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Young’srule717●YYYeager,ErnestBill(Sep.,,Orange,NJ,USA–Mar.,,Cleve-land,OH,USA)Americanelectrochemist;BAMont-clairStateUniversity;MSWesternReserveUniver-sity,Cleveland;PhDinphysicalchemistryWesternReserveUniversity,Cleveland.AtWesternReserveUniversity,associateprofessor(–)thenprofes-sor(–)and,since,FrankHovorkaProfes-sor;founderofCaseCenterofElectrochemicalSciences;retired;morethanpublications;editororco-editorofbooks;researchonelectrodekinetics,spectroelectrochemistry,electrocatalysis,ultrasound.Ref.:()TheElectrochemicalSociety,Interface():RHYoung’sequation—Adropletofaliquidlayingonaplanesolidandsurroundedbythevaporphase(cf.Fig.),willexhibitacontactangleΘthatfollowsfromtheequationγS–V=γS–L−γL–VcosΘ,whereγdenotestheinterfacialtensionbetweenthedifferentphases(S–V:solid–vapor,S–L:solid–liquid,L–V:liquid–vapor)(Young’srule).Thisequationisbasedontheassumptionofequilibriumbetweenallthreephases.Th.Young(June,,Milverton(Somer-set)UK–May,,London,UK;Britishphysician,physicist,andEgyptologist)hadstatedthisrelationinverbalformavoidingmathematicalformulaein[i].Duprédetermineditinalgebraicformin[ii].TheequationisreferredtoasYoung’sequation.Incaseofanoildropletsuspendedonthesurfaceofwater(Fig.)theequationhastheform:γW–VcosΘ=γO–VcosΘ+γW–OcosΘ,Young’sequation—Figure.Adropletofaliquidislayingontheflatsurfaceofasolidandsurroundedbythevaporphasewhereγdenotestheinterfacialtensionbetweenthedif-ferentphases(W–V:water–vapor,O–V:oil–vapor,W–O:water–oil).Thisequationisbasicfordeterminationofinterfa-cialtensionandforexplainingphenomenaofwettingandadhesion[iii–v]includingelectrochemicalexperi-ments[vi].Fortheworkofadhesionbetweentwoim-miscibleliquidsseeDupréequation.Refs.:[i]YoungT()PhilosTrans;[ii]DupréA()TheorieMecaniquedelaChaleur.Gauthier-Villars,Paris;[iii]AdamsonAW,GastAP()Physicalchemistryofsurfaces.Wiley,NewYork,pp;[iv]HunterRJ()Foundationsofcolloidscience,ndedn.OxfordUniversityPress,Oxford,pp;[v]PayneR()Thestudyofionicdoublelayerandadsorptionphenomena.In:YeagerE,SalkindAJ(eds)Techniquesofelectrochemistry,vol..Wiley-Interscience,NewYork,pp;[vi]IvoševićN,ŽutićV()Langmuir:FSYoung’srule—Aliquiddropletformedonaflat,solidforeignsubstrate(Fig.)hasavolumeV(R,θ),V(R,θ)=()πR(−cosθ+cosθ)()whereasthetotalsurfacefreeenergy¯Φ(R,θ)ofthethree-phaseelectrochemicalsystemsolution()–droplet()–foreignsubstrate()isgivenby¯Φ(R,θ)=πR(−cosθ)γ+πRsinθ(γ−γ).()InEqs.andRistheradiusofthehomogeneouslyformedsphere,θisthewettingangleandγ,γ,andγarethespecificfreesurfaceenergiesattheso-lution/substrate,solution/droplet,andsubstrate/dropletinterfaceboundaries,respectively.Theconditionforanequilibriumform(equilib-riumformofcrystalsanddroplets)inthissystemisob-tainedfromtherequirementforaminimumof¯Φ(R,θ)ataconstantvolumeV(R,θ),(d¯Φ=,dV=),andaf-terdifferentiationofEqs.andoneobtainsthesimplerelationγ−γ+γcosθ=()knownasYoung’srule(Young’sequation).718YSZ(yttriastabilizedzirconia)●YYoung’sequation—Figure.AnoildropletsuspendedonawatersurfaceYoung’srule—Figure.CrosssectionofaliquiddropletformedonaflatforeignsubstrateEquationshowsthatthewettingangleθofacap-shapedliquiddropletonaflatforeignsubstrateisexactlydeterminedbythevaluesofthespecificfreesurfaceen-ergiesγ,γ,andγ.Iftheforeignsubstrateplaystheroleofaworkingelectrode,thespecificfreesurfaceener-giesarepotential-dependentquantitiesandthewettingangleθisafunctionoftheelectrodepotential,too.Ref.:[i]YoungT()PhilosTransRSoc:AMYSZ(yttriastabilizedzirconia)stabilizedzirconia