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Vo��tchovsky, K. (2016) 'E�ect of temperature on the viscoelastic properties of nano-con�ned liquid mixtures.',Nanoscale., 8 (40). pp. 17472-17482.

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Nanoscale, 2016, DOI: 10.1039/C6NR05879E.

Nanoscale

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Received00thJanuary20xx,Accepted00thJanuary20xx

DOI:10.1039/x0xx00000x

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Effectoftemperatureontheviscoelasticpropertiesofnano-confinedliquidmixturesKislonVoïtchovskya

The behaviour of fluids confined in nanoscale gaps plays a central role in molecular science and nanofluidics, with applications ranging from biological function to multiscale printing, osmosis and filtration, lab-on-chip technology and friction reduction. Here atomic force microscopy is used to shear five different mixtures of hexadecane and squalane confined between the tip apex and atomically flat graphite. The shearing amplitudes are typically < 2 nm, hence reflecting highly localised information at the interface. The evolution of each mixture’s viscoelastic properties is studied as a function of temperature, between 20 °C and 100 °C. The results, complemented by sub-nanometre resolution images of the interface, show that spatial organisation of the liquid molecules at the surface of graphite largely dominates the measurements. Squalane presents a higher affinity for the surface and forms a robust self-assembled layer in all mixtures. This results in a step-like change of the viscous and elastic response of the confined liquid as the confining pressure increases. In contrast, measurements in pure hexadecane show a continuous and linear increase in the apparent viscosity with pressure at all temperatures. This is interpreted as a more fragile interfacial layer and images show that it can be completely removed at high temperatures. Depending on the mixture composition, measurements can be strongly location-dependent which suggests molecular clustering and nanoscale phase separation at the interface.

IntroductionNano-confined liquids play an important role in countlesssystems, ranging from transport in protein channels andpores1,2 to nanofluidics-based technologies3,4 such as 3Dprinting5,6waterpurification7,singlemoleculedetectors8,9,lab-on-chip technology10 and the control of dynamical contactsbetween macroscopic solids in motion11-14. In most real-lifesystems, the confined liquid is not pure, but composed of amixture of molecules15. The liquid is typically sheared withrespect to a solid wall, either by force flow in nano-channels2,8,10,16orbyconfinementbetweenmovingsolids11,17.Thebehaviouroftheshearedliquidhasalargeimpactontheefficiency of the process considered. This behaviour cangenerally not be inferred from measurements on the bulkliquid. Given the small scale of a nano-confined system, thespecificmolecular details, the geometry and structure of theconfining solids, the presence of impurities as well as themeasurement itself can all considerably influence theresults12,18-20.Several studies have reported confinement-induced phasetransitions21,22 and increases of several orders ofmagnitudesin the ‘effective’ viscosity of the liquids under shear23-27.Despiteongoingdebates regarding thebehaviourofconfined

water18,24-26,28, it is generally accepted that nano-confinedliquids tend to exhibit structural and viscoelastic propertiesdramatically different from those of their bulk counterpart29.The interface between the liquid and the confining surfacesoften dominate the nano-confined liquid’s behaviour11, inparticular for systemswhere the liquid exhibits some affinityfor the surface27. At the macroscopic scale, when a liquidmeetsasolid,theresultinginterfacecanbequantifiedthroughso-calledcontact-anglemeasurements30,31 thatdeterminethewetting properties of the liquid – that is, its affinity for thesurface. At the molecular level, a wetting liquid indicatesstrong interactions between the liquids molecules and thesolid32,33.Theseinteractionsaswellasreducedconfigurationalentropyresult inthe interfacial liquidmoleculesbeingusuallymore ordered and less mobile than in bulk liquid34,35. A‘wetting’ interfacial liquid often exhibit molecular layeringparalleltothesolid’ssurface,butthisstructuredecaysrapidlyinto the bulk liquid, usually over a few moleculardiameters19,36. Inthecaseofnano-confinedliquids,twosolid-liquid interfaces are brought in close proximity and thethickness of the resulting liquid film becomes comparable tothatoftheinterfaces.Asaresult,bothconfinedinterfacesmaysignificantly influence the behaviour of the confined liquidmoleculeswhich tend tobecomegloballymoreordered thanbulkliquid,oftenforminglayers27,37-43.Intuitively,thisiscanbeunderstood by combining the properties of both interfaceswith a further loss of entropy for the liquid molecules. Thisconfers the confined properties close to that of a solid or aglass, reflected by an important increase in the apparent

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viscosity and elasticity24,26,27,39,44. Although intuitive, thisunderstandingisfarfromabsolutesincemoleculardetailssuchasorientation, structuralarrangementorability to slipat thesurfaceofthesolidcanplayasignificantrole12,20,45.Computersimulationscanprovideinvaluableinsightsintosuchdetails,inparticularfordynamicsystems29,46.Experimentally, nano-confined liquids are typically studiedwiththesurfaceforceapparatus11,21,22,28,39,43,47-52(SFA)orwithatomic forcemicroscopy (AFM)25-27,40,42,53-66. The SFA offers aclose to perfect confinement geometry between atomicallyflat surfaces, an absolute measurement of the distancesbetweenthesurfacesdowntosub-angstromresolutionanditcan apply shear forces to the system. In contrast, the AFMconfines liquids between a poorly controlled nanoscale tip(typically 5-10nm radius) anda flat surface, but it allows forconsiderablyhigherappliedpressures,imagingoftheinterfacewith sub-nanometre spatial resolution32,33,58,67, and a highflexibilitywiththechoiceofconfiningmaterials.Here AFM is used to compare five different mixtures ofhexadecaneandsqualaneshearedbetweenasiliconoxidetipandanatomicallyflathighlyorderedpyrolyticgraphite(HOPG)surfaceasafunctionoftemperatureandconfiningforce.Thesetupusedfortheexperimentisinspiredbythatdevelopedinthe lab of E. Riedo26,68, and allows formeasurement of boththe viscous and elastic response of the confined liquid as afunction of the tip-sample distance. Significantly, the shearamplitude is relatively small (<2nm)whichallows forahighspatialresolutionoftheshearmeasurement.Thisisimportantin liquidmixturesoronunevensurfaceswheredifferences inliquid behaviour can occur between different locations. Theresultsconfirmthattheorganisationoftheliquidmoleculesatthe surfaceof graphite largelydominates themeasurements.Whenpresentintheliquid,squalanemoleculesformarobustinterfacialmonolayerwith themoleculesorienteduprightonthe surface. The layer, which remains intact even at 100 °C,inducesastep-likeincreaseoftheviscousandelasticresponseof the confined liquid as the confining pressure increases. Incontrast, measurements in pure hexadecane show acontinuous and linear increase in the apparent viscositywithpressure at all temperatures, with little elastic response.Depending on the liquid mixture, measurements can bestrongly location-dependent which suggests molecularclustering and nanoscale phase separation at the interfacewithHOPG.The results illustrate the importance ofmolecular details fordetermining the viscoelastic properties of confined liquids.This is particularly true for liquid mixtures where a singlecomponentcancompletelydominatetheobservedbehaviour,ornanoscalephaseseparationcaninducestronglocaleffects.

