Surface freezing and a two-step pathway of the isotropic-sme(3)

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.3:Imageofamacroscopicallyphaseseparatedisotropic-nematicinterfaceswhichexhibitsurfacefreez-ing.Theconcentrationsofthecoexistingisotropicandnematicphasesarecfd=242mg/mlandcdex=51.5mg/ml.Densenematicphaseisbelowtheimageplanewhiletheisotropicphaseisabovetheimage.Thethick-nessofthesurfaceinducedsmecticphaseisfewhundrednm.Thesurfacestructureshow

hereisidenticaltothesurfaceoftactoidsshowninFig.2d.Apairofdisloca-tiondefectsisclearlyvisibleintheimage.Scalebarsindicate5µm.

hasacorrugatedI-Ninterfacewherethelengthofeachridgealongthedroplet’slongaxisisapproxi-matelyoneviruslong.Asthetactoidscoalesceandincreaseinsize,thesurfacecorrugationsarealwayscon nedtoanarrowlayerofwellde nedthicknesslocatedattheI-Ninterface.Thisimpliesthattheformationofcorrugationsisapurelysurfacee ect.Theseobservationsleadustoconclusionthatthereexistsasurface-inducedquasi2DsmecticphasethatwetstheI-Ninterface.Theridgesobservedattheinterfaceareindividuallayersofthesurface-inducedsmecticphase.Aschematicrepresentationofasec-tionofacorrugatedtactoidisshowninFig.2d.Thesurfacesmecticphaseisobservedaboveanfdcon-centrationof235mg/mlwhilethebulkI-Sphasetransition(region4)isobservedat255mg/ml.

Afterafewhours,thefd/Dextranmixturepre-paredinregion2completelyphaseseparateswithdensernematictactoidscoalescingandsettlingtothebottomofthesample.InthiscaseamacroscopicI-Ninterfaceisformed.Thismakesitpossibletofo-cusontheinterfaceanddirectlyobservethesurfaceinducedsmecticphase(Fig.3).Weconcludeourdescriptionofthesysteminregion2bynotingthattherearenotheoreticalpredictionsofthesurface-inducedsmecticphaseinrod/polymermixture.Weexpectthatsuchphaseisaresultofnon-monotonicdensitypro lesacrosstheI-Ninterface[24].Addi-tionally,inthefd/polymersystemrodsinthesurfacefrozenlayerlieintheplaneoftheinterface.Thisisincontrasttomolecularsystemswhichexhibitsur-facefreezingwhereanisotropicmoleculesareeithertiltedorperpendiculartotheinterface[12,13].Wenowturnourattentiontoregion3ofthephasediagram.Rightaftermixingthesample,inadditiontotheformationofnematicdropletswithasurfacesmectic,weobserveself-assemblyofrodsintodisk-likeorribbon-likestructures(Fig.2eandf).Thethicknessofthediskcorrespondstothelengthofa

FIG.4:Imagesofstructuresobservedinregion3ofthephasediagramafterthesamplehasbeenequilibratedforafewdays.Forimagesaandbcfd=254mg/mlandcdex=53.5mg/mlwhileforimagescanddcdex=56mg/mlandcfdisundetermined.(a)Nematicdropletwithasurfacefrozensmecticphase.Surfacesmecticphaseactsasanucleationsitefortheformationofcol-loidalmembranes.(b)Twistedsmecticribbonnucleatesatthesurfacesmecticphaseandgrowsintotheisotropicbulkphase(c)DICimageofalarge(35µmdiameter)isolatedcolloidalmembraneinwhichrodslieperpendic-ulartotheimageplane.Correspondinglythemembraneshowsnobirefringenceundercrossedpolarizers.(d)Po-larizationimageofacolloidalmembraneinwhichrodslieintheplaneoftheimage.Directionsofpolarizerandanalyzerareindicatedbywhitearrows.Scalebarsindicate5µm.

singlerod.Whenviewedfromaboveadiskshowsnobirefringencewhilefromthesideitshowsmaxi-mumbirefringencewhenorientedat45owithrespecttothepolarizerandanalyzer(Fig.4d).Therefore,polarizationmicroscopyshowsthatdisksarecom-posedofamonolayerofalignedrodsinthesmectic-Acon guration.Wecalltheseself-assembleddiskscolloidalmembranesbecauseoftheirsimilaritytoamphiphilicmembranes.Smallhomogeneouslynu-cleatedmembranes(Fig.2e)growbycoalescinglat-erallytoformlarge40µdiameterisolatedmem-branes(Fig.4c)[9].Thissuggeststhatanisolatedcolloidalmembraneandnotabulksmecticphaseistheequilibriumstructureinregion3.Polarizationmicroscopyindicatesthattwistedribbonsareiden-ticaltodisksexceptthattheyhaveatwistalongtheirlongaxisduetothechiralnatureoffd[25].Weexpectthatthefreeenergydi erencebetween