Spontaneous water adsorption-desorption oscillations in mesoporous thin films 2019 Raúl Urteaga.pdf

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AcceptedManuscript
Spontaneouswateradsorption-desorptionoscillationsinmesoporousthinfilms
RaúlUrteaga,MagalíMercuri,RocíoGimenez,,.
Berli
PII:S0021-9797(18)31369-9
DOI:/
Reference:YJCIS24320
Toappearin:JournalofColloidandInterfaceScience
ReceivedDate:7October2018
RevisedDate:12November2018
AcceptedDate:13November2018
Pleasecitethisarticleas:,,,,,Spontaneouswater
adsorption-desorptionoscillationsinmesoporousthinfilms,JournalofColloidandInterfaceScience(2018),doi:
/

,typesetting,and

errorsmaybediscoveredwhichcouldaffectthecontent,:.
Spontaneouswateradsorption-desorptionoscillationsin
mesoporousthinfilms
RaúlUrteaga,1MagalíMercuri,2RocíoGimenez,,2,*.
Berli3,*
1
IFIS-Litoral(UniversidadNacionaldelLitoral-CONICET),Güemes3450,3000Santa
Fe,Argentina.
2
InstitutodeNanocienciayNanotecnología(CNEA-CONICET),,
SanMartín,BuenosAires,Argentina.
3INTEC(UniversidadNacionaldelLitoral-CONICET),PredioCCTCONICETSanta
Fe,RN168,3000,SantaFe,Argentina.
Abstract
Understandingfluidtransportandphasechangesinnanoporestructuresisofgreat
interesttomanyapplicationfields,
workdiscussesthespontaneousoscillationsofthewatersaturationofmesoporousthin
films,inthezoneadjacenttoasessilewaterdrop,-
frontdynamicsontothefilmisdescribedbyconsideringthreecoexistingphenomena:
infiltrationfromthewaterdrop,condensationfromairvapor,andevaporationtothe
-evaporation
imbalances,whicharegovernedbythehystereticcharacteroftheadsorption-desorption

complexinterplaybetweenwaterandnanoporestructures,whichhaspractical
implicationsforthehandlingofhumidmicroenvironmentsinlab-on-a-chiptechnology,
aswellasformanyprocessesthattakepartofthecycleofwaterinnature.
Keywords:nanopores,saturation,condensation,evaporation,complexdynamics
*E-mail:(MGB)******@;(CLAB)******@santafe-
:.
Introduction
Inspiteoftherecentemerginginterestontheinteractionbetweenwaterand
nanostructures,moststudiessofarhavebeenconductedinsetupswithartificially
controlledambientconditions,,thereisa
lackofmechanisticunderstandingofthecomplexdynamicsofwaterinnanopore
,thereisagenuineneed,frombothfundamentaland
appliedviewpoints,toelucidatetheoutdoornanostructure–waterbehaviorinsimple
,aremarkableadvantagemesoporousfilmsinopenpitsetupsis
thatawaterdropincontactwiththefilmgeneratesamicroenvironmentwherethebulk,
infiltrated,andvaporwatercoexists[1,2].Thisscenarioallowsonetostudythe
dynamicequilibriumofcapillaryfilling,evaporationandcondensation,afundamental
problemthathaspracticalimplicationsforthehandlingofhumidsettingsinmicroand
nanotechnology[3],aswellasformanyprocessesthattakepartofthecycleofwaterin
nature[4].
Moreprecisely,whenawaterdropisdepositedontoamesoporousfilm,thetypical
behavioristhatwaterradiallyinfiltratesintotheporoussubstrate,andthewetannulus
reachesasteadystatewidthascapillaryinfiltrationandevaporationcounterbalance[1].
Thewettingfrontisclearlyobservedduetothefactthatlightreflectancelocally
changesuponwatersaturation,providinganetcontrastbetweenwetanddryzonesof
thefilm,,inthecaseofmesoporousfilmswitha
specificwormlikenanostructure(),anotablephenomenoncanbevividly
observed:theouterannulusborderdisplaysaspontaneousoscillatorymovementinstead
ofmaintainingasteadyposition().Wehadreportedthisphenomenonasanopen
probleminapreviouswork[1].Further,theseoscillationswerelaterexploitedto
generatesmallelectriccurrentsonelectrodesintegratedtothesamemesoporous
material[5].Here,aftercarefuldataanalysis,wepresentacomprehensivetheoretical
-
frontdisplacementobeystospontaneouscondensation-evaporationimbalances,which
inturnfollowthehystereticcharacteroftheadsorption-desorptionisothermsofthe
,itmakesan
attractiveplatformfortheexaminationofcomplexquestionsconcerningthemultiple
:.
Inaddition,arelevantaspectofourstudyisthatmesoporousfilmsactasreal-time
ultra-sensitivesensorsofthespontaneousvariationsofvaporconcentrationarounda
dropletinopenair,whichisalsorelatedtotheactualdynamicsofwaterevaporation.
Thisbasicproblemhasemergedintheliteraturehundredyearsago[6],howeverthe
topicisexperiencingarenascencenowadays[7-10]andoutstandingapplicationsare
emerging[3].Asanadditionaloutcome,thisworkimplicitlypresentsapractical
methodtoexploretheinstantaneousdynamicsofthehumidityenvironmentaround
watersourcesinopenair,asubjectofmuchimportanceincountlessnaturaland
technicalfields[11].
(a)Experimentaldataofthewetting-frontpositionasafunctionoftimefortwodifferentsilica
films:wormlike(WM)andordered(OM)mesoporousstructures;theinsetisaschematic(topview)
representationofthesessiledropoverthefilm,showingthewetannulusandthepositionofthe
wetting-front.(b)SEMimageoftheOMsilicafilm.(c)SEMimageoftheWLsilicafilm;theinset
.
:.
Materialandmethods

