g)Ananisotropic-tiltedpillarrowbyrotatingthemagneticeldinplane.
Scalebars,100m.
AllSEMviewangles,60°.
5www.
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KGaA,Weinheimwww.
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comandrecedingangle.
Slidingangle(α)wasrecordedbymeasuringthetiltingofsubstrateforthemovementofa5Ldropletonit.
Eachangledatawasanaverageofatleastveindependentmeas-urementsondifferentpositionsofthesamesample.
MoredetaileddataareshowninFigureS1,SupportingInformation.
PrintingDroplets:WeprintedtheM-inkdropletsontotheas-preparedcompositesurfacebyajetprintingsystemusingthepiezoelectricvalvewithdispensingvolumeof2nL(PicoDotEFD,Nordson,USA).
AcubicNdFeBmagnetwasplacedverticallyuponthedropletswithouttouchingthem,inducingthedroplettodeformandretraceonthesurfacetogeneratefreestanding3Dmicrostruc-tures.
Thepositionandmovementofdropletsandarrayswereaccuratelycontrolledby4-axissteppingmotorswithpositioningaccuracyof1m(JR2200NDesktopRobot,Nordson,USA).
MagnetandMagneticField:AcubicNdFeBpermanentmagnetwithsizeof5*2.
5*5cm3wasused.
Magneticeldintensity(H)wasmeasuredasafunctionofthedistancefromthesurfaceofthemagnetbyaGaussmeter(DigitalMeasurementSystemSG-3M,China).
Verticalmagneticeldgradient(gradH)wasobtainedfromthederivationofthemeasuredmagneticeldintensity.
DetaileddataareshowninFigureS8,SupportingInformation.
Characterization:Themorphologiesofthe3Dmicrostructureswerecharacterizedbyaeld-emissionscanningelectronmicroscope(SEM,JEOL,JSM-7500F,Japan).
AlltheSEMphotographsweretakenataviewangleof60°unlessstated.
ThedropletdeformationprocesswasacquiredbyaCCD-camerawithcapturespeedof25frames1andresolutionof768*576pixel2equippedofawhite-light-emittinglampandazoominglens(SCA40,Dataphysics,Germany).
SupportingInformationSupportingInformationisavailablefromtheWileyOnlineLibraryorfromtheauthor.
AcknowledgementsTheauthorsthankthenancialsupportbytheNationalNatureScienceFoundation(GrantNos.
51173190and21121001),the973Program(Nos.
2013CB933004,2011CB932303,and2011CB808400),andthe"StrategicPriorityResearchProgram"oftheChineseAcademyofSciences(GrantNo.
XDA09020000).
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Received:November12,2014Publishedonline:small2015,DOI:10.
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201403355