ExperimentalSamplepreparationandexperimentalconditions

Hexadecane (Hd) and squalane (Sq) were purchased fromSigma-Aldrich (Dorset, UK). The stated purity of both liquidswas >99% and they were used without further purification.

The fusionand vaporisation temperaturesof thepure liquids(Tf andTv respectively)areTf =18 °CandTv =287 °C forHd,and are Tf = -38 °C and Tv = 176 °C for Sq. Five differentsolutionswerepreparedfrombinaryHd:Sqmixtures:1:0(pureHd),3:1,1:1,1:3,and0:1(pureSq).Allthemixturesappearedcompletelysolubleinbulkatalltemperatures.Measurementsin the pure liquids served as reference. Each mixture wassonicated at 30 °C until optically homogenous and kept atroomtemperature.The substrate, highly ordered pyrolytic graphite (HOPG) (SPIsupplies, West Chester, PA) was glued to a steal disc withepoxy resin (Araldite, Basel, Switzerland), ensuring directcontactbetweenthegraphiteandthemetal.Thisisnecessarytoachievegoodthermalconductionthroughoutthesubstrateand favour accurate temperature settings during themeasurement.TheHOPGwasfreshlycleavedbeforeeachsetofmeasurementusingadhesivetapeandimmediatelycoveredwiththesolutionstudied.Foreachofthe5liquidsprobed(Hd,Hd3:1Sq,Hd1:1Sq,Hd1:3Sq,andSq),thestudywasconductedat20°C,30°C,40°C,50°C,60°C,70°C,80°C,90°C,100°C(always±0.1°C),andrepeatedatleast3timeswithanewtip.Temperature control. In order to allow precise andreproducible temperature-controlled measurement, aheating/coolingsystemwithprecisionbetter than±0.1°Cwasused. The system is integrated in the AFM and accessiblethrough the AFM control software. The temperature ismeasured in the metallic sample support. The use of aconductingHOPGsubstrategluedtoametaldiscenablesgoodthermalconductivitythroughthesample.Afteranychange intemperature,thesystemwasallowedtosettlesoastoensureminimal drift and thermal equilibrium before measurementswere conducted. The heating/cooling rates imposed by thetemperatureregulationloopweremonitoredagainsttime,anditwasestimated that thermal equilibriumwas reachedwhenthe rates became constantwithin ±1%. This strategy allowedsub-nanometreresolutionimagingatalltemperatures.

AFMmeasurements

All the measurements were carried out on a commercialCypher ES AFM system (Oxford Instruments, Santa Barbara,CA) equipped with temperature control. Two types ofmeasurements were conducted complementarily: shearingand imaging. All measurements were conducted withArrowUHF-AuD cantilevers (Nanoworld, Neuchâtel,Switzerland, nominal vertical spring constant of𝑘! ~ 2N/m).Each cantilever was calibrated (𝑘!) using its thermalspectrum69. Torsional calibration of the cantilever’s inverseoptical lever sensitivity and spring constant 𝑘! are moredemanding.Giventhetriangularshapeofthecantilever,thereis currently no simple calibration method available. Thecalibrationwasthereforeconductedonlyonafewcantileverssoastodetermineanorderofmagnitude.Theprocedureusedwasadaptedfromestablishedtechniques70,71andisdescribedindetail insupplementaryinformation(seeFig.S1). Ityieldeda value of ~200 N/m for 𝑘!. This is however an estimateresting on important simplifications (see Supplementary

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Methods) and the results hence display only the shearamplitude,notforce.Themeasurementswereconductedwithboth the cantilever and the HOPG substrate fully immersedinto the liquid (Fig. 1a). Each set of measurement wasrepeatedat three timeswithadifferent cantilever/tipand innoparticularordersoastoensurestatistical reliabilityof theresults.

Fig.1CartoonrepresentationoftheAFMtipandcantileveratthesurfaceofHOPGinliquid(a).Whenusedforimaging(b),thecantileverisexternallydrivenatitsresonancefrequency(bluearrow)resultinginanetverticaloscillationofthetip(redarrow).Thetip oscillation is damped as the tip interacts with the interfacial liquid near thesubstrate.Inshearexperiment(c),thesubstrateisoscillatedlaterally(bluearrow).Asthe tip approaches the interface, liquid-mediated tip-substrate coupling can inducetorsionofthecantilever(redarrow).

