LubricantsinConfinedConditions

Understandingandcontrollingthefundamentalmechanismunderlyinglubricationisofgreatinterest foroilandgascompanies.Confininghydrocarbonsunderhighshearandpressureconditions gives rise to a number of experimental1 and theoretical2 phenomena that areboth complex and intriguing. This case study is based on the assumption that atomisticsimulationsbasedonmoleculardynamicscanachieveadeeperinsightintosuchphenomenaand enable fundamental understanding of the physical behaviour of lubricants, helped byincreasingcomputationalpowerandnoveldata-andIT-basedmethods.

Thecomplexityofreallubricantsisdrivenbythemultiplerolestheoilmustplaywithintheengine e.g., protecting metal surfaces, preventing wear, reducing friction, cooling theengine,anddispersingsootandothercontaminants.Theserolesmustbecarriedoutunderdifferent temperatureandpressureconditionsandawide rangeofadditivechemistries isrequiredinordertofulfilthem.Bridgingthegapbetweentheidealisedsystemapproachableby modelling and the real behaviour of complex mixtures in a range of environmentalconditions remains therefore a challenge. The present case study is also based on thefurtherassumptionthatitispreciselyinsuchasituationthatinformationefficiencybasedonsystematicdataarchivingandcollectivedatare-usagebypurpose-developedanalyticstoolsismostwarranted.TrainingaprivatesectorR&Doperatortoopeninghermethodologytoadata-basedapproachisalsopartoftheplan,totestourassumptionsinrealconditions.

TheKCLgroup, incollaborationwithBP, isworkingonproducingthedataanddevelopingpost-processing analytic tools for tribological applications connected with their UK-basedindustrially relevant lubricant research (and more generally, relevant for any industrially-hostedmodellingofhydrocarbon-basedfluids).Inparticular,MsRachelFort,aBPemployeeattheirTechnologyCentre inPangbourne,UK,willbedevoting50%ofhertimeunderthejoined supervision of Prof. Alessandro De Vita (NOMAD PI) and Dr Sorin Filip (an ExpertResearchTechnologistforBPFormulatedProductsTechnology,workinginBPPangbourne).Support to fund Ms Fort’s work was independently obtained as an Industrial FellowshipawardedbytheRoyalCommissionfortheExhibitionof1851.

MsFortisworkingwithDrMartinaStella(basedatKCL),investigatingthephysicalbehaviourofhydrocarbon-based lubricants.Theirworkaimsatpredictingthe lubricantviscositywithparticularinterestforextremeconditions(i.e.highpressure,hightemperature).ThisimpliesdatagenerationusingtheMDprogramLAMMPS,whoseNOMADparserhasbeendevelopedinKCLbyDrAdamFekete,andwhereaLAMMPSexpert,DrChrisLorenz,willbeinvolvedinplanningproductioncalculations.Atthetimeofwriting,theviscositypropertiesofsqualane(2,6,10,15,19,23-Hexamethyltetracosane) are being investigated in bulk and confinedconditions.Various simulationsconditionswillbeexplored,namely including roughnessof

the slabs and presence of modifiers on the surfaces. Figure 1 below shows an examplerepresentationofthetypeofsystemsstudied.

LongMDsimulations,inthethousandatomsizescaleorabove,areverydata-intense.Post-processing is also quite demanding, and data-analysis carried out by industrial R&D usersworkinginisolationcanbechallenging.TheNOMADKCLgroupwilldevelopsupportingtoolsfor the analysis of this data.Namely,DrAdam Fekete andDrMartina Stellawill teamupwithMsFort andmake themselves available to respond to anydata-analysis request.Weexpectsoftwaretoolswillbenecessaryforfacilitatingandspeedinguptheextrapolationoftrends and statistical properties (e.g. macroscopic friction coefficients, shearing velocityprofiles) particularly for classical MD simulations. Predicting the viscosity of mixtures(complex lubricant compositions) on the basis of the results of MD calculation is alsoexpectedtorequiremachinelearningbasedtoolsthat industryR&DuserssuchasMsFortwouldbeunlikelytodevelopontheirown,butcouldeasilyuseafterappropriatetraining.The data produced by this study will be uploaded on the NOMAD repository, and anyresultinganalytictoolwillbealsomadeavailableontheNOMADwebsite.

Fig1.Examplesystemforatomistictribologysimulationofconfinedlubricants.ThesearetwoslabsofFeO2,stericacidassurfacemodifiersanddecaneaslubricant.Both2D(left)andprospective(right)representationsare

shown.

1)R.G.HornandJ.N.Israelachvili,J.Chem.Phys.75,1400(1981).

2)J.P.Gao,W.D.Luedtke,andU.Landman,Phys.Rev.Lett.79,705(1997).

Pz = 0.5 GPa

+U /2

-U /2

z

x

y

decane

stearic acid

iron oxide

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Tribology applications