Dynamic-Effects-Induced-Transition-of-Droplets-on-

9208DOI:10.1021/la900761uLangmuir2009,25(16),9208–9218PublishedonWeb05/14/2009pubs.acs.org/Langmuir©2009AmericanChemicalSocietyDynamicEffectsInducedTransitionofDropletsonBiomimeticSuperhydrophobicSurfacesYongChaeJungandBharatBhushan*NanoprobeLaboratoryforBio-andNanotechnologyandBiomimetics(NLB2),TheOhioStateUniversity,201West19thAvenue,Columbus,Ohio43210-1142ReceivedMarch3,2009.RevisedManuscriptReceivedApril14,2009Superhydrophobicsurfaceshaveconsiderabletechnologicalpotentialforvariousapplicationsbecauseoftheirextremewater-repellentproperties.Dynamiceffects,suchasthebouncingandvibrationofadroplet,candestroythecompositesolid-air-liquidinterface.Theimpactpressureofabouncingdropletandtheinertiaforceofavibratingdropletaffectthetransitionfromasolid-air-liquidinterfacetoasolid-liquidinterface.Therefore,itisnecessarytostudythedynamiceffectofdropletsundervarioussystemparameters(impactvelocityandfrequencyandamplitudeofvibration).Anewmodelforthepredictionofthewettinganddewettingprocessduringdropletvibrationbasedontherelationshipbetweentheadhesionforceandtheinertiaforceofadropletisproposed.Toinvestigatewhethermicro-,nano-,andhierarchicalstructurescanresistthedestabilizingfactorsresponsibleforthetransition,astudyofbouncingandvibrationofawaterdropletissystematicallyconductedonvarioussurfaces.Thephysicsofwettingphenomenaforwaterdropletstudiesisoffundamentalimportanceinthegeometricaldesignofsuperhydrophobicsurfaces.1.IntroductionSuperhydrophobicandself-cleaningsurfaceswithahighstaticcontactangleabove150
andlowcontact-anglehysteresis(thedifferencebetweentheadvancingandrecedingcontactangles)playanimportantroleintechnicalapplicationsrangingfromself-cleaningwindowglasses,paints,textiles,solarpanels,andapplica-tionsrequiringantifoulingandincludeareductionofdragforfluidflowtoenergyconservation.1-3Atalowvalueofcontactanglehysteresis,thedropletsmayrollinadditiontoslide,whichfaci-litatesremovalofcontaminantparticles.Surfaceswithlowcontact-anglehysteresishavealowwaterroll-off(tilt)angle,whichdenotestheangletowhichasurfacemustbetiltedforrolloffofwaterdrops.Condensationofwatervaporfromtheenvironmentand/orprocessliquidfilmcanformmenisci,leadingtohighadhesionindevicesrequiringrelativemotion.4-6Superhydrophobicsurfacesareneededtominimizeadhesionbetweenasurfaceandliquid.Amodelsurfaceforsuperhydrophobicityandself-cleaningisprovidedbytheleavesoftheLotusplant(Nelumbonucifera).7-11Theleafsurfaceisveryroughbecauseofso-calledpapilloseepidermalcells,whichformasperitiesorpapillae.Inadditiontothemicroscaleroughness,thesurfaceofthepapillaeisalsoroughwithsub-micrometer-sizedasperitiescomposedof3Depicuticu-larwaxes.ThewaxesofLotusaretubules,butonotherleaves,waxesalsoexistintheformofplateletsorothermorphologies.10,11Lotusleaveshavehierarchicalstructures,whichhavebeenstudiedbyBurtonandBhushan8andBhushanandJung.9Thewaterdropletsonthesesurfacesreadilysitontheapexofnanostructuresbecauseairbubblesfillinthevalleysofthestructureunderthedroplet.Therefore,theseleavesexhibitconsiderablesuperhydro-phobicity.Wateronsuchasurfaceformsasphericaldroplet,andboththecontactareaandtheadhesiontothesurfacearedramaticallyreduced.1,2,12Anumberofartificialroughness-inducedhydrophobicsur-faceshavebeenfabricatedwithhierarchicalstructuresusingelectrodeposition,colloidalparticles,photolithography,plasmatreatment,self-assembly,andimprinting.13-20Moldingisalowcostandreliablewayofsurface-structurereplicationandcanprovideaprecisionontheorderof10nm.21,22Bhushanetal.3,23,24andKochetal.25developedanewmethodtofabricatehierarch-icalstructuredsurfaces.Moldingwasusedtoreplicateleafandsiliconmicrostructures,andself-assemblyofwaxwasusedtocreatenanostructuresontopofthemicrostructurestorealizehierarchicalstructures.Thistwo-stepprocessprovidesflexibilityinthefabricationofavarietyofhierarchicalstructures.*Towhomcorrespondenceshouldbeaddressed.E-mail:[e-mail].(1)Bhushan,B.;Jung,Y.C.J.Phys.:Condens.Matter2008,20,225010.(2)Nosonovsky,M.;Bhushan,B.MultiscaleDissipativeMechanismsandHierarchicalSurfaces:Friction,Superhydrophobicity,andBiomimetics;Springer-Verlag:Heidelberg,Germany,2008.(3)Bhushan,B.;Jung,Y.C.;Koch,K.Philos.Trans.R.Soc.,A2009,367,1631.(4)Bhushan,B.IntroductiontoTribology;Wiley:NewYork,2002.(5)Bhushan,B.J.Vac.Sci.Technol.,B:Microelectron.Nanometer2003,21,2262.(6)Bhushan,B.NanotribologyandNanomechanics;AnIntroduction,2nded.;Springer-Verlag:Heidelberg,Germany,2008.(7)Barthlott,W.;Neinhuis,C.Planta1997,202,1.(8)Burton,Z.;Bhushan,B.Ultramicroscopy2006,106,709.(9)Bhushan,B.;Jung,Y.C.Nanotechnology2006,17,2758.(10)Koch,K.;Bhushan,B.;Barthlott,W.SoftMatter2008,4,1943.(11)Koch,K.;Bhushan,B.;Barthlott,W.Prog.Mater.Sci.2009,54,137.(12)Nosonovsky,M.;Bhushan,B.Mater.Sci.Eng.,R2007,58,162.(13)Shirtcliffe,N.J.;McHale,G.;Newton,M.I.;Chabrol,G.;Perry,C.C.Adv.Mater.2004,16,1929.(14)Ming,W.;Wu,D.;vanBenthem,R.;deWith,G.NanoLett.2005,5,2298.(15)Sun,M.;Luo,C.;Xu,L.;Ji,H.;Ouyang,Q.;Yu,D.;Chen,Y.Langmuir2005,21,8978.(16)Chong,M.A.S.;Zheng,Y.B.;Gao,H.;Tan,L.K.Appl.Phys.Lett.2006,89,233104.(17)delCamp