High-resolutionimaging.Whenusedforimaging,theAFMwasoperated in amplitude modulation (AM-AFM). In AM thecantilever is oscillated vertically at a frequency close to itsnatural resonance.Adetection system records theoscillationamplitude 𝐴 and phase 𝜑 of tip in the liquid. Given therelatively high viscosity and poor thermal conductivity of thefluid, the cantilever oscillation was driven acoustically. Awayfromthe substrate, the tiposcillates in thebulk liquidwithaconstantamplitude𝐴! (freeamplitude).Thephase𝜑! =±90°duetoviscousdampingofthecantilevermotionintheliquid.As the tip is brought closer to the sample, the oscillationamplitude decreases due to tip-interface interactions so that𝐴 < 𝐴! (Fig. 1b). Imaging is achieved by raster scanning thesurface while maintaining the amplitude to a set value 𝐴!(setpoint amplitude) using a feedback loop that constantlyreadjust the average tip-sample distance. The phase lag 𝜑!betweenthedrivingsignalandtiposcillationisallowedtovaryfreely and changes depending on the local tip-sampleinteractions. Topographic images are obtained from thevertical corrections applied to average tip-sample distancewhen keeping the setpoint amplitude constant. In this study,the typical setpoint amplitudes were 𝐴! < 1 nm andsetpoint/free amplitude ratio were kept relatively high(𝐴!/𝐴! ~ 0.7-0.9)soastoensureimagingconditionspreservingthesample.Undertheseconditions,the imagesaremainlyofthe liquid structured at the interface rather than the solidsubstrate underneath32,33,67,72. The phase, recorded for eachposition scanned, carries information about local changes inthe viscoelastic properties of the interfacial liquid layersimaged and is hence sensitive to variations in the structuralarrangement of the molecules images, often with sub-nanometreresolution32,67,73-75.Shear-force spectroscopy. The shearing measurements wereconducted using custom-made adaptations of the AFMsoftware that re-assigned a lock-in amplifier to the torsional

signal of the cantilever. During a typical measurement, theimmersed HOPG substrate was oscillated laterally at afrequencyof1.1kHzandanamplitudeofabout2nmusingtheAFM scanner (Fig. 1c). This frequencywas below the naturalresonance of the scanner (around 2 kHz). Standard force-distance curves were then acquired while simultaneouslyrecordingthetipverticaldeflectionand lateraltorsionduetoliquid-mediatedcouplingwiththesubstrate.Theamplitude𝐴!andphaseφtofthetorsionsignalwereextractedusingoneofthe lock-in amplifiers of the AFM. The magnitude of theshearing force 𝐹! is directly proportional to 𝐴!. For smalltorsionamplitudes(as ishere)wecanwrite𝐹! = 𝑘!𝐴!with𝑘!the torsional spring constantof the cantilever. Thephase lag𝜑!betweenthelateraloscillationofthesampleandthatofthecantilever torsion caries information about the nature of thetip-substratecouplingthroughthesheared liquid: if𝜑! = 90°,thecouplingispurelyviscouswhilefor𝜑! = 0°thecantileverisperfectly coupled to the substrate and the sheared liquidbehavespurelyelastically(Fig.1c).Shearingmeasurementsare‘blind’ in the sense that they are done over a particularlocation of the sample without knowledge of the localtopography. Complementary high-resolution imagingconductedatthelocationimmediatelybeforeoraftershearingmeasurements allows this problem to be overcome. It alsoensures that a suitable location is selected for themeasurement, for example away from surface defects andsingularities which might artificially affect the results. Morethan 100 shear curves were acquired over five randomlyselectedlocationsforeachliquid,ateachtemperatureprobed.Theresultswereanalysedusing routinesprogrammed in IgorPro(Wavemetrics,LakeOswego,OR).Thecurveswerealignedandaveragedforpure liquids. Inmixtures, itwascommontoobserve different behaviour depending on the measuringlocation.Insuchcase,thecurvesweresortedinsub-groupsofsimilarbehaviour.It is worth noting that the experimental approach used hereallows for rapid switching between imaging and shearingmeasurements without requiring making any change oradjustmenttothesystem,thereforeensuringthatbothmodesprobethesameregionofthesample.

ResultsanddiscussionResults are presented sequentially for each liquid, startingfrom the pure liquids which serve as references forinterpretingresultsinthemixtures.Puresqualane

AnexampleofasingleshearcurveacquiredinSqispresentedin Fig. 2. The vertical deflection of the cantilever is recordedsimultaneously to the shear amplitude and phase, andprovidesadirectmeasurementof theconfining forceappliedby the tip. The shear amplitude, initially close to zero,increases in a step-like fashion as the tip approaches thesurface.This indicates that theconfined liquid isorganised incohesivelayersattheinterfacewithHOPG.Thesestepsdonot

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follow a progressive behaviour, suggesting that the layeringexists even in the absence of the confining tip. Past theseinitial steps, the shear amplitude tends to increase moreregularlywiththeappliedforce.Theshearphaseisoftenmoresensitivethantheamplitude,asvisibleinFig.2.Theshearing,initially only viscous in bulk liquid (𝜑! = 90°) becomesviscoelastic when the liquid is confined. The phase is notdefined if the shear amplitude is zero, far away from thesurface.This isvisiblehere is the large fluctuationsvisible for𝑧 > 10 nm. Interestingly, after an initial step increase, thephase remains constant with increasing confining force,indicatingasteadyshearingregime.

Fig2ExamplesetofcurvesacquiredduringasingleshearmeasurementinSqat40°C.Theverticaldeflection(a),theshearamplitude(b)andtheshearphase(c)arerecordedsimultaneously as the tip approaches the substrate. Thehorizontal axis indicates theextension of the base of the cantilever towards the surface. Several events can bedistinguishedasthetipnears thesurface (fromtheright).Around8nm(i),asuddenjump in phase suggests that the tip starts interacting with the surface, but withoutsignificant change in amplitude. Around 6nm (ii), the cantilever starts deflectingvertically.Thisishowever‘soft’withthedeflectionreachingaplateauandchangesinamplitudeandphaseonlystartingfromaround2.5nm.At0nm(iii),ahardcontactisestablished (deflection), defining arbitrarily the zero on the horizontal axis. Thiscoincides with a jump in both phase and amplitude. As the tip-applied pressureincreases(iv),asuddenjumpinallchannelsindicatestheruptureofamolecularlayerconfinedbetweenthetipandthesample.

Fig 3 Representation of the shear amplitude and phase in Sq as a function of theconfiningforcefordifferenttemperatures.ThecurvesinredarethesameaspresentedinFig.2.Themainstepinamplitude(labelledivinFig.2)ishighlightedwithanarrow.Theshearamplitudeisstillsizeableat80°Cwithinallcasesaninitialrapidamplitudeincrease instepsfollowedbyamoreprogressive increasewiththeappliedforce.Thephase also exhibits a characteristic step-decrease at contact (0 nN) followed by aplateau.Thevalueoftheplateauevolveswithtemperaturefromviscoelasticat20°Ctopurely viscous at 80 °C. (A purely elastic responsewould yield𝜑! = 0° and a purelyviscousbehaviour𝜑! = 0°).TheevolutionofamplitudeandphasewithtemperatureismonotonicinSq(seesupplementaryFig.S2).

The data presented in Fig. 2 canbe combined so as to placetheemphasisontheregionwherethetip iscoupledwiththesample.Thisisdonebyplottingtheshearamplitudeandphaseas a function of the cantilever vertical deflection, with thelatterconvertedintheconfiningforce(Fig.3).This presentation of the data has two advantages: first itallows foranestimateof theconfiningpressurebetweenthetip and theHOPG surface. Considering a typical tip radius of10-20 nm, the confinement area varies is about 10-100 nm2,yieldinginincreaseinpressureintheorderof10-100MPapernanoNewton applied. Second, it condenses the entire regionofthecurvewithnoforceappliedtoasinglepoint,thereforeplacing the emphasis on the relevant part of the data. Fig. 3alsoshowsrepresentativeshearamplitudeandphasesat20°Cand 80 °C for comparison. The average behaviour of shearcurves for every temperature probed is available insupplementaryinformation(Fig.S2)InFig.3,acoherentandcontinuousevolutioncanbeseen inbothamplitudeandphaseas the temperature increases. Thedata shows more variability past 60 °C. As the temperatureincreases, the shearing amplitude (and hence shear force)progressivelydecreases.TheevolutionoftheshearphasewithtemperatureindicatesthattheshearedSqlayerchangesfromviscoelasticat20°Ctomostlyviscousbeyond80°C.However,sizeable shear amplitudes are observed at all temperatures,indicatingtheexistenceofastableandcohesiveSqmolecularlayer on the surface of HOPG. This is also consistent withprevious results probingOMCTS/Sqmixtures at the interfacewithHOPGandreportingasubstantiallayeringoftheliquid37.The effect of temperature on the Sq-HOPG interface can bevisualized directly in high-resolution images of the interfaces(Fig.4). Initially,a thick layer iscoveringthesurfaceofHOPG(20°C)withoccasionaldefectfollowingregularlines,indicativeof a crystallinemolecular packing of Sq. As the temperatureincreases, the thick layer leavesbehindadisorderedmeshofgel-like filaments, suggestingasuperficial ‘melting’of thetopSq layer exposed to the bulk liquid. Further temperatureincrease to 40°C makes the superficial Sq layer completelydisappear, revealing an ordered row structure underneath.

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The spacingbetweenadjacent rows is~4.5nm,probablydueto Sqmolecules lying flaton the surface.Occasionalholesorcracks inthestructure(notshown)indicatethattherowsareformed on top of a thicker Sq layer and not directly on theHOPG substrate. At 70 °C the row structure disappears,revealing a hexagonal lattice with ~8.3 nm periodicityunderneath. The presence of the hexagonal lattice suggeststhattheSqmoleculesarenotarrangedparalleltothesurface,but rather perpendicularly in a self-assembledmonolayer. Inthis interpretation, the apparent periodicity results from amismatch between the HOPG lattice and that of thehexagonallypackedSqmonolayer,hencecreatingahexagonalMoiré pattern. The phenomenon has previously beenobserved in similar systems76,77, but usually with the self-assembled molecules lying flat on the HOPG surface. Here,given the periodicity of the HOPG lattice (2.46 Å), theintermolecular spacing of the Sq layer must be ~3.5 Å toexplaintheobservedMoiré.

Fig4AFMimagesofSqattheinterfacewithHOPGatdifferenttemperatures.At20°Ca thick layer (cyan arrow) is present and progressively ‘melts’ as the temperatureincreases (white arrow). At 40 °C, a relatively fragile row structure appears (alignedwiththewhitearrow)underneaththe‘melted’toplayer.Therowspacingisabout4.5nm.At70°C,ahexagonallattice(about8.3nmperiodicity)appearsbeneaththerows.ThelatticecanbeexplainedbyamonolayerofSqself-assembleddirectlyontheHOPGsurface. The hexagonal pattern is a Moiré induced by a mismatch between theperiodicitiesoftheHOPGlattice(2.46Å)andthatoftheself-assembledSqlayer(~3.5Å).Thelayerremainsstableupto90°Cwheredifferenttypesofmolecularassembliescoexist in domains: the cyan arrow point to a lattice region and thewhite arrow toanother lessorderedregion.The imagingphaseover thesamearea (inset)exhibitsaclearcontrast,indicatingthatthetworegionshavedifferentmolecularassemblies.Alltopographic imagesare shown in theblack-purple-yellowcolour scale and thephaseimageisshowninbluescale.Thescalebarsare60nm(20°C),40nm(40°C),40nm(70°C),and40nm(90°C).Thecolourscalesrepresentatotalheightvariationof7nm(20°C),2nm(40°C),0.1nm(70°C),and2nm(90°C),andaphasevariationof4°(90°C).

Thissuggestsstronglayercohesionduetointer-molecularvander Waals interactions, possibly strengthened by lateralinterlockingoftheSqside-chains.Evenpast90°C,thelatticeisstill present with different types of domains that can bedistinguished from local variations in the imaging phase 𝜑!(Fig.4).Throughout all experiments conducted in liquids thatcontained Sq, the tip was never able to reach the HOPGsurface.Thefactthataviscousbehaviourwasobservedat90°CdespitethetipnotreachingtheHOPGsurfaceindicatesthatthe behaviour of the last Sq layer was never probed; theshearing tip was probably sliding over it. This is in starkcontrastwithresultsobtainedinpureHd.Purehexadecane

The shear experiments conducted in pure Hd indicate aconsistent and gradual decrease of the effective viscosity ofthe confined liquid with temperature. Figure 5 presentsamplitude and phase curves as a function of the applied tipforce taken at 20 °C, 40 °C and 80 °C. The complete set ofaverageevolutionforeachtemperatureprobedisavailableinsupplementaryinformation(Fig.S3).Increasesintheconfiningpressure result in an almost linear increase of the shearamplitude,exceptat20°Cwhereaninitialsteparound4nN,isvisible. It can be explained by the tip interacting with themultiplelayersformedbyHdonHOPGatroomtemperature78-80. Themeasured shear amplitudes become vanishingly smallat higher temperatures rendering the phase considerablynoisier.Thephaseshowslittledependenceontemperature,atrendconfirmed forall temperatures (see supplementaryFig.S3). This is characteristic of Hd and reflects a largely viscousbehaviour of the confined liquid, with little evolution as afunctionoftheappliedpressureortemperature.The evolution of the Hd shear curves with increasingtemperature is progressive, with a steady decrease of theshearingamplitudeandnosignificantchangeinthephase(seeFig. S3). Unlike for Sq, the average behaviour of Hd ishomogenous across the whole interface. Occasionally,substantial variations from the average behaviourwere seenfor individual curves at lower temperatures, suggesting somemolecular organisation of the sheared liquid. These eventswerenot commonanddisappeared almost entirely at highertemperatures.

Fig 5 Representative shear amplitude and phase curves acquired in pure Hd at thesurfaceofHOPGat20 °C (green),40 °C (red)and80 °C (blue).Past40 °C, the shearamplitude become so small that the phase is hardly detectable, resulting in noisyreadings.Thephaseishoweverconsistentlycentredaround𝜑! = 90°(exceptat20°C)indicatingapurelyviscousbehaviour(seealsoFig.S3).

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High-resolution images (Fig. 6) indicate a relatively highmobilityof theHdmolecules at the interface consistentwiththeshearobservations.At20°C,Hdmoleculesformaregulararray of epitaxially-induced rows on the surface, oftenspanning several layers on top of each other. The distancebetweenadjacentrowsis~2.4nmbutoccasionallywiderrowstructures are visible near surface singularities (atomic steps,layeredges).TheHdlayeringatroomtemperatureandlateralorganisation in rows when in contact with HOPG is a well-documented phenomenon78,79 further confirmed here. At 50°C,mostoftherowstructurehas‘melted’inthesensethatitis less clear and more disordered with only a single layerpartiallyvisible.At70°Cnomorestructureisvisible;onlyauniformdisorderedinterfacial layer with hints of temporary epitaxial effects.Finally,past90°C,thescanningAFMtipcanfullyremovetheHdmoleculesfromthesurface,revealingtheunderlyingHOPGlattice.Thisoccursdespitetheverygentle imagingconditions(comparedtoshearingexperiments)indicatingahighmobilityoftheinterfacialHdmolecules.

Fig6TheinterfacebetweenHOPGandHdimagedatdifferenttemperatures.At20°Cclearrowsarevisibleonthesurface,buildinguponseverallevels.Thebluearrow indicates thedirectionof~2.4nmspaced rows.Thewhitearrowshowswiderrowsoccasionallyobservedonsmallregionsofthesurface.Theinsetisamagnificationoverapartofthe imagehighlightedbythebluearrow.At50°C,therowsarevisiblemostlyonasinglelevel(directlyontheHOPG)andareeasilydisrupted by the scanning AFM tip. At 70 °C, a homogenous and disorderedmolecular layer is visible, but there are hints of epitaxial effects. At 90°C theunderlying HOPG lattice is occasionally visible as lines with~0.25 nm spacing(alongthewhitearrows) indicatingthatall theHdcanbefullyremovedbythescanningtip.The image ishowevernoisyduetotheHdmoleculesoccasionallyleft under the tip. The inset is amagnificationover a regionof the image andsomeHOPGatomicrowsarehighlighted.ThecolourscaleisasinFig.4.Thescalebarsare40nm(20°C),20nm(20°Cinset),10nm(50°C),10nm(70°C),and2nm(90°Candinset).Thecolourscalesrepresentatotalheightvariationof0.8nm(20°C),0.6nm(50°C),0.1nm(70°C),and0.8nm(90°C).

Squalane:Hexadecane3:1mixture

Resultsacquired inthe liquidmixturespresentamuchhighervariability than in pure liquids, likely due to occasionalnanoscale phase separation at the interface and molecularclustering within the sheared liquid. This is not so surprisinggiven the strikingly different behaviour of Hd and Sq both inbulkandundershear81,butitmakesitdifficulttopresentandmeaningful average picture. This ‘molecular segregation’,confirmed by high-resolution imaging, often allowsinterpretation of the results as combination of Hd and Sqresults.ShearforcecurvesweremorereproducibleinSq3:1Hdthan inothermixtures, inparticularbelow60 °Cwheremostofthevariabilitybetweenprobedlocationsisusuallyobserved(See Fig. S4 and S5). As the confining pressure increases, aninitial sharp step in shear amplitude is followedby a plateaushowing little change. Past 60 °C two main types of curveswere observed depending on the location probed (Fig. S4),eventuallyconvergingtoasingletypeofcurvespast80°C–90°C(Fig.7).Images in acquired in Sq3:1Hd did not reveal any particularstructuresuchas rowsor lattice.Acohesive layer ishoweverpresentatall timesonthesurface.DifferenttypesofdomainsimilartothoseobservedinpureSqarealreadyvisibleat20°Cwithaclearcontrastinthephaseimage(Fig.8).The behaviour of the shear force curves in Sq3:1Hd is morereproduciblethaninothermixturesandclosetothatobservedinpureSqsuggestingthattheHdcanbeseenmainlyasasmallperturbation in the Sq-dominated interface. Interestingly, noorderedstructurecouldbe imagedbutamolecular layerwaspresentforalltemperaturesattheinterface.

Fig 7 Selected curves representing shear amplitude and phase as a function of theappliedtipforceat20°Cand80°CinSq3:1Hd.Theshearamplitudeexhibitsastep-likeincrease at all temperature, but with a markedly lower magnitude at highertemperature. The phase curves also exhibit the step-decrease at contact (0nN),characteristicofmeasurements inpureSq(seeFigs.5andS2).Thetwophasecurvespresent a~90° offset at 0nN, but they tend to converge towards a same value of𝜑! = 90°with increasingforce.Thisbehaviour isqualitativelyreminiscentofpureSq,butwithconvergencetowardsacompletelyviscousshearing.

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Fig8 AFMimagesofHOPG inSq3:1Hdatdifferent temperatures.At20°Casmoothlayerisvisibleintopography(mainframe)withoccasionalholes(upperleft).Differentmolecular organisations coexist in organised domains within the layer as in pure Sq(blueandwhitearrow), confirmed in thephase image (inset). Littleevolution is seenwithtemperature,withmolecularislands(whitearrow)orgrooves(bluearrow)visibleat30°Candsimilarlyat100°C.ThecolourscaleisasinFig.4.Thescalebarsare40nm (20 °C), 10 nm (30 °C), and 10 nm (100 °C). The colour scales represent a totalheightvariationof0.5nm(20°C),0.16nm(30°C),and0.2nm(100°C),andaphasevariationof4°(20°C).

Squalane:Hexadecane1:1mixture

The shear forces curves in Sq1:1Hd show more variabilitybetweendifferentlocationsthanallotherliquidsandmixturesstudied (see also supplementary Fig. S5). At 20°C a typicalamplitude curve shows an initial step followed by a regularincrease in amplitude (Fig. 9) reminiscent of observations inpure Sq (Fig. 4). As the temperature increase, the pressureapplied by the tip affects mostly the shear phase with littlechanges in amplitude, apart for an initial step around 0 nN(seenat80 °C inFig.9).Thestrongdependenceof theshearcurvesonlocationwasobservedineachsetofdata(withnewcantilevers) suggesting a wide range of different molecularorganisationsattheinterfaceratherthanexperimentalerrors.This is confirmed by the fact that the shear curves werereproducible with different cantilevers in pure liquids. Thephaseshowedandinversionofitsevolutionwithappliedforcebetweenlowandhighertemperatures(Fig.9).Thisbehaviourwasalsoreproduced.

Fig9 Selected curves representing shear amplitudeandphaseas a functionof theapplied tip force at 20 °C and 80 °C in Sq1:1Hd. The shear amplitude exhibits anevolutionsimilartoinpureSqat20°Cbutonlyastep-likeincreaseat80°Cwithlimited reproducibility. The phase curves always exhibit the step-change atcontact (0nN),characteristicofmeasurements inpureSq (Figs.3andS2).Theshapeofthephasecurvesat20°Cand80°Careinvertedfromeachother,withno obvious explanation. The inversion occurs progressively with temperature,confirming it results fromtemperature-inducedchangeofmolecularbehaviourattheinterface.Ifweacceptthatthebehaviouroftheshearedliquidat20°Cismostlyelastic,thentheevolutionwithtemperatureisconsistentwiththeliquidbecomingalmostcompletelyviscouspast80°C.

Fig 10 High-resolution AFM images of HOPG in Hd1:1Sq at differenttemperatures.At20°Crowswithvariousspacingandorientationsappearwithoccasional aggregates (white protrusion at the top). Hd-like rows exhibit aspacingof~2.2nm(whitearrow)whilewiderrowspresenta~11.5nmspacing(bluearrow).Therowsarerelativelystableupto40°C.At60°Cmolecularislands(whitearrow)andgrooves(cyanarrows)arevisiblewithhintsofepitaxialeffects(parallel to the arrows). At 80 °C large aggregates are visible (blue arrow)surrounded by stable rows/longitudinal grooves in the molecular layer (whitearrow).ThecolourscaleisasinFig.4.Thescalebarsare100nm(20°C,left),10nm(20°C,right),4nm(60°C),and10nm(80°C).Thecolourscalesrepresentatotalheightvariationof0.8nm(20°C, left),0.15nm(20°C,right),0.1nm(60°C),and0.5nm(80°C).

High-resolution imaging confirmed the broad range ofinterfacialassemblies,especiallyattemperaturesabove50°C.Representative images of the HOPG surface in Sq1:1Hd areshown in figure 10. At 20 °C the surface is relativelyhomogenous despite several aggregates present on thesurface. Several row structures coexist with an inter-rowspacing ranging between about 2.2 nm and 11.5 nm.Comparisonwithresultsinpureliquids(Fig.4and6)suggeststhatbothspeciesareinvolvedintheassembly.Thestructuresare stable up to 40 °C, beyond which they tend to ‘melt’leaving instead loosely epitaxial molecular structures withislands andgrooves.Near surface features suchof theHOPGsuch as atomic steps, the liquid appears to form largeaggregates that can resist temperature beyond 80 °C. Theseaggregates act as anchoring points for the interfacialassemblies with rows and stable linear grooves appearing intheir vicinity. No obvious molecular structure could beidentifiedpast80°C.Inallcasesandupto100°C,atleastonelayer remained on the HOPG surface which could not beimaged.Squalane:Hexadecane1:3mixture

Shear force curves acquired in Sq1:3Hd exhibit a behaviourthat combines features of Hd and Sq in their evolution withtemperature and pressure (Fig. 11). The shear amplitude is

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consistent with the evolution observed for Hd, showing analmostlinearincreasewithtipforce,relativelylowvaluesevenat20°C,andsteadydecreasewithtemperature.Thephase,incontrast,exhibitstheinitialstepat0nNfollowedbyaplateau,consistentwithresultsinSq.Theshearcurvesareremarkablyreproducible between different locations of the sample,almostasinpuresolvents.However,somevariabilitycouldbeseen between different cantilevers. Overall, these resultssuggest thatat the interfacewithHOPG,Sq1:3Hdbehavesaspure Hd in contact with an ordered Sq surface. The goodstability of Sq at the surface of HOPG seems to inducepreferentialSqadsorptionalthoughtheseresultsalonecannotexclude that this cohesive Sq-based layer also includes Hdmolecules. Interestingly, the shear curves are relativelyhomogenous over the surface suggesting a reproduciblemolecular organisation at the interface. The shear phaseexhibitsasubstantialelasticcomponentcomparabletothatofpureSqat20°C,butasthetemperatureincreasestheelasticbehaviour is counter-intuitively reinforced. This could beexplainedbypinningofthetipintheSq-basedlayer,thelatterbeing weakened by the presence of Hd. This explanation ishowever speculative and experiments conducted withdifferenttipsshowedsomevariations,althoughalwayswithasignificantelasticbehaviour.Overall, the shear results are supported by high-resolutionimagingoftheinterface(Fig12).At20°Caclearrowstructureis present in some area of the surface (2.1 nm spacing)suggesting that it is formed by Hd. The row structure ishowever not formed directly on the HOPG surface, butprobably on a cohesive Sq layer, as confirmed by results athigher temperature. The rows are also fragile, unlike in pureHd,furtherindicationthattheyareontopofothermolecularlayer(s) assembled at the interface. Longitudinal islands arestill visible at 30 °Cwith but tend to vanish past 40 °C oftenwith disordered regions suggesting partialmelting of the toplayer. At 50 °C, different phase contrasts are visible acrossdifferentregionsofthesurface(notshown),exactlyasinpureSq at 90°C (Fig. 4), indicating the existence of well-definedmolecular layers on the HOPG surface, and with differentmoleculararrangements.

Fig 11 Representative curves of shear amplitude and corresponding phase as afunctionof theapplied tip forceat20 °Cand80 °C.The shearamplitudeexhibitsanevolutionsimilartothatinpureHd(comparewithFig.5andS3),butthephaseexhibitsastep-increaseatcontact(0nN),characteristicofmeasurementsinpureSq(Figs.4andS2).However, thephasevalues indicatea strongelastic component in theshear,amplifiedat80°C(takingintoaccounta180°offsetcomparedtoFig.3).

Fig 12 Selectedhigh-resolutionAFM imagesofHOPG in Sq1:3Hdat differenttemperatures.At20°Crows(spacing~2.1nm)arevisibleonthesurface(arrow).Therowsarefragileandeasilyremovedbythetipalreadyat30°Cwheretheyare hardly visible (arrow). At 70 °C,molecular islands are seen on the surfacewith hints of epitaxial effects suggesting an underlying hexagonal lattice(directionofthearrows).At80°Candhigher,anunstructuredlayerremainsonthesurface,demonstratedbythepresenceofmolecularpointdefects(arrows).

Athighertemperatures,aggregatesformnearatomicstepsofthe HOPG (not shown) and epitaxial ‘molecular islands’ arevisibleonthesurface.Astable layer isalwayspresentontheHOPGsurface,suggestingittobedominatedbySq. Generaldiscussion

Pure liquids exhibit a continuous evolutionwith temperaturewhen under confinement, but their behaviour completelydepends on their molecular structure. Sq and Hd are bothhydrophobic liquids that are composed of linear chains ofcarbon, and both show a strong affinity for HOPG with Hdachieving completewetting82. Themain difference resides inthesidebranchingofSq,whichcompletelyaltersSqinterfacialbehaviourunderconfinement.Previouswork81hashighlightedimportant differences between the two pure liquids undershear even without confinement. Not only the shape of theinterface,butthemoleculardetailsoftheliquidswereshowntoplayakeyrole intheobservedbehaviour.Here,nanoscaleimaging shows that Sqmolecules create robust layers at thesurfaceofHOPG that can resist temperaturesbeyond100 °Cunderharshshearingconditions(pressuresintheorderof0.1GPa). Studies of flat lying Sq monolayers on graphite haveshownameltingtemperature increasedby>90°CcomparedtobulkSq83.Presentresultsshowthatcontactoftheinterfacewith liquid Sq allows for more molecules to pack upright,hencecreatingstronginter-molecularinteractionspresumablydue interlocking of the Sq side branches. This hindersmolecularmobilityandconfertheadsorbedlayersasolid-likebehaviour. Shearing experiments support this view, with an

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important elastic response of the confined layer thatprogressivelydecreaseswithincreasingtemperaturebutnevercompletely disappears. Comparison with results in Hd3:1Sqsuggests that the cohesive Sq monolayer effectively becomethe confining surface with limited contribution to theviscoelasticbehaviourobserved.Incontrast,Hdmoleculesaremoremobileandtheformationof self-assembled structures at the surface of HOPG can beeasily undone by the scanning AFM tip, even using gentleimagingconditions.Above90°C, itwasnotpossibleto imagethe interface without completely displacing the moleculesbetween the tip and the HOPG, occasionally revealing theatomic details of the HOPG surface. The shearing results inpureHdsuggestalargelyviscousbehaviouroftheliquidatalltemperatures, with the shear force decreasing as thetemperature increases. The AFM results do not necessarilyindicatethattheinteractionbetweenHOPGandHdmoleculesis weaker than between HOPG and Sq molecules. The moreviscous behaviour of Hd comes from the absence of specificinteractionsbetweenadjacentmolecules,thuspreventingtheformation of a robust monolayer at all temperatures. Theapparent strong affinity of the Sq monolayer for the HOPGsurfacecanbeexplainedbyagroupeffectwhereacohesiveSqpatchbindstotheHOPG.Results obtained in liquid mixtures exhibit a rich variety ofbehaviours that can rarely be explained by a composition-weighted combination of the Sq and Hd behaviours. In thebulk,HdandSqappearedfullysolubleatalltheconcentrationsprobed. However, when Sq molecules are present in thesolution, they appear to dominate the behaviour of theconfined liquid and replace Hd molecules at the surface ofHOPG.This ishardlysurprisinggiventherobustnessoftheSqlayersundershear,comparedtoHd.InSq3:1HdandSq1:1Hd,the behaviour of the liquid under shear displays similarcharacteristics to pure Sq with an initial step change inamplitudeandphaseastheconfiningpressure increases.Thefinal shear phase is however different, suggesting that thecohesiveSqlayercanbealteredbythepresenceofHd,atleastpartially.InSq1:3Hd,thebehaviourisclosertothatofpureHd,butastablemolecularlayerattributedtoSqisstillpresentonthe surface at all temperatures. This is not simply a case ofhigheraffinityofSqfortheHOPGsurfacethanHd,butrelatesto more complex intermolecular interactions: despite HOPGbeingatomically flat, theshearingamplitudesandphasesarefound to often depend strongly on the location probed,suggesting the presence of differentmolecular arrangementsat the interface, including possible clustering and nanoscalede-mixing of the solution. This variability is less marked forhigher temperatures where viscous behaviour usuallydominatesbothinSqandHd.Thedynamicsoftheliquidmoleculesinthelayerimmediatelyadjacent to the HOPG substrate depends on both liquidcomposition and temperature. High-resolution imaging andshear measurements indicate that the layer is solid-like atlowertemperaturesbutcan‘melt’athighertemperatures,asexemplifiedinHd.Afrozenlayertypicallyallowsreproducibleimagingofmolecularstructureswhiletheimagesbecomeless

clearuponpartialorcompletemeltingofthelayer.Thisisdueto the diffusion timescale of individual molecules becomingcomparable to that of the AFMmeasurement. However, thefact that shear measurements always detect changes inamplitude and phase upon confinement indicate that theeffectiveviscosityofthislayerishigherthanthatofbulkliquid.It iswellknownthatliquidmoleculesareoftenmoreorderedand less mobile at the interface with solids than in thebulk34,35, inparticular if theyhave someaffinity for the solid.This interfacial effect dramatically influences the liquid’sbehaviour under confinement27. The results of this studyhighlightthefactthatnotonlysolid-liquidbutalsoliquid-liquidmolecularinteractionsdeterminetheviscoelasticbehaviourofthe confined solution. Details of the shape, properties andorganisationof themoleculesat the interfacearealsocrucialinunderstanding theshearingbehaviour.Workingwith liquidmixtures adds considerable complexity to the system,especially if the interface can drive local de-mixing orclustering.Theeffectcanbecomeevenmorepronounced forinterfaces where the confining solids present chemical andtopologicalsingularitiessuchasroughness.Itshouldbenotedthattheexperimentitself,inparticularthenanoscale structure84 and chemistry85,86 of the tip, and theshearing speeds11 are likely to play a role in the shearingobservations but these aspects are not explored here, partlydue to experimental constraints. Given the higher affinity ofthe liquids for theHOPGsurface thanthesiliconoxideof thetip and the high curvature of the latter87, it is reasonable toassume that the measurements are dominated by theinteractionsbetweentheliquidsandtheHOPGsurface.Itwasnot possible to calculate the storage and loss moduli of thesheared liquid since the slip boundary conditions26 are notknown for each liquid nor their evolution with temperature.Theshearamplitudeandphasenonethelessprovidesareliableand quantitative point of comparison between the differentliquidsstudied.

ConclusionsThisstudyexaminedthemolecularorganisationandbehaviourof a dual paraffinic system under nano-confinement and atdifferent temperatures using AFM. Shear experiments andhigh-resolution imaging of mixtures of Sq and Hd at theinterfacewith atomically flatHOPG revealeda rich varietyofbehaviour that strongly depends on the local molecularorganisation of the liquid. Pure Sq is viscoelastic underconfinement at lower temperatures due to inter-molecularbondsthatcreatecohesiveinterfaciallayers.Incontrast,pureHd is largely viscous at all temperatures. Generally, thebehaviour of mixtures is dominated by Sq, but it cannot besimply inferred from that of the pure components due toimportantlocalvariationsinthemoleculararrangementatthenanoscale.Asaresult,measurementsdependonthelocationprobeddespitetheconfiningsurfacesremainingidentical.Thisvariability diminishes at higher temperatures where Sqbecomesmoreviscous.

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Liquidmixturesoftenexhibitsomenanostructureatinterfacesdue to differences in affinity between the surface and itscomponent.Examplesincludedmonolayer,clustersorsponge-like molecular arrangements. This study highlights theimportanceofsuchstructuresindeterminingthebehaviourofnano-confined liquids where molecular details takeprecedenceoverbulkproperties.

AcknowledgementsThisstudywasfundedbyBPFormulatedProductsTechnology,whichisgratefullyacknowledged.CommentsandproofreadingbyMr.WilliamTrewbyisalsoacknowledged.

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