NanostructuredsurfacesInterfacialtension

We2ng

γ =N(Newton)m(meter)

=Force/unitlength=Energy/unitsurface

Crea;ngsurfacescostsenergy

Defini;on:Interfacialtension

Energycostforcrea;ngthissurface:E=Aγ=2Bγ

B

TotalsurfaceAcreated:A=2B

Spheresaretheshapewiththesmallestsurfaceforagivenvolume

hHp://www.123rf.com

Crea;ngsurfacescostsenergy

γwater/air=72mN/mγwater/oil=50mN/mγmercury/air=490mN/m

hHp://www.kidsdiscover.com

INTERFACES:liquid-liquidgas-liquidsolid-solidsolid-liquid:WETTING

Wikipedia

θc:contactangle

θc<90°:we2ngθc>90°:nonwe2ng

We2ng

Youngequa;on:

Terminology:Whentheliquidiswater:Wespeakabouthydrophilicsurface(likeswater)whenthedropofwaterspreadsonthesurface:andhydrophobicsurfacewhenthedropofwaterdoesnotspreadonthesurface

θc>90

θc<90

ExerciseLet’sconsideradropofwateronTeflon.ThesurfacetensionforTeflon-airis92mM/m,thesurfacetensionofwater-airis72mN/mandthesurfacetensionofTeflon-airis20mN/m.CalculatethecontactangleofthedropontheTeflonsurface.Drawthedrop.Answer:Youngequa;on:

RoughsurfacesMicro/nanostructuredsurfaces

ProgressinMaterialsScience56(2011)1–108

Rf=surfaceroughnessfactor=Surfaceareasolid-liquid/flatprojectedarea

RoughsurfacesMicro/nanostructuredsurfaces

Ifyouhaveahydrophilicmaterial(e.g.cleanglass),addingmicroandnanostructurestothesurfacewillmakethesurfaceevenmorehydrophilic.->thesesurfacearecalledsuperhydrophilicsurfaces(θc=0°)Ifyouhaveahydrophobicmaterial(e.g.polystyrene),addingmicroandnanostructurestothesurfacewillmakethesurfaceevenmorehydrophobic.->thesesurfacesarecalledsuperhydrophobicsurfaces.(θc≈180°)

Takealeafofladiesmantle.Pourasmalldropofwater(diameter≤1mm)onanundamagedandcleanareaoftheleaf.-Whatisthecontactangleofthewaterdropontheleaf?-Tilttheleafbackandforthandobservehoweasyitistogetridofthewaterfromtheleaf.-Rubyourfingeronapartoftheleafandrepeattheexperiment.-Repeattheabovewithwatercontainingsoap.TakeamicroscopeglassslidePourasmalldropofwater(diameter≤1mm)onacleanareaoftheslide.-Whatisthecontactangleofthewaterdropontheslide?-Tilttheslidebackandforthandobservehoweasyitistogetridofthewaterfromtheslide.Whatisthedifferencebetweenthelady’smantleleafandthemicroscopeslide?

Superhydrophilicsurfaces:Contactangle=0Applica;on:defogging

hHps://www.youtube.com/watch?v=bX9TP53gSy0

Superhydrophobicsurfaces:Contactangle>150°androll-offangle<10°Selfcleaningeffect

www.youtube.com

Lotusleaf

Inspiredresearchon:-newgenera;onoffabrics-coa;ngs,windowsetc…

Howtomakeinterfacesmorestable?

Howtomakeinterfacesmorestable?

Surfactants(=ac=ngonsurfaces)Moleculesthatlowertheinterfacialenergy

Headthatlikeswater

Tailthatlikesoil/air ≈1-5nm

=Polarhead,hydrophilichead

=Apolartail,hydrophobictail

Surfactants(=ac=ngonsurfaces)Moleculesthatlowertheinterfacialenergy

air

water

γ = 72 mN/m

air

water

γ ≈ 25 mN/m

Surfactants

Neutral

Anionic

Ca;onic

zwiHerionic

Surfactants

Sodiumstearate(soap)

Phospholipids:examplePhospha;dylcholine(componentofcellmembrane)Foundineggyolk,soy…

Surfactantapplica;ons

Emulsiondefini;on:mixtureof2nonmiscibleliquidsOneliquidisdispersedintotheother

Emulsionstabiliza;on

hHp://sparror.cubecinema.com/

DispersedPhaseCon;nuousphase

Surfactantapplica;ons

Exercise:Let’sconsideranoilinwateremulsion.Thevolumeofwateris1landthevolumeofoilis10%ofthewatervolume.Theoildroppletsare1µminradiusandtheinterfacialtensionbetweenoilandwateris50mN/m.Calculatetheoil-watertotalinterfacialenergyofthisemulsion.

Answer:

Surfactantapplica;ons

Exercise:Let’sconsidertheoilinwateremulsionfrombefore.(Thevolumeofwateris1landthevolumeofoilis10%ofthewatervolume.Theoildroppletsare1µminradiusandtheinterfacialtensionbetweenoilandwaterwithsurfactantpresentis≈10mN/m.Howmuchistheinterfacialenergynow?Answer:3JWilltheemulsionbemoreorlessstable?Answer:morestable(lowerenergy)

Surfactantapplica;ons

Examples:VinaigreHe(Unstable)Mayonnaise(stabilizedbyeggyolk)Cosme;cs,lo;ons(stabilizedbypolymersetc…)

Emulsionstabiliza;on

Howaresurfactantsposi;onnedinasoapbubble?

Surfactants

WaterfilmAir

Air

Surfactants

WaterlayerAir

Air

SurfactantsMicelles

hHp://w

ww.aH

ension.com

hHp://www.piercen

et.com

Air

Water

Cri;calmicellarconcentra;on:Surfactantconcentra;onabovewhichmicellesforminthebulk(surface=full).

Surfactantapplica;onsLaundrydetergent,soapetc….

Water

Fabricwithfatstain

fat

fat

micelle

SurfactantsLiposomes(ar;ficialvesicles):Plentyofpossibili;esfordrugdelivery

-  Canbeloadedwithmolecules/drugs

-  Protectthedrugfrominvivocondi;ons(stomach,blood,immunecells)-  Surfacecanbefunc;onalized(targetcells)

-  Somecandeliverdruginsidecellsbymembranefusion(difficultforhydrophilicdrugstocrossthecellmembrane)-somereleasetheircargoonspecificcondi;ons(pH,pressure,temperature….)

hHp://w

ww.britannica.com

10nm-10µm

SurfactantsLiposomeapplica;ons

www.nature.com/nnano/journal/v7/n8/pdf/nnano.2012.84.pdf

Localdeliveryofvasodilatorsfortrea;ngatherosclerosis(UniversityofGeneva,2012)Atherosclerosis:-nospecificbiomarkersiden;fied-highshearstressduetothenarrowingofthebloodvessel

Vesiclesbreakandreleasetheircontentathighshearstress

AB

CD

EF G

H

Whichofthefollowingobjectcontainssurfactant?

AB

CD

EF G

H

Whichofthefollowingobjectcontainssurfactant?

Anethole:solubleinethanol-notinwater

Milkproteins Eggyolk(phospholipids) NosurfactantsLotsofsurfactants!

Lotsofsurfactants!Surfactantstopreventlungcollapse

Nosurfactants

Ouzoeffect:Surfactant–freestableemulsion!

Anethole:solubleinethanol-notinwater

Youneed3liquids:Liquid1:waterLiquid2:ethanol–solubleinwaterLiquid3:anethole(organicflavoringsubstance≈oil)-Solubleinethanol,butnotinwater.

Beforepouringthewater,theanetholeisinethanol,bothliquidsaremiscible,thesolu;onistransparent.Aweraddingwater,theanetholformssmalldroplets(≈1µm)surroundedbyethanolandsuspendedinwater.Thedropletsarestable.

anethole

ethanol

water

LaplaceLaw:Pressuredifferenceacrosstheinterfaceof2fluids.Forasphericaldropofonefluidintoanother:P=2γ/rwhereγistheinterfacialtensionbetweenthe2fluidsandristhedropradius.

r

Lungsurfactants

LaplaceLaw:P=2γ/r:pressureneededtomaintainanairinwatersphericalbubbleThealveolicanbeapproximatedbyaair-in-waterbubbleswithaninterfacialenergyγ)Whentheairisexpelled:rbecomesverysmall.Iftherearenosurfactants,thepressureneededtopreventlungcollape(=tomaintainthesmallairbubbleisverybig).Iftherearesurfactantsattheinterface,theinterfacialtensionbecomesmaller(γ)andthepressureneededtopreventlungcollapseissmaller.

Thesurfactantpreventlungcollapseuponairexpulsion

r