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Publié par | johannes_gutenberg-universitat_mainz |
Publié le | 01 janvier 2009 |
Nombre de lectures | 19 |
Langue | English |
Poids de l'ouvrage | 52 Mo |
Extrait
ChargedColloidalSuspensions
inConfinedGeometries
DissertationThesis
toobtainthedegree
“DoktorderNaturwissenschaften”
attheFachbereich08,
Physik,MathematikundInformatik
oftheJohannesGutenbergUniversität,Mainz
Submittedby
AnaBarreiraFontecha
borninValencia,Spain
Mainz,March2009
AFerxu
Abstract
Thebehaviourofchargedcolloidalsuspensionsundergeometricalconfinementhasbeenstudied.
We used different wedgelike geometries which allow a continuous variation of the distance
betweentheplates.Atlowsaltconcentrationstheconfinementofsuspensionsinfluidphasecells
revealed that the particles accumulate in the cusp of the wedge forming crystalline ordered
structures. We present systematic experiments to understand the accumulation effect in the
narrow part of the cell. We conclude that the effect is due to electrostatic trapping, which is
confirmedbyasimpletheoreticalmodelproposedbyLöwenetal..Crystallinestructuresappear
formingacharacteristicsequencewithincreasingplateseparation.Thisstructuralsequencehas
beenalreadyobservedpreviouslyinconfinementatwedgegeometries,however,thesmallwedge
anglesachievedinourexperimentsallowedtheobservationofnewstructures.Someofthesenew
structuresshowexoticarrangementsandhavenoatomiccounterpart.Wealsoproposedifferent
transition mechanism models between the different structures based in the experimental
observations. For that we assume that the particles behave as hard spheres subject to a high
pressure.Finally,acellwithvariableheightwasdesignedinordertostudythecompletephase
behaviour of charged spheres confined between parallel plates. The preliminary results are
comparedwiththeoreticalpredictions.
Index
Pag.
Listofterms .................................................................................................................................. 1
1 Introduction.......................................................................................................................... 3
2 ChargedSphereSystems..................................................................................................... 7
2.1 Particleinteractionsinchargedspheresystems ............................................................ 7
2.1.1Interactionpotentialbetweenchargedparticles ................................................... 8
2.1.2Phasediagramofchargedcolloidsinthebulk................................................... 10
2.2Colloidalcrystals ........................................................................................................... 12
2.2.1Crystallinestructuresinconfinement................................................................. 15
3 Formerresultsingeometricalconfinement..................................................................... 17
3.1Firstevidences:Thestructuralsequence. ...................................................................... 18
3.2Modelofhardspheresinthehighpressurelimit .......................................................... 21
3.3Thehexagonalandsquarestructuresinthestructuralsequence ................................... 22
3.4Otherstructures.............................................................................................................. 24
3.4.1Fromthemonolayertothebilayer ..................................................................... 25
3.4.1.1TheBucklingPhase .............................................................................. 27
3.4.1.2.TheRhombicphase.............................................................................. 29
3.4.2Highernumberoflayers:Theprismphases....................................................... 31
3.5Thecompletephasediagramforabilayersystemofchargedspheres.......................... 33
3.6Finalremarks ................................................................................................................. 35
4 Experimentaltechniques................................................................................................... 37
4.1BightFieldMicroscopy................................................................................................. 38
4.2Fouriermicroscopy........................................................................................................ 47
4.2.1FundamentalConceptsinLightScattering ........................................................ 47
4.2.2.LightscatteringintheRayleighGansDebyeapproximation........................... 50
4.2.3StaticLightScattering........................................................................................ 50
4.2.4.Lightscatteringfromperiodicstructures:diffractionincrystals...................... 52
4.2.5.Workingwiththemicroscopeinconoscopicmode .......................................... 59
4.3Othertechniques ............................................................................................................ 62
4.3.1AtomicForceMicroscope(AFM)...................................................................... 63
4.3.2ScanningElectronMicroscope(SEM)............................................................... 64
5 Samplespreparationandconfinementcelldesigns........................................................ 67
5.1Preparationandcharacterizationofthestocksuspensions............................................ 68
5.2Pumpingsetup ............................................................................................................... 71
5.3Cellsusedintheexperiments ........................................................................................ 75
5.3.1Wedgegeometrycells ....................................................................................... 75
5.3.1.1Rectangularcells................................................................................... 76
5.3.2Sphericalgeometrycells.................................................................................... 79
5.3.3Cellwithvariableheight.................................................................................... 81
5.3.3.1Experimentalproceeding...................................................................... 84
6 Effectofconfiningchargedparticlesinwedge-likegeometries .................................... 93
6.1Generaleffect ................................................................................................................ 94
6.2Testingevaporationcurrents ......................................................................................... 96
6.3Testingthegravitystrength ......................................................................................... 101
6.4Testingtheinfluenceofthechargeatthecellplates................................................... 104
6.5Testingtheinfluenceofsaltgradients......................................................................... 106
6.6Discusion ..................................................................................................................... 107
7 Experimentalphasediagrams ........................................................................................ 111
7.1Comparisonbetweenthetheoreticalandexperimentalphasebehaviourin.............. 112
closedwedgecells. ...................................................................................................... 112
7.2Resultsobtainedwiththecellwithparallelplates. ..................................................... 115
7.3Comparisonanddiscussionoftheexperimentalphasediagrams ............................... 118
8 Structuralsequenceofcolloidalcrystalsinconfinement............................................. 121
8.1Introduction:Thestructuralsequence ......................................................................... 122
8.2Then→(n+1)transition .................................................................................... 127
8.2.1Thebucklingstructure ..................................................................................... 129
8.2.2Thehcp⊥structure........................................................................................... 130
8.2.3Thehexagonalprismstructure......................................................................... 139
8.2.4Squareprismstructure,P A.......................................................................... 148
8.3Then→ntransition ............................................................................................ 153
8.3.1Therhombicphase,nR. ................................................................................. 158
8.3.2Thesquareprismphase,nP B ..................................................................... 162
8.3.3The(101)hcpphase,nhcp—............................................................................ 165
8.3.4Otherarrangementsinthen→ntransition .............................................. 167
8.3.5Transitionmodelforhardspheresysteminthehighpressurelimit.....