elsewhere[1,5]andadditionaldataisincludedhereintheSupplementaryMaterial.
Briefly,silicamesoporousfilmswithtwotypesofporemorphologieswereused:
orderedmesostructures(OM)andwormlikemesostructures(WM).Inbothcases,thin
filmsweredip-coatedonsiliconsubstrates,withfilmthicknessaround100nm.
Micrographs()wereobtainedusingaZEISSLEO982GEMINIfield
emissionelectronmicroscopeinthesecondary-electronmodeandtransmissionelectron
-
adsorptionanddesorptionisotherms()weremeasuredbyenvironmental
ellipsometricporosimetry(EEP)byusingaSOPRAGES5Aellipsometer[12].Results
showthatporeandnecksizesare7-8nmand3-5nm,respectively,howeverthepore
morphologysubstantiallychangesfromOMtoWMfilms(detailsaregiveninthe
SupplementaryMaterial).
-frontdynamicsresultingfromthecoexisting
capillaryfilling,condensation,andevaporationphenomenainmesoporousfilmswas
investigatedthroughasimpleexperiment:asessiledropofwaterwasdepositedoverthe
filmandtheadvancementofthefluidfrontintotheporousstructurewasoptically
-evolutionofwatersaturationinthe
mesoporousfilmswasfollowedbyusingaLeicaDM2700Mopticalmicroscope.
Imageswererecordedbyusingahigh-
operatedinthelabatmosphere,undercontrolledconditionsoftemperatureandrelative
humidity:25°Cand46%,respectively.

amountofwaterinthefilms,whichisbasedonthefollowinglinearapproximationfor
theeffectiverefractiveindex:,whereniandfiare,respectively,the
refractiveindexandvolumefractionofeachcomponent,namelymesoporoussilica,
water,andair[13].Thisapproximationimpliesthattherelationshipbetweenwater
uptakeandlightreflectanceisratherlinear,asitwasrecentlysuggestedformesoporous
siliconlayers[14].TherecordedimageswereprocessedinMATLAB,whereascript
wasimplementedtoextractthereflectedlightintensityprofilesatdifferenttime
intervals,:.
differentpositionsalongthex-direction,inanelongateddomainthatincludesthewet
annulusandthefilmzonefar-offthedroplet().Theinformation
consideredwasthatfromthegreenchannel(RGBsystem)forWMfilmsandred
-dependentlightintensityprofilesweredeterminedas
follows:eachdatapointinthex-directionwasdefinedastheaverageintensity
takenover70pixelsintransversey-direction,
reflectedfromthefilmbeforethedepositionofthedropletwasselectedtobethe
referencelevel().Thentherelativeintensitywasdefinedas,
takingintoaccountthatthereflectedintensitydecreaseswiththelocalamountofwater
inthefilm().
Resultsanddiscussions

exhibitedbythewetting-frontwhenasessiledropisdepositedonthewormlike
-front,,asa
,thewetannuluspresentsasteadywidth,
whichisgovernedbyinfiltrationandevaporation,aspreviouslydescribedbytheoryand
experiments[1].InthecaseoftheWMfilms,theouterpartoftheannulusis
alternativelysubjectedtowettinganddryingprocesses,dependingontherelative
dominanceofvaporcondensationorevaporation,-frontis
observedduetothelocalchangeoflightreflectanceassociatedtothedecreaseofwater
,thesaturation
ofthemesoporousfilmisdefinedasthefractionoftheporespaceoccupiedbythe
liquid,thusfor,wherethefilmisfullywet,andfor
,wherethefilmhasthesaturationlevelinequilibriumwithambient
(namely,fluctuationsofthereflectedlight
intensity)areobservedasforwardandbackwardmovementsofthewetting-front[1].
Waterevaporationfromdroplet-airinterfacecreatesamicroclimateofincreasedvapor

thedropissaturatedinvapor,thuspdecreasesfromthesaturatedvaporpressurepsaton
thedropsurfacetotheambientvaporpressurepa,formingapressuregradientwiththe
,hereweadopttheclassicaldescription::.
dropletevaporationtakesplaceinaquasy-steadystate,inisothermalconditions,and
vapordistributionintotheairisgovernedbymoleculardiffusion[11,15].Thisvapor
fieldconstitutesarelativehumidity()gradientthatspansalongthefilm
surfaceandfavorsvaporcondensationinthemesoporousmatrix.
.(a)Schematicrepresentationofvaporpressurecontoursaroundasessiledrop(sideview)
depositedonamesoporousfilmoverasiliconsubstrate.(b)Zoomoftheregionofthesaturation
oscillationsshowingthepositionofthewetting-front;condensationtakesplaceinthezonewhere
thevaporpressurepishigherthanthecondensationpressurepcatthenanopores.(c)Zoomoftheporous
matrixillustratingtheunsaturatedzone()thatformsabroadenwetting-frontinthepore
space.(d)Alterationofthevaporfieldaroundthesessiledropduetonaturaladvectionoffreshair(from
lefttoright);thedotted-linerectangledenotestheregionoftheoscillatingwetting-front,whichis
temporallysubjectedtotheambientvaporpressurepa(lowerthanpc),andthusevaporationprevailsover
(a)and(d)isattheoriginoftheoscillationsof.
Itisworthnotingthat,whilecondensationonaflatsurfacerequiresvaporat,on
porousmaterialsthephenomenonoccursatsubstantiallylowervaporpressures[16,17].
AccordingtotheKelvin-Laplaceequation,thecondensationpressureis
,whereismolarvolumeoftheliquid,isthegasconstant,
andistheabsolutetemperature[14,17,18].Alsointhisexpression,isthe
capillarypressureattheliquidmeniscusformedinthenanopore