Surface freezing and a two-step pathway of the isotropic-sme(2)

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

FIG.1:Thecoexistenceconcentrationsofanimmisci-blefd-Dextranmixture.TheY-axesshowsconcentra-tionandosmoticpressureofDextraninthephasethatcoexistswithrod-richliquidcrystallinephase(droplets)whoseconcentrationisshownontheX-axis.Numbers1through4indicateregionswheredi erentphasebehav-iorsareobserved.Imagesof

thestructuresobservedintheseregionsareshowninFig.2.Stablesurfacesmec-ticphasewetstheisotropic-nematicinterfaceinregion2.Colloidalmembranesarestableinregion3.Inset:Thecompletephasediagramoffd/Dextranmixture.Tielinesalongwhichthephaseseparationproceedsareindi-catedbydashedlines.Regionsoftheisotropic-nematic(I-N)andisotropic-smectic(I-S)coexistenceareindi-cated.

adsorbingpolymerDextran.ThephasediagramofthismixtureisshownintheinsetofFig.1.Addingnon-adsorbingpolymertofdsuspensionproducesef-fectiveattractiveinteractionsbetweenfdrods[22].ThemainconsequenceofthisattractivepotentialonthephasebehaviorofarodlikesystemistowidentheI-Chcoexistenceconcentrationswiththepolymerpreferentiallypartitioningintotheisotropicphase[23].Sincetheinteractionsinthefd/polymermixturesaretemperatureindependent,allphasetransitionsareentropicallydriven.Inthe rstpartofthepaperwedescribetheequilibriumstructuresrelatedtothesurfacefreezingobservedinregion2ofthephasediagram.Inthesecondpartofthepa-perwedescribeoneofthekineticpathwaysofphaseseparationobservedinregion3.

Atrodconcentrationsbelow235mg/ml(region1

FIG.2:Imagesandschematicrepresentationsofdi er-entstructuresobservedinthefd/Dextranmixture.(a)Anisotropicnematicdropletinthepolymerrichback-ground.Con gurationofrodsinshownin gurec.(b)Nematicdropletwithsurfacesmecticphase.Imageisformedbyfocusingonthemidplaneofthetactoids.The3Dstructureisanobjectofrevolutionaboutthelongaxis.Aschematicrepresentationofanematicdropletwithsurfaceinducedsmecticphaseisshowninimaged.(e)Colloidalmembraneswhichhomogeneouslynucleatefromisotropicphase.Thebottomleftdiskliesintheplaneoftheimagewhiletheothertwolineperpendicu-lartotheplaneoftheimage(f)Twistedribbonwhichisidenticaltocolloidalmembraneexceptthatthatitiselongatedalongthetwistdirections(g)Bulkisotropic-smecticphasecoexistence.Scalebarsindicate3µm.

inFig.1aandb),nematicdroplets(tactoids)forminanisotropicbackground(Fig.2a).Polarizationmicroscopyindicatesthatthecon gurationofrodsinthenematictactoidisasshowninFig.2c.Whencon nedtoasmallvolumethecholestericorderisnotabletodevelop;thereforeweobserveonlyun-woundnematicphasewithinaindividualtactoid.Athigherrodconcentrations(region2inFig.1)weobservedropletsthathavethesameanisotropicshape.Microscopyindicatesthattheinteriorofthesedropletsisstillnematic.However,eachdroplet