Surface freezing and a two-step pathway of the isotropic-sme

We study the kinetics of the isotropic-smectic phase transition in a colloidal rod/polymer mixture by visualizing individual smectic layers. First, we show that the bulk isotropic-smectic phase transition is preceded by a surface freezing transition in whi

Surfacefreezingandatwo-steppathwayoftheisotropic-smecticphase

transitionincolloidalrods

ZvonimirDogic

IFF/WeicheMaterie,ForschungszentrumJ¨ulich,J¨ulich,D-52425Germany

(Dated:February2,2008)

Westudythekineticsoftheisotropic-smecticphasetransitioninacolloidalrod/polymermixturebyvisualizingindividualsmecticlayers.First,weshowthatthebulkisotropic-smecticphasetran-sitionisprecededbyasurfacefreezingtransitioninwhichaquasitwo-dimensionalsmecticphasewetstheisotropic-nematicinterface.Next,weidentifyatwostepkineticpathwayfortheformationofabulksmecticphase.Inthe rststepametastableisotropic-nematicinterfaceisformed.Thisinterfaceiswettedbythesurfaceinducedsmecticphase.Inthesubsequentstep,smecticlayersnucleateatthissurfacephaseandgrowintotheisotropicbulkphase.

PACSnumbers:64.70.Md82.70.D

arXiv:cond-mat/0308556v1 [cond-mat.soft] 26 Aug 2003

Colloidswithhardcorerepulsiveinteractionsareoftenstudiedduetothesimplicityandgeneralityoftheirintermolecularpotential.Asaresultofthesestudies,theequilibriumphasediagramofhardrodsandspheresiswellunderstoodatthepresenttime[1,2,3,4].However,muchlessisknownaboutthekineticpathwaysofphasetransitionsinthesesystemsDirectvisualizationofcolloidsinasystemundergoingphasetransitionhaveprovidedapowerfultooltostudygeneralaspectsofphasetransitionkinetics8].Inthispaperwestudythekineticsoftheisotropic-smecticphasetransitionbydirectlyvisualizingindividualsmecticlayersinaphaseseparatingsample.Asamodelsystemofcol-loidalrodsweuseamonodispersesuspensionoffdvirusWeelucidateakineticpathwayofunex-pectedcomplexity.Theexistenceofsurfacefreezingandametastableisotropic-cholestericphasetransi-tionsisdiscoveredandtheirin uenceonthekineticpathwayisdiscussed.Becausethebehaviorofthefd/Dextranmixtureisdeterminedbystericinterac-tionsandsinceallmoleculesincludinglowmolecu-larweightthermotropicshaveastericcorethere-sultsreportedinthispaperarelikelytobequitegeneral.Inaddition,ourresultsmightbepertinenttounderstandingthedynamicsofamphiphilicmem-branes2Dsmecticsystemssurfacefreezingandwettingtransitionsandselfassem-blednano-structures[15].

Ithasbeenknownforalongtimethatsurfacefreezing/meltingcandramaticallyalterthenucle-ationrateandthekineticpathwayofaphasetran-sition.Ononehand,mostsubstancesexhibitsur-facemelting.Inthiscasealiquidsurfacewetsthecrystallinebulkphase.Itfollowsthatcrys-talsmeltfromthesurfaceinwardsandthereforeitisdi culttoprepareasuperheatedmetastablesolidOntheotherhand,surfacefreez-ingisobservedinveryfewsystems,mostnotablythermotropicliquidcrystals,alkanesandsurfactantmesophases[12,13,Uponsupercoolingthese

materials,theorderedphasenucleatesatthefrozeninterfaceandpropagatestowardsthebulkphase.Therefore,itisdi culttosupercoolliquidsthatex-hibitsurfacefreezing[19].

Anotherfactorthatcana ectthenucleationrateofatransitionsisthepresenceofmetastablephasesForexample,recentsimulationspredictthatthefreeenergybarrierforthefor-mationofproteincrystalsisgreatlyreducedwhenametastablegas-liquidphasetransitionislocatedinavicinityofastableliquid-solidphasebound-aryInthiscasethenucleationofproteincrys-talsproceedsintwosteps.Inthe rststepadensemetastabledropletassociatedwiththegas-liquidphasetransitionisformed,whileinthesubsequentsteptheproteincrystalnucleateswithinthisdroplet.Inthispaperweshowthatbothsurfacefreezingandmetastablephasesareimportantforunderstandingthekineticsoftheisotropic-smecticphasetransition.Bacteriophagefdisasemi- exibleviruswithcon-tourlengthof880nm,diameterof7nmandper-sistencelengthof2200nm.Itwaspreparedaspreviouslydescribedanddialyzedagainstbu erofknownionicstrength(190mMNacl,10mMTris,pH=8.10).Thephasediagramoftherod-polymermixturewasmeasuredaccordingtothepublishedprocedure[9].Allthesamplesarepreparedinametastable/unstableisotropicphasebyshearmelt-inganyexistingstructureandsamplesareplacedintorectangularcapillaries(VitroCom,MountainLakes,NJ).Nucleationandgrowthoftheorderphaseisobservedwithanopticalmicroscope(ZeissAxioPlan2)equippedwithDICoptics.AllimagesarerecordedwithacooledCCDcamera(AxioCamZeiss)

Atzeropolymerconcentrationfdisagoodmodelsystemofhardrodsandformsastableisotropic(I),cholesteric(Ch)andsmectic(S)phaseswithincreas-ingconcentrationinagreementwiththeoreticalpre-dictions21].EquilibriumI-Sphasetransitionisobservedinamixtureofrod-likefdvirusesandnon-