DROP-IMPACT-DYNAMICS-Splashing--Spreading--Recedin

AR266-FL38-07ARI11November200516:55DropImpactDynamics:Splashing,Spreading,Receding,Bouncing...A.L.YarinFacultyofMechanicalEngineering,Technion—IsraelInstituteofTechnology,Haifa32000,Israel;email:[e-mail].2006.38:159–92TheAnnualReviewofFluidMechanicsisonlineatfluid.annualreviews.orgdoi:10.1146/annurev.fluid.38.050304.092144Copyrightc
2006byAnnualReviews.Allrightsreserved0066-4189/06/0115-0159$20.00KeyWordscrownformation,recoil,rebound,jetting,fingering,wettability,roughness,freerim,cuspformation,secondarydropletsAbstractThereviewdealswithdropimpactsonthinliquidlayersanddrysurfaces.Theim-pactsresultingincrownformationarereferredtoassplashing.Crownsandtheirpropagationarediscussedindetail,aswellassomeadditionalkindred,albeitnon-splashing,phenomenalikedropspreadinganddeposition,receding(recoil),jetting,fingering,andrebound.Thereviewbeginswithanexplanationofvariouspracticalmotivationsfeedingtheinterestinthefascinatingphenomenaofdropimpact,andtheabove-mentionedtopicsarethenconsideredintheirexperimental,theoretical,andcomputationalaspects.159Annu.Rev.FluidMech.2006.38:159-192.Downloadedfrom:551.INTRODUCTION“Inthelandofsplashes,whatthescientistknowsasInertiaandSurfaceTensionarethesculptorsinliquids,andfashionfromthemdelicateshapesnonethelessbeautifulbecausetheyaretooephemeralforanyeyebutthatofthehigh-speedcamera.”–Edgerton&Killian(1954),slightlymodifiedDropimpactsonsolidandliquidsurfacesareakeyelementofawidevarietyofphe-nomenaencounteredintechnicalapplications,suchasink-jetprinting,rapidspraycoolingofhotsurfaces(turbineblades,rollsinrollingmillsforsteelproduction,lasers,semiconductorchips,andelectronicdevices),annealing,quenchingofalu-minumalloysandsteel,firesuppressionbysprinklers,internalcombustionengines(intakeductsofgasolineenginesorpistonbowlsindirect-injectiondieselengines),incinerators,spraypaintingandcoating,plasmaspraying,andcropspraying.Mi-crofabricationofstructuredmaterials,solderbumpsonprintedcircuitboards,andelectriccircuitsinmicroelectronicsproducedbyprecisionsolder-dropdispensing,aswellasliquidatomizationandcleaning,andiceaccumulationonpowerlinesandaircraftalsoinvolvedropimpacts.Thelatterarealsoimportantincriminalforensics,indevelopmentofnonwettableorfullywettablesurfaces,inhigh-accuracyactiva-tionorpassivationofsubstratesbymicrodrops,intransportofsurfacecontaminantsintobulkliquids,andingasentrapment.Understandingtheaccompanyingphysicalphenomenaisofutmostimportanceinformulatingreliableboundaryconditionsinnumericalcodesforspraysimulation.Suchlarge-scaleandwidespreadnaturalphe-nomenaasaerationofthesurfacelayersoflakes,seas,andoceansdependonairbubbleentrainmentduetoraindropimpacts.Theseimpactsatoceansurfacesleadtoformationofupwardjetsandsecondarydroplets,whichevaporateandformsaltcrystals.Thelatterserveasnucleationsitesinclouds,withtheattendantrelevancetometeorology.Erosionofsoil,dispersalofsporesandmicro-organisms,andunderwa-ternoiseduringrainsarethreeadditionalnaturalphenomenainvolvingdropimpact.Nail-likejetsandbubblesareafamiliarspectacleduringrainfallingonpuddlesandponds.Worthington(1908)wasoneofthefirsttoinvestigatetheseimpactssystematicallyandhisbookcontainsmanyfascinatingphotographsofthephenomenaaccompanyingdrop-andsolid-ballimpactsondeepliquidpools.Inspiteofitscommonness,andofmorethan100yearsofresearch,thephenomenonisstillfarfrombeingfullyunderstoodandcontinuestoattractphysicists,engineers,andmathematicians.Itevenattractsthegeneralpublicandmotivatespotentialcustomers,giventhenumberofcommercialsbasedondropimpactscenesairedontelevisionandshownonpostcards(Figure1).Theaccompanyingphenomenaareextremelydiverse,involved,andsurprising.Adropmaybesphericalorelliptic(duetooscillations)atthemomentofimpact.Itmayimpactonthefreesurfaceofaliquidinadeeppool,onathinliquidfilmonawall,oronadrysolidsurface.Theimpactmaybenormal(perpendicular)oroblique,inairorinvacuum.TheliquidmaybeNewtonianornon-Newtonian(e.g.,aviscoelasticpolymerorasurfactantsolution).Theliquidsofthedropand160YarinAnnu.Rev.FluidMech.2006.38:159-192.Downloadedfrom:55Figure1AnAmericanpostcardwithafamousEdgerton’sphotograph.CourtesyofHarold&EsterEdgertonFoundation(2005)andPalmPress,Inc.pool/filmmaybemiscibleorimmiscible.Thesolidsurfacemaybehardorsoft,roughorsmooth,chemicallyhomogeneousorheterogeneous.Itmayalsobeporous,flatorcurved,atatemperaturedifferentfromthatofthedroporthesame.Onliquidsurfaces,pre-existingorgeneratedwavesmayaffecttheflowpattern.Theimpactmayresultinthedropspreadingoverthesolidsurface,receding,rebound-ing,orevenlevitatingiftheevaporationnearahotwallissufficientlystrongfortheLeidenfrosteffect.Acratermayformintheliquidbulkinapoolandlateroncollapse,leadingtoformationoftheso-calledWorthingtonjetflowingoutfromitscenterandbeingsubjectedtocapillarybreakup.Theimpactonaliquidfilmmayresultincrownformation,prop