DTIC ADA583397: Using Amphiphilic Nanostructures to Enable Long-Range Ensemble Coalescence and Surface Rejuvenation in Dropwise Condensation

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A
Using Amphiphilic Nanostructures To
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Enable Long-Range Ensemble
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Coalescence and Surface Rejuvenation
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in Dropwise Condensation
DavidM.Anderson,†ManeeshK.Gupta,‡AndreyA.Voevodin,§ChadN.Hunter,§ShawnA.Putnam,§),
VladimirV.Tsukruk,‡andAndreiG.Fedorov†,^,*
†G.W.WoodruffSchoolofMechanicalEngineeringand‡SchoolofMaterialsScienceandEngineering,GeorgiaInstituteofTechnology,Atlanta,Georgia30332,UnitedStates,
§AirForceResearchLaboratory,ThermalSciencesandMaterialsBranch,Wright-PattersonAFB,Ohio45433,UnitedStates,)UniversalTechnologyCorporation,Dayton,OH
45432,UnitedStates,and^ParkerH.PetitInstituteofBioengineeringandBioscience,GeorgiaInstituteofTechnology,Atlanta,Georgia30332,UnitedStates
T
houghwettabilityhaslongbeenun-
ABSTRACT
derstoodtodependonsurfacechem-
istry and morphology,1 intriguing
opportunitiestocontrolnanostructureand
createsurfaceenergygradientswithnovel
nanofabrication techniques have recently
generated a surge of research interest in
this field.2(cid:1)4 Such surfaces elucidate pre-
viouslyunknownmodesofdropletdynamics
duringcondensationandchallengethecur- Controllingcoalescenceeventsinaheterogeneousensembleofcondensingdropletsonasurfaceis
rent understanding of physical mechanisms anoutstandingfundamentalchallengeinsurfaceandinterfacialsciences,withabroadpractical
atmultiplelengthscales5thatunderliediffer- importanceinapplicationsrangingfromthermal managementofhigh-performanceelectronic
entstagesoftheprocess,especiallyrelevant
devices to moisture management in high-humidity environments. Nature-inspired superhydro-
tocollectivedropletdynamicsduringgrowth
phobicsurfaceshavebeenactivelyexploredtoenhanceheatandmasstransferratesbyachieving
andcoalescence.6Thesenewinsightscanbe
favorabledynamicsduringdropwisecondensation;however,theeffectivenessofsuchchemically
harnessedtodesign“smart”nanostructured
interfaceswhichcanintrinsically(i.e.,without homogeneoussurfaceshasbeenlimitedbecausecondensingdropletstendtoformaspinned
externalstimuli)controlthedynamicsofdrop- WenzeldropsratherthanmobileCassieones.Here,weintroduceanamphiphilicnanostructured
let nucleation, growth, and coalescence surface, consisting of a hydrophilic base with hydrophobic tips, which promotes the periodic
events to maximize heat and mass transfer regenerationofnucleationsitesforsmalldroplets,thusrenderingthesurfaceself-rejuvenating.This
rates. Such advances are critical for emerg- uniqueamphiphilicnanointerfacegeneratesanarrangementofcondensedWenzeldropletsthat
ingmicro/nanotechnologies,includinghigh- arefluidicallylinkedbyawettedsublayer,promotingpreviouslyunobservedcoalescenceevents
performance microprocessors, high-energy
where numerous droplets simultaneously merge, without direct contact. Such ensemble
densitybatteries,andoptoelectronicdevices,
coalescencesrapidlycreatefreshnucleationsites,therebyshiftingtheoverallpopulationtoward
which are becoming thermally limited and
requiremajoradvancesinourabilitytocon- smallerdropletsandenhancingtheratesofmassandheattransferduringcondensation.
trolphase-changephenomenaonthenano-
KEYWORDS: amphiphilicnanostructures.environmentalscanningelectron
scale.
microscopy.nanoscalewatercondensation.dropletcoalescencedynamics
The manner in which condensed liquid
interactswiththeunderlyingsubstratehas
adramaticeffectontheachievablerateof
heatandmasstransferonasurface.Drop- andgrowthprocess,whichismostefficient *Addresscorrespondenceto
wise condensation,inwhich thesurface is formoisture/heatmanagement,10,11incon- [email protected].
not uniformly wetted, provides heat and trast to filmwise condensation where heat
ReceivedforreviewJanuary13,2012
masstransfer coefficientsover anorder of andmasstransferisrestrictedbyagrowing andacceptedMarch28,2012.
magnitude higher than its filmwise coun- condensedliquidlayerwithlimitedthermal
PublishedonlineMarch28,2012
terpart.7(cid:1)9 Droplets are continually swept conductivity. Droplets typically leave the
10.1021/nn300183d
off a surface by gravity or vapor shear to surface by a rolling mechanism; therefore,
allow repetition of the droplet nucleation contact line pinning and contact angle C2012AmericanChemicalSociety
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4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Using Amphiphilic Nanostructures To Enable Long-Range Ensemble
5b. GRANT NUMBER
Coalescence and Surface Rejuvenation in Dropwise Condensation
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
5e. TASK NUMBER
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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
Georgia Institute of Technology,School of Materials Science and REPORT NUMBER
Engineering,Atlanta,GA,30332
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13. SUPPLEMENTARY NOTES
14. ABSTRACT
Controlling coalescence events in a heterogeneous ensemble of condensing droplets on a surface is an
outstanding fundamental challenge in surface and interfacial sciences, with a broad practical importance
in applications ranging from thermal management of high-performance electronic devices to moisture
management in high-humidity environments. Nature-inspired superhydrophobic surfaces have been
actively explored to enhance heat and mass transfer rates by achieving favorable dynamics during
dropwise condensation; however, the effectiveness of such chemically homogeneous surfaces has been
limited because condensing droplets tend to form as pinned Wenzel drops rather than mobile Cassie ones.
Here, we introduce an amphiphilic nanostructured surface, consisting of a hydrophilic base with
hydrophobic tips, which promotes the periodic regeneration of nucleation sites for small droplets, thus
rendering the surface self-rejuvenating. This unique amphiphilic nanointerface generates an arrangement
of condensed Wenzel droplets that are fluidically linked by a wetted sublayer, promoting previously
unobserved coalescence events where numerous droplets simultaneously merge, without direct contact.
Such ensemble coalescences rapidly create fresh nucleation sites, thereby shifting the overall population
toward smaller droplets and enhancing the rates of mass and heat transfer during condensation.
15. SUBJECT TERMS
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF
ABSTRACT OF PAGES RESPONSIBLE PERSON
a. REPORT b. ABSTRACT c. THIS PAGE Same as 7
unclassified unclassified unclassified Report (SAR)
Standard Form 298 (Rev. 8-98)
Prescribed by ANSI Std Z39-18
A
hysteresis must be minimized to promote easy re- R
moval.So-called“Cassie”droplets,restingonahetero-
T
geneous interface consisting of air and the tips of
roughnessfeatures,aresubstantiallymoremobilethan I
C
“Wenzel” droplets, which penetrate the roughness
featurestoformahomogeneoussolid(cid:1)liquidinterface L
andthussufferfromcontactlinepinning.12,13Surfaces
E
which wet with mobile droplets exhibiting minimal
contact angle hysteresis are termed superhydropho-
bic,andtheextensivebodyofdropwisecondensation
researchhas focusedonthis type ofsurface andthe
associated rolling mechanism.14(cid:1)20 A widely recog-
nized challenge with superhydrophobic surfaces, as
observedinnatureonthelotusleaf21andonsynthetic
surfaces,22,23isthattheyareoftenrenderedineffective
during condensation because the Wenzel state is
typically at a lower interfacial free energy than the
Cassiestate.24(cid:1)26Asaresult,metastableCassiedrop-
lets may initially form on the surface, but eventually
dropletscondensedfromthesurroundingvaporphase
will revert to the pinned Wenzel state, as opposed to
depositeddropletswhichassumetheCassiestateandroll
easily off the surface. The only notable exceptions are Figure1. (a)Sideand(b)overheadSEMmicrographsshow-
surfaceswhichrequirecomplex,two-tierroughness.15(cid:1)17 ingnanostructuredsurfacemorphology.
Here,weintroduceanamphiphilicsurfaceconsist-
ingofdenselypackednanowiresmadeofhydrophilic goldatthebase.Thisarrangementofverticalsurface
base material with hydrophobic tips which demon- energygradientwithlowenergyattheroughnesstips
strate the ability to produce long-range, noncontact withhighenergyatthebaseonacondensingsurfaceis
coalescence events within a heterogeneously sized adeparturefrommosteffortstodesignrobustdrop-
droplet ensemble to periodically regenerate nucle- wisecondensingsurfaces,whichutilizeuniformhydro-
ation sites, without requiring condensed droplets to phobiccoatingmethods.Studiesthathavevariedthe
remain stable in the mobile Cassie state! To demon- surfaceenergyhavedonesowitharadialgradient28or
strate this intriguing phenomenon, we performed hydrophilictipsonhydrophobicbasefeatures.18
complementaryopticalvisualizationwithvisiblelight The array is closely packed with less than 25 nm
and environmental scanning electron microscopy separating adjacent wires and an estimated packing
(ESEM) imaging of droplet condensation on a sub- fraction of 85%. The wires are also not uniform in
cooledamphiphilicnanostructureandreportamode height, creating a secondary roughness scale which
of coalescence involving numerous droplets simulta- allowsdropletsthatdorestonitsroughnessfeaturesto
neously merging with no direct contact above the only contact the tallest asperities and display a high
surface.Weproposeanewmechanismofcoalescence, contact angle by minimizing the amount of solid(cid:1)
involving Laplace pressure imbalance between adja- liquid contact area. This secondary roughness scale is
centfluidicallylinkeddroplets,fortheseeventswhich criticaltothesurface'scondensationperformance,aswill
rapidlyopenupfreshnucleationsitesthusrejuvenat- be discussed later. The depth of penetration of the
ingthesurfaceandgeneratingafavorabledistribution hydrophobic fluoropolymer coating is roughly 10% of
ofsmallerdropletsattractivetoheat/masstransfer. thetotalwirelength,asdeterminedbyESEMvisualization
ofthewettingofthehydrophilicbaseofthewireswhen
RESULTSANDDISCUSSION
the surface is cooled below the vapor saturation tem-
Amphiphilic Nanostructured Surface. The nanowire ar- peraturebyaPeltiercooler(webenhancedobjectS1).
ray, shown in Figure 1, consists of 200 nm diameter Droplet Size Distribution during Condensation. The tem-
hydrophilic gold wires that are coated from the top poralevolutionofthecondenseddropletsizedistribution
with a hydrophobic fluoropolymer using plasma- on a subcooled amphiphilic nanostructured surface is
enhanced chemical vapor deposition (PECVD).27 Be- comparedtothatofasmoothsiliconsurfacewithhydro-
cause PECVD is a directional process, the fluoropoly- phobiccoating.Opticalvisualizationisused,ratherthan
mer only coats the tips of the nanowires, creating a ESEM,forthiscomparisontoobtainalargerfieldofview.
surface energy gradient of low-energy hydrophobic Ambienttemperatureisheldwithin(0.5(cid:2)Candrela-
coating on the nanowire tips and native hydrophilic tive humidity within (2.5%, and to ensure consistent
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Figure2. Temporalevolutionofdropletsizedistributionduringcondensationon(a)amphiphilicnanostructuredand(b)
smoothhydrophobicsurface.
subcoolingbetweentests,DCpowertoaPeltierstage remains relatively high. Lastly, long-range multidroplet
beneaththesampleiscontrolledmanuallytocoolthe collectivecoalescenceeventstakeplaceduringthethird
surface 4 (cid:2)C below saturation temperature for the stage,causingthedropletdistributionsat10and13min
correspondingambientconditions. toflattenoutwithsignificantreductioninthesmalldrop-
Representative images at various times past the letcount,thuscreatingfreshnucleationsites.Thesurface
onsetofcondensationareshowninFigure2.During isableto“self-rejuvenate”inthismanner,andbecause
the initial transient growth, condensation appears priorstudieshaveshownthatdropletswithdiametersless
similar on both the smooth hydrophobic and the than10μmcontributethemosttoheattransferduring
amphiphilic nanostructured surfaces. After the third dropwisecondensation,10thisbehaviorisexpectedtoen-
minuteofcondensation,differentsizedropletsbegin hanceheattransferbycontinuallynucleatingnewmicro-
toformontheamphiphilicsurfaceandcoalescences meter-sizeddroplets.
simultaneouslyinvolvingmanydropletswhicharenot Proposed Mechanism of Droplet Growth and Coalescence.
indirectcontactabovethesurfaceareobserved.These ESEMimagingisemployedtocomplementtheoptical
uniquecoalescenceeventsaremostvividlyobserved visualization results and investigate the long-range
inwebenhancedobjectS2.Asnewdropletsnucleate ensemblecoalescenceeventsreportedabove,taking
in the space cleared behind coalesced droplets, the advantageoftheenhanceddepthofviewandspatial
processrepeatsitselfinaperiodicmanner.Incontrast, resolutionofferedbythistechnique.30,31Thesequence
thedropletsonthesmoothhydrophobicsurfacemostly ofimagesinFigure4showsthat,uponinitialcooling
grow through further condensation, with coalescences below saturation temperature, a layer of condensate
onlyoccurringinatraditionalmannerbyadjacentdrop- forms at the base of the wires (Figure 4a,b) due to
lets physically growing into one another.29 As a result, preferentialnucleationandcapillarycondensationon
during15minofcondensation,themeandropletsizeon thehigh-energyhydrophilicsurface.Wealsoconducted
the amphiphilic surface only reaches approximately anESEMinvestigationintocondensationonanuncoated
20 μm, while for the smooth hydrophobic surface, it goldnanowirearraywithuniformhighsurfaceenergy.In
continuestogrowupabove100μm. thiscase,theliquidsublayercontinuestogrowuntilthe
The histograms in Figure 3 provide statistical in- nanowiresarecompletelycoatedwithnodropletsform-
formationontheperiodicnatureofcondensationon ingonthesurface.Avideooftheprocessisavailablein
theamphiphilicsurfacebylookingatevolutionofthe theSupportingInformation.
dropletdistributionfortwoconsecutivecyclesinthe Oncethehydrophilicportionisfullywetted,drop-
periodicrejuvenationregime(seealsowebenhanced lets nucleate on the hydrophobic wire tips. These
objectS3).Thefirststageofthisperiodicbehavior,a droplets are initially in the Cassie state, as indicated by
large spike in small droplets, occurs immediately fol- thefactthattheyarehighlyspherical,exhibitveryhigh
lowingamajorcoalescenceeventasnewdropletsnucle- (>150(cid:2)) contact angles and grow in a constant contact
ateinthespaceleftopenbehindthesmallerdroplets.This angle mode (Figure 4c,d). The intrinsic contact angle,
initialstagecanbeseeninthedropletdistributionat8and θ ,isroughly108(cid:2)forafluoropolymeronasmooth
young
11minpasttheonsetofcondensation.Inthenextstage, surface,32meaningthatinordertohaveaCassie(cid:1)Baxter
visiblerespectivelyat9and12min,thetallpeakofsmaller contact angle of over 150(cid:2) the area underneath the
dropletsinthedistributionbeginstoflattenoutandshift dropletbasethatcontactssolidmustbelessthan20%
totheright(i.e.,towardfewbutlargerdroplets),asthe ofthetotaldropletbasearea.Thiscanbedeterminedby
smallerdropletscoalescewiththeirnearestneighbors.In solvingtheCassie(cid:1)Baxterequation,33cosθ =f(cid:3)cos
CB
thissecondstageonly2(cid:1)3dropletscoalesceatoncein θ þ(f(cid:1)1),wherefisthefractionalsolidcontactarea
young
the traditional direct-contact induced manner, as indi- underneath the droplet.This observationhighlights the
cated by the fact that the overall droplet count still importance of the secondary roughness caused by the
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Figure 3. (a) Droplet size distributions in the 0(cid:1)60 μm
rangeat8,9,and10minpasttheonsetofcondensation
ontheamphiphilicnanostructuredsurface.(b)Sizedistri-
bution11,12,and13minpastcondensationonset.(c)Time
historyofmeandropletdiameterontheamphiphilicsur-
face;timeregionsfromthe histograms in(a)and(b) are
labeled to highlight periodic behavior. Inset in top right Figure4. bW ESEMimagesofcondensationwettingstageson
includesmeandropletdiameterdataonthesmoothhydro-
amphiphilicsurface.(a)Dryamphiphilicsurfacepriortosub-
phobicsurfacefordirectcomparison.
cooling. (b) Subcooling the surface leads to nucleation at
hydrophilicbaseofnanowiresandformationofwettedsub-
difference inwire heightsbecause,withawire packing layer.(c)Formationand(d)constantcontactanglegrowthof
Cassiedropletsonhydrophobicnanowiretips.(e)Coalescence
fractionofapproximately85%,theinitialCassiedroplets ofadjacentCassiedropletsthroughdirectcontact.(e)Result-
mustonlycontactafractionofthe(tallest)wiresbeneathit ingcoalesceddropisinWenzelstate,connectedtowetted
sublayer,asindicatedbyconstantbaseareagrowth.Seeweb
toachievea20%areafraction.Further,roughnessfeatures
enhancedobjectS4foravideooftheentireprocess.
musttrulybenanosizedtosupportdropletsontheorder
of 10 μm because a high number of wires at different at least one coalescence, droplets transition to a
heightsmustbepresentbeneaththissmalldropletforthe Wenzelstateandarethusfluidicallylinkedviathewetted
above-describedbehaviortooccur. sublayerbetweenthehydrophilicportionsofthenano-
Eventually, adjacent Cassie droplets coalesce by wires.Intherejuvenationregime,thereisanassortment
growingintooneanother,andindoingsotransition ofWenzelstatedropletsofvaryingdiameterwithinclose
totheWenzelstatewhereintheyareconnectedtothe proximityofoneanother.Thermodynamically,droplets
underlyingfluidlayer.Thistransitioncanbeseenbya onasurfacewillalwaysprefertocoalesceintoonelarger
post-coalescence reduction in contact angle and by droplettominimizethetotalsurfacefreeenergy;how-
thefactthatacoalesceddropletgrowsinaconstant ever,onatypicalsurface,thereisnopathtothislower
base area mode with a high degree of contact line energystate.Incontrast,ontheamphiphilicsurface,the
pinning(Figure4e,f). Laplace pressure difference between adjacent droplets
TheinsightgainedfromESEMvisualizationofthe with different radii of curvature supplies the driving
wetted sublayer development and initial droplet dy- force for coalescence and the fluidic linkage provides
namicsoffersamorecompleteunderstandingofthe the pathway. A representative coalescence sequence
phenomenon occurring in the periodic rejuvenation withanillustratedschematicishighlightedinFigure5.
regimedepictedinFigures2and3.Afterundergoing It is clear that the droplets do coalesce into one,
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Figure5. (a)Pre-andpost-coalescenceopticalimagesofaLaplacepressure-drivenmultidropletlong-rangecoalescence
event.EncircleddropletsareintheCassiestateandthereforedonotinteractwiththesurroundingcoalescingdroplets.(b)
Sketchedschematicofthecoalescenceprocess.
rather than moving radially outward from the field of condensed droplets to remain stable in the Cassie
view,becausearegionofdropletsfullysurroundingeach state such that they can roll off the surface. High-
coalescencezoneremainsunaffectedduringtheevent. speed visualization could provide further insight into
Furthermore,thefactthattwosmalldroplets,presumably thisnewphenomenon,potentiallycapturingtherapid
intheCassiestateduetotheirsmallsize,areunaffected transient interaction between droplets and the sub-
bythecoalescenceofthesurroundingdropletsprovides strate.Thistypeofamphiphilicsurfacehasfabrication
strongevidencesupportingtheproposedmechanismof advantageous in that the roughness is generated in a
nondirect coalescence through the wetted sublayer. If singlestep,ratherthanrequiringfabricationofmicroscale
thelargerWenzeldropletswereinsteadcoalescedabove roughnessfollowedbysubsequentgrowthofnanostruc-
thesurfacebyacascadeofadjacentcontactingdroplets, turesasutilizedinotherstudies.15(cid:1)17
theseCassiedropletswouldundoubtedlybesweptupin Theissueofremovingthefewlarge“sticky”Wenzel
theprocess.Itisimportanttonote,however,thatboth dropletsremainsandisimportantforpracticalapplica-
contact-basedandLaplacepressure-drivencoalescences tionsofdesigningheattransferequipment.However,
canoccursimultaneously,asindicatedinFigure5,bythe thefactthatthewettedsublayerprovidesapathfor
othersmallCassiedropletswhicharewithinthefootprint dropletcoalescencefromlongerrangemeansthatthe
ofthefinalcoalesceddropandthereforedoparticipatein largestdropletscanmorerapidlycollectvolumeand
thecoalescence. growtoasizewhereremovalbecomespractical.For
example, on a vertically oriented surface, eventually,
CONCLUSION gravitationalforceswillbecomesignificantrelativeto
In summary, we have reported a novel nanostruc- surfaceforcesatwhichpointthelargestdropletscan
tured amphiphilic surface with a vertical surface en- slide off the surface. Alternatively, other techniques
ergy gradient which enables long-range collective such as vibration-induced dewetting34 may be em-
coalescence events resulting in periodic generation ployed to remove the largest droplets at a greater
of fresh nucleation sites for small droplets during frequency.Regardlessofremovalmechanism,during
dropwise condensation. ESEM visualization indicates thetimeinwhichthelargestdropletsgrowtoasize
that this regenerative behavior is achieved by practicalforremoval,aheattransferenhancementis
Laplacepressure-drivencoalescenceofWenzeldroplets expected in the area of the surface immediately
through the wetted sublayer, rather than requiring surrounding these droplets as moderately sized
ANDERSONETAL. VOL.6 ’ NO.4 ’ 3262–3268 ’ 2012 3266
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Wenzel droplets are drawn in by the Laplace pres- condensationtothatoftraditionalsuperhydropho- R
sure imbalance and fresh nucleation sites are cre- bicsurfaces,thusopeningnewavenuesforabroad
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ated in their wake. With further exploration and rangeofcriticalmoistureandthermalmanagement
optimization, this type of amphiphilic surface applications in emerging thermally limited nano- I
C
could provide an alternative mode of dropwise technologies.
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Conflict of Interest: The authors declare no competing
17. Chen,X.;Wu,J.;Ma,R.;Hua,M.;Koratkar,N.;Yao,S.;Wang,
financialinterest.
Z.NanograssedMicropyramidalArchitecturesforContin-
uousDropwiseCondensation.Adv.Funct.Mater.2011,21,
Acknowledgment. AFOSRBIONICCenter(AwardNo.FA9550- 4617–4623.
09-1-0162)providedfinancialsupportforthiswork.Theauthors 18. Varanasi, K. K.; Hsu, M.; Bhate, N.; Yang, W. S.; Deng, T.
thankA.WaiteatAirForceResearchLaboratoryforassistancewith SpatialControlintheHeterogeneousNucleationofWater